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Title: The Wright Brothers' Engines and Their Design
Author: Leonard S. Hobbs
Release Date: February 2, 2012 [EBook #38739]
Language: English
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SERIAL PUBLICATIONS OF THE SMITHSONIAN INSTITUTION
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S. DILLON RIPLEY
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Smithsonian Institution
The Wright Brothers' Engines
And Their Design
[Illustration: Kitty Hawk Flyer with original Wright engine poised on
launching rail at Kill Devil Hill, near Kitty Hawk, North Carolina, 24
November 1903, the month before the Wrights achieved man's first
powered and controlled flight in a heavier-than-air craft.]
[Illustration: Reproduction of the first engine, built by Pratt &
Whitney, as displayed in Wright Brothers National Memorial at Kitty
Hawk. Engine is mounted in a reproduction of the Wrights' Flyer built
by the National Capital Section of the Institute of the Aeronautical
Sciences (now the American Institute of Aeronautics and Astronautics).
Engine and plane were donated in 1963 to the National Park Service
Cape Hatteras National Seashore.]
SMITHSONIAN ANNALS OF FLIGHT * NUMBER 5
SMITHSONIAN INSTITUTION * NATIONAL AIR AND SPACE MUSEUM
The Wright Brothers' Engines
And Their Design
_Leonard S. Hobbs_
SMITHSONIAN INSTITUTION PRESS
CITY OF WASHINGTON
1971
_Smithsonian Annals of Flight_
Numbers 1-4 constitute volume one of _Smithsonian Annals of Flight_.
Subsequent numbers will bear no volume designation, which has been
dropped. The following earlier numbers of _Smithsonian Annals of
Flight_ are available from the Superintendent of Documents as
indicated below:
1. The First Nonstop Coast-to-Coast Flight and the Historic T-2
Airplane, by Louis S. Casey. 1964. 90 pages, 43 figures, appendix,
bibliography. Out of print.
2. The First Airplane Diesel Engine: Packard Model DR-980 of
1928, by Robert B. Meyer. 1964. 48 pages, 37 figures, appendix,
bibliography. Price 60¢.
3. The Liberty Engine 1918-1942, by Philip S. Dickey. 1968.
110 pages, 20 figures, appendix, bibliography. Price 75¢.
4. Aircraft Propulsion: A Review of the Evolution of Aircraft Piston
Engines, by C. Fayette Taylor. 1971 viii + 134 pages,
72 figures, appendix, bibliography of 601 items. Price $1.75.
For sale by Superintendent of Documents, Government Printing Office
Washington, D.C. 20402--Price 60 cents
Foreword
In this fifth number of _Smithsonian Annals of Flight_ Leonard S.
Hobbs analyzes the original Wright _Kitty Hawk Flyer_ engine from the
point of view of an aeronautical engineer whose long experience in the
development of aircraft engines gives him unique insight into the
problems confronting these remarkable brothers and the ingenious
solutions they achieved. His review of these achievements also
includes their later vertical 4-and 6-cylinder models designed and
produced between 1903 and 1915.
The career of Leonard S. (Luke) Hobbs spans the years that saw the
maturing of the aircraft piston engine and then the transition from
reciprocating power to the gas turbine engine. In 1920 he became a
test engineer in the Power Plant Laboratory of the Army Air Service at
McCook Field in Dayton, Ohio. There, and later as an engineer with the
Stromberg Motor Devices Corporation, he specialized in aircraft engine
carburetors and developed the basic float-type to the stage of utility
where for the first time it provided normal operation during airplane
evolutions, including inverted flight.
Joining Pratt & Whitney Aircraft in 1927 as Research Engineer, Hobbs
advanced to engineering manager in 1935 and in 1939 took over complete
direction of its engineering. He was named vice president for
engineering for all of United Aircraft in 1944, and was elected vice
chairman of United Aircraft in 1956, serving in that capacity until
his retirement in 1958. He remained a member of the board of directors
until 1968. Those years saw the final development of Pratt & Whitney's
extensive line of aircraft piston engines which were utilized by the
United States and foreign air forces in large quantities and were
prominent in the establishment of worldwide air transportation.
In 1963 Hobbs was awarded the Collier Trophy for having directed the
design and development of the J57 turbojet, the country's first such
engine widely used in both military service and air transportation.
He was an early fellow of the Institute of Aeronautical Sciences
(later the American Institute of Aeronautics and Astronautics), served
for many years on the Powerplant Committee of the National Advisory
Committee for Aeronautics, and was the recipient of the Presidential
Certificate of Merit.
FRANK A. TAYLOR, _Acting Director_
_National Air and Space Museum_
_March 1970_
Contents
Foreword v
Acknowledgments ix
The Beginnings 1
The Engine of the First Flight, 1903 9
The Engines With Which They Mastered the Art of Flying 29
The Four-Cylinder Vertical Demonstration Engine and the First
Production Engine 34
The Eight-Cylinder Racing Engine 47
The Six-Cylinder Vertical Engine 49
Minor Design Details and Performance of the Wright Engines 57
Appendix 62
Characteristics of the Wright Flight Engines 62
The Wright Shop Engine 64
Bibliography 69
Index 71
Acknowledgments
As is probably usual with most notes such as this, however short,
before completion the author becomes indebted to so many people that
it is not practical to record all the acknowledgments that should be
made. This I regret extremely, for I am most appreciative of the
assistance of the many who responded to my every request. The mere
mention of the Wright name automatically opened almost every door and
brought forth complete cooperation. I do not believe that in the
history of the country there has been another scientist or engineer as
admired and revered as they are.
I must, however, name a few who gave substantially of their time and
effort and without whose help this work would not be as complete as it
is. Gilmoure N. Cole, A. L. Rockwell, and the late L. Morgan Porter
were major contributors, the latter having made the calculations of
the shaking forces, the volumetric efficiency, and the connecting rod
characteristics of the 1903 engine. Louis P. Christman, who was
responsible for the Smithsonian drawings of this engine and also
supervised the reconstruction of the 1905 Wright airplane, supplied
much information, including a great deal of the history of the early
engines. Opie Chenoweth, one of the early students of the subject, was
of much assistance; and I am indebted to R. V. Kerley for the major
part of the data on the Wrights' shop engine.
Also, I must express my great appreciation to the many organizations
that cooperated so fully, and to all the people of these organizations
and institutions who gave their assistance so freely. These include
the following:
Air Force Museum, Wright-Patterson Air Force Base, Ohio
Carillon Park Museum, Dayton, Ohio
Connecticut Aeronautical Historical Association, Hebron, Connecticut
Fredrick C. Crawford Museum, Cleveland, Ohio
Historical Department, Daimler Benz A. G., Stuttgart-Untertürkheim,
West Germany
Engineers Club, Dayton, Ohio
Deutsches Museum, Munich, West Germany
Educational and Musical Arts, Inc., Dayton, Ohio
Henry Ford Museum, Dearborn, Michigan
Franklin Institute, Philadelphia, Pennsylvania
Howell Cheney Technical School, Manchester, Connecticut
Library of Congress, Washington, D.C.
Naval Air Systems Command, U.S. Navy, Washington, D.C.
Science Museum, London, England
Victoria and Albert Museum, London, England
In particular, very extensive contributions were made by the
Smithsonian Institution and by the United Aircraft Corporation through
its Library, through the Pratt & Whitney Aircraft Division's entire
Engineering Department and its Marketing and Product Support
Departments, and through United Aircraft International.
The Beginnings
The general history of the flight engines used by the Wright Brothers
is quite fascinating and fortunately rather well recorded.[1] The
individual interested in obtaining a reasonably complete general story
quickly is referred to three of the items listed in the short
bibliography on page 69. The first, _The Papers of Wilbur and Orville
Wright_, is a primary source edited by the authority on the Wright
brothers, Marvin W. McFarland of the Library of Congress; a compact
appendix to volume 2 of the _Papers_ contains most of the essential
facts. This source is supplemented by the paper of Baker[2] and the
accompanying comments by Chenoweth, presented at the National
Aeronautics Meeting of the Society of Automotive Engineers on 17 April
1950. Aside from their excellence as history, these publications are
outstanding for the manner in which those responsible demonstrate
their competence and complete mastery of the sometimes complex
technical part of the Wright story.
[Footnote 1: An extensive bibliography, essentially as complete at
this time as when it was compiled in the early 1950s, is given on
pages 1240-1242 of volume 2 of _The Papers of Wilbur and Orville
Wright_, 1953.]
[Footnote 2: Max P. Baker was a technical adviser to the Wright estate
and as such had complete access to all of the material it contained.]
The consuming interest of the Wrights, of course, was in flight as
such, and in their thinking the required power unit was of only
secondary importance. However, regardless of their feeling about it,
the unit was an integral part of their objective and, due to the
prevailing circumstances, they very early found themselves in the
aircraft engine business despite their inexperience. This business was
carried on very successfully, against increasingly severe competition,
until Orville Wright withdrew from commercial activity and dissolved
the Wright Company. The time span covered approximately the twelve
years from 1903 to 1915, during the first five years of which they
designed and built for their own use several engines of three
different experimental and demonstration designs. In the latter part
of the period, they manufactured and sold engines commercially, and
during this time they marketed three models, one of which was
basically their last demonstration design. A special racing engine was
also built and flown during this period. Accurate records are not
available but altogether, they produced a total of something probably
close to 200 engines of which they themselves took a small number for
their various activities, including their school and flying exhibition
work which at one time accounted for a very substantial part of their
business. A similar lack of information concerning their competition,
which expanded rapidly after the Wright's demonstrations, makes any
comparisons a difficult task. The Wrights were meticulous about
checking the actual performance of their engines but at that time
ratings generally were seldom authenticated and even when different
engines were tried in the same airplane the results usually were not
measured with any accuracy or recorded with any permanency. There is
evidence that the competition became effective enough to compel the
complete redesign of their engine so that it was essentially a new
model.
For their initial experimentation the Wrights regarded gravity as not
only their most reliable power source but also the one most economical
and readily available, hence their concentration on gliding. They had
correctly diagnosed the basic problem of flight to be that of control,
the matter of the best wing shapes being inherently a simpler one
which they would master by experiment, utilizing at first gravity and
later a wind tunnel. Consequently, the acquisition of a powerplant
intended for actual flight was considerably deferred.
Nevertheless, they were continuously considering the power requirement
and its problems. In his September 1901 lecture to the Western Society
of Engineers, Wilbur Wright made two statements: "Men also know how to
build engines and screws of sufficient lightness and power to drive
these planes at sustaining speed"; and in conjunction with some
figures he quoted of the required power and weight: "Such an engine is
entirely practicable. Indeed, working motors of one-half this weight
per horsepower [9 pounds per horsepower] have been constructed by
several different builders." It is quite obvious that with their
general knowledge and the experience they had acquired in designing
and building a successful shop engine for their own use, they had no
cause to doubt their ability to supply a suitable powerplant when the
need arose. After the characteristics of the airframe had been
settled, and the engine requirements delineated in rather detailed
form, they had reached the point of decision on what they termed the
motor problem. Only one major element had changed greatly since their
previous consideration of the matter; they had arrived at the point
where they not only needed a flight engine, they wanted it quickly.
Nothing has been found that would indicate how much consideration they
had given to forms of power for propulsion other than the choice they
had apparently made quite early--the internal-combustion,
four-stroke-cycle piston engine. Undoubtedly, steam was dismissed
without being given much, if any, thought. On the face of it, the
system was quite impractical for the size and kind of machine they
planned; but it had been chosen by Maxim for his experiments,[3] and
some thirty-five or forty years later a serious effort to produce an
aviation engine utilizing steam was initiated by Lockheed. On the
other hand internal-combustion two-stroke-cycle piston engines had
been built and used successfully in a limited way. And since, at that
time, it was probably not recognized that the maximum quantity of heat
it is possible to dissipate imposed an inherent limitation on the
power output of the internal-combustion engine, the two-stroke-cycle
may have appeared to offer a higher output from a given engine size
than the four-stroke-cycle could produce. Certainly, it would have
seemed to promise much less torque variation for the same output,
something that was of great importance to the Wrights. Against this,
the poor scavenging efficiency of the two-stroke operation, and most
probably its concurrent poor fuel economy, were always evident; and,
moreover, at that time the majority of operating engines were
four-stroke-cycle. Whatever their reasoning, they selected for their
first powered flight the exact form of prime mover that continued to
power the airplane until the advent of the aircraft gas turbine more
than forty years later.
[Footnote 3: In the 1890s the wealthy inventor Sir Hiram Stevens Maxim
conducted an experiment of considerable magnitude with a flying
machine that utilized a twin-cylinder compound steam powerplant. It
was developed to the flight-test stage.]
The indicated solution to their problem of obtaining the engine--and
the engine that would seem by all odds most reliable--would have been
to have a unit produced to their specifications by one of the best of
the experienced engine builders, and to accomplish this, the most
effective method would be to use the equivalent of a bid procedure.
This they attempted, and sent out a letter of inquiry to a fairly
large number of manufacturers. Although no copy of the letter is
available, it is rather well established that it requested the price
of an engine of certain limited specifications which would satisfy
their flight requirements, but beyond this there is little in the
record.
A more thorough examination of the underlying fundamentals, however,
discloses many weaknesses in the simple assumptions that made the
choice of an experienced builder seem automatic. A maximum requirement
limited to only one or two units offered little incentive to a
manufacturer already successfully producing in his field, and the
disadvantage of the limited quantity was only accentuated by the basic
requirement for a technical performance in excess of any standard of
the time. Certainly there was no promise of any future quantity
business or any other substantial reward. Orville Wright many times
stated that they had no desire to produce their own engine, but it is
doubtful that they had any real faith in the buying procedure, for
they made no attempt to follow up their first inquiries or to expand
the original list.
Whatever the reasoning, their judgment of the situation is obvious;
they spent no time awaiting results from the letter but almost
immediately started on the task of designing and building the engine
themselves. Perhaps the generalities were not as governing as the two
specific factors whose immediate importance were determining: cost and
time. The Wrights no doubt realized that a specially designed,
relatively high performance engine in very limited hand-built
quantities would not only be an expensive purchased article but would
also take considerable time to build, even under the most favorable
circumstances. So the lack of response to their first approach did not
have too much to do with their ultimate decision to undertake this
task themselves.
The question of the cost of the Wrights' powerplants is most
intriguing, as is that of their entire accomplishment. No detailed
figures of actual engine costs are in the record, and it is somewhat
difficult to imagine just how they managed to conduct an operation
requiring so much effort and such material resources, given the income
available from their fairly small bicycle business. The only evidence
bearing on this is a statement that the maximum income from this
business averaged $3,000 a year,[4] which of course had to cover not
only the airplane and engine but all personal and other expenses. Yet
they always had spare engines and spare parts available; they
seemingly had no trouble acquiring needed materials and supplies, both
simple and complex; and they apparently never were hindered at any
time by lack of cash or credit. The only mention of any concern about
money is a statement by Wilbur Wright in a letter of 20 May 1908 when,
about to sail for France for the first public demonstrations, he
wrote: "This plan would put it to the touch quickly and also help ward
off an approaching financial stringency which has worried me very much
for several months." It is a remarkable record in the economical use
of money, considering all they had done up to that time. The myth that
they had been aided by the earnings of their sister Katherine as a
school teacher was demolished long ago.
[Footnote 4: Fred C. Kelly, _Miracle at Kitty Hawk_, 1951.]
The decision to build the engine themselves added one more
requirement, and possibly to some extent a restriction, to the design.
