Today the airplane is part of everyday life, whether we see one gracefully winging overhead, fly in one, or receive someone or something (package, letter, etc.) that was delivered by one. The invention and development of the airplane is arguably one of the three most important technical developments of the twentieth century—the other two being the electronics revolution and the unleashing of the power of the atom.
The first practical airplane was invented by Orville and Wilbur Wright, two bicycle shop proprietors from Dayton, Ohio. On December 17, 1903, the Wright Flyer lifted from the sand of Kill Devil Hill near Kitty Hawk, North Carolina, and with Orville at the controls, flew a distance of 120 ft above the ground, staying in the air for 12 sec. It was the first successful, sustained flight of a heavier-than-air piloted airplane. The photograph of the Flyer as it is lifting off the ground, with Wilbur running alongside to keep the right wing tip from digging into the sand, is the most famous photograph in the annals of the history of aeronautics. There were three more flights that morning, the last one covering a distance of 852 ft above the ground, and remaining in the air for 59 sec. At that moment, the Wright brothers knew they
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had accomplished something important—a feat aspired to by many before them, but heretofore never achieved. But they had no way of knowing the tremendous extent to which their invention was to dominate the course of events in the twentieth century—technically, socially, and politically.
The history of the technical development of the airplane can be divided into four eras: pre-Wright; the strut-and-wire biplane; the mature propeller-driven airplane, and the jet-propelled airplane. We will organize our discussion in this article around these four eras.
THE PRE-WRIGHT ERA
Before the Wright brothers' first successful flight, there were plenty of attempts by others. Indeed, the Wrights did notinvent the first airplane. They inherited a bulk of aeronautical data and experience achieved by numerous would-be inventors of the airplanes over the previous centuries. In many respects, when the Wright brothers began to work on the invention of the practical airplane, they were standing on the shoulders of giants before them.
From where and whom did the idea of the modern configuration airplane come? The modern configuration that we take for granted today is a flying machine with fixed wings, a fuselage, and a tail, with a separate mechanism for propulsion. This concept was first pioneered by Sir George Cayley in England in 1799. Cayley is responsible for conceiving and advancing the basic idea that the mechanisms for lift and thrust should be separated, with fixed wings moving at an angle of attack through the air to generate lift and a separate propulsive device to generate thrust. He recognized that the function of thrust was to overcome aerodynamic drag. In his own words, he stated that the basic aspect of a flying machine is "to make a surface support a given weight by the application of power to the resistance of air."
To key on Cayley's seminal ideas, the nineteenth century was full of abortive attempts to actually build and fly fixed-wing, powered, human-carrying flying machines. Cayley himself built several full-size aircraft over the span of his long life (he died in 1857 at the age Page 33 | Top of Article of eighty-three), but was unsuccessful in achieving sustained flight. Some of the most important would-be inventors of the airplane were William Samuel Henson and John Stringfellow in England, Felix Du Temple in France, and Alexander Mozhaiski in Russian. They were all unsuccessful in achieving sustained flight. In regard to the nature of airplane performance and design, we note that these enthusiastic but unsuccessful inventors were obsessed with horsepower (or thrust). They were mainly concerned with equipping their aircraft with engines powerful enough to accelerate the machine to a velocity high enough that the aerodynamic lift of the wings would become large enough to raise the machine off the ground and into the air. Unfortunately, they all suffered from the same circular argument—the more powerful the engine, the more it weighs; the heavier the machine is, the faster it must move to produce enough lift to get off the ground; the faster the machine must move, the more powerful (and hence heavier) the engine must be—which is where we entered this circular argument. A way out of this quandary is to develop engines with more power without an increase in engine weight, or more precisely, to design engines with large horsepower-to-weight ratios. The thrust-to-weight ratio, T/W, for the entire aircraft, is a critical parameter in airplane performance and design. In the nineteenth century, inventors of flying machines functioned mainly on the basis of intuition, with little quantitative analysis to guide them. They knew that, to accelerate the aircraft, thrust had to be greater than the drag; that is, T - D had to be a positive number. And the larger the thrust and the smaller the drag, the better things were. In essence, most of the nineteenth-century flying machine inventors were obsessed with brute force—given enough thrust (or horsepower) from the engine, the airplane could be wrestled into the air. The aviation historians call such people "chauffeurs." They were so busy trying to get the flying machine off the ground that they paid little attention to how the machine would be controlled once it got into the air; their idea was that somehow the machine could be chauffeured in the air much as a carriage driven on the ground. This philosophy led to failure in all such cases.
The antithesis of the chauffeur's philosophy was the "airman's" approach. In order to design a successful flying machine, it was necessary to first get up in the air and experience flight with a vehicle unencumbered by a power plant; that is, you should learn to fly before putting an engine on the aircraft. The person who introduced and pioneered the airman's philosophy was Otto Lilienthal, a German mechanical engineer, who designed and flew the first successful gliders in history. Lilienthal first carried out a long series of carefully organized aerodynamic experiments, covering a period of about twenty years, from which he clearly demonstrated the aerodynamic superiority of cambered (curved) airfoils in comparison to flat, straight surfaces. His experiments were extensive and meticulously carried out. They were published in 1890 in a book entitled "Der Vogelflug als Grundlage der Fliegekunst" ("Bird Flight as the Basis of Aviation"); this book was far and away the most important and definitive contribution to the budding science of aerodynamics to appear in the nineteenth century. It greatly influenced aeronautical design for the next fifteen years, and was the bible for the early work of the Wright brothers. Lilienthal's aerodynamic research led to a quantum jump in aerodynamics at the end of the nineteenth century.
