Introduction to aeronautics: a design perspective




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INTRODUCTION TO AERONAUTICS: A DESIGN PERSPECTIVE
CHAPTER 9: CASE STUDIES AND FUTURE AIRCRAFT DESIGNS
[Concerning] engines of war, the invention of which has long since reached its limit, and for the improvement of which I see no further hope in the applied arts ...”
Sextus Julius Frontius, Roman engineer

In the development of air power, one has to look ahead and not backward and figure out what is going to happen, not too much of what has happened.”


Billy Mitchell


9.1 INTRODUCTION
In this final chapter, you are invited to consider several examples of the design method as it has been applied to several interesting and important aircraft, and to thoughts of future aircraft. The knowledge and skills in aeronautics and the design method which you have gained by working through the previous chapters should have given you a greater ability to appreciate the material which follows. These design examples or case studies include descriptions of the influence of technology and customer needs on each design, and of the many decisions and compromises which had to be made in order to bring each aircraft into existence. Each case study also shows that each aircraft was shaped by many iterations through the design cycle, often involving progressive improvement of earlier designs into the final form of the aircraft.
Chapter 1 gave you an overview of the design method with an example, a description of engineering design in an aeronautical context, and a brief introduction to the history of aircraft design. Figures 1.3 and 1.7 should have served to inform you that design can be thought of as a cyclical process that requires creative synthesis, that it is heavily dependent on detailed analysis and that, in practice, it must converge on a solution. Engineering design is not only a matter of focusing on the right questions -- decisions and corresponding solutions are required!
Chapters 2 through 8 were created to introduce you to some of the terminology, procedures, and issues of aeronautics in the context of aircraft design. In this last chapter, we want to leave you with some background on design successes of the past as well as some of the challenges and opportunities that exist in aircraft design today. Recall the following quote from Theodore von Karman: “A scientist discovers that which exists. An engineer creates that which never was.” The central theme of this book is that design, and the creative work that it entails, is the essence of engineering.

9.2 CASE STUDY #1 - The 1903 Wright Flyer
The body of written material on the Wright brothers is immense. Their epic achievement which culminated in the first sustained flight of a manned powered aircraft is truly one of the turning-points in human history. The next few paragraphs will attempt only to identify how the Wright brothers used the scientific (design) method described in Chapter 1 to make the many breakthroughs which ushered in the aviation age. For some extremely interesting and inspiring reading in more detail on the Wrights and their work see Miracle at Kitty Hawk1 and The Wright Brothers2 by Fred C. Kelly, Wilbur and Orville3 by Fred Howard, The Bishop’s Boys4 by Tom D. Crouch, or any of the many other books on this subject. The comments that follow are distilled from the four sources just listed.
Though their education did not extend beyond secondary school and they earned their living by running a bicycle shop, the Wright brothers were true scientists and engineers. They followed the steps of the design method faithfully and methodically, making many iterations through the design cycle, and successively solving each of the problems they encountered. The paragraphs that follow will list some of the activities and accomplishments of the Wrights in each step of the design process. As a result, this case study will follow a topical rather than a chronological organizational pattern. It is hoped that this arrangement of the material will make it easier for you to appreciate how faithfully the Wrights followed the design method, and how remarkable an achievement they produced as a result.

Defining the problem(s)

The Wrights, especially Wilbur, had been interested in the idea of human flight since their boyhood. They initially framed the problem by stating the fact that various types of animals and insects were able to fly with apparent ease, and that they believed that with the right combination of knowledge, skill, and technology, man could be enabled to do the same. The problem was to determine how this could be done. As they studied the problem further, they broke it down into many sub-problems, the first being how to shape wings so that they generated lift when moved through the air with minimum drag penalty. They needed to determine what camber, location of maximum camber, and thickness to use for their airfoils, and what aspect ratio, planform shape, and structure to use for their wings. They wondered whether a monoplane, biplane, or triplane was a more efficient design concept for their airplane. They also identified the need for an engine/propeller propulsion system which could generate sufficient power while not weighing too much. These considerations were in the thinking of most aeronautical researchers of the day. One unique aspect of the Wrights’ work was in their recognition of the need for adequate control of their aircraft about all three axes. In fact they stated on several occasions that they believed that solving the problems of stability and control was the key to successful manned flight. Perhaps their extensive experience with designing, building, and riding bicycles gave them a better appreciation for the interaction between humans and machines, and for the role of control and stability in the design of a successful man-operated machine.