They undoubtedly desired to machine as much of the engine as possible
in their own shop, and the very limited equipment they had would
affect the variety of features and constructions that could be
utilized, although experienced machine shops with sophisticated
equipment were available in Dayton and it is obvious that the Wrights
intended to, and did, utilize these when necessary. The use of their
own equipment, of course, guaranteed that the parts they could handle
themselves would be more expeditiously produced. They commenced work
on the design and construction shortly before Christmas in 1902.
The subject of drawings of the engine is interesting, not only as
history but also because it presents several mysteries. Taylor[5]
stated, "We didn't make any drawings. One of us would sketch out the
part we were talking about on a piece of scrap paper ..." Obviously
somewhere in the operation some dimensions were added, for the design
in many places required quite accurate machining. Orville Wright's
diary of 1904 has the entry, "Took old engine apart to get
measurements for making new engine." Finally, no Wright drawings of
the original engine have been seen by anyone connected with the
history or with the Wright estate. In the estate were two drawings
(now at the Franklin Institute), on heavy brown wrapping paper,
relating to one of the two very similar later engines built in 1904;
one is of a cylinder and connecting rod, the other is an end view of
the engine. Thus even if the very ingenious drafting board now in the
Wright Museum at Carillon Park was available at the time there is no
indication that it was used to produce what could properly be called
drawings of the first engine.
[Footnote 5: Charles E. Taylor (Charley Taylor to the many who knew
him) was in effect the superintendent of and also the only employee to
work in the original small machine shop. A most versatile and
efficient mechanic and machine operator, he made many parts for all of
the early engines, and in the manner of the experimental machinist,
worked mainly from sketches. He also had charge of the bicycle shop
and its business in the absence of the Wrights.]
There are in existence, however, two complete sets of drawings, both
of which purport to represent the 1903 flight engine. One set was made
in England for the Science Museum in the two years 1928 and 1939. The
1928 drawings were made on receipt of the engine, which was not
disassembled, but in 1939 the engine was removed from the airplane,
disassembled, the original 1928 drawings were corrected and added to,
and the whole was made into one very complete and usable set. The
other set was prepared in Dayton, Ohio, for Educational and Musical
Arts, Inc.,[6] and was donated to the Smithsonian Institution. This
latter set was started under the direction of Orville Wright, who died
shortly after the work had been commenced.
[Footnote 6: This is a charitable agency set up by the late Colonel
and Mrs. E. A. Deeds primarily for the purpose of building and
supporting the Deeds Carillon and the Carillon Park Museum in Dayton,
Ohio.]
The two sets of drawings, that is, the one of the Science Museum and
that made in Dayton for the Smithsonian Institution, cannot be
reconciled in the matter of details. Hardly any single dimension is
exactly the same and essentially every part differs in some respect.
Many of the forms of construction differ and even the firing order of
the two engines is not the same, so that in effect the drawings show
two different engines.
[Illustration: Figure 1.--First flight engine, 1903, valve side.
(Photo courtesy Science Museum, London.)]
The primary trouble is, of course, that the exact engine which flew in
1903 is no longer in existence, and since no original drawings of it
exist, there is considerable doubt about its details. The engine had
its crankcase broken in an accident to the airframe (this was caused
by a strong wind gust immediately following the last of the first
series of flights at Kitty Hawk), and when it was brought back to
Dayton it was for some inexplicable reason completely laid aside, even
though it presumably contained many usable parts. When the engine was
disassembled to obtain measurements for constructing the 1904 engines,
again apparently no drawings were made. In February 1906 Orville
Wright wrote that all the parts of the engine were still in existence
except the crankcase; but shortly after this the crankshaft and
flywheel were loaned for exhibition purposes and were never recovered.
In 1926 the engine was reassembled for an exhibition and in 1928 it
was again reassembled for shipment to England. The only parts of this
particular engine whose complete history is definitely known are the
crankshaft and flywheel, which were taken from the 1904-1905 flight
engine. This latter engine, now in the restored 1905 airplane in the
Carillon Park Museum in Dayton, does not contain a crankshaft, and in
its place incorporates a length of round bar stock.
[Illustration: Figure 2.--First flight engine, 1903, underside and
flywheel end. (Photo courtesy Science Museum, London.)]
In late 1947 work on the Educational and Musical Arts drawings was
initiated under the direction of Louis P. Christman and carried
through to completion by him. Christman has stated that Orville Wright
was critical of the Science Museum drawings but just what he thought
incorrect is not known. Whatever his reasons, he did encourage
Christman to undertake the major task of duplication. Christman worked
directly with Orville Wright for a period of six weeks and had access
to all the records and parts the Wrights had preserved. The resultant
drawings are also very complete and, regardless of the differences
between these two primary sets, both give a sufficiently accurate
picture of the first engine for all purposes except that of exact
reproduction in every detail.
There exists a still unsolved puzzle in connection with what seems to
be yet another set of drawings of the first engine. In December 1943,
in writing to the Science Museum telling of his decision to have the
airplane and engine brought back to the United States, Orville Wright
stated, "I have complete and accurate drawings of the engine. I shall
be glad to furnish them if you decide to make a replica."[7] No trace
of these particular drawings can be found in any of the museums,
institutions, or other repositories that normally should have acquired
them and the executors of Orville Wright's estate have no record or
knowledge of them. The date of his letter is four years before the
Dayton drawings were commenced; and when Christman was working on
these with Orville Wright they had copies of the Science Museum
drawings, with complete knowledge of their origin, yet Christman has
no knowledge of the drawings referred to in Orville's letter to the
Museum. Finally, the evidence is quite conclusive that there were no
reproducible or permanent drawings made at the time the first engine
was constructed, and, of course, the reconstructed engine itself was
sent to England in 1928 and not returned to this country until
1948.[8]
[Footnote 7: The Science Museum expressed a desire to have these but
never received them. There is a reference to them in a letter to the
Museum from the executors of his estate dated 20 February 1948, but is
seems rather obvious from the text that by this time the drawings
mentioned by Orville Wright in his 1943 letter had become confused
with those being prepared by Christman for the Smithsonian
Institution. The Science Museum did have constructed from its own
drawings a very fine replica which is completely operable at this
time.]
[Footnote 8: There is a third set of drawings prepared by the Ford
Motor Company also marked as being of the 1903 engine and these are
rather well distributed in various museums and institutions. What this
set is based on has been impossible to determine but it is indicated
from the existence of actual engines and parts and the probable date
of their preparation (no date is given on the drawings themselves)
that they were copied from drawings previously made, and therefore add
nothing to them. The Orville Wright-Henry Ford friendship originated
rather late, considering Ford's avid interest in history and
mechanical things. This tardiness could possibly have been the result
of Wright coolness--a coolness caused by a report, at the time the
validity of the Wright patents was being so strongly contested, that
Ford had advised some of those opposing the Wrights to persevere and
to obtain the services of his patent counsel who had been successful
in overturning the Selden automobile patent. If this barrier ever
existed it was surmounted, and Ford spent much effort and went to
considerable expense to collect the Wright home and machine shop for
his Dearborn museum. The shop equipment apparently had been widely
scattered and its retrieval was a major task. It is most likely that
the drawings resulted from someone's effort to follow out an order to
produce a set of Ford drawings of the original engine. A small scale
model of the 1903 flight engine, constructed under the supervision of
Charles Taylor, is contained in the Dearborn Museum.]
The Engine of the First Flight, 1903
In commencing the design of the first engine, the first important
decision arrived at was that of the number and size of the cylinders
to be employed and the form in which they would be combined, although
it is unlikely that this presented any serious problem. In a similar
situation Manly, when he was working on the engine for the Langley
Aerodrome,[9] was somewhat perturbed because he did not have access to
the most advanced technical knowledge, since the automobile people who
were at that time the leaders in the development of the internal
combustion engine, tended for competitive reasons to be rather
secretive about their latest advancements and designs. But although
the standard textbooks may not have been very helpful to him, there
were available such volumes as W. Worby Beaumont's _Motor Vehicles and
Motors_ which contained in considerable detail descriptions and
illustrations of the best of the current automobile engines. The
situations of Manly and the Wrights differed, however, in that whereas
the Wrights' objective was certainly a technical performance
considerably above the existing average, Manly's goal was that of
something so far beyond this average as to have been considered by
many impossible. Importantly, the Wrights had their own experience
with their shop engine and a good basic general knowledge of the size
of engine that would be necessary to meet their requirements.
[Footnote 9: Charles L. Manly was engaged in the development of the
engine for the Langley Aerodrome. See also footnote to Table on page
62.]
Engine roughness was of primary concern to them. In the 1902
description of the engine they sent to various manufacturers, they had
stated: "... and the engine would be free from vibration." Even though
their requirement for a smooth engine was much more urgent than merely
to avoid the effect of roughness on the airplane frame, they were
faced, before they made their first powered flight, with the basic
problem with which the airplane has had to contend for over
three-quarters of its present life span: that is, it was necessary to
utilize an explosion engine in a structure which, because of weight
limitations, had to be made the lightest and hence frailest that could
possibly be devised and yet serve its primary purpose. However great
the difficulty may have appeared, in the long view, the fault was
certainly a relatively minor one in the overall development of the
internal combustion engine--that wonderful invention without which
their life work would probably never have been so completely
successful while they lived, and which, even aside from its
partnership with the airplane, has so profoundly affected the nature
of the world in which we live.
It seems quite obvious that to the Wrights vibration, or roughness,
was predominantly if not entirely caused by the explosion forces, and
they were either not completely aware of the effects of the other
vibratory forces or they chose to neglect them. Although crankshaft
counterweights had been in use as far back as the middle 1800s, the
Wrights never incorporated them in any of their engines; and despite
the inherent shaking force in the 4-inline arrangement, they continued
to use it for many years.
The choice of four cylinders was obviously made in order to get, for
smoothness, what in that day was "a lot of small cylinders"; and this
was sound judgment. Furthermore, although the majority of automobiles
at that time had engines with fewer than four cylinders, for those
that did the inline form was standard and well proven, and, in fact,
Daimler was then operating engines of this general design at powers
several times the minimum the Wrights had determined necessary for
their purpose.
What fixed the exact cylinder size, that is, the "square" 4×4-in.
form, is not recorded, nor is it obvious by supposition. Baker says it
was for high displacement and low weight, but these qualities are also
greatly affected by many other factors. The total displacement of just
over 200 cu in. was on the generous side, given the horsepower they
had determined was necessary, but here again the Wrights were
undoubtedly making the conservative allowances afterwards proven
habitual, to be justified later by greatly increased power
requirements and corresponding outputs. The Mean Effective Pressure
(MEP), based on their indicated goal of 8 hp, would be a very modest
36 psi at the speed of 870 rpm at which they first tested the engine,
and only 31 psi at the reasonably conservative speed of 1000 rpm. The
4×4-in. dimension would provide a cylinder large enough so that the
engine was not penalized in the matter of weight and yet small enough
to essentially guarantee its successful operation, as cylinders of
considerably larger bore were being utilized in automobiles. That
their original choice was an excellent one is rather well supported by
the fact that in all the different models and sizes of engines they
eventually designed and built, they never found it necessary to go to
cylinders very much larger than this.
[Illustration: _Figure 3._--First flight engine, 1903, installed in
the Kitty Hawk airplane, as exhibited in the Science Museum. (Photo
courtesy the Science Museum, London.)]
A second basic determination which was made either concurrently or
even possibly in advance of that of the general form and size was in
the matter of the type of cylinder cooling to adopt. Based on current
practice that had proven practical, there were three possibilities,
all of which were in use in automobiles: air, water, or a combination
of the two. It is an interesting commentary that Fernand Forest's[10]
proposed 32-cylinder aircraft engine of 1888 was to be air-cooled,
that Santos-Dumont utilized an air-cooled Clement engine in his
dirigible flights of 1903, and that the Wrights had chosen air cooling
for their shop engine. With the promise of simplicity and elimination
of the radiator, water and piping, it would seem, offhand, that this
would be the Wrights' choice for their airplane; but they were
probably governed by the fact that not only was the water-cooled type
predominant in automobile practice, but that the units giving the best
and highest performance in general service were all water cooled. In
their subsequent practice they never departed from this original
decision, although Wilbur Wright's notebook of 1904-1907 contains an
undated weight estimate by detailed parts for an 8-cylinder air-cooled
engine. Unfortunately, the proposed power output is not recorded, so
their conception of the relative weight of the air-cooled form is not
disclosed.
[Footnote 10: Fernand Forest, _Les Bateaux Automobiles_, 1906.]
One of the most important decisions relating to the powerplant--one
which was probably made long before they became committed to the
design itself--was a determination of the method of transmission of
power to the propeller, or propellers. A lingering impression exists
that the utilization of a chain drive for this purpose was a natural
inheritance from their bicycle background. No doubt this experience
greatly simplified the task of adaptation but a merely cursory
examination shows that even if they had never had any connection with
bicycles, the chain drive was a logical solution, considering every
important element of the problem. The vast majority of automobiles of
the time were chain driven, and chains and sprockets capable of
handling a wide range of power were completely developed and
available. Further, at that time they had no accurate knowledge of
desirable or limiting propeller and engine speeds. The chain drive
offered a very simple and inexpensive method of providing for a
completely flexible range of speed ratios. The other two possibilities
were both undesirable: the first, a simple direct-driven single
propeller connected to the crankshaft, provided essentially no
flexibility whatsoever in experimentally varying engine or propeller
speed ratios, it added an out-of-balance engine torque force to the
problem of airplane control, and, finally, it dictated that the pilot
would be in the propeller slipstream or the airflow to it; the second,
drive shafts and gearing for dual propellers, would have been very
heavy and expensive, and most probably would have required a long-time
development, with every experimental change in speed ratios requiring
a complete change in gears. Again, their original choice was so
correct that it lasted them through essentially all their active
flying years.
The very substantial advantages of the chain drive were not, however,
obtained at no cost. Torque variations in the engine would tend to
cause a whipping action in the chain, so that it was vulnerable to
rough running caused by misfiring cylinders and, with the right timing
and magnitude of normal regular variations, the action could result in
destructive forces in the transmission system. This was the basic
reason for the Wrights' great fear of "engine vibration," which
confined them to the use of small cylinders and made a fairly heavy
flywheel necessary on all their engines. When they were requested to
install an Austro-Daimler engine in one of their airplanes, they
designed a flexible coupling which was interposed between the engine
and the propeller drive and this was considered so successful that it
was applied to the flywheel of some engines of their last model, the
6-70, "which had been giving trouble in this regard."[11]
[Footnote 11: Grover Loening, letter of 10 April 1963, to the
Smithsonian Institution.]
Although flat, angled, and vertical engines had all been operated
successfully, the best and most modern automotive engines of the time
were vertical, so their choice of a horizontal position was probably
dictated either by considerations of drag or their desire to provide a
sizable mounting base for the engine, or both. There is no record of
their ever having investigated the matter of the drag of the engine,
either alone or in combination with the wing. The merit of a vertical
versus a horizontal position of the engine was not analogous to that
of the pilot, which they had studied, and where the prone position
undoubtedly reduced the resistance.
Having decided on the general makeup of their engine, the next major
decision was that of just what form the principal parts should take,
the most important of these being the cylinders and crankcase. Even at
this fairly early date in the history of the internal combustion
engine various successful arrangements and combinations were in
existence. Individual cylinder construction was by far the most used,
quite probably due to its case of manufacture and adaptability to
change. Since 4-cylinder engines were just coming into general use (a
few production engines of this type had been utilized as early as
1898), there were few examples of en-bloc or one-piece construction.
