The Wright Brothers' Engines and Their Design Part 1

The Wright Brothers' Engines and Their Design.

by Leonard S. Hobbs.

Foreword

In this fifth number of _Smithsonian Annals of Flight_ Leonard S.

Hobbs a.n.a.lyzes 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 quant.i.ties 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 Inst.i.tute of Aeronautical Sciences (later the American Inst.i.tute 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 s.p.a.ce Museum_

_March 1970_

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 a.s.sistance 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 a.s.sistance; 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 inst.i.tutions who gave their a.s.sistance so freely. These include the following:

Air Force Museum, Wright-Patterson Air Force Base, Ohio Carillon Park Museum, Dayton, Ohio Connecticut Aeronautical Historical a.s.sociation, Hebron, Connecticut Fredrick C. Crawford Museum, Cleveland, Ohio Historical Department, Daimler Benz A. G., Stuttgart-Unterturkheim, West Germany Engineers Club, Dayton, Ohio Deutsches Museum, Munich, West Germany Educational and Musical Arts, Inc., Dayton, Ohio Henry Ford Museum, Dearborn, Michigan Franklin Inst.i.tute, Philadelphia, Pennsylvania Howell Cheney Technical School, Manchester, Connecticut Library of Congress, Was.h.i.+ngton, D.C.

Naval Air Systems Command, U.S. Navy, Was.h.i.+ngton, D.C.

Science Museum, London, England Victoria and Albert Museum, London, England

In particular, very extensive contributions were made by the Smithsonian Inst.i.tution 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 circ.u.mstances, they very early found themselves in the aircraft engine business despite their inexperience. This business was carried on very successfully, against increasingly severe compet.i.tion, 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 compet.i.tion, 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 compet.i.tion 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 quant.i.ty 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 a.s.sumptions 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 quant.i.ty 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 quant.i.ty 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 quant.i.ties would not only be an expensive purchased article but would also take considerable time to build, even under the most favorable circ.u.mstances. 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 sc.r.a.p 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 Inst.i.tute), 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 disa.s.sembled, but in 1939 the engine was removed from the airplane, disa.s.sembled, 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 Inst.i.tution. 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 Inst.i.tution, 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.

[Ill.u.s.tration: 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 disa.s.sembled 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 rea.s.sembled for an exhibition and in 1928 it was again rea.s.sembled for s.h.i.+pment 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.

[Ill.u.s.tration: 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.