As their work progressed, the Wrights identified and solved countless other problems; where to conduct their flying experiments, what kind of control surfaces and mechanisms to use, how to transmit the engine power to the propellers, even how to transport their airplane from Dayton, Ohio to Kitty Hawk, North Carolina. In each case, they tackled the problem or question deliberately and methodically, in close adherence to the steps of the design method.
Gathering Information

The spark of interest in flight which the Wrights had held since their boyhood was kindled into a flame when they read in a newspaper in 1896 of the death after a flying accident of Otto Lilienthal, a German scientist and flight pioneer, and at the time the world’s most experienced glider builder and pilot. They gradually came to the opinion that sustained manned flight was possible and began pursuing the problem in earnest. Their experience in the bicycle shop gave the brothers a solid engineering background and good mechanical skills, but they had very little background in flight-related topics. They began reading encyclopedias and library books about flight in nature and anything else flight-related which they could obtain. As they read and discussed the many ideas stirred by their reading, the brothers’ interest and enthusiasm grew. In 1899, Wilbur wrote a letter to the Smithsonian Institution seeking a list of good books on manned flight. The list of authors and written works which he received in return read like a who’s who of recent aeronautical research. Included in the list were a pamphlet by Lilienthal, a pamphlet and a book by Samuel P. Langley, secretary of the Smithsonian, (who had built and successfully flown a small unmanned steam-powered aircraft in 1896) and a comprehensive review of the aeronautical state of the art by Octave Chanute, a French-born American civil engineer who was the foremost promoter of aviation in the USA at the time.




Figure 9.1 Otto Lilienthal Flying One of His Gliders in 1894 (Photo Courtesy of the National Air and Space Museum)



Figure 9.2 A Biplane Glider Designed, Built, and Tested by Octave Chanute in 1896 (Photo Courtesy of the National Air and Space Museum)

The book by Chanute gave the Wrights a great deal of information , and they were apparently impressed by Chanute’s broad knowledge and experience, so in 1900 Wilbur wrote to Chanute asking for his recommendation for a good place to conduct their flying experiments. Chanute suggested several good sites in reply, and over the ensuing years provided the Wrights with considerable encouragement, some equipment, and a great deal of (often unwanted) advice. The Wrights also wrote to the U.S. Weather Service, and based on the information they received selected the beaches and dunes near Kitty Hawk, North Carolina as a site with reliable winds and good weather during late autumn and early winter. Chanute, at the Wrights’ invitation, visited Kitty Hawk several times while they were conducting their experiments.


The Wrights’ careful reading and studying of the aeronautical literature of the day brought their knowledge of the basic theories and technologies of flight to a level of understanding which was equal to that of anyone in the world at the time. As they read and discussed (frequently argued) the various issues raised by these books and pamphlets, the Wrights identified gaps in their knowledge, and doggedly sought out the missing information. They paid particular attention to the writings of Otto Lilienthal, and came to agree with him that actual flight experience was essential to their mastery of the problem. Perhaps, once again, their experiences designing and building bicycles had taught them how important it was for a vehicle designer to be an experienced operator of such vehicles, in order to appreciate all the factors which contributed to good performance and handling qualities.
As the Wrights began using the wealth of information they had obtained, they discovered errors, even in the data supplied by Lilienthal. As a consequence, they were forced to make their own tests of airfoils and wings. They built a wind tunnel with a delicate and intricate force balance to measure lift and drag. They also performed ground tests of engines and propellers, and flight tests of a variety of kites and gliders to obtain the information they required. In all cases their research was extremely methodical, meticulous, and systematic, so that the data they obtained was the most accurate of its type at the time.



Figure 9. 3 A Mockup of the Wright Brothers’ Wind Tunnel (Photo Courtesy of the National Air and Space Museum)
Creating Design Concepts

The overall configurations of the Wrights’ airplanes were initially quite similar to those of other researchers, and in fact resembled several biplane gliders which had been built and flown by Chanute. However , they made many original inventions both on their airplanes and in the equipment they built to obtain test data. Their wind tunnel and force balance, so essential to obtaining accurate aerodynamic data, were all of their own design. The Wrights also designed and built their own engine, propellers, and drive system.


The fact of Lilienthal’s fatal crash, in which his ability to control his glider’s attitude by shifting his weight was insufficient to recover from the affects of a wind gust, haunted the Wrights’ research and discussions, and challenged them to create a more effective control system. The control system design which resulted, especially wing warping for roll control, was so completely new that the Wrights were able to obtain a patent on it. The roll control system in their 1902 and 1903 aircraft included a rudder which moved in concert with the wing warping to help coordinate the turn. This control system, which gave the Wrights the ability to maneuver like a bird, was the feature unique to the Wright aircraft which made the planes not only possible but practical.