The original German Daimler Company undoubtedly was at this time the
leader in the development of high-output internal-combustion engines,
and in 1902, as an example of what was possible, had placed in service
one that possibly approximated 40 hp, which was an MEP of 70 psi.
(Almost without exception, quoted power figures of this period were
not demonstrated quantities but were based on a formula, of which the
only two factors were displacement and rpm.) The cylinders of this
Daimler engine were cast iron, the cylinder barrel, head, and water
jacket being cast in one piece. The upper part of the barrel and the
cylinder head were jacketed, but, surprisingly, the bottom 60 percent
of the barrel had no cooling. The cylinders were cast in pairs and
bolted to a two-piece aluminum case split at the line of the
crankshaft. Ignition was make-and-break and the inlet valves were
mechanically actuated. Displacement was 413 cu in. and the rpm was
1050.
Although a few examples of integral crankcase and water jacket
combinations were in use, the Wrights were being somewhat radical when
they decided to incorporate all four cylinders in the one-piece
construction, particularly since they also proposed to include the
entire crankcase and not just one part of it. It was undoubtedly the
most important decision that they were required to make on all the
various construction details, and probably the one given the most
study and investigation. Many factors were involved, but fundamentally
everything went back to their three basic requirements: suitability,
time, and cost. There was no obvious reason why the construction would
not work, and it eliminated a very large number of individual parts
and the required time for procuring, machining, and joining them.
Probably one very strong argument was the advanced state of the
casting art, one of the oldest of the mechanical arts in existence and
one the Wrights used in many places, even though other processes were
available. What no doubt weighed heavily was that Dayton had some
first-class foundries. The casting, though intricate and not
machinable in their own shop, could be easily handled in one that was
well outfitted. The pattern was fairly complex but apparently not
enough to delay the project or cause excessive cost.
[Illustration: _Figure 4._--First flight engine, 1903, left side and
rear views, with dimensions. (Drawing courtesy Howell Cheney Technical
School.)
LEFT SIDE VIEW.]
[Illustration: REAR VIEW]
The selection of aluminum for the material was an integral part of
the basic design decision. Despite the excellence and accuracy of the
castings that could be obtained, there was nevertheless a minimum
dimension beyond which wall thickness could not be reduced; and the
use of either one of the two other proven materials, cast iron or
bronze, would have made the body, as they called it, prohibitively
heavy. The use of aluminum was not entirely novel at this time, as it
had been utilized in many automobile engine parts, particularly
crankcases; but its incorporation in this rather uncommon combination
represented a bold step. There was no choice in the matter of the
alloy to be used, the only proven one available was an 8 percent
copper 92 percent aluminum combination.
By means of the proper webs, brackets and bosses, the crankcase would
also carry the crankshaft, the rocker arms and bearings, and the
intake manifold. The open section of the case at the top was covered
with a screw-fastened thin sheet of cold-rolled steel. The main
bearing bosses were split at a 45° angle for ease of assembly. The
engine support and fastening were provided by four feet, or lugs, cast
integral on the bottom corners of the case, and by accompanying bolts
(Figure 2). Although the crankcase continued to be pretty much the
"body" of the internal combustion aircraft engine throughout its life,
the Wrights managed to incorporate in this original part a major
portion of the overall engine, and certainly far more than had ever
previously been included.
The design of the cylinder barrel presented fairly simple problems
involving not much more than those of keeping the sections as thin as
possible and devising means of fastening it and of keeping the water
jacket tight. They saved considerable weight by making the barrel
quite short, so that in operation a large part of the piston extended
below the bottom of it; but this could be accepted, as there were no
rings below the piston pin (Figure 6). The barrel material, a good
grade of cast iron, was an almost automatic choice. In connection with
these seemingly predetermined decisions, however, it should be
remembered that their goal was an engine which would work without
long-time development, and that, with no previous experience in
lightweight construction to guide them they were nevertheless
compelled to meet a weight limit, so that the thickness of every wall
and flange and the length of every thread was important.
With the separate cylinder barrel they were now almost committed to a
three-piece cylinder. It would have been possible to combine the
barrel and head in a one-piece casting and then devise a method of
attachment, but this would have been more complex and certainly
heavier. For housing the valves, what was in effect a separate
cylindrical, or tubular, box was decided upon. This would lie across
the top of the cylinder proper at right angles to the cylinder axis,
and the two valves would be carried in the two ends of this box. The
cylinder barrel would be brought in at its head end to form a portion
of the cylinder head and then extended along its axis in the form of a
fairly large boss, a mating boss being provided on one side of the
valve box. The cylinder barrel would then be threaded into the valve
box and the whole tightened or fastened to the crankcase by means of
two sets of threads, one at each end of the barrel proper. This meant
that three joints had to be made tight with only two sets of threads.
This was accomplished by accurate machining and possibly even hand
fitting in combination with a rather thick gasket at the head end, one
flat of which bore against two different surfaces. This can be seen in
Figure 6, where the circular flange on the valve box contacts both the
crankcase and the cylinder barrel. Altogether it was a simple, light,
and ingenious solution to a rather complex problem.
At this point the question arises: Why was the engine layout such that
the exhaust took place close to the operator's ears? It would have
been possible, starting with the original design, to turn the engine
around so that the exhaust was on the other side. This would have
little effect on the location of the center of gravity, and the two
main drive chains would then have been of more equal length. However,
of the many factors involved, probably one of the principal
considerations in arriving at their final decision was the location of
the spark-advance control, which was in effect the only control they
had of engine output, except for complete shutoff. In their design
this was immediately adjacent to the operator; with a turned-around
engine, an extension control mechanism of some sort would have been
required. The noise of the exhaust apparently became of some concern
to them, as Orville's diary in early 1904 contains an entry with a
sketch labeled "Design for Muffler for Engine," but there is no
further comment.
The problem of keeping joints tight, and for that matter the entire
construction itself, were both greatly simplified by their decision to
water-jacket only a part of the cylinder head proper, and the valve
box not at all. This was undoubtedly the correct decision for their
immediate purpose, as again they were effecting savings in time, cost,
complexity, and weight. There is nothing in the record, however, to
show why they continued this practice long after they had advanced to
much greater power outputs and longer flight times. Their own
statements show that they were well aware of the effect of the very
hot cylinder head on power output and they must also have realized its
influence on exhaust-valve temperature.
The cylinder assembly was made somewhat more complicated by their
desire to oil the piston and cylinder by means of holes near the
crankshaft end in what was, with the engine in the horizontal
position, the upper side of the cylinder barrel. This complication was
no doubt taken care of by not drilling the holes until a tight
assembly had been made by screwing the barrel into place, and by
marking the desired location on the barrel. Since this position was
determined by a metal-to-metal jam fit of the crankcase and cylinder
barrel flange, the barrel would reassemble with the holes in very
nearly the same relative position after disassembly.
With the valve box, or housing, cylindrical, the task of locking and
fastening the intake and exhaust valve guides and seats in place was
easy. The guide was made integral with and in the center of one end of
a circular cage, the other end of which contained the valve seat (see
Figure 5). Four sections were cut out of the circular wall of the cage
so that in effect the seat and guide were joined by four narrow legs,
the spaces between which provided passages for the flow of the
cylinder gases. These cages were then dropped into the ends of the
valve boxes until they came up against machined shoulders and were
held in place by internal ring nuts screwed into the valve box. The
intake manifold or passage was placed over the intake valves so that
the intake charge flowed directly into and through the valve cage
around the open valve and into the cylinder. The exhaust gas, after
flowing through the passages in the valve cage, was discharged
directly to the atmosphere through a series of holes machined in one
side of the valve box.
[Illustration: _Figure 5._--First flight engine, 1903, assembly.
(Phantom cutaway by J. H. Clark, with key, courtesy _Aeroplane_.)
KEY
1 and 2. Bearing caps in one piece with plate 3.
3. Plated screwed over hole 4 in crankcase end.
4. Key-shaped hole as hole 5 in intermediate ribs.
6. Inter-bearings cap (white-metal lined) and screwed to inter-rib
halves 7.
8. Splash-drip feed to bearings.
9. Return to pump from each compartment of crankcase base ("sump") via
gallery 10 and pipe to pump 11 underneath jacket.
12. Oil feed from pump via rubber tube 13.
13. Drip feeds to cylinders and pistons.
14. Gear drive to pump.
15. Big-end nuts, lock-strip, and shims.
16. Gudgeon-pin lock.
17. Piston-ring retainer pegs.
18. Cylinder liner screwing into jacket.
19. Open-ended "can" admits air.
20. Fuel supply.
21. (Hot) side of water jacket makes surface carburetter.
22. Sparking plug (comprising positive electrode 23 and
spark-producing make-and-break 24).
25. Lever attached to lever 26 via bearing 27 screwed into chamber
neck 28.
26. Levers with mainspring 29 and inter-spring 30, and rocked by "cam"
31.
31. Cam with another alongside (for adjacent cylinder).
32. Positive busbar feed to all four cylinders.
33. Assembly retaining-rings.
34. Sealing disc.
35. Exhaust outlet ports.
36. Camshaft right along on underside of jacket and also driving oil
pump 11 via 14.
37. Spring-loaded sliding pinion drives make-and-break shaft 38
through peg in inclined slot 39.
40. Cam to push pinion 37 along and so alter its angular relation with
shaft 38 (to vary timing).
41. Exhaust-valve cams bear on rollers 42 mounted in end of
rocker-arms 43.
44. Generator floating coils.
45. Friction-drive off flywheel.
46. Sight-feed lubricator (on stationary sleeve).
47. Hardwood chain tensioner.]
The intake and exhaust valves were identical and of two-piece
construction, with the stems screwed tightly into and through the
heads and the protruding ends then peened over. This construction was
not novel, having had much usage behind it, and it continued for a
long time in both automobile and aircraft practice. One-piece cast
and forged valves were available but here again it was a choice of the
quick, cheap, and proven answer.
The entire valve system, including guides and seats, was of cast iron,
a favorite material of the Wrights, except for the valve stems, which
were, at different times, of various carbon steels. Ordinary
cold-rolled apparently was used in those of the original engine, but
in later engines this was changed to a high-carbon steel.
The piston design presented no difficulty. In some measure this was
due to the remarkable similarity that seems to have existed among all
the different engines of the time in the construction of this
particular part, for, although there were some major variations, it
was, in fact, almost as if some standard had been adopted. Pistons all
were of cast iron and comparatively quite long (it was a number of
years before they evolved into the short ones of modern practice);
they were almost invariably equipped with three wide piston rings
between the piston pin and the head; and, although there were in
existence a few pistons with four rings, no oil wiper or other ring
seems to have been placed below the piston pin. The Wrights' piston
was typical of the time, with the rings pinned in the grooves to
prevent turning and the piston pin locked in the piston with a
setscrew. In designing this first engine they were, however,
apparently somewhat unsure about this latter feature, as they provided
the rod with a split little end and a clamping bolt (see Figure 6), so
that the pin could be held in the rod if desired; but no examples of
this use have been encountered.
The Wrights' selection of an "automatic" or suction-operated inlet
valve was entirely logical. Mechanically operated inlet valves were in
use and their history went back many years, but the great majority of
the engines of that time still had the automatic type, and with this
construction one complete set of valve-operating mechanisms was
eliminated. They were well aware of the loss of volumetric efficiency
inherent in this valve, and apparently went to some pains to obtain
from it the best performance possible. Speaking of the first engine,
Orville Wright wrote, "Since putting in heavier springs to actuate the
valves on our engine we have increased its power to nearly 16 hp and
at the same time reduced the amount of gasoline consumed per hour to
about one-half of what it was."[12]
[Footnote 12: Assuming a rich mixture, consumption of all the air, and
an airbrake thermal efficiency of 24.50% for the original engine, the
approximate volumetric efficiency of the cylinder is calculated to
have been just under 40%.]
Why they continued with this form on their later engines is a question
a little more difficult to answer, as they were then seeking more and
more power and were building larger engines. The advantages of
simplicity and a reduced number of parts still existed, but there also
was a sizable power increase to be had which possibly would have more
than balanced off the increased cost and weight. They did not utilize
mechanical operation until after a major redesign of their last engine
model. Very possibly the answer lies in the phenomenon of fuel
detonation. This was only beginning to be understood in the late
1920s, and it is quite evident from their writings that they had
little knowledge of what made a good fuel in this respect. It is
fairly certain, however, that they did know of the existence of
cylinder "knock," or detonation, and particularly that the compression
ratio had a major effect on it. The ratios they utilized on their
different engines varied considerably, ranging from what, for that
time, was medium to what was relatively high. The original flight
engine had a compression ratio of 4.4:1. The last of their service
engines had a compression ratio about twenty percent under that of the
previous series--a clear indication that they considered that they had
previously gone too high. Quite possibly they concluded that
increasing the amount of the cylinder charge seemed to bring on
detonation, and that the complication of the mechanical inlet valve
was therefore not warranted.
[Illustration: _Figure 6._--First flight engine, 1903, cross section.
(Drawing courtesy Science Museum, London.)]
The camshaft for the exhaust valves (101, Figure 6), was chain driven
from the crankshaft and was carried along the bottom of the crankcase
in three babbit-lined bearings in bearing boxes or lugs cast integral
with the case. Both the driving chain and the sprockets were standard
bicycle parts, and a number of bicycle thread standards and other
items of bicycle practice were incorporated in several places in the
engine, easing their construction task. The shaft itself, of mild
carbon steel, was hollow and on each side of an end bearing sweated-on
washers provided shoulders to locate it longitudinally. Its location
adjacent to the valves, with the cam operating directly on the rocker
arm, eliminated push rods and attendant parts, a major economy. The
cams were machined as separate parts and then sweated onto the shaft.
Their shape shows the principal concern in the design to have been
obtaining maximum valve capacity--that is, a quite rapid opening with
a long dwell. This apparent desire to get rid of the exhaust gas
quickly is manifested again in the alacrity with which they adopted a
piston-controlled exhaust port immediately they had really mastered
flight and were contemplating more powerful and more durable engines.
This maximum-capacity theory of valve operation, with its neglect of
acceleration forces and seating velocities, may well have been at
least partially if not largely the cause of their exhaust-valve
troubles and the seemingly disproportionate amount of development they
devoted to this part, as reported by Chenoweth, although it is also
true that the exhaust valve continued to present a problem in the
aircraft piston engine for a great many years after, even with the
most scientific of cam designs.
The rocker arm (102, Figure 6) is probably the best example of a small
part which met all of their many specific requirements with an extreme
of simplicity. It consisted of two identical side pieces, or walls, of
sheet steel shaped to the desired side contour of the assembly, in
which were drilled three holes, one in each end, to carry the roller
axles, and the third in the approximate middle for the rocker axle
shaft proper. This consisted of a piece of solid rod positioned by
cotter pins in each end outside the side walls (see Figure 5). The
assembly was made by riveting over the ends of the roller axles so
that the walls were held tightly against the shoulders on the axles,
thus providing the correct clearance for the rollers. The construction
was so light and serviceable that it was essentially carried over to
the last engine the Wrights ever built.
The basic intake manifold (see Figure 5) consisted of a very low flat
box of sheet steel which ran across the tops of the valve boxes and
was directly connected to the top of each of them so that the cages,
and thus the valves, were open to the interior of the manifold.