The last, and perhaps the most dramatic, failure of the pre-Wright era was the attempt by Samuel P. Langley to build a flying machine for the U.S. government. Intensely interested in the physics and technology of powered flight, Langley began a series of aerodynamic experiments in 1887, using a whirling arm apparatus. At the time, he was the director of the Allegheny Observatory in Pittsburgh. Within a year he seized the opportunity to become the third Secretary of the Smithsonian Institution in Washington, D.C. Langley continued with his aeronautical experiments, including the building and flying of a number of elastic-powered models. The results of his whirling arm experiments were published in 1890 in his book Experiments in Aerodynamic. In 1896, Langley was successful in flying several small-scale, unmanned, powered aircraft, which he called aerodromes. These 14-ft-wingspan, steam-powered aerodromes were launched from the top of a small houseboat on the Potomac River, and they flew for about a minute, covering close to 1 mi over the river. These were the first steam-powered, heavier-than-air machines to successfully fly—a historic event in the history of aeronautics that is not always appreciated today.
This was to be the zenith of Langley's success. Spurred by the exigency of the Spanish-American War, Langley was given a $50,000 grant from the War Page 34 | Top of Article Department to construct and fly a full-scale, person-carrying aerodrome. He hired an assistant, Charles Manly, who had just graduated from the Sibley School of Mechanical Engineering at Cornell University. Together, they set out to build the required flying machine. The advent of the gasoline-powered internal-combustion engine in Europe convinced them that the aerodrome should be powered by a gasoline-fueled reciprocating engine turning a propeller.
By 1901 Manly had assembled a radically designed five-cylinder radial engine. It weighed 200 lb, produced a phenomenal 52.4 hp, and was the best airplane power plant designed until the beginning of World War I. The full-scale aerodrome, equipped with his engine, was ready in 1903. Manly attempted two flights; both resulted in the aerodrome's falling into the water moments after its launch by a catapult mounted on top of a new houseboat on the Potomac River.
Langley's aerodrome and the fate that befell it are an excellent study in the basic aspects of airplane design. Despite excellent propulsion and adequate aerodynamics, it was the poor structural design that resulted in failure of the whole system.
ERA OF STRUT-AND-WIRE BIPLANES
The 1903 Wright Flyer ushered in the era of successful strut-and-wire biplanes, an era that covers the period from 1903 to 1930. There is no doubt in this author's mind that Orville and Wilbur Wright were the first true aeronautical engineers in history. With the 1903 Wright Flyer, they had gotten it all right—the propulsion, aerodynamic, structural, and control aspects were carefully calculated and accounted for during its design. The Wright brothers were the first to fully understand the airplane as a whole and complete system, in which the individual components had to work in a complementary fashion so that the integrated system would perform as desired.
Let us dwell for a moment on the Wright Flyer as an airplane design. The Wright Flyer possessed all the elements of a successful flying machine. Propulsion was achieved by a four-cylinder in-line engine designed and built by Orville Wright with the help of their newly hired mechanic in the bicycle shop, Charlie Taylor. It produced close to 12 hp and weighed 140 lb, barely on the margin of what the Wrights had calculated as the minimum necessary to get the flyer into the air. This engine drove two propellers via a bicycle-like chain loop. The propellers themselves were a masterpiece of aerodynamic design. Wilbur Wright was the first person in history to recognize the fundamental principle that a propeller is nothing more than a twisted wing oriented in a direction such that the aerodynamic force produced by the propeller was predominately in the thrust direction. Wilbur conceived the first viable propeller theory in the history of aeronautical engineering; vestiges of Wilbur's analyses carry through today in the standard "blade element" propeller theory. The Wrights had built a wind tunnel, and during the fall and winter of 1901 to 1902, they carried out tests on hundreds of different airfoil and wing shapes. Wilbur incorporated these experimental data in his propeller analyses; the result was a propeller with an efficiency that was close to 70 percent (propeller efficiency is the power output from the propeller compared to the power input to the propeller from the engine shaft). This represented a dramatic improvement of propeller performance over contemporary practice. For example, Langley reported a propeller efficiency of only 52 percent for his aerodromes. Today, a modern, variable-pitch propeller can achieve efficiencies as high as 85 to 90 percent. In 1903, the Wrights' propeller efficiency of 70 percent was simply phenomenal. It was one of the lesser-known but most compelling reasons for the success of the Wright Flyer. With their marginal engine linked to their highly efficient propellers, the Wrights had the propulsion aspect of airplane design well in hand.
The aerodynamic features of the Wright Flyer were predominately a result of their wind tunnel tests of numerous wing and airfoil shapes. The Wrights were well aware that the major measure of aerodynamic efficiency is the lift-to-drag ratio L/D. They knew that the lift of an aircraft must equal its weight in order to sustain the machine in the air, and that almost any configuration could produce enough lift if the angle of attack was sufficiently large. But the secret of "good aerodynamics" is to produce this lift with as small a drag as possible, that is, to design an aircraft with as large an L/D value as possible. To accomplish this, the Wrights did three things:
- 1. They chose an airfoil shape that, based on the collective data from their wind tunnel tests, would give a high L/D. The airfoil used on the Wright Flyer was a thin, cambered shape, with a camber ratio (ratio of maximum camber to chord length) of 1/20, with the maximum camber near the quarter-chord location. (In contrast, Lilienthal favored airfoils that were circular arcs, i.e., with maximum camber at midchord.) It is interesting that the precise airfoil shape used for the Wright Flyer was never tested by the Wright brothers in their wind tunnel. By 1903, they had so much confidence in their understanding of airfoil and wing properties that, in spite of their characteristic conservative philosophy, they felt it unnecessary to test that specific shape.
