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SERIAL PUBLICATIONS OF THE SMITHSONIAN INSTITUTION
The emphasis upon publications as a means of diffusing knowledge was
expressed by the first Secretary of the Smithsonian Institution. In
his formal plan for the Institution, Joseph Henry articulated a
program that included the following statement: "It is proposed to
publish a series of reports, giving an account of the new discoveries
in science, and of the changes made from year to year in all branches
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thousands of titles in serial publications under the Smithsonian
imprint, commencing with _Smithsonian Contributions to Knowledge_ in
1848 and continuing with the following active series:
_Smithsonian Annals of Flight_
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In these series, the Institution publishes original articles and
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Institution Press as long as stocks are available.
S. DILLON RIPLEY
_Secretary_
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 60c.
3. The Liberty Engine 1918-1942, by Philip S. Dickey. 1968.
110 pages, 20 figures, appendix, bibliography. Price 75c.
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-Untertuerkheim,
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" 4x4-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
4x4-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 deg. 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.