Through an opening in the side toward the engine the manifold was
connected to a flat induction chamber (21, Figure 5) which served to
vaporize the fuel and mix it with the incoming air. This chamber was
formed by screw-fastening a piece of sheet steel to vertical ribs cast
integral with the crankcase, the crankcase wall itself thus forming
the bottom of the chamber. A beaded sheet-steel cylinder resembling a
can (73, Figure 6) but open at both ends was fastened upright to the
top of this chamber. In the absence of anything else, this can could
be called the carburetor, as a fuel supply line entered the cylinder
near the top and discharged the fuel into the incoming air stream,
both the fuel and air then going directly into the mixing chamber. The
can was attached near one corner of the chamber, and vertical baffles,
also cast integral with the case, were so located that the incoming
mixture was forced to circulate over the entire area of exposed
crankcase inside the chamber before it reached the outlet to the
manifold proper, the hot surface vaporizing that part of the fuel
still liquid.
[Illustration: _Figure 7._--First flight engine, 1903: cylinder, valve
box, and gear mechanism; below, miscellaneous parts. (Photos courtesy
Science Museum, London, and Louis P. Christman.)]
Fuel was gravity fed to the can through copper and rubber tubing from
a tank fastened to a strut, several feet above the engine. Of the two
valves placed in the fuel line, one was a simple on-off shutoff cock
and the other a type whose opening could be regulated. The latter was
adjusted to supply the correct amount of fuel under the desired flight
operating condition; the shutoff cock was used for starting and
stopping. The rate of fuel supply to the engine would decrease as the
level in the fuel tank dropped, but as the head being utilized was a
matter of several feet and the height of the supply tank a matter of
inches, the fuel-air ratio was still maintained well within the range
that would ignite and burn properly in the contemplated one-power
condition of their flight operation.
This arrangement is one of the best of the many illustrations of how
by the use of foresight and ingenuity the Wrights met the challenge of
a complex requirement with a simple device, for while carburetors were
not in the perfected stage later attained, quite good ones that would
both control power output and supply a fairly constant fuel-air
mixture over a range of operating conditions were available, but they
were complex, heavy, and expensive. The arrangement, moreover, secured
at no cost a good vaporizer, or modern "hot spot." In their subsequent
engines they took the control of the fuel metering away from the
regulating valve and gravity tank combination and substituted an
engine-driven fuel pump which provided a fuel supply bearing a fairly
close relationship to engine speed.
The reasons behind selection of the type of ignition used, and the
considerations entering into the decision, are open to speculation, as
are those concerning many other elements that eventually made up the
engine. Both the high-tension spark plug and low-tension
make-and-break systems had been in wide use for many years, with the
latter constituting the majority in 1902. Both were serviceable and
therefore acceptable, and both required a "magneto". The art of the
spark plug was in a sense esoteric (to a certain extent it so remains
to this day), but the spark-plug system did involve a much simpler
combination of parts: in addition to the plug and magneto there would
be needed only a timer, or distributor, together with coils and
points, or some substitute arrangement. The make-and-break system, on
the other hand, required for each cylinder what was physically the
equivalent of a spark plug, that is, a moving arm and contact point
inside the cylinder, a spring-loaded snap mechanism to break the
contact outside the cylinder, and a camshaft and cams to actuate the
breaker mechanism at the proper time. Furthermore, as the Wrights
applied it, the system required dry cells and a coil for starting,
although these did not accompany the engine in flight. And finally
there was the problem of keeping tight the joint where the
oscillating shaft required to operate the moving point in the spark
plug entered the cylinder.
This is one of the few occasions, if not the only one, when the
Wrights chose the more complex solution in connection with a major
part--in this particular case, one with far more bits and pieces.
However, it did carry with it some quite major advantages. The common
spark plug, always subject to fouling or failure to function because
of a decreased gap, was not very reliable over a lengthy period, and
was undoubtedly much more so in those days when control of the amount
of oil inside the cylinder was not at all exact. Make-and-break
points, on the other hand, were unaffected by excess oil in the
cylinder. Because of this resistance to fouling, the system was
particularly suitable for use with the compression-release method of
power control which they later utilized, although they probably could
not have been looking that far ahead at the time they chose it.
High-tension current has always, and rightfully so, been thought of as
a troublemaker in service; in Beaumont's 1900 edition of _Motor
Vehicles and Motors_, which seems to have been technically the best
volume of its time, the editor predicted that low-tension
make-and-break ignition would ultimately supersede all other methods.
And finally, the large number of small parts required for the
make-and-break system could all be made in the Wright Brothers' shop
or easily procured, and in the end this was probably the factor, plus
reliability, that determined the decision which, all things
considered, was the correct one.
There was nothing exceptional about the exact form the Wrights
devised. It displayed the usual refined simplicity (the cams were made
of a single small piece of strip steel bent to shape and clamped to
the ignition camshaft with a simple self-locking screw), and
lightness. The ignition camshaft (38, Figure 5), a piece of
small-diameter bar stock, was located on the same side as the exhaust
valve camshaft, approximately midway between it and the valve boxes,
and was operated by the exhaust camshaft through spur gearing. That
the Wrights were thinking of something beyond mere hops or short
flights is shown by the fact that the ignition points were
platinum-faced, whereas even soft iron would have been satisfactory
for the duration of all their flying for many years.
The control of the spark timing was effected by advancing or retarding
the ignition camshaft in relation to the exhaust valve camshaft. The
spur gear (37, Figure 5) driving the ignition camshaft had its hub on
one side extended out to provide what was in effect a sleeve around
the camshaft integral with the gear. The gear and integral sleeve were
slidable on the shaft and the sleeve at one place (39, Figure 5) was
completely slotted through to the shaft at an angle of 45° to the
longitudinal axis of the shaft. The shaft was driven by a pin tightly
fitted in it and extending into the slot. The fore-and-aft position of
the sleeve on the shaft was determined by a lever-operated cam (40,
Figure 5) on one side and a spring on the other. The movement of the
sleeve along the shaft would cause the shaft to rotate in relation to
it because of the angle of the slot, thus providing the desired
variation in timing of the spark. The "magneto" was a purchased item
driven by means of a friction wheel contacting the flywheel, and
several different makes were used later, but the original is indicated
to have been a Miller-Knoblock (see Figure 5).
The connecting rod is another example of how, seemingly without
trouble, they were able to meet the basic requirements they had set
for themselves. It consisted of a piece of seamless steel tubing with
each end fastened into a phosphor-bronze casting, these castings
comprising the big and little ends, drilled through to make the
bearings (See Figures 5 and 6). It was strong, stiff and light.[13]
Forged rods were in rather wide use at the time and at least one
existing engine even had a forged I-beam section design that was
tapered down from big to little end. The Wrights' rod was obtained in
little more time than it took to make the simple patterns for the two
ends. The weight was easily controlled, no bearing liners were
necessary, and a very minimum of machining was required. Concerning
the big-end material, there exists a contradiction in the records:
Baker, whose data are generally most accurate, states that these were
babbited, but this must be in error, as the existing engine has
straight bronze castings without babbiting, and there is no record, or
drawing, or other indication of the bearings having been otherwise.
[Footnote 13: A rather thorough stress analysis of the rod shows it to
compare very favorably with modern practice. In the absence of an
indicator card for the 1903 engine, if a maximum gas pressure of five
times the MEP is assumed, the yield-tension factor of safety is
measurably higher than that of two designs of piston engines still in
wide service, and the column factor of safety only slightly less. The
shear stresses in the brazed and threaded joints are so low as to be
negligible.]
Different methods of assembling the rod were used. At one time the
tube ends were screwed into the bronze castings and pinned, and at
another the ends were pinned and soldered. There is an indication that
at one time soldering and threads were used in combination. One of the
many conflicts between the two primary sets of drawings exists at this
point. The Smithsonian drawings show the use at each end of adapters
between the rod and end castings, the adapters being first screwed
into the castings and pinned and then brazed to the inside of the
tube. The Science Museum drawings show the tube section threaded and
screwed into the castings. The direct screw assembly method called for
accurate machining and hand fitting in order to make the ends of the
tubing jam against the bottom of the threaded holes in the castings,
and at the same time have the end bearings properly lined up. The
weakness of the basic design patently lies in the joints. It is an
attempt to utilize what was probably in the beginning a combination
five-piece assembly and later three, in a very highly stressed part
where the load was reversing. It gave them considerable trouble from
time to time, particularly in the 4-cylinder vertical engines, and was
abandoned for a forged I-beam section type in their last engine model;
but it was nevertheless the ideal solution for their first engine.
The crankshaft was made from a solid block of relatively high carbon
steel which, aside from its bulk and the major amount of machining
required, presented no special problems. It was heat-treated to a
machinable hardness before being worked on, but was not further
tempered. The design was an orthodox straight pin and cheek
combination and, as previously noted, there were no counterweights to
complicate the machining or assembly. A sizable bearing was provided
on each side of each crank of the shaft, which helped reduce the
stiffness requirement.
Their only serious design consideration was to maintain the desired
strength and still keep within weight limitations. A fundamental that
every professional designer knows is that it is with this particular
sort of part that weight gets out of control; even an additional 1/16
in., if added in a few places, can balloon the weight. With their
usual foresight and planning, the Wrights carefully checked and
recorded the weight of each part as it was finished, but even this
does not quite explain how these two individuals, inexperienced in
multicylinder engines--much less in extra-light construction--could,
in two months, bring through an engine which was both operable and
somewhat lighter than their specification.
In one matter it would seem that they were quite fortunate. The
records are not complete, but with one exception there is no
indication of any chronic or even occasional crankshaft failure. This
would seem to show that it apparently never happened that any of their
designs came out such that the frequency of a vibrating force of any
magnitude occurred at the natural frequency of the shaft. Much later,
when this type of vibration became understood, it was found virtually
impossible, with power outputs of any magnitude, to design an
undampened shaft, within the space and weight limitations existing in
an ordinary engine, strong enough to withstand the stress generated
when the frequency of the imposed vibration approximated the natural
frequency of the shaft. The vibratory forces were mostly relatively
small in their engines, so that forced vibration probably was not
encountered, and the operating speed range of the engines was so
limited that the natural frequency always fell outside this range.
The flywheel was about the least complex of any of their engine parts
and required little studied consideration, although they did have to
balance its weight against the magnitude of the explosion forces which
would reach the power transmission chains, with their complete lack of
rigidity, a problem about which they were particularly concerned. The
flywheel was made of cast iron and was both keyed to and shrunk on the
shaft.
Some doubt still exists about the exact method of lubricating the
first engine. The unit presently in the airplane has a gear-type oil
pump driven by the crankshaft through a worm gear and cross shaft, and
the Appendix to the _Papers_ states that it was lubricated by a small
pump; nevertheless Baker says, after careful research, that despite
this evidence, it was not. Also, the drawings prepared by Christman
(they were commenced under the supervision of Orville Wright) do not
show the oil pump. In March 1905 Wilbur Wright wrote to Chanute,
"However we have added oiling and feeding devices to the engine ...";
but this could possibly have referred to something other than an oil
pump. But even if a pump was not included originally, its presence in
the present engine is easily explained. Breakage of the crankcase
casting caused the retirement of this engine, which was not rebuilt
until much later, and the pattern for this part had no doubt long
since been altered to incorporate a pump. It was therefore easier in
rebuilding to include than to omit the pump, even though this required
the addition of a cross shaft and worm gear combination. On later
engines, when the pump was used, oil was carried to a small pipe,
running along the inside of the case, which had four small drill holes
so located as to throw the oil in a jet on the higher, thrust-loaded
side of each cylinder. The rods had a sharp scupper on the outside of
the big end so placed as also to throw the oil on this same thrust
face. Some scuppers were drilled through to carry oil to the rod
bearing and some were not.
The first engine was finished and assembled in February 1903 and given
its first operating test on 22 February. The Wrights were quite
pleased with its operation, and particularly with its smoothness.
Their father, Bishop Wright, was the recorder of their satisfaction
over its initial performance, but what he noted was probably the
afterglow of the ineffable feeling of deep satisfaction that is the
reward that comes to every maker of a new engine when it first comes
to life and then throbs. They obtained 13 hp originally: later figures
went as high as almost 16, but as different engine speeds were
utilized it is rather difficult to settle on any single power figure.
The most realistic is probably that given in the _Papers_ as having
been attained later, after an accurate check had been made of the
power required to turn a set of propellers at a given rpm. This came
out at approximately 12 hp, the design goal having been 8. Following
exactly the procedure that exists to this day, the engine went through
an extended development period, and it was the end of September 1903
before it was taken, with the airplane, to Kitty Hawk where the
historic flights, which have had such a profound effect on the lives
of all men, were made on 17 December 1903.
The Engines With Which They Mastered The Art of Flying
Two more engines of this first general design were built but they
differed somewhat from each other as well as from the original.
Together with a third 8-cylinder engine these were begun right after
the first of the year in 1904, shortly after the Wrights' return from
Kitty Hawk. In planning the 8-cylinder engine they were again only
being forehanded, but considerably so, in providing more power for
increased airplane performance beyond that which might possibly be
obtained from the 4-cylinder units. Progress with the 4-cylinder
engines was such that they fairly quickly concluded that the
8-cylinder size would not be necessary, and it was abandoned before
completion. Exactly how far it was carried is not known. The record
contains only a single note covering the final scrapping of the parts
that had been completed; and apparently there were no drawings, so
that even its intended appearance is not known with any exactness. It
was probably a 90° V-type using their original basic cylinder
construction.
The changes carried through in the two 4-cylinder engines were not
major. The water-cooled area of the cylinder barrel was increased by
nearly ten percent but the head remained only partially cooled. In
hindsight, this consistent avoidance of complete cylinder-head cooling
presents the one most inexplicable of the more important design
decisions they made, as it does not appear logical. In the original
engine, where the factors of time and simplicity were of paramount
importance, this made sense, but now they were contemplating
considerably increased power requirements, knowing the effect of
temperature on both the cylinder and the weight of cylinder charge,
and knowing that valve failure was one of their most troublesome
service problems. Nor does it seem that they could have been avoiding
complete cylinder cooling through fear of the slightly increased
complexity or the difficulty of keeping the water connections and
joints tight, for they had faced a much more severe problem in their
first engine, where their basic design required that three joints be
kept tight with only two sets of threads, and had rather easily
mastered it; so there must have been some much more major but not
easily discernible factor which governed, for they still continued to
use the poorly cooled head, even carrying it over to their next engine
series. Very probably they did not know the effect on detonation of a
high-temperature fuel-charge.
One of the new engines was intended for use in their future
experimental flying and has become known as _No. 2._ It had a bore of
4-1/8 in., incorporated an oil pump, and at some time shortly after
its construction a fuel pump was added. The fuel pump was undoubtedly
intended to provide a metering system responsive to engine speed and
possibly also to eliminate the small inherent variation in flow of the
original gravity system.
This engine incorporated a cylinder compression release device not on
the original. The exact reason or reasons for the application of the
compression release have not been determined, although the record
shows it to have been utilized for several different purposes under
different operating conditions. Whatever the motivation for its
initial application, it was apparently useful, as it was retained in
one form or another in subsequent engine models up to the last
6-cylinder design. Essentially it was a manually controlled mechanism
whereby all the exhaust valves could be held open as long as desired,
thus preventing any normal charge intake or compression in the
cylinder. Its one certain and common use was to facilitate starting,
the open exhaust valves easing the task of turning the engine over by
hand and making priming easy. In flight, its operation had the effect
of completely shutting off the power. The propellers would then
"windmill" and keep the engine revolving. One advantage stated for
this method of operation was that when power was required and the
control released, the engine would be at fairly high speed, so that
full power was delivered immediately fuel reached the engine. It is
also reported to have been used both in making normal landings and in
emergencies, when an instant power shutdown was desired. Although it
is not clear whether the fuel shutoff cock was intended to be
manipulated when the compression release was used for any of these
reasons, over the many years of its availability, undoubtedly at one
time or another every conceivable combination of operating conditions
of the various elements was tried. Because of the pumping power
required with at least one valve open during every stroke, the
windmilling speed of the engine was probably less than with any other
method of completely stopping power output, but whether this
difference was large enough to be noticeable, or was even considered,
is doubtful.