- 2. They chose an aspect ratio of 6 for the wings. (Aspect ratio is defined as the square of the wing span divided by the wing area; for a rectangular wing, the aspect ratio is simply the ratio of the span to the chord length.) They had experimented with gliders at Kitty Hawk in the summers of 1900 and 1901, and they were quite disappointed in their aerodynamic performance. The wing aspect ratio of these early gliders was 3. However, their wind tunnel tests clearly indicated that higher-aspect-ratio wings produced higher values of L/D. (This was not a new discovery; the advantage of high-aspect-ratio wings had been first theorized by Francis Wenham in 1866. Langley's whirling arm data, published in 1890, proved conclusively that better performance was obtained with higher-aspect-ratio wings. Based on their own wind tunnel results, the Wrights immediately adopted an aspect ratio of 6 for their 1902 glider, and the following year for the 1903 flyer. At the time, the Wrights had no way of knowing about the existence of induced drag; this aerodynamic phenomenon was not understood until the work of Ludwig Prandtl in Germany fifteen years later. The Wrights did not know that, by increasing the aspect ratio from 3 to 6, they reduced the induced drag by a factor of 2. They only knew from their empirical results that the L/D ratio of the 6-aspect-ratio wing was much improved over their previous wing designs.
- 3. The Wrights were very conscious of the importance of parasite drag, which in their day was called head resistance. They used empirical formulas obtained from Octave Chanute to estimate the head resistance for their machines. (Octave Chanute was a well-known civil and railroad engineer who had become very interested in aeronautics. In 1893 he published an important survey of past aeronautical work from around the world in a book entitled "Progress in Flying Machines." It has become a classic; you can still buy reprinted copies today. From 1900, Octave Chanute was a close friend and confidant of the Wright brothers, giving them much encouragement during their intensive inventive work in 1900 to 1903.) The Wrights choice of lying prone while flying their machines, rather than sitting up, or even dangling underneath as Lilienthal had done, was a matter of decreasing head resistance. In early 1903, they even tested a series of wooden struts in an airstream in order to find the cross-sectional shape that gave minimum drag. Unfortunately, they did not appreciate the inordinately high drag produced by the supporting wires between the two wings.
The Wrights never quoted a value of L/D for their 1903 Wright Flyer. Modern wind tunnel tests of models of the Wright Flyer carried out in 1982 and 1983 as reported by Culick and Jex at Cal Tech indicate a maximum L/D of 6. This value is totally consistent with values of (L/D)max measured by Gustave Eiffel in 1910 in his large wind tunnel in Paris for models of a variety of aircraft of that time. It has been estimated that the Fokker E-111, an early World War I aircraft had an (L/D)max of 6.4. In 1903 the Wrights had achieved a value of (L/D)max with their flyer that was as high as that for aircraft designed 10 years later.
The control features of the Wright Flyer are also one of the basic reasons for its success. The Wright brothers were the first to recognize the importance of flight control around all three axes of the aircraft. Pitch control, obtained by a deflection of all or part of the horizontal tail (or the forward canard such as the Wright Flyer), and yaw control, obtained by deflection of the vertical rudder, were features recognized by investigators before the Wrights; for example, Langley's aerodrome had pitch and yaw controls. However, no one except the Wrights appreciated the value of roll control. Their novel idea of differentially warping the wing tips to control the rolling motion of the airplane, and to jointly control roll and yaw for coordinated turns, was one of their most important Page 36 | Top of Article contributions to aeronautical engineering. Indeed, when Wilbur Wright finally carried out the first public demonstrations of their flying machines in LeMans, France, in August 1908, the two technical features of the Wright machines most appreciated and immediately copied by European aviators were their roll control and their efficient propeller design.
Finally, the structural features of the Wright Flyer were patterned partly after the work of Octave Chanute and partly after their own experience in designing bicycles. Chanute, inspired by the gliding flights of Lilienthal, carried out tests of gliders of his own design beginning in 1896. The most important technical feature of Chanute's gliders was the sturdy and lightweight Pratt-truss method of rigging a biplane structure. The Wright brothers adopted the Pratt-truss system for the Wright Flyer directly from Chanute's work. Other construction details of the Wright Flyer took advantage of the Wrights' experience in designing and building sturdy but lightweight bicycles. When it was finished, engine included, the empty weight of the Wright Flyer was 605 lb. With a 150-lb person on board, the empty weight-gross weight ratio was 0.8. By comparison, the empty weight of the Fokker E-111 designed 10 years later was 878 lb, and the empty weight-gross weight ratio was 0.65, not greatly different from that of the Wright Flyer. Considering that 10 years of progress in aircraft structural design had been made between the 1903 flyer and the Fokker E-111, the structural design of the 1903 Wright Flyer certainly seems technically advanced for its time. And the fact that the flyer was structurally sound was certainly well demonstrated on December 17, 1903.