Figure 9.4 The Wright Brothers’ 1902 Glider at Launch (Photo Courtesy of the Library of Congress)


Figure 9.5 The Wright 1903 Flyer, Ready for a Flight Attempt on December 14, 1903 (Photo Courtesy of the Library of Congress)

Unfortunately, in the area of pitch control, the brothers over-corrected for the inadequacies of Lilienthal’s machine. They placed their pitch control surface forward of the wing to give it more control authority, but they also made their aircraft statically unstable in pitch. The result was an aircraft which required constant pilot control inputs to remain in level flight, and which was prone to sudden pitch-ups and dives which caused numerous crashes. So, though their bank control system and coordinated rudder made their aircraft the first really practical flying machine, their pitch control system made it relatively difficult and dangerous to fly.


Unable to find a willing manufacturer, the Wrights designed and built their own engine. It was extremely simple, and did not put out much power for its weight, but it was adequate. The brothers also designed and built their own propellers and the system to transmit engine power to them. Not too surprisingly, their power transmission system used bicycle chains and sprockets.


Figure 9.6 The Wright 1903 Flyer, Damaged After its Fourth Flight on December 17, 1903, Showing Clearly the Engine, Chain Drives, and Hand-Carved Propellers (Photo Courtesy of the Library of Congress)

Analysis

The books by Lilienthal, Langley, and Chanute taught the brothers how to predict the lift and drag which would be generated by a wing, and what amount of engine power would be needed to give an aircraft a specified performance. However, as they used these equations in designing their first aircraft, they discovered to their dismay that the predictions they made were incorrect. By carefully testing many wing shapes in their wind tunnel and by building a series of kites and gliders for flight test, they determined more accurate values for the coefficients in the equations they were using. When they finally built their first powered aircraft, they knew exactly how much power they needed from their engine, how fast their aircraft would be able to fly, and how much range it would have on the tiny amount of fuel they carried.



Decisions

Hesitantly at first, but with increasing confidence, the Wright brothers made a series of design decisions which shaped their aircraft. The initial choice of a configuration similar to Chanute’s was a conservative one. The biplane arrangement offered the ability to build a very light truss-type structure with bracing wires between the wings. The tail in the rear gave Chanute’s and Lilienthal’s gliders natural stability, but the Wrights, concerned about the lack of control which caused Lilienthal’s death, put a moveable control surface in front of the wings, reducing stability but vastly increasing control. As the design and construction process continued through wind tunnel tests, concept demonstrator and prototype construction, and flight tests, the Wrights made countless design decisions. Each decision was based on careful analysis and hours of discussion. The result was one of the most incredible achievements in history.


9.3 CASE STUDY #2 - The Douglas DC-3
In the entire history of commercial air transportation, one aircraft stands out above all others, the DC-3, famous third model in the Commercial series of the Douglas Aircraft Company. The DC-3 has been used for every conceivable purpose to which an aircraft can be put, but above all it surely must be considered as the most successful airliner ever built.

Figure 9.7 The Douglas DC-3 (Photo Courtesy of the National Air and Space Museum)

Air Transport in the 1920s

Great strides were being made towards improving the world economy at this time. An important factor was the increasingly intensive operation of transport aircraft. Immediately after World War One the highest aircraft utilization rates attained in practice were usually in the range of 500-800 hours per year. Often they were a great deal less than this. In the late 1920s, however, rates rose towards 1000 hours and, in the 1930s, still further to between 1,000 and 2,000 hours. The actual values varied between operators.


From about 1922, it began to be realized that a primary technical requirement for future transport aircraft was that they should be freed, as soon as possible, of the need to land immediately in the event of engine failure. None of the early twins had acceptable single engine performance. Furthermore, there was no prospect, at the stage then reached in aircraft development, of producing a twin that would fly satisfactorily on one engine while carrying a commercial load. The obvious solution was a three-engined aircraft.
The technical developments which led to the ‘modern’ airliner

As with many innovative, record-breaking aircraft, the DC-3 was made possible and successful by advances in aviation technology which gave it cost, safety, and performance advantages. Each of these advances made significant improvements in the capabilities of all aircraft which made use of them, but they came together most effectively in the DC-3. The synergy of these technological advances being brought together in a single new design produced a quantum jump in air transport capability which made the DC-3 one of the most prolific and long-lived aircraft ever built.


Engine Developments

All through the 1920s and into the 1930s steady progress was made improving the reliability and economy of airline operations. Apart from the switch from single- and twin- to three-engine types, much was being done to improve the powerplants themselves. Some indication of this is given by the gradual lengthening of the running times authorized between overhauls, which stood at between 15 and 50 hours during World War One. In the early 1920s liquid-cooled engines, exclusively used at that time, had overhaul lives of 120-150 hours. These were gradually superseded by the air-cooled radial engine of much improved reliability and economy but not of greatly increased power. By the middle to late 1920s, when air-cooled radials were coming into general use, overhaul lives had increased to 200-300 hours, as shown in Table 9.1.