Since a simple ignition switch was all that was required to stop the
power output, regardless of whether a fuel-control valve or a
spark-advance control was used, it must be concluded that the primary
function of the compression release was to facilitate starting, and
any other useful result was something obtained at no cost. The
compression release was later generally abandoned, and until the
advent of the mechanical starter during the 1920s, starting an engine
by "pulling the propeller through" could be a difficult task. With the
Wrights' demonstrated belief that frugality was a first principle of
design, it is hardly conceivable that they would have accepted for any
other reason the complication of the compression-release mechanism if
a simple ignition switch would have sufficed.
The compression-release mechanism was kept relatively simple,
considering what it was required to accomplish. A small non-revolving
shaft was located directly under the rocker arm rollers that actuated
the exhaust valves. Four slidable stops were placed on this shaft,
each in the proper location, so that at one extreme of their travel
they would be directly underneath the rocker roller and at the other
extreme completely in the clear. They were positioned along the shaft
by a spring forcing them in one direction against a shoulder integral
with the shaft, and the shaft was slidable in its bearings, its
position being determined by a manually controlled lever. When the
lever was moved in one direction the spring pressure then imposed on
the stops would cause each of them to move under the corresponding
rocker roller as the exhaust valve opened, thus holding the exhaust
valve in the open position. When the shaft was moved in the other
direction the collar on the shaft would mechanically move the stop
from underneath the roller, allowing the valve to return to normal
operation.
[Illustration: _Figure 8._--Development engine No. 3, 1904-1906,
showing auxiliary exhaust port, separate one-piece water-jacket block.
(Photo by author.)]
If the 1903 engine is the most significant of all that the Wrights
built and flew, then certainly the _No. 2_ unit was the most useful,
for it was their sole power source during all their flying of 1904 and
1905 and, as they affirmed, it was during this period that they
perfected the art, progressing from a short straightaway flight of 59
seconds to a flight controllable in all directions with the duration
limited only by the fuel supply. It is to be greatly regretted that no
complete log or record was kept of this engine.
The Wrights again exhibited their engineering mastery of a novel basic
situation when, starting out to make flight a practical thing, they
provided engine _No. 3_ to be used for experimental purposes. In so
doing they initiated a system which continues to be fundamental in the
art of providing serviceable aircraft engines to this day--one that is
expensive and time consuming, but for which no substitute has yet been
found. Their two objectives were: improvement in performance and
improvement in reliability, and the engine was operated rather
continuously from early 1904 until well into 1906. Unfortunately,
again, no complete record exists of the many changes made and the
ideas tested, although occasional notes are scattered through the
diaries and notebooks.
In its present form--it is on exhibition at the Engineers Club in
Dayton, Ohio--the _No. 3_ engine embodies one feature which became
standard construction on all the Wright 4-cylinder models. This was
the addition of a number of holes in a line part way around the
circumference of the cylinder barrel so that they were uncovered by
the piston at the end of its stroke toward the shaft, thus becoming
exhaust ports (see Figure 9). This arrangement, although not entirely
novel, was just beginning to come into use, and in its original form
the ports exhausted into a separate chamber, which in turn was
evacuated by means of a mechanically operated valve, so that two
exhaust valves were needed per cylinder. Elimination of this chamber
and the valve arrangement is typical of the Wrights' simplifying
procedure, and it would seem that they were among the very first to
use this form.[14]
[Footnote 14: Rankin Kennedy, _Flying Machines--Practice and Design_,
1909.]
The primary purpose of the scheme was to reduce, by this early release
and consequent pressure and temperature drop, the temperature of the
exhaust gases passing the exhaust valve, this valve being one of their
main sources of mechanical trouble. It is probable that with the
automatic intake valves being used there was also a slight effect in
the direction of increasing the inlet charge, although with the small
area of the ports and the short time of opening, the amount of this
was certainly minor. With the original one-piece crankcase and
cylinder jacket construction, the incorporation of this auxiliary
porting was not easy, but this difficulty was overcome in the
development engine by making different castings for the crankcase
itself and for the cylinder jacket and separating them by several
inches, so that room was provided between the two for the ports.
This engine demonstrated the most power of any of the flat 4s,
eventually reaching an output of approximately 25 hp, which was even
somewhat more than that developed by the slightly larger
4-1/8-in.-bore flight engine, with which 21 hp was not exceeded.
Indicative of the development that had taken place, the performance of
the _No. 3_ engine was twice the utilized output of the original
engine of the same size, an increase that was accomplished in a period
of less than three years.
The Wrights were only twice charged with having plagiarized others'
work, a somewhat unusual record in view of their successes, and both
times apparently entirely without foundation. A statement was
published that the 1903 flight engine was a reworked Pope Toledo
automobile unit, and it was repeated in an English lecture on the
Wright brothers. This was adequately refuted by McFarland but
additionally, it must be noted, there was no Pope Toledo company or
car when the Wright engine was built. This company, an outgrowth of
another which had previously manufactured one-and two-cylinder
automobiles, was formed, or reformed, and a Pope license arrangement
entered into during the year 1903.
The other incident was connected with Whitehead's activities and
designs. Whitehead was an early experimenter in flying, about the time
of the Wrights, whose rather extraordinary claims of successful flight
were published in the 1901-1903 period but received little attention
until very much later. His first engines were designed by a clever
engineer, Anton Pruckner, who left at the end of 1901, after which
Whitehead himself became solely responsible for them. It was stated
that the Wrights visited the Whitehead plant in Bridgeport,
Connecticut, and that Wilbur remained for several days, spending his
time in their machine shop. This was not only categorically denied by
Orville Wright when he heard of it but it is quite obvious that the
1903 or any other of the Wright engine designs bears little
resemblance to Pruckner's work. In fact, its principal design features
are just the opposite of Pruckner's, who utilized vertical cylinders,
the 2-stroke cycle, and air-cooling, which Whitehead at some point
changed to water-cooling.[15]
[Footnote 15: Considerable doubt surrounds Whitehead's actual flight
accomplishments, but Pruckner's engines were certainly used, as
several were sold to early pioneers, including Charles Wittemann. It
is probable that the specific power output was not very great, for the
air-cooled art of this time was not very advanced and Pruckner had a
rather poor fin design. But the change to water cooling eliminated
this trouble, and the engines were most simple, should have been
relatively quite light, and with enough development could probably
have been made into sufficiently satisfactory flying units for that
period.]
The Four-Cylinder Vertical Demonstration Engine and the First
Production Engine
In 1906, while still doing general development work on the flat
experimental engine, the Wrights started two new engines, and for the
first time the brothers engaged in separate efforts. One was "a
modification of the old ones" by Wilbur and the other, "an entirely
new pattern" by Orville. There is no record of any of the features of
Wilbur's project or what was done in connection with it. Two months
after the experimental operation of the two designs began, an entry in
Wilbur's diary gives some weight and performance figures for the "4" x
4" rebuilt horizontal," and since Orville's design was vertical the
data clearly refer to Wilbur's; but since the output is given only in
test-fan rpm it does not serve to indicate what had been accomplished
and there is no further mention of it.
Orville's design became the most used of any model they produced. It
saw them through the years from 1906 to 1911 or 1912, which included
the crucial European and United States Army demonstrations, and more
engines of this model were manufactured than any of their others
including their later 6-cylinder. Although its ancestry is traceable
to the original 1903 engine, the design form, particularly the
external configuration, was considerably altered. Along with many
individual parts it retained the basic conception of four medium-size
cylinders positioned in line and driving the propellers through two
sprocket wheels. From the general tenor of the record it would seem,
despite there being no specific indication, that from this time on
Orville served as the leader in engine design, although this occurred
with no effect whatsoever on their finely balanced, exactly equal
partnership which endured until Wilbur's death in 1912.
The first major change from the 1903 design, putting the engine in an
upright instead of flat position, was probably done primarily to
provide for a minimum variation in the location of the center of
gravity with and without a passenger. Whether or not it had any
influence, the vertical cylinder arrangement was becoming predominant
in automobile powerplants by this time, and the Wright engines now
began to resemble this prevailing form of the internal combustion
engine--a basic form that, in a wide variety of uses, was to endure
for a long time.
Over the years, the Wrights seem to have made many changes in the
engine: the bore was varied at different times, rod assembly methods
were altered, and rod ends were changed from bronze to steel.
Chenoweth states that on later engines an oil-control ring was added
on the bottom of the piston, necessitating a considerable increase in
the length of the cylinder barrel. This arrangement could not have
been considered successful, as it apparently was applied to only a
limited number of units and was not carried over to the later
6-cylinder engine model. There was much experimentation with cam
shapes and most probably variations of these got into production.
With the crankcase, they did not go all the way to the modern
two-piece form but instead retained the one-piece construction.
Assembly was effected through the ends and a detachable plate was
provided on one side for access to the interior. It is clear that they
regarded this ability to get at the interior of the case without major
disassembly as a valuable characteristic, and later featured it in
their sales literature. They were apparently willing to accept the
resultant weakening of the case and continued the construction through
their last engine model. The integrally cast cylinder water jackets
were abandoned and the top of the crankcase was machined flat to
provide a mounting deck for individual cylinders. The use of aluminum
alloy was continued, and the interior of the case was provided with
strengthening webs of considerable thickness, together with supporting
ribs. The cam shaft was supported directly in the case.
The individual cylinder design was of extreme simplicity, a single
iron casting embodying everything except the water jacket. The valves
seated directly on the cast-iron cylinder head and the guides and
ports were all contained in an integral boss on top of the head. The
exhaust valve location on the side of the engine opposite the pilot
was a decided advantage over that of the 1903 design, where the
exhaust was toward the pilot. A four-cornered flange near the bottom
of the cylinder provided for fastening it to the crankcase, and a
threaded hole in the top of the head received a vertical eyebolt which
served as the rocker-arm support. The cylinder was machined all over;
two flanges, one at the bottom and the other about two-thirds of the
way down provided the surfaces against which the water jacket was
shrunk. The jacket was an aluminum casting incorporating the necessary
bosses and double shrunk on the barrel; that is, the jacket itself was
shrunk on the cylinder-barrel flanges and then steel rings were shrunk
on the ends of the jacket over the flanges. The jacket thickness was
reduced by machining at the ends, making a semigroove into which the
steel shrink rings fitted. These rings insured the maintenance of a
tight joint despite the tendency of the aluminum jacket to expand away
from the cast-iron barrel.
[Illustration: _Figure 9._--4-Cylinder vertical engine: a, magneto
side; b, valve port side with intake manifold removed; c, flywheel end
of engine at Carillon Park Museum, Dayton, Ohio; d, magneto side with
crankcase cover removed. (Photos: a, Smithsonian A-3773; b, d, Pratt &
Whitney D-15003, 15007; c, by A. L. Rockwell.)]
Why the one-piece crankcase and cylinder jacket combination of the
1903 engine was abandoned for the individual cylinder construction can
only be surmised. The difference in weight was probably slight, as the
inherent weight advantage of the original crankcase casting was
largely offset by the relatively heavy valve boxes, and the difference
in the total amount of machining required, because of the separate
valve boxes, cages, and attaching parts, also was probably slight.
Although the crankcase had shown itself to be structurally weak, this
could have been cared for by proper strengthening. The 1903 design did
have some fundamental disadvantages: it required a fairly complex
pattern and expensive casting, plus some difficult machining, part of
which had to be very accurate in order to maintain both gas and water
joints tight; and the failure of any one cylinder that affected the
jacket meant a complete crankcase replacement.
It seems probable that a change was initially made mandatory by their
intention to utilize the ported exhaust feature, the value of which
they had proved in the experimental engine. The separate one-piece
water jacket construction they had arrived at in this engine was
available, but once the decision to change was made, the individual
cylinder with its shrunk-on jacket had much to commend it--simplicity,
cost, ease of manufacture and assembly and attachment, and
serviceability. The advantages of the auxiliary, or ported, exhaust
were not obtained without cost, however, as the water jacket around
the barrel could not very easily be extended below the ports. Thus,
even though the water was carried as high as possible on the upper
end, a large portion of the barrel was left uncooled, and the lack of
cooling at the lower end, in conjunction with the uncooled portion of
the head, meant that only approximately half the entire cylinder
surface was cooled directly.
The piston was generally the same as in the 1903 engine, except that
six radial ribs were added on the under side of the head, tapering
from maximum thickness at the center to nothing near the wall. They
were probably incorporated as an added path for heat to flow from the
center of the piston toward the outside, as their shape was not the
best use of material for strength. The piston pin was locked in the
piston by the usual set screw, but here no provision was made for the
alternate practice of clamping the rod on the pin. This piston-pin
setscrew construction had become a standard arrangement in automobile
practice. The piston rings were the normal wide design of that time,
with what would now be considered a low unit pressure.
Quite early in the life of this engine model the practice was
initiated of incorporating shallow grooves in the surface of the more
highly loaded thrust face of the piston below the piston pin to
provide additional lubrication. This development apparently proceeded
haphazardly. Figure 10c shows three of the pistons from an engine of
low serial number--the first of this model to be delivered to the U.S.
Navy--and it will be noted that one has no grooves, another has one,
and the other has three. The eventual standardized arrangement
provided three of these grooves, approximately 1/16 in. wide,
extending halfway around the piston, and, although the depth was only
a few thousandths of an inch, the amount of oil carried in them was
apparently sufficient to assist in the lubrication of the face, as
they were used in both the 4-and 6-cylinder engines.
Each cylinder was fastened to the crankcase by four nuts on studs
driven into the aluminum case. Valves and rocker arms were similar to
those of the early engines, the automatic inlet valve being retained.
The continued use of the two-piece valve is not notable, even though
one-piece forgings were available and in use at this time; the
automobile continued for many years to use this construction. The
camshaft was placed at the bottom of the engine, inside the crankcase,
and the rocker arms were actuated by pushrods which were operated by
hinged cam followers. The pushrod was fastened in the rocker by a pin,
about which it operated, through its upper end and was positioned near
the bottom by a guide in the crankcase deck. The lower end of the rod
bore directly on the flat upper surface of the cam follower, and valve
clearance adjustment was obtained by grinding this end. The camshaft
and magneto were driven by the crankshaft through a three-member train
of spur gears (see Figures 9, 10 and 11).
The built-up construction of the connecting rod was carried over from
the first engine, and in the beginning apparently the same materials
were used, except that the big end was babbited. Later the rod ends
were changed from bronze to steel. The big end incorporated a small
pointed scupper on one side for lubrication, as with the original, and
this was sometimes drilled to feed a groove which carried oil to the
rod bearing, but where the drilling was omitted, the only function the
scupper then could perform was, as in the original engine, to throw a
small amount of oil on the cylinder wall.
The crankshaft and flywheel were similar in design to those on the
1903 engine, except that the sharp corners at the top and bottom of
the crank cheeks were machined off to save weight (see Figure 10f). An
oil pump and a fuel pump were mounted side by side in bosses cast on
the valve side of the crankcase; they were driven from the camshaft by
worm gears and small shafts crossing the case.