In summary, the Wright brothers had gotten it right. All the components of their system worked properly and harmoniously—propulsion, aerodynamics, control, and structures. There were no fatal weak links. The reason for this was the natural inventiveness and engineering abilities of Orville and Wilbur Wright. The design of the Wright Flyer is a classic first study in good aeronautical engineering. There can be no doubt that the Wright brothers were the first true aeronautical engineers.
The Wright Flyer ushered in the era of strut-and-wire biplanes, and it basically set the pattern for subsequent airplane design during this era. The famous World War I fighter airplanes—such as the French Nieuport 17 and the SPAD XIII, the German Fokker D. VII, and the British Sopwith Camel—were in many respects "souped-up" Wright flyers.
First, the wing warping method of roll control used by the Wrights was quickly supplanted by ailerons in most other aircraft. (The idea of flaplike surfaces at the trailing edges of airplane wings can be traced to two Englishmen: M. P. W. Boulton, who patented a concept for lateral control by ailerons in 1868; and Richard Harte, who also filed for a similar patent in 1870). Ailerons in the form of triangular "winglets" that projected beyond the usual wingtips were used in 1908 by Glenn Curtiss on his June Bug airplane; flying the June Bug, Curtiss won the Scientific American Prize on July 4, 1908, for the first public flight of 1,000 m or longer. By 1909, Curtiss had designed an improved airplane, the Gold Bug, with rectangular ailerons located midway between the upper and lower wings. Finally, in 1909 the Frenchman Henri Farman designed a biplane named the Henri Farman III, which included a flaplike aileron at the trailing edge of all four wingtips; this was the true ancestor of the conventional modern-day aileron. Farman's design was soon adopted by most designers, and wing warping quickly became passé. Only the Wright brothers clung to their old concept; a Wright airplane did not incorporate ailerons until 1915, six years after Farman's development.
Second, the open framework of the fuselage, such as seen in the Wright Flyer, was in later designs enclosed by fabric. The first airplane to have this feature was a Nieuport monoplane built in 1910. This was an attempt at "streamlining" the airplane, although at that time the concept of streamlining was only an intuitive process rather than the result of real technical knowledge and understanding about drag reduction.
Third, the demands for improved airplane performance during World War I gave a rebirth to the idea of "brute force" in airplane design. In relation to the thrust minus drag expression T – D, designers of World War I fighter airplanes, in their quest for faster speeds and higher rates of climb, increased the thrust rather than decreasing the drag. The focus was on more powerful engines. The SPAD XIII, one of the best and most famous aircraft from World War I, had a Hispano-Suiza engine that produced 220 hp—the most powerful engine used on a fighter aircraft at that time. Because of this raw power, the SPAD XIII had a maximum velocity of 134 mph which made it one of the fastest airplanes during the war. The SPAD XIII Page 37 | Top of Article typifies the strut-and-wire biplane: the struts and wires produced large amounts of drag, although this was not fully understood by most airplane designers at that time. In fact, in the March 1924 issue of the Journal of the Royal Aeronautical Society, the noted British aeronautical engineer Sir Leonard Bairstow was prompted to say, "Our war experience showed that, whilst we went forward as regard to horsepower, we went backwards with regard to aerodynamic efficiency." Aircraft design during World War I was an intuitive "seat-of-the-pants" process. Some designs were almost literally marked off in chalk on the concrete floor of a factory, and the completed machines rolled out the front door two weeks later.
ERA OF THE MATURE, PROPELLER-DRIVEN AIRPLANE
The period from 1930 to 1950 can be classified as the era of the mature, propeller-driven airplane. During this time, airplane design matured, new technical features were incorporated, and the speed, altitude, efficiency, and safety of aircraft increased markedly. The 1930s are considered by many aviation historians as the "golden age of aviation" (indeed, there is currently a gallery at the National Air and Space Museum with this title). Similarly, the 1930s might be considered as a golden age for aeronautical engineering—a period when many improved design features, some gestating since the early 1920s, finally became accepted and incorporated on "standard" aircraft of the age.
The maturity of the propeller-driven airplane is due to nine major technical advances, all of which came to fruition during the 1930s.
Hugo Junkers in Germany during World War I and Anthony Fokker in Holland during the 1920s pioneered the use single-wing aircraft (monoplanes). This was made possible by the introduction of thick airfoils, which among other advantages allowed room for a large cantilevered spar that structurally supported the wing internally. This eliminated the need for the biplane box structure with its external supporting struts and wires. Consequently, the drag of monoplanes was less than that of comparable strut-andwire biplanes.
The All-Metal Airplane
The vast majority of airplanes before 1930 were constructed from wood and fabric, with some having a steel tube frame mechanism for the fuselage, over which fabric was stretched. Although Hugo Junkers designed and built the first all-metal airplane in 1915, this design feature was not adopted by others for many years. The case for the all-metal airplane was strengthened when the famous Notre Dame football coach Knute Rockne was killed on March 31, 1931, in the crash of a Fokker tri-motor transport. This shook the public's faith in the tri-motor design, and essentially led to its demise in the United States. Such concern was misdirected. Later investigation showed that the wooden wing spar (the entire wing of the Fokker tri-motor was made from wood) had rotted, and the crash was due to this structural failure. What better case could be made for all-metal construction?