Table 9.1 Engine Characteristics 6


Type and date

Aircraft

Take-off power (bhp)

Cruising sfc (lb/bhp/hr)

Weight (lb/bhp)

Fuel Octane number

Initial overhaul life (hr)

P&W Wasp

R-1340C (1930)



Ford 5-ATC Northrop Alpha

450

0.55

1.7

73

300

Wright Whirlwind

J-6 (1930)



Fokker F-7

Ford 4-ATC



300

0.55

1.8

73

250

Bristol Jupiter XIF

(1931)


Handley-Page HP.42

525

0.53

1.84

73

200

P&W Wasp

R-1340 SIDI (1933)



Boeing 247

550

0.526

1.79

80

N/A

Wright Cyclone

R-1820-F3 (1934)



Douglas DC-2

710

0.57

1.45

87

N/A

P&W Twin Wasp

R-1830B (1936)



Douglas DC-3

1,000

0.46

1.67

87

500

American engine makers were beginning to get substantial civil sales by 1930, and they began to modify development procedures to give the airlines what they wanted. In 1931 Pratt & Whitney (P&W) decided to add 50 hours of running at high temperature to the 150 hour bench test that the government required for new engines to prove durability. The next year, engines for the Boeing 247 were put through even more stringent tests to prove their reliability, including 100 hours of full-throttle running and 500 hours at cruising power. This made the tests three to four times more severe than for military engines. Maintenance expenses for engines fall almost in direct relation to the time between overhauls. Long engine overhaul lives and high reliability were (and still are) vitally important to the economy of airline operations. This is indicated by the fact that no less than 40-50% of the variable operating costs at that time were attributed to engine maintenance and overhaul.


With the emphasis almost entirely on improved reliability and durability, there was no marked increase in power output. Indeed, until the ‘modern-type’ monoplane transports began to appear, in the early 1930s, increases in aircraft performance remained very modest. Cruising speeds continued to be around 100mph, the fastest being 125mph. To get a Certificate of Airworthiness, all civil aircraft were supposed to be capable of reaching a height of 66ft (20m) on take-off within a distance of 656ft (200m) at full load in still air conditions. This was the so-called ‘ICAN screen’ requirement. However, Junkers and other European monoplane designers soon started exploiting the advantages of higher wing loadings. This meant that airfield performance of their aircraft tended to be significantly inferior to that of the more traditional biplanes. Even so, major airports did not need to have runs of more than about 3,000ft, and grass runways were still in common use.
Developments in Structural Design

For the first few years after 1919 the biplane transport aircraft were of the traditional ‘stick and string’ form of construction, which had been used since the earliest days. Airframes consisted essentially of wooden, wire-braced, frameworks that were covered by fabric. The only really important structural advance in biplane designs was the substitution, in the 1920s, of metal for wood in their framework structures. These normally had a practical life of about 5,000 flying hours.


Two technical developments, both of which first appeared during World War I, were to lead to the first revolution in transport aircraft design. These were the Junkers all-metal cantilever low wing monoplane, and the Fokker high cantilever wing of all-wood construction mounted on a fabric-covered fuselage of welded metal tubes. Junkers produced his first low-wing monoplane transport, the F.13, in 1919 and this started an evolutionary process that was to prove outstandingly successful between the wars. The F.13 and a number of other designs were built in quantity and widely used in many countries. They used a new type of all-metal construction with a corrugated aluminum alloy skin. The metal skin contributed a little to the strength of the structure, by helping the internal spars to carry the shear loads, but it was not stressed to the extent of the ‘modern-type’ airliner of the 1930s. Airframe lives increased to 20,000 hours or more. The corrugations imposed a drag penalty so that the advantages of the robust and durable metal structure were to some extent offset by their aerodynamic ‘dirtiness’.
It was not until 1924 that Junkers produced the first of his three-engine monoplanes which, in a later (1932) version, the Ju52/3M, persisted in widespread use until 1945. Junkers was followed by Fokker, who flew his famous Trimotor in 1925. Then, in 1926, Henry Ford in the United States used a derivative of the Junkers all-metal corrugated structure for a high-wing monoplane of broadly Fokker configuration. The Ford 5-AT was put into intensive production, with over four per week being produced during 1929. It was the first successful airliner built in America, but its real significance was that Henry Ford was marketing it. When there were virtually no passenger services in the US, he at first used the aircraft to operate a freight service, but encouraged airlines to think of carrying passengers. The Ford was faster than the Fokker or Junkers, having more power, and it may have been slightly better streamlined.
The trimotor monoplanes were a parallel development to biplanes and had similar characteristics, except that they required rather bigger airfields, were a bit faster (cruising speeds up to 125mph) and proved to have more durable structures. These were the first transport aircraft to be built in quantity: about 200 each of the Fokker and Ford and eventually about 4,000 of the Junkers, mostly for the Lufwaffe.
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