[Illustration: _Figure 10._--4-Cylinder vertical engine: a, cylinder
assembly with valve mechanism parts; b, cylinder disassembled, and
parts; c, pistons and connecting rods; d, bottom side of piston; e,
crankshaft, flywheel and crankcase end closure; f, crankcase, with
compression release parts. (Pratt & Whitney photos D-14996, 15001,
14998, 14994, 14999, 14989, respectively.)]
The camshaft construction was considerably altered from the 1903
design. Although the reason is not entirely clear, one indication
suggests that breakage or distortion of the shaft may have been
encountered: whereas in the 1903 engine there had been no relationship
between the location of the cams and the camshaft bearings, in this
engine the exhaust valves were carefully positioned so that all cams
were located very close to the supporting bearings in the crankcase.
Also, the camshaft was solid, although it would seem that the original
hollow shaft construction could have provided equal stiffness with
less weight. The final decision was possibly determined by the
practicality that there existed no standard tubing even approximating
the size and wall thickness desired.
There still was no carburetor, a gear pump metering the fuel in the
same manner as on the 1904-1905 engine. Basically, the intake charge
was fed to the cylinders by a round gallery manifold running alongside
the engine. This was split internally by a baffle extending almost
from end to end, so that the fuel mixture entering the manifold on one
side of the baffle was compelled to travel to the two ends before it
could return to the inside cylinder, this feature being a copy of
their 1903 general intake arrangement. Apparently various shapes and
positions of entrance pipes with which to spray the fuel into the
manifold were used; and the injection arrangement seems also to have
been varied at different times. The fuel pump was not necessarily
always used, as the engine in some of the illustrations did not
incorporate one, the fuel apparently being fed by gravity, as on the
original engine. Chenoweth describes an arrangement in which exhaust
heat was applied to the inlet manifold to assist the fuel vaporization
process, but it is believed that this was one of the many changes made
in the engine during its lifetime and not necessarily a standard
feature.
A water circulation pump was provided, driven directly by the
crankshaft through a two-arm universal joint intended to care for any
misalignment between the shaft and the pump. The water was piped to a
horizontal manifold running along the cylinders just below the intake
manifold, and a similar manifold on the other side of the engine
collected it for delivery to the radiator. It is a little difficult to
understand why it was not introduced at the bottom of the water
jackets.
The crankcase was a relatively strong and well proportioned structure
with three heavy strengthening ribs running from side to side, its
only weakness being the one open side. A sheet-iron sump was fastened
to the bottom by screws and it would appear from its design, method of
attachment, and location of the engine mounting pads that this was
added some time after the crankcase had been designed; but if so it
was apparently retrofitted, as engines with quite low serial numbers
have this part.
The ignition was by high-tension magneto and spark plug and this
decision to change from the make-and-break system was undoubtedly the
correct one, just as adoption of the other form originally was logical
under the circumstances that existed then. The high-tension system was
simpler and had now collected more service experience. The magneto
was driven through the camshaft gear, and a shelf, or bracket, cast as
an integral part of the case, was provided for mounting it. The spark
advance control was in the magneto and, since spark timing was the
only means of regulating the engine power and speed, a wide range of
adjustment was provided.
The engine had the controllable compression release which had been
added to the _No. 2_ and _No. 3_ flat engines, although mechanically
it was considerably altered from the original design. Instead of the
movable stop operating directly on the rocker roller to hold the
exhaust valve open, it was located underneath a collar on the pushrod.
This stop was hinged to the crankcase and actuated by a small rod
running along and supported by the crankcase deck. Longitudinal
movement of this rod in one direction would, by spring pressure on
each stop, push them underneath the collars as the exhaust valves were
successively opened. A reverse movement of the rod would release them
(see Figure 10f). Why they retained the method of manually operating
the compression release, which was the same as had been used in the
1904-1905 engine, is not quite clear. That is, the mechanism was put
into operation by pulling a wire running from the pilot to a lever
actuating the cam which moved the control rod. When normal valve
operation was subsequently desired, the pilot was compelled to reach
with his hand and operate the lever manually, whereas a second wire or
push-pull mechanism would have obviated the necessity for both the
awkward manual operation of the lever and the gear guard which was
added to protect the pilot's hand, the lever being located close to
the camshaft gear.
The 4-cylinder vertical engine was a considerable improvement over the
previous designs. They had obtained a power increase of about 40
percent, with a weight decrease of 10 percent, and now had an engine
whose design was almost standard form for good internal combustion
engines for years to come. In fact, had they split the crankcase at
the crankshaft center line and operated the inlet valves mechanically,
they would have had what could be termed a truly modern design. They
needed more cylinder cooling, both barrel and head, particularly the
latter, and an opened-up induction system for maximum power output,
but this was not what they were yet striving for. They had directly
stated that they were much more interested in reliability than light
weight.
This engine model was the only one of the Wright designs to be
licensed and produced abroad, being manufactured in Germany by the
Neue Automobil-Gesellschaft and by Bariquand et Marré in France. The
latter was much more prominent and their engines were used in several
early European airplanes.
[Illustration: _Figure 11._--4-Cylinder vertical engine assembly,
Bariquand et Marré version. (Drawing courtesy Bristol Siddeley
Engines, Ltd.)]
[Illustration: THE WRIGHT BROTHERS AERO ENGINE]
The French manufacturer, without altering the basic design, made a
number of changes of detail which seem to have greatly annoyed
Wilbur Wright, although some of them could probably be listed as
improvements, based on several features of later standard design. One
consisted of an alteration in the position of the fuel and oil pumps,
the latter being lowered to the level of the sump. The crankcase was
drilled to provide forced-feed lubrication to the connecting rod big
end and crankshaft main bearings. Strengthening ribs were added to the
pistons running from the upper side of the pin bosses to the piston
wall, and the crankcase studs holding down the cylinders were replaced
with bolts having their heads inside the case. The hinged cam follower
was omitted and the pushrod bore directly on the cam through a roller
in its end. The magneto was moved toward the rear of the engine a
considerable distance and an ignition timing control device was
introduced between it and its driving gear. Instead of the magneto
being mounted directly on the special bracket integral with the
crankcase, a wooden board running from front to rear of the engine was
used and this was fastened to the two engine support pads, the magneto
bracket being omitted entirely.
Despite his criticism of the French motor and the quality of its
manufacture, Wilbur was compelled to install one in his own exhibition
airplane during his early French demonstrations at Le Mans after rod
failure had broken his spare crankcase, and much of his subsequent
demonstration flying was made with the French product.
The Eight-Cylinder Racing Engine
By 1909 regular and special air meets and races were being held and
various competitions for trophies conducted. Among these the Gordon
Bennett Cup Race for many years was considered a major event. For the
1910 competition it was decided to enter a Wright machine and, since
this was a race with speed the sole objective, the available
4-cylinder engine, even in a version pushed to its maximum output, was
deemed too small. They built for it a special 8-cylinder unit in a
90°V form. They were thus resorting to one of their 1904
concepts--modifying and enlarging a version known and proved in
use--as the proper method of most quickly increasing output.
Unfortunately again, there are essentially no detailed drawings
available, so that the design cannot be studied.[16]
[Footnote 16: A drawing of the camshaft is held by The Franklin
Institute.]
Only one engine is historically recorded as having been built,
although in view of the Wrights' record of foresight and preparation
it is almost certain that at least one spare unit, assembled or in
parts, was provided. In any case, the airplane--it was called the
_Baby Grand Racer_--and engine were wrecked just before the race, and
no physical parts were retained, so that the sole descriptions come
from external photographs, memory, and hearsay. McFarland thinks that
possibly Orville Wright, particularly, was somewhat discomfited over
the accident that eliminated the machine, as he had previously flown
it quite successfully at a speed substantially higher than that of the
ultimate winner, and he wanted to get it out of sight and mind as
quickly as possible. The Air Force Museum at Wright Field, Dayton,
Ohio, has an incomplete set of drawings of a 90°V, 8-cylinder Wright
engine, but it is quite obvious from the basic design and individual
features, as well as from at least one date on the drawings, that this
conception is of a considerably later vintage than that of the _Baby
Grand Racer_.
The racing engine was in essence a combination of two of the standard
4s on a redesigned crankcase utilizing as many of the 4-cylinder
engine parts as possible. The rods were reported to have been placed
side by side, and the regular 4-cylinder crankshaft, with alterations
to accommodate the rods, was utilized. A single cam operated all the
exhaust valves. It was compact and light, its only fundamental
disadvantage being the inherent unbalance of the 90°V-8. The
arrangement provided a much higher powered unit in the cheapest and
quickest manner, and one that could be expected to operate
satisfactorily with the least development.
The Six-Cylinder Vertical Engines
Shortly after the construction of the 8-cylinder engine the Wrights
were again faced with the ever-recurrent problem of providing a higher
powered standard production engine for their airplanes, which were now
being produced in some numbers. By this time, 1911, there had been a
relatively tremendous growth in both flying and automotive use of the
internal combustion engine and as a result many kinds and sizes had
been produced and utilized, so that numerous choices were presented to
them. But if they were both to make use of their past experience and
retain the simplicity they had always striven for, the more practical
possibilities narrowed down to three: they could increase the cylinder
size in the 4-cylinder combination, or they could go either to 6 or 8
cylinders in the approximate size they had previously used.
[Illustration: _Figure 12._--Original 6-cylinder engine: a, push-rod
side; b, valve-port side; c, crankcase with sump removed. (Photos:
Smithsonian A-3773A, 45598; Pratt & Whitney D-15015, respectively.)]
The 4-in. cylinder in combination with a 5-in. stroke would provide in
four cylinders about the displacement they wanted. Strokes of 6 in.
were not uncommon and cylinders of 6-in. bore had been very
successfully utilized in high-output automobile racing engines many
years before this, so there was seemingly no reason to doubt that the
5-in. cylinder could be made to operate satisfactorily, but it is not
difficult to imagine the Wrights' thoughts concerning the roughness of
an engine with cylinders of this diameter. The question of the grade
of available fuel may possibly have entered into their decision to
some extent, but it seems far more likely that roughness, their
perennial concern, was the predominant reason for not staying with the
more simple 4-cylinder form (as we have seen, roughness to them meant
the effect of the cylinder explosion forces). Actually, of course,
they never went larger than a 4-3/8-in. cylinder bore, and later
aircraft engine experience would seem generally to confirm their
judgment, for with the piston engine it has always been much more
difficult to make the larger bores operate satisfactorily at any given
specific output.
While the 90°V, 8-cylinder arrangement would have enabled them to
utilize a great number of the 4-cylinder-engine parts, it would have
given them a somewhat larger engine than was their apparent desire,
unless they reduced the cylinder size. And while they had had some
limited experience in building and operating this kind of engine, and
twice had chosen it when seeking more power, both of these choices
were greatly influenced by the desire to obtain quickly an engine of
higher power. It is also possible that something in their experience
with the V-8 moved them away from it; the unbalanced shaking force
inherent in the arrangement may well have become evident to them. What
probably also helped them to their final conclusion was the
fundamental consideration that the V-8 provided two extra cylinders
which were not really needed.
The eventual selection of the 6-cylinder was a slight compromise. In
order to get the desired output the cylinder displacement was
increased, but this was done by lengthening the stroke--the first time
this had been altered since the original design. The increase (from 4
to 4-1/2 in.) was only 1/2 in., and the bore, the more important
influence on fuel performance, was kept the same. Overall, the choice
was quite logical. They were utilizing the in-line construction upon
which almost all of their now considerable experience had been based,
and the sizes of and requirements for parts also conformed to this
experience. They could, in fact, use many of the same parts. The
natural balance of the 6-cylinder arrangement gave them a very smooth
engine, and had they stiffened the shaft and counter-weighted the
cranks, they would have produced the smoothest engine that could have
been built at that time.
In the literature are two references to a Wright 6-cylinder engine
constructed around the cylinders of the vertical 4. One of these is in
Angle's _Airplane Engine Encyclopedia_, published in 1921, and the
other is in _Aerosphere 1939_, published in 1940. The wording of the
latter is essentially identical with that of the former; it seems a
reasonable conclusion that it is a copy. Although it is possible that
such an engine was built at some time, just as the 8-cylinder racing
engine was cobbled up out of parts from the 4-cylinder vertical, no
other record, no engines, and no illustrations have been found. It is
thus quite certain that no significant quantity was ever manufactured
or utilized.
The crankcase was considerably changed from that of the vertical 4,
and was now in two pieces, with the split on the crankshaft center
line. However, the shaft was not supported by the lower half of the
case, as eventually became standard practice, but by bearing caps
bolted to the ends of the upper case and, in between, to heavy ribs
running across the upper case between the cylinders. The lower half of
the case thus received none of the dynamic or explosion loads, and,
serving only to support the engine and to provide for its mounting,
was lightly ribbed. In it were incorporated integral-boss standpipe
oil drains which discharged into a bolted-on sump. The upper half of
the case was again left open on one side, giving the desired access to
the interior, and, additionally, the design was altered to provide a
method of camshaft assembly that was much simpler than that of the
vertical 4 (see p. 42).
The cylinder was also greatly altered from that of the vertical 4. It
was made in three parts, a piece of seamless steel tubing being shrunk
on a cast-iron barrel to form the water jacket, with a cast-iron
cylinder head shrunk on the upper end of the barrel. This construction
compelled the use of long studs running from the cylinder head to the
case for fastening down the cylinder (see Figures 12a-c). For the
first time the cylinder heads were water-cooled, cored passages being
provided, and more barrel surface was jacketed than previously,
although a considerable area at the bottom was still left uncooled,
obviously by direct intent, as the ported exhaust arrangement was no
longer employed.
Also for the first time one-piece forged valves were used, but just
when these were incorporated is not certain and, surprisingly, they
were applied to the inlet only, the exhaust valve being continued in
the previous two-piece screwed and riveted construction. The reasoning
behind this is not evident. If a satisfactory two-piece exhaust valve
had finally been developed it would be logical to carry it over to
the new design; but exhaust valves normally being much more
troublesome, it would seem that a good exhaust valve would make an
even better inlet valve and, in the quantities utilized, the two-piece
design should have been much cheaper. In the original 6-cylinder
engine the inlet valves operated automatically as in all previous
models, but at the time of a later extensive redesign (1913) this was
changed to mechanical actuation, and the succeeding engines
incorporated this feature. All the valve-actuating mechanism was
similar to that of the vertical 4, and the engine had the usual
compression-release mechanism, the detail design being carried over
directly from the 4-cylinder.
Design of the piston followed their previous practice, with wide rings
above the pin and shallow grooves below the pin on the thrust face,
and with the pin fastened in the piston by a set screw. The piston had
four ribs underneath the head (see Figure 13b) radiating from the
center and with the two over the pin bosses incorporating
strengthening webs running down and joining the bosses. The piston
length was reduced by 1 in., thus giving a much less clumsy appearance
and, with other minor alterations, a weight saving of 40 percent (see
Figures 13b and c). The rods were for the first time made of I-section
forgings, a major departure, machined on the sides and hand
finished at the ends, with a babbit lining in the big end, the piston
pin bearing remaining steel on steel.
[Illustration: _Figure 13._--Original 6-cylinder engine: a, cylinder
assembly and valve parts; b, bottom side of piston; c, piston, piston
pin and connecting rod; d, valve mechanism; e, crankshaft and
flywheel. (Pratt & Whitney photos D-15012, 15017, 15013, 15018,
respectively.)]