Air-Cooled Engines and the NACA Cowling
Propeller-driven airplanes have two types of reciprocating engines—liquid cooled, or air-cooled engines. Since the early days of flight, liquid-cooled engines had the advantage of being longer and thinner, allowing them to be enclosed in relatively streamlined housings with less frontal drag. However, such engines were more vulnerable to damage during combat—a bullet through any part of the liquid cooling system would usually spell failure of the engine. Also, liquid-cooled engines were heavy due to all the machinery and cooling jackets that were associated with the liquid cooling mechanism. In contrast, air-cooled engines, where the cylinder heads are directly exposed to, and cooled by, the airstream over the airplane, are lighter. They require fewer moving parts, and therefore tend to be more reliable. The development of the powerful and reliable Pratt and Whitney Wasp series and the Curtiss-Wright Cycline series of air-cooled radial engines during the late 1920s and the 1930s resulted in the widespread adoption of these engines. But with the cylinders exposed directly to the airstream, the drag created by these was inordinately large.
This set the stage for a major technical development during this era, namely the National Advisory Committee for Aeronautics (NACA) cowling for radial piston engines. Such engines have their pistons arranged in a circular fashion about the crankshaft, and the cylinders themselves are cooled by airflow over the outer finned surfaces. Until 1927, these cylinders were usually directly exposed to the main airstream of the airplane, causing inordinately high
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drag. Engineers recognized this problem, but early efforts to enclose the engines inside an aerodynamically streamlined shroud (a cowling) interfered with the cooling airflow, and the engines overheated. One of the earliest aeronautical engineers to deal with this problem was Colonel Virginius E. Clark (for whom the famous Clark-Y airfoil is named). Clark designed a primitive cowling in 1922 for the Dayton-Wright XPSI airplane; it was marginal at best, and Clark had no proper aerodynamic explanation as to why a cowling worked. The first notable progress was made by H. L. Townend at the National Physical Laboratory in England. In 1927, Townend designed a ring of relative short length that wrapped around the outside of the cylinders. This resulted in a noticeable decrease in drag, and at least it did not interfere with engine cooling. Engine designers who were concerned with the adverse effect of a full cowling on engine cooling were more ready to accept a ring.
The greatest breakthrough in engine cowlings was due to the National Advisory Committee for Aeronautics in the United States. Beginning in 1927, at the insistence of a group of U.S. aircraft manufacturers, the NACA Langley Memorial Laboratory at Hampton, Virginia, undertook a systematic series of wind tunnel tests with the objective of understanding the aerodynamics of engine cowlings and designing an effective shape for such cowlings. Under the direction of Fred E. Weick at Langley Laboratory, this work quickly resulted in success. Drag reduction larger than that with a Townend ring was obtained by the NACA cowling. In 1928, Weick published a report comparing the drag on a fuselage-engine combination with and without a cowling. Compared with the uncowled fuselage, a full cowling reduced the drag by a stunning 60 percent. By proper aerodynamic design of the cowling, the airflow between the engine and the inside of the cowling resulted in enhanced cooling of the engine. Hence, the NACA cowling was achieving the best of both worlds. One of the first airplanes to use the NACA cowling was the Lockheed Vega, shown in Figure 1. Early versions of the Vega without a cowling had a top speed of 135 mph; after the NACA cowling was added to later versions, the top speed increased to 155 mph. The Lockheed Vega went on to become one of the most successful airplanes of the 1930s. The Vega 5, equipped with the NACA cowling and a more powerful engine, had a top speed of 185 mph. It was used extensively in passenger and corporate service. In addition, Amelia Earhart and Wiley Post became two of the most famous aviators of the 1930s—both flying Lockheed Vegas. Not only is the Vega a classic example of the new era of mature propeller-driven airplanes, but its aesthetic beauty supported the popular adage "If an airplane looks beautiful, it will also fly beautifully."
Variable-Pitch and Constant-Speed Propellers
Before the 1930s, a weak link in all propeller-driven aircraft was the propeller itself. For a propeller of fixed orientation, the twist of the propeller is designed so that each airfoil section is at its optimum angle of attack to the relative airflow, usually that angle of attack that corresponds to the maximum lift-to-drag ratio of the airfoil. The relative airflow seen by each airfoil section is the vector sum of the forward motion of the airplane and the rotational motion of the propeller. When the forward velocity of the airplane is changed, the angle of attack of each airfoil section changes relative to the local flow direction. Thus a fixed-pitch propeller is operating at maximum efficiency Page 39 | Top of Article only at its design speed; for all other speeds of the airplane, the propeller efficiency decreases.
The solution to this problem was to vary the pitch of the propeller during the flight so as to operate at near-optimum conditions over the flight range of the airplane—a mechanical task easier said than done. The aerodynamic advantage of varying the propeller pitch during flight was appreciated as long ago as World War I, and H. Hele-Shaw and T. E. Beacham patented such a device in England in 1924. The first practical and reliable mechanical device for varying propeller pitch was designed by Frank Caldwell of Hamilton Standard in the United States. The first production order for Caldwell's design was placed by Boeing in 1933 for use on the Boeing 247 transport. The 247 was originally designed in 1932 with fixed-pitch propellers. When it started flying in early 1933, Boeing found that the airplane had inadequate takeoff performance from some of the airports high in the Rocky Mountains. By equipping the 247 with variable-pitch propellers, this problem was solved. The new propellers increased its rate of climb by 22 percent and its cruising velocity by over 5 percent. Later in the 1930s, the variable-pitch propeller, which was controlled by the pilot, developed into the constant-speed propeller, where the pitch was automatically controlled so as to maintain constant rpm over the flight range of the airplane. Because the power output of the reciprocating engine varies with rotational speed, by having a propeller in which the pitch is continuously and automatically varied to constant engine speed, the net power output of the engine-propeller combination can be maintained at an optimum value.
High-Octane Aviation Fuel
Another important advance in the area of propulsion was the development of high-octane aviation fuel, although it was eclipsed by the more visibly obvious breakthroughs in the 1930s such as the NACA cowling, retractable landing gear, and the variable-pitch propeller. Engine "pinging," an audible local detonation in the engine cylinder caused by premature ignition, had been observed as long ago as 1911. An additive to the gasoline, tetraethyl lead, was found by C. F. Kettering of General Motors Delco to reduce this engine knocking. In turn, General Motors and Standard Oil formed a new company, Ethyl Gasoline Corporation, to produce "ethyl" gasoline with a lead additive. Later, the hydrocarbon compound of octane was also found to be effective in preventing engine
knocking. In 1930, the Army Air Corps adopted 87-octane gasoline as its standard fuel; in 1935, this standard was increased to 100 octane. The introduction of 100-octane fuel allowed much higher compression ratios inside the cylinder, and hence more power for the engine. For example, the introduction of 100-octane fuel, as well as other technological improvements, allowed Curtiss-Wright Aeronautical Corporation to increase the power of its R-1820 Cycline engine from 500 to 1,200 hp in the 1930s.
When a new airplane is designed, the choice of wing area is usually dictated by speed at takeoff or landing (or alternatively by the desired takeoff or landing distances along a runway). The wing area must be large enough to provide sufficient lift at takeoff or landing; this criterion dictates the ratio of airplane weight to wing area, that is, the wing loading W/S—one of the most important parameters in airplane performance and design. After the airplane has taken off and accelerated to a much higher cruising speed, the higher-velocity Page 40 | Top of Article airflow over the wing creates a larger pressure difference between the upper and lower wing surfaces, and therefore the lift required to sustain the weight of the airplane can be created with a smaller wing area. From this point of view, the extra wing required for takeoff and landing is extra baggage at cruising conditions, resulting in higher structural weight and increased skin friction drag. The design of airplanes in the era of strut-and-wire biplanes constantly suffered from this compromise. A partial solution surfaced in the late 1920s and 1930s, namely, the development of high-lift devices such as flaps, slats, and slots. Figure 2 illustrates some of the standard high-lift devices employed on aircraft since the 1920s, along with a scale of lift coefficient indicating the relative increase in lift provided by each device. By employing such high-lift devices, sufficient lift can be obtained at takeoff and landing with wings of smaller area, allowing airplane designers the advantage of high wing loadings at cruise. High-lift devices were one of the important technical developments during the era of the mature propeller-driven airplane.
Another technical development of the late 1930s is the advent of the pressurized airplane. Along with the decrease in atmospheric pressure with increasing altitude, there is the concurrent decrease in the volume of oxygen necessary for human breathing. The useful cruising altitude for airplanes was limited to about 18,000 ft or lower. Above this altitude for any reasonable length of time, a human being would soon lose consciousness due to lack of oxygen. The initial solution to the problem of sustained high-altitude flight was the pressure suit and the auxiliary oxygen supply breathed through an oxygen mask. The first pilot to use a pressure suit was Wiley Post. Looking like a deep-sea diver, Post set an altitude record of 55,000 ft in his Lockheed Vega in December 1934. This was not a practical solution for the average passenger on board an airliner. The answer was to pressurize the entire passenger cabin of the airplane, so as to provide a shirtsleeve environment for the flight crew and passengers. The first airplane to incorporate this feature was a specially modified and structurally strengthened Lockheed IOE Electra for the Army Air Corps in 1937. Designated the XC-35, this airplane had a service ceiling of 32,000 ft. It was the forerunner of all the modern pressurized airliners of today.
Superchargers for Engines
Along with pressurization for the occupants, high-altitude aircraft needed "pressurization" for the engine. Engine power is nearly proportional to the atmospheric density; without assistance, engine power dropped too low at high altitudes, and this was the major mechanical obstacle to high-altitude flight. Assistance came in the form of the supercharger, a mechanical pump that compressed the incoming air before it went into the engine manifold. Supercharger development was a high priority during the 1930s and 1940s; it was a major development program within NACA. All high-performance military aircraft during World War II were equipped with superchargers as a matter of necessity.
One of the most important developments in the era of the mature propeller-driven airplane was the appreciation of the need for streamlining the airframe. The rather box-like shape of the World War I vintage SPAD was characteristic of airplanes of that day. There was little if any attempt to shape the airplane into a streamlined configuration. The Douglas DC-3, however, was designed and began airline service in the mid-1930s. Here is streamlining personified. By comparison, the zero-lift-drag coefficient for the SPAD is 0.04, whereas that for the DC-3 is about 0.025, a considerable improvement. Part of the concept of streamlining was to retract the landing gear flush with the external airframe.
The Douglas DC-3 epitomizes the mature, propeller-driven aircraft of the 1930s. Here you see a cantilever wing monoplane powered by radial engines enclosed in NACA cowlings, and equipped with variable-pitch propellers. It is an all-metal airplane with retractable landing gear, and it uses flaps for high lift during takeoff and landing. For these reasons, the 1930s can indeed be called the golden age of aeronautical engineering.