At least two different general carburetion and induction systems were
utilized, possibly three. One, and most probably the original,
consisted of a duplicate of the injection pump of the 4-cylinder
fitted to a manifold which ran the length of the engine, with three
takeoffs, each of which then split into two, one for each cylinder. Of
this arrangement they tried at least two variations involving changes
in the location and method of injecting the fuel into the manifold;
and there seems to have been an intermediate manifold arrangement,
using fuel-pump injection at the middle of the straight side, or
gallery, manifold, which was fed additional air at both ends through
short auxiliary inlet pipes. This would indicate that with the
original arrangement, the end cylinders were receiving too rich a
mixture, when the fuel in the manifold was not properly vaporized.
Although the exhaust was on the same side of the engine as the inlet
system, no attempt was made to heat the incoming charge at any point
in its travel. An entirely different system adopted at the time of the
complete redesign in 1913 consisted of two float-feed Zenith
carburetors each feeding a conventional three-outlet manifold. This
carburetor was one of the first of the plain-tube type, that is, with
the airflow through a straight venturi without any spring-loaded or
auxiliary air valves, and was the simplest that could be devised. When
properly fitted to the engine, it gave a quite good approximation of
the correct fuel and air mixture ratio over the speed-load running
range, although it is considerably more than doubtful that this was
maintained at altitude, as is stated in one of the best descriptions
of the engine published at the time the carburetors were applied.
The compression ratio of this engine was lowered by almost 20 percent
from that of the vertical 4. This, in combination with the low
bore-to-stroke ratio, the unheated charge, and the later mechanically
operated inlet valve, indicates that the Wrights were now attempting
for the first time to secure from an engine something approaching the
maximum output of which it was capable.
As the engine originally came out, it continued to utilize only one
spark plug in each cylinder. The high-tension magneto had a wide range
of spark advance adjustment, which again provided the only control of
the engine when equipped with the original fuel pump injection.
The location of the valves and pushrods was similar to that in the 4,
so that the cams were immediately adjacent to the camshaft bearings,
which were carried in the crankcase ends and in the heavy webs. The
camshaft was gear-driven and the cam shape was similar to that of the
last 4s, with a quite rapid opening and closing and a long dwell,
leaving the valve opening accelerations and seating velocities still
quite high.
The crankshaft was a continuation of their basic design of rather
light construction, particularly in the webs. The cheeks were even
thinner (by 1/4 in.) than those of the 4 although the width was
increased by 1/8 in. (see Figure 13e). For the first time they went to
a forging, the rough contour type of the time, and utilized a
chrome-nickel alloy steel.
Lubrication was by means of the usual gear pump, and the piston and
rod bearings continued to be splash-fed. The rod big-end bearing
carried a small sharp undrilled boss at the point where, on the other
engines, had been located scuppers whose purpose was apparently still
to throw lubricating oil on the cylinder wall carrying the more highly
loaded side of the piston. The rod big-end bearing was lubricated by a
hole on the top of the big-end boss catching some of the crankcase
splash, which was then carried to the bearing by a groove.
When the 6-cylinder engine was completely redesigned in 1913 this led
to the introduction in late fall of that year of a new model called
the 6-60, the 60 designating the rating in horsepower. There is little
in the Wright records to show why such a radical revision was thought
necessary, but the general history of the period gives a rather clear
indication. The competition had caught up to the Wrights in
powerplants. Other engines were being installed in Wright airplanes,
and Navy log books show these other engines being used interchangeably
with those of the Wrights.
Most of the descriptions of the new model published at the time it was
introduced concentrate on the addition of the two carburetors and the
mechanical operation of the inlet valves, but these were only two of
many major changes. The cylinder was completely revised, the intake
being moved to the camshaft side of the engine from its position
adjacent to the exhaust, so that the two ports were now on opposite
sides of the cylinder. By proper positioning of the rocker-arm
supports and choice of their length and angles, all valves were made
operable from a single camshaft. The shrunk-on steel water jacket
cylinder was retained, but the water connections were repositioned so
that the water entered at the bottom and came out at the top of the
cylinder. Over the life of the 6-cylinder engine several different
valve types were used but the published specifications for the model
6-60 called for "cast iron heads"--the old two-piece construction. The
piston pins were case hardened and ground and the crankshaft pins and
journals were heat treated and ground.
The fuel and oil pumps were removed from the side of the crankcase and
a different ignition system was applied, although still of the
high-tension spark-plug type which by this time had become general
practice on all so-called high-speed internal-combustion engines. A
second threaded spark-plug hole was provided in the cylinder head and
despite its more common use for other purposes, it is evident that the
intention was to provide two-plug ignition. It is doubtful that at the
specific output of this engine any power difference would be found
between one-and two-plug operation, so that the objective was clearly
to provide a reserve unit in case of plug failure. However, it was
also used for the installation of a priming cock for starting and
because of the prevalence of single-wire ignition systems on existing
and illustrated engines, it seems to have been used mostly in this
manner, even though dual-ignition systems later became an unvarying
standard for aircraft engines.
Viewed externally, the only part of the engine that appears the same
as the original 6 is the small lower portion of the crankcase; but
what is more visually striking is the beauty of the new lines and
extreme cleanness of the exterior design (see Figures 14 and 15). Many
of their individual parts had shown the beauty of the sparse design of
pure utility but it was now in evidence in the whole. Despite the
proven practical value of their other models, this is the only one
that can be called a good-looking engine, instantly appealing to the
aesthetic sense, even though the vertical 4 is not an ugly engine. The
appearance of their final effort, in a field they were originally
reluctant to enter and concerning which they always deprecated the
results of their own work, was a thing of which a technically trained
professional engine designer could be proud.
The 6-60 was continued in production and development until it became
the 6-70, and indications are that it eventually approached an output
of 80 horsepower.
[Illustration: _Figure 14._--6-Cylinder 6-60 and 6-70 engine, right
rear intake side. (Pratt & Whitney photo.)]
[Illustration: _Figure 15._--6-Cylinder 6-70 engine, incorporating
flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)]
Minor Design Details and Performance of the Wright Engines
In the Wright brothers' various models were many minor design items
which altogether required a great deal of consideration, but which did
not materially affect overall engine performance. The results
generally could all be classed as good practice; however, one of these
utilized in the 4-cylinder vertical engine was rather unorthodox and
consisted of offsetting the cylinders with relation to the crankshaft.
This arrangement, which can be seen in the drawing (Figure 11) was
apparently an attempt to reduce the maximum side load on the piston
during the power stroke, but since the peak gas loading usually occurs
at about 10 to 15 percent of the power stroke, this probably did not
have much effect, and it was not carried over to the 6-cylinder
design.
All engine bearings were of the plain sleeve type and, except for the
bronze and steel bearings in the connecting rod, were of babbit. The
advantages of babbit for bearings were discovered very early in the
development of the mechanical arts, and apparently the Wrights never
encountered a bearing loading sufficiently high to cause a structural
breakdown in this relatively weak material.
Valve openings show no variation through the successive production
engines, although the Wrights most probably experimented with
different amounts. The 1903 engine and the vertical 4-and 6-cylinder
all had lifts of 5/16 in., but the valve-seat angles varied somewhat;
the records show included angles of 110° to 90°--not a large
difference.
The valve-operating mechanism was the same from the first vertical 4
onward. The high side thrust caused by the cam shape required for the
very rapid valve opening they chose was, no doubt, the reason for the
use of the hinged cam follower, and since the same general cam design
was used in their last engine, the 6-cylinder, the same method of
operation which had apparently proved very serviceable was continued.
How satisfactory was the considerably simpler substitute used in the
Bariquand et Marré version of the 4-cylinder engine is not known.
Possibly it was one of the alterations in the Wrights' design that
Wilbur Wright objected to, although in principle it more closely
conforms to the later fairly standard combination valve tappet and
roller construction: The available drawings do indicate, however, that
the cam of the Bariquand et Marré engine was also altered to give a
considerably less abrupt valve opening than the Wright design, so that
there was less side thrust. For the Wright 6-cylinder engine their
4-cylinder cam was slightly altered to provide a rounding off near the
top of the lobe, thus providing some reduction in the velocity before
maximum opening was reached. All their cam designs indicate a somewhat
greater fear of the effect of seating velocities than of opening
accelerations.
Since the range of cylinder diameters utilized did not vary greatly,
the valve sizes were correspondingly fairly uniform. The diameter of
the valves for the original 4-in.-bore cylinder was 2 in., while that
for the 4-3/8-in. bore used in the 6-cylinder engine was actually
slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the
automatic inlet valve, although it did serve them well; but possibly,
as has been previously noted, there were valid reasons for continuing
its use despite the inherently low volumetric efficiency this
entailed.
The inherent weakness in the joints of the three-piece connecting rod
has been pointed out, but aside from this, the design was excellent,
for all the materials and manufacturing methods required were readily
available, and structurally it was very sound. Tubular rods were still
in use in aircraft engines in the 1920s.
The Wrights had a surprisingly thorough grasp of the metallurgy of the
time, and their choice of materials could hardly have been improved
upon. Generally they relied upon the more simple and commonly used
metals even though more sophisticated and technically better alloys
and combinations were available.[17] Case hardening was in widespread
use in this period but their only utilization of it was in some parts
of the drive chains purchased completely assembled and in the piston
pins of their last engine. The treatment of the crankshafts of all
their engines except the final 6-cylinder was typical of their
uncomplicated procedure: the particular material was chosen on the
basis of many years of experience with it, hardening was a very simple
process, and the expedient of carrying this to a point just below the
non-machinable range gave them bearing surfaces that were sufficiently
hard, yet at the same time it eliminated the possibility--present in a
heat-treating operation--of warping the finished piece.
[Footnote 17: Baker states that the first crankshaft was made from a
slab of armor plate and if this is correct the alloy was a rather
complex one of approximately .30-.35 carbon, .30-.80 manganese, .10
silicon, .04 phosphorus, .02 sulphur, 3.25-3.50 nickel, 0.00-1.90
chromium; however, all the rest of the evidence, including Orville
Wright's statement to Dr. Gough, would seem to show that it was made
of what was called tool steel (approximately 1.0 carbon).]
In the entire 1903 engine only five basic materials--excepting those
in the purchased "magneto" and the platinum facing on the
ignition-system firing points--were used: steel, cast iron, aluminum,
phosphor bronze, and babbit. The steels were all plain carbon types
with the exception of the sheet manifold, which contained manganese,
and no doubt this was used because the sheet available came in a
standard alloy of the time.
Overall, the Wright engines performed well, and in every case met or
exceeded the existing requirements. Even though aircraft engines then
were simpler than they became later and the design-development time
much shorter, their performance stands as remarkable. As a result, the
Wrights never lacked for a suitable powerplant despite the rapid
growth in airplane size and performance, and the continual demand for
increased power and endurance.
Few service records dating from before 1911, when the military
services started keeping log books, have been found. Some of those for
the period toward the end of their active era have been preserved, but
for that momentous period spanning the first few years when the
Wrights had the only engines in actual continuous flight operation,
there seems to be essentially nothing--perhaps because there were no
standard development methods or routines to follow, no requirements to
be met with respect to pre-flight demonstrations or the keeping of
service records. Beginning in 1904, however, and continuing as long as
they were actively in business, they apparently had in progress work
on one or more developmental or experimental engines. This policy, in
combination with the basic simplicity of design of these engines,
accounted in large measure for their ability to conduct both
demonstrations and routine flying essentially whenever they chose.
Time between engine overhauls obviously varied. In mid 1906 an engine
was "rebuilt after running about 12 hours." This is comparatively
quite a good performance, particularly when it is remembered that
essentially all the "running" was at full power output. It was
considerably after 1920 before the Liberty engine was redesigned and
developed to the stage where it was capable of operating 100 hours
between overhauls, even though it was being used at cruising, or less
than full, power for most of this time.
The Wrights of course met with troubles and failures, but it is
difficult, from the limited information available, to evaluate these
and judge their relative severity. Lubrication seems to have been a
rather constant problem, particularly in the early years. Although
some bearing lubrication troubles were encountered from time to time,
this was not of major proportions, and they never had to resort to
force-feed lubrication of the main or rod big-end bearings. The piston
and cylinder-barrel bearing surfaces seem to have given them the most
trouble by far, and examination of almost any used early Wright engine
will usually show one or more pistons with evidence of scuffing in
varying degrees, and this is also apparent in the photographs in the
record. This is a little difficult to understand inasmuch as most of
the time they had the very favorable operating condition of cast iron
on cast iron. Many references to piston seizure or incipient seizure,
indicated by a loss of power, occur, and this trouble may have been
aggravated by the very small piston clearances utilized. Why these
small clearances were continued is also not readily explainable,
except that with no combination of true oil-scraper rings, which was
the basic reason why the final form of aviation piston engine was able
to reach its unbelievably low oil consumptions, their large and rather
weak compression rings were probably not doing an adequate job of oil
control, and they were attempting to overcome this with a quite tight
piston fit.[18] In any event, they did encounter scuffing or seizing
pistons and cylinder over-oiling at the same time. As late as 4 May
1908 in the Wright _Papers_ there appears the notation: "The only
important change has been in the oiling. The engine now feeds entirely
by splash...."
[Footnote 18: Their intended piston ring tension is not known.
Measurements of samples from the 4-and 6-cylinder vertical engines
vary greatly, ranging from less than 1/2 lb per sq in. to almost 1-1/4
lb. The validity of these data is very questionable as they apply to
parts with unknown length of service and amount of wear. It seems
quite certain, however, that even when new the unit tension figure
with their wide rings was only a small fraction of that of the modern
aircraft piston engine.]
Their troubles tended to concentrate in the cylinder-piston
combination, as has been true of almost all piston engines. References
to broken cylinders are frequent. These were quite obviously cylinder
barrels, as replacement was common, and this again is not readily
explainable. The material itself, according to Orville Wright, had a
very high tensile strength, and in the 1903 engine more than ample
material was provided, as the barrel all the way down to well below
the attachment to the case was 7/32 in. thick. The exact location of
the point of failure was never recorded, but in its design are many
square corners serving as points of stress concentration. Also, of
course, no method was then available for determining a faulty casting,
except by visual observation of imperfections on the surface, and this
was probably the more common cause. It is interesting, however, that
the engine finally assembled in 1928 for installation in the 1903
airplane sent to England has a cracked cylinder barrel, the crack
originating at a sharp corner in the slot provided at the bottom of
the barrel for screwing it in place.
Valve failures were also a continuing problem, and Chenoweth reports
that a large proportion of the operating time of the 1904-1906
development engine was concentrated on attempts to remedy this
trouble. None of their cams, including those of the 6-cylinder engine,
evidence any attempt to effect a major reduction in seating
velocities. United States Navy log books of 1912 and 1913 record many
instances of inlet valves "broken at the weld," indicating that some
of the earlier 6-cylinder engines were fitted with valves of welded
construction.