ERA OF THE JET-PROPELLED AIRPLANE
The jet engine was invented independently by two people: Frank Whittle in England and Dr. Hans von Ohain in Germany. In 1928, as a student at the Royal Air Force technical college at Cranwell, Frank Whittle wrote a senior thesis entitled "Future
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Developments in Aircraft Design" in which he expounded on the virtues of jet propulsion. It aroused little interest. Although Whittle patented his design for a gas-turbine aircraft engine in 1930, it was not until five years later that he formed, with the help of friends, a small company to work on jet engine development. Named Power Jets Ltd., this company was able to successfully bench-test a jet engine on April 12, 1937—the first jet engine in the world to successfully operate in a practical fashion. It was not the first to fly. Quite independently, and completely without the knowledge of Whittle's work, Dr. Hans von Ohain in Germany developed a similar gas-turbine engine. Working under the private support of the famous airplane designer Ernst Heinkel, von Ohain started his work in 1936. On August 27, 1939, a specially designed Heinkel airplane, the He 178, powered by von Ohain's jet engine, successfully flew; it was the first gas turbine-powered, jet-propelled airplane in history to fly. It was strictly an experimental airplane, but von Ohain's engine of 838 lb of thrust pushed the He 178 to a maximum speed of 360 mph. It was not until almost two years later that a British jet flew. On May 15, 1941, the specially designed Gloster E.28/39 airplane took off from Cranwell, powered by a Whittle jet engine. It was the first to fly with a Whittle engine. With these first flights in Germany and Britain, the jet age had begun.
The era of jet-propelled aircraft is characterized by a number of design features unique to airplanes intended to fly near, at, or beyond the speed of sound. One of the most pivotal of these design features was the advent of the swept wing. For a subsonic airplane, sweeping the wing increases the airplane's critical Mach number, allowing it to fly closer to the speed of sound before encountering the large drag rise caused by the generation of shock waves somewhere on the surface of the wing. For a
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supersonic airplane, the wing sweep is designed such that the wing leading edge is inside the Mach cone from the nose of the fuselage; if this is the case, the component of airflow velocity perpendicular to the leading edge is subsonic (called a subsonic leading edge), and the resulting wave drag is not as severe as it would be if the wing were to lie outside the Mach cone. In the latter case, called the supersonic leading edge, the component of flow velocity perpendicular to the leading edge is supersonic, with an attendant shock wave generated at the leading edge. In either case, high subsonic or supersonic, an airplane with a swept wing will be able to fly faster than one with a straight wing, everything else being equal.
The concept of the swept wing for high-speed aircraft was first introduced in a public forum in 1935. At the fifth Volta Conference, convened on September 30, 1935, in Rome, Italy, the German aerodynamicist Adolf Busernann gave a paper in which he discussed the technical reasons why swept wings would have less drag at high speeds than conventional straight wings. Although several Americans were present, such as Eastmann Jacobs from NACA and Theodore von Karman from Cal Tech, Busernann's idea went virtually unnoticed; it was not carried back to the United States with any sense of importance. Not so in Germany. One year after Busemann's presentation at the Volta Conference, the swept-wing concept was classified by the German Luftwaffe as a military secret. The Germans went on to produce a large bulk of swept-wing research, including extensive wind tunnel testing. They even designed a few prototype swept-wing jet aircraft. Many of these data were confiscated by the United States after World War II, and made available to U.S. aircraft companies and government laboratories. Meanwhile, quite independently of this German research, Robert T. Jones, a NACA aerodynamicist,
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had worked out the elements of swept-wing theory toward the end of the war. Although not reinforced by definitive wind tunnel tests in the United States at that time, Jones's work served as a second source of information concerning the viability of swept wings.
In 1945, aeronautical engineers at North American Aircraft began the design of the XP-86 jet fighter; it had a straight wing. The XP-86 design was quickly changed to a swept-wing configuration when the German data, as well as some of the German engineers, became available after the war. The prototype XP-86 flew on October 1, 1947, and the first production P-86A flew with a 35° swept wing on May 18, 1948. Later designated the F-86, the swept-wing fighter had a top speed of 679 mph, essentially Mach 0.9—a stunning speed for that day. Shown in Figure 3, the North American F-86 Sabre was the world's first successful operational swept-wing aircraft.
By the time the F-86 was in operation, the sound barrier had already been broken. On October 14, 1947, Charles (Chuck) Yeager became the first human being to fly faster than the speed of sound in the Bell X-1 rocket-powered airplane. In February 1954, the first fighter airplane capable of sustained flight at Mach 2, the Lockheed F-104 Starfighter, made its first appearance. The F-104, Figure 4, exhibited the best qualities of good supersonic aerodynamics—a sharp, pointed nose, slender fuselage, and extremely thin and sharp wings. The airfoil section on the F-104 is less than 4 percent thick (maximum thickness compared to the chord length). The wing leading edge is so sharp that protective measures must be taken by maintenance people working around the aircraft. The purpose of these features is to reduce the strength of shock waves at the nose and trailing edges, thus reducing supersonic wave drag. The F-104 also had a straight wing with a very low aspect ratio rather than a swept wing. This exhibits an
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|Aircraft||Payload Capacity||Top Speed||Cruising Speed||Fuel Consumption (1000 mile flight)||4D Ratio|
|737||128 passengers||M=0.89||M=0.83||1600 gal.||17|
|737 Stretch||188 passengers||0.89||0.83||1713||17|
|SST Concorde||126 passengers||2.2||2.0||6400||8|
|Turboprop DHC-8||37 passengers||325 MPH||305 MPH||985||15|
|Fighter Plane F-15||M=2.5||750||6|
|Military Cargo Plane C-17||120,000 lbs.||0.80||0.76||5310||14|
alternative to supersonic airplane designers; the wave drag on straight wings of low aspect ratio is comparable to that on swept wings with high aspect ratios. Of course, this low-aspect-ratio wing gives poor aerodynamic performance at subsonic speeds, but the F-104 was point-designed for maximum performance at Mach 2. With the F-104, supersonic flight became an almost everyday affair, not just the domain of research aircraft.
The delta wing concept was another innovation to come out of Germany during the 1930s and 1940s. In 1930, Alexander Lippisch designed a glider with a delta configuration; the leading edges were swept back by 20°. The idea had nothing to do with high-speed flight at that time; the delta configuration had some stability and control advantages associated with its favorable center-of-gravity location. When Busemann introduced his swept-wing ideas in 1935, Lippisch and his colleagues knew they had a potential high-speed wing in their delta configuration. Lippisch continued his research on delta wings during the war, using small models in German supersonic wind tunnels. By the end of the war, he was starting to design a delta wing ramjet-powered fighter. Along with the German swept-wing data, this delta wing technology was transferred to the United States after the war; it served as the basis for an extended wind tunnel test program on delta wings at NACA Langley Memorial Laboratory.
The first practical delta wing aircraft was the Convair F-102. The design of this aircraft is an interesting story in its own right—a story of the interplay between design and research, and between industry and NACA. The F-102 was designed as a supersonic airplane. Much to the embarrassment and frustration of the Convair engineers, the prototype F-102 being tested at Edwards Air Force Base during October 1953 and then again in January 1954 exhibited poor performance and was unable to go supersonic. At the same time, Richard Whitcomb at NACA Langley was conducting wind tunnel tests on his "area rule" concept, which called for the cross-sectional area of the fuselage to be reduced in the vicinity of the wing. By so doing, the transonic drag was substantially reduced. The Convair engineers quickly adopted this concept on a new prototype of the F-102, and it went supersonic on its second flight. Convair went on to produce 975 F-102s; the practical delta wing airplane was a finally a reality.
The area rule was one of the most important technical developments during the era of jet-propelled airplanes. Today, almost all transonic and supersonic aircraft incorporate some degree of area rule. For his work on the area rule, Whitcomb received the Collier Trophy, the highest award given in the field of aeronautics.
One of the most tragic stories in the annals of airplane design occurred in the early 1950s. Keying on England's early lead in jet propulsion, de Havilland Aircraft Company designed and flew the first commercial jet transport, the de Havilland Comet. Powered by four de Havilland Ghost jet engines, the Page 45 | Top of Article Comet carried 36 passengers for 2,000 mi at a speed of 460 mph, cruising at relatively high altitudes near or above 30,000 ft. The passenger cabin was pressurized; indeed, the Comet was the first pressurized airplane to fly for extended periods at such high altitudes. Inasmuch as good airplane design is an evolutionary process based on preceding aircraft, the de Havilland designers had little precedent on which to base the structural design of the pressurized fuselage. The Comet entered commercial service with BOAC (a forerunner of British Airways) in 1952. In 1954, three Comets disintegrated in flight, and the airplane was quickly withdrawn from service. The problem was later found to be structural failure of the fuselage while pressurized. De Havilland used countersunk rivets in the construction of the Comet; reaming the holes for the rivets produced sharp edges. After a number of pressurization cycles, cracks in the fuselage began to propagate from these sharp edges, leading eventually to catastrophic failure. At the time, de Havilland had a massive lead over all other aircraft companies in the design of commercial jet aircraft. While it was in service, the Comet was very popular with the flying public, and it was a moneymaker for BOAC. Had these failures not occurred, de Havilland and England might have become the world's supplier of commercial jet aircraft rather than Boeing and the United States.
In 1952, the same year as the ill-fated de Havilland Comet went into service, the directors of Boeing Company made a bold and risky decision to privately finance and build a commercial jet prototype. Designated the model 367-80, or simply called the Dash 80 by the Boeing people, the prototype first flew on July 15, 1954. It was a bold design that carried over to the commercial field Boeing's experience in building swept-wing jet bombers for the Air Force (the B-47 and later the B-52). Later renamed the Boeing 707, the first production series of aircraft were bought by Pan American Airlines and went into service in 1958. The Boeing 707 (Figure 5), with its swept wings and podded engines mounted on pylons below the wings, set the standard design pattern for all future large commercial jets. The design of the 707 was evolutionary because it stemmed from the earlier experience at Boeing with jet bombers. But it was almost revolutionary in the commercial field, because no other airliner had ever (not even the Comet) looked like that. Boeing's risky gamble paid off, and it transformed a predominately military aircraft company into the world's leader in the design and manufacture of commercial jet transports.
Boeing made another bold move on April 15, 1966, when the decision was made to "go for the big one." Boeing had lost the Air Force's C-5 competition to Lockheed; the C-5 at the time was the largest transport airplane in the world. Taking their losing design a few steps further, Boeing engineers conceived of the 747, the first wide-body commercial jet transport. Bill Allen, president of Boeing at that time, and Juan Trippe, president of Pan American Airlines, shared the belief that the large, wide-body airplane offered economic advantages for the