For the engineer particularly, the fascination of the Wrights' engine
story lies in its delineation of the essentially perfect engineering
achievement by the classic definition of engineering--to utilize the
available art and science to accomplish the desired end with a minimum
expenditure of time, energy, and material. Light weight and
operability were the guiding considerations; these could be obtained
only through constant striving for the utmost simplicity. Always
modest, the Wrights seem to have been even more so in connection with
their engine accomplishments. Although the analogy is somewhat
inexact, the situation is reminiscent of the truism often heard in the
aircraft propulsion business--few people know the name of Paul
Revere's horse. Yet, as McFarland has pointed out, "The engine was in
fact far from their meanest achievement." With hardly any experience
in this field and only a meagerly equipped machine shop, they designed
and assembled an internal combustion engine that exceeded the
specifications they had laid down as necessary for flight and had it
operating in a period of about two months elapsed time. The basic form
they evolved during this unequalled performance carried them through
two years of such successful evolutionary flight development that
their flying progressed from a hop to mastery of the art. And the
overall record of their powerplants shows them to have been remarkably
reliable in view of the state of the internal combustion engine at
that time.
Appendix
Characteristics of the Wright Flight Engines
-------------------------------------------------------------------------
_1903 _1904-1905 _1908-1911 _1911-1915
First flight Experimental Demonstrations service_
engine[a]_ flights_ and
service_
-------------------------------------------------------------------------
Cyl./Form 4/flat 4/flat 4/vertical 6/vertical
Bore and stroke (in.) 4×4 4-1/8×4 4-3/8×4 4-3/8×4-1/2
Displacement (cu. in.) 201 214 240 406
Horsepower 8.25-16 15-21 28-42 50-75
RPM 670-1200 1070-1360 1325-1500 1400-1560
MEP 49-53 52-57 70-87 70-94
Weight (lb) 140-180 160-170 160-180 265-300
-------------------------------------------------------------------------
[Footnote a: Concurrently with the Wrights' first engine work, Manly
was developing the engine for the Langley Aerodrome, and a comparison
of the Wrights' engine development with that of Manly is immediately
suggested, but no meaningful comparison of the two efforts can be
drawn. Beyond the objective of producing a power unit to accomplish
human flight and the fact that all three individuals were superb
mechanics, the two efforts had nothing in common. The Wrights' goal
was an operable and reasonably lightweight unit to be obtained quickly
and cheaply. Manly's task was to obtain what was for the time an
inordinately light engine and, although the originally specified power
was considerably greater than that of the Wrights, it was still
reasonable even though Manly himself apparently increased it on the
assumption that Langley would need more power than he thought. The
cost and time required were very much greater than the Wrights
expended. He ended up with an engine of extraordinary performance for
its time, containing many features utilized in much later important
service engines. His weight per horsepower was not improved upon for
many years. The Wrights' engine proved its practicability in actual
service. The Manly engine never had this opportunity but its
successful ground tests indicated an equal potential in this respect.
A description of the Langley-Manly engine and the history of its
development is contained in _Smithsonian Annals of Flight_ number 6,
"Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44
figures; Smithsonian Institution Press, 1971)]
It is not possible to state the exact quantities of each engine that
the Wrights produced up to the time that their factory ceased
operation in 1915. Chenoweth gives an estimate, based on the
recollection of their test foreman, of 100 vertical 4s and 50 6s. My
estimate (see page 2) places the total of all engines at close to 200.
Original Wright-built engines of all four of these basic designs are
in existence, although they are rather widely scattered. The
Smithsonian's National Air and Space Museum has examples of them all,
including, of course, the unique first-flight engine. Their condition
varies, but many are operable, or could easily be made so. Among the
best are the first-flight engine and the last vertical 6, at the
Smithsonian, the first vertical 6, at the United States Air Force
Museum, and the vertical 4, at the Carillon Park Museum.
The Wrights were constantly experimenting and altering, and this in
connection with the lack of complete records makes it almost
impossible to state with any certainty specific performances of
individual engines at given times. Weights sometimes included
accessories and at others did not. Often they were of the complete
powerplant unit, including radiator and water and fuel, with no
clarification. In the table, performance is given in ranges which are
thought to be the most representative of those actually utilized.
Occasionally performances were attained even beyond the ranges given.
For example, the 4×4-in. flat development engine eventually
demonstrated 25 hp at an MEP of approximately 65 psi.
One important figure--the horsepower actually utilized during the
first flight--is quite accurately known. In 1904 the 1904-1905 flight
engine, after having been calibrated by their prony-brake test-fan
method, was used to turn the 1903 flight propellers, and Orville
Wright calculated this power to be 12.05 bhp by comparing the
calibrated engine results with those obtained with the flight engine
at Kitty Hawk when tested under similar conditions. However, since the
tests were conducted in still air with the engine stationary, this did
not exactly represent the flight condition. No doubt the rotational
speed of the engine and propellers increased somewhat with the forward
velocity of the airplane so that unless the power-rpm curve of the
engine was flat, the actual horsepower utilized was probably a small
amount greater than Orville's figures. The lowest power figure shown
for this engine is that of its first operation.
No fuel consumption figures are given, primarily because no
comprehensive data have been found. This is most probably because in
the early flight years, when the Wrights were so meticulously
measuring and recording technical information on the important factors
affecting their work, the flights were of such short duration that
fuel economy was of very minor importance. After success had been
achieved, they ceased to keep detailed records on very much except
their first interest--the flying machine itself--and when the time of
longer flights arrived, the fuel consumption that resulted from their
best engine design efforts was simply accepted. The range obtained
became mostly a matter of aerodynamic design and weight carried.
Orville Wright quotes an early figure of brake thermal efficiency for
the 1903 engine that gives a specific fuel consumption of .580 lb of
fuel per bhp/hr based on an estimate of the heating value of the fuel
they had. This seems low, considering the compression ratio and
probable leakage past their rather weak piston rings, but it is
possible. In an undated entry, presumably in 1905, Orville Wright's
notebook covered fuel consumption in terms of miles of flight; one of
the stated assumptions in the entry is, "One horsepower consumes .60
pounds per horsepower hour"--still quite good for the existing
conditions. Published figures for the 6-60 engine centered around .67
lb/hp hr for combined fuel and oil consumption.
The Wright Shop Engine
Despite the fact that the Wright shop engine was not a flight unit, it
is interesting both because it was a well designed stationary
powerplant with several exceedingly ingenious features, and because
its complete success was doubtless a major factor in the Wrights'
decision to design and build their own first flight engine. Put in
service in their small shop in the fall of 1901, it was utilized in
the construction of engine and airframe parts during the vital years
from 1902 through 1908 and, in addition, it provided the sole means of
determining the power output of all of their early flight engines. By
means of a prony brake, its power output was carefully measured and
from this the amount of power required for it to turn certain fans or
test clubs was determined. These were then fitted to the flight
engines and the power developed calculated from the speed at which the
engines under test would turn the calibrated clubs. Although a
somewhat complex method of using power per explosion of the shop
engine was made necessary by the basic governor control of the engine,
the final figures calculated by means of the propeller cube law seem
to have been surprisingly accurate.[19] Restored under the personal
direction of Charles Taylor, it is in the Henry Ford Museum in
Dearborn, Michigan, together with the shop machinery it operated.
[Footnote 19: _The Papers of Wilbur and Orville Wright_, volume 2,
Appendix.]
The engine was a single cylinder, 4-stroke-cycle "hot-tube" ignition
type. The cylinder, of cast iron quite finely and completely finned
for its day, was air-cooled, or rather, air-radiated, as there was no
forced circulation of air over it, the atmosphere surrounding the
engine simply soaking up the dissipated heat. Although this was
possibly a desirable adjunct in winter, inside the small shop in
Dayton, the temperature there in summer must have been quite high at
times. The operating fuel was city illuminating gas, which was also
utilized to heat, by means of a burner, the ignition tube. This part
was of copper, with one completely closed end positioned directly in
the burner flame; the other end was open and connected the interior of
the tube to the combustion chamber. The inlet valve was of the usual
automatic type while the exhaust valve was mechanically operated. The
fuel gas flow was controlled by a separate valve mechanically
connected to the inlet valve so that the opening of the inlet valve
also opened the gas valve, and gas and air were carried into the
cylinder together.
[Illustration: _Figure 16._--Shop engine, 1901, showing governor and
exhaust valve cam. (Photo courtesy R. V. Kerley.)]
The engine was of normal stationary powerplant design, having a heavy
base and two heavy flywheels, one on each side of the crank. These
were necessary to ensure reasonably uniform rotational speed, as, in
addition to having only one cylinder, the governing was of the
hit-and-miss type. It had a 6×7-in. bore and stroke and would develop
slightly over 3 hp at what was apparently its normal operating speed
of 447 rpm, which gives an MEP of 27 psi.
The engine is noteworthy not only for its very successful operation
but also because it incorporated two quite ingenious features. One was
the speed-governing mechanism. As in the usual hit-and-miss operation,
the engine speed was maintained at a constant value, the output then
being determined by the number of power strokes necessary to
accomplish this. The governor proper was a cylindrical weight free to
slide along its axis on a shaft fastened longitudinally to a spoke of
one of the flywheels. A spring forced it toward the center of the
wheel, while centrifugal force pulled it toward the rim against the
spring pressure. After each opening of the valve the exhaust-valve
actuating lever was automatically locked in the valve-open position by
a spring-loaded pawl, or catch. The lever had attached to it a small
side extension, or bar, which, when properly forced, would release the
catch and free the actuating lever. This bar was so positioned as to
be contacted by the governor weight when the engine speed was of the
desired value or lower, thus maintaining regular valve operation; but
an excessive speed would move the governor weight toward the rim and
the exhaust valve would then be held in the open position during the
inlet stroke, so no cylinder charge would be ingested. Since the
ignition was not mechanically timed, the firing of the charge was
dependent only on the compression of the inlet charge in the cylinder,
so it made no difference whether the governor caused the engine to
cease firing for an odd or even number of revolutions, even though the
engine was operating on a 4-stroke cycle at all times.
[Illustration: _Figure 17._--Shop engine, 1901, showing operation of
exhaust valve cam. (Pratt & Whitney drawing.)]
The exhaust valve operating cam was even more ingenious. To obtain
operation on a 4-stroke cycle and still avoid the addition of a
half-speed camshaft, a cam traveling at crankshaft speed was made to
operate the exhaust valve every other revolution (see Figure 17). It
consisted of a very slim quarter-moon outline fastened to a disc on
the crankshaft by a single bearing bolt through its middle which
served as the pivot about which it moved. Just enough clearance was
provided between the inside of the quarter-moon and the crankshaft to
allow the passage of the cam-follower roller. The quarter-moon,
statically balanced and free to move about its pivot, basically had
two positions. In one the leading edge was touching the shaft (Figure
17b), so that when the cam came to the cam follower, the follower was
forced to go over the top of the cam, thus opening the exhaust valve.
When the cam pivot point had passed the roller, the pressure of the
exhaust valve spring forced the following edge of the cam into
contact with the shaft and this movement, which separated the leading
edge of the cam from the shaft, provided sufficient space between it
and the shaft for the roller to enter (Figure 17c). Thus, when the
leading edge of the cam next reached the roller, the roller, being
held against the crankshaft by the valve spring pressure (Figure 17d),
entered the space between the cam and the shaft and there was no
actuation of the valve. In exiting from the space, it raised the
trailing edge of the cam, forcing the leading edge against the shaft
(Figure 17a) so that at the next meeting a normal valve opening would
take place. The cam was maintained by friction alone in the position
in which it was set by the roller, but since the amount of this could
be adjusted to any value, it could be easily maintained sufficient to
offset the small centrifugal force tending to put the cam in a neutral
position.[20]
[Footnote 20: The Wrights apparently never applied for an engine
patent of any kind. This no doubt grew out of their attitude of
regarding the engine as an accessory and deprecating their work in
this field. A reasonably complete patent search indicates that this
particular cam device has never been patented, although a much more
complex arrangement accomplishing the same purpose was patented in
1900, and a patent application on a cam-actuating mechanism
substantially identical to that of the Wrights and intended for use in
a golf practice apparatus is pending at the present time.]
Bibliography
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BAKER, MAX P. The Wright Brothers as Aeronautical Engineers. _Annual
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30, 1950_, pages 209-223, 4 figures, 9 plates.
BEAUMOUNT, WILLIAM WORBY. _Motor Vehicles and Motors: Their Design,
Construction, and Working by Steam, Oil, and Electricity._ 2 volumes.
Philadelphia: J. B. Lippincott, 1901-1902.
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FOREST, FERNAND. _Les Bateaux automobiles._ Paris: H. Dunod et E.
Pinat, Éditeurs, 1906.
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KELLY, FRED C. _Miracle at Kitty Hawk; the Letters of Wilbur and
Orville Wright._ New York: Farrar, Straus and Young, 1951.
---------- _The Wright Brothers, a Biography Authorized by Orville
Wright._ New York: Harcourt, Brace & Co., 1943.
KENNEDY, RANKIN. _Flying Machines: Practice and Design. Their
Principles, Construction and Working._ 158 pages. London: Technical
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Conference, Washington, D.C., 12-14 December 1928, Papers Submitted by
the Delegates for Consideration by the Conference. Washington:
Government Printing Office, 1928.
MCFARLAND, MARVIN W. _The Papers of Wilbur and Orville Wright._ 2
volumes. New York: McGraw Hill Book Co., 1953.
RENSTROM, ARTHUR G. Wilbur and Orville Wright: A Bibliography
Commemorating the Hundredth Anniversary of the Birth of Wilbur Wright,
April 16, 1867. Washington, D.C.: The Library of Congress [Government
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The 6-Cylinder 60-Horsepower Wright Motor. _Aeronautics_ (November
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Index
Angle, Glenn D., 51
_Baby Grand Racer_, 47
Baker, Max P. 1, 10, 26, 28
Bariquand et Marré, 43, 44-45, 57-58
Beaumount, William Worby, 9, 25
Bristol Siddeley Engines, Ltd., 44-45
Carillon Park Museum, Dayton, Ohio, ix, 5n, 7, 37
Chanute, Octave, 28
Chenoweth, Opie, ix, 22, 35, 42, 63
Christman, Louis P., ix, 7, 8, 28
Cole, Gilmoure N., ix
Clarke, J. H., 18
Daimler-Benz A. G., ix, 10, 13
Engineers Club, Dayton, Ohio, ix, 32
Ford, Henry, 8
Ford, Henry, Museum, Dearborn, Michigan, 8, 64
Forest, Fernand, 11
Franklin Institute, Philadelphia, Pennsylvania, ix, 47
Gough, Dr. H. J., 58n
Howell Cheney Technical School, Manchester, Connecticut, x, 14, 15
Kelly, Fred C, 4n
Kerley, R. V., ix, 65
_Kitty Hawk Flyer_, ii, 3
Langley [Samuel P.] Aerodrome, 9, 62
Loening, Grover C, 13n
Manly, Charles L., 9, 62
Maxim, Sir Hiram Stevens, 3
McFarland, Marvin W., 1, 33, 47, 61
Miller-Knoblock Manufacturing Co., South Bend, Indiana, 26
National Park Service, Cape Hatteras National Seashore, ii, ix
Neue Automobil-Gesellschaft, 43
Porter, L. Morgan, ix
Pratt & Whitney Aircraft Corp., v, x, 37, 40-41, 49, 52, 53, 67
Pruckner, Anton, 33
Rockwell, A. L., ix, 37
Santos-Dumont, Alberto, 11
Science Museum, London, x, 5, 6, 7, 8, 11, 21, 23, 26
Taylor, Charles E., 5, 64
United Aircraft Corp., v, x
Western Society of Engineers, 2
Whitehead, Gustave, 33
Wittemann, Charles, 33n
Wright, Bishop Milton (father), 28
Wright, Katherine (sister), 4
Zenith carburetor, 52
*U.S. GOVERNMENT PRINTING OFFICE: 1971--397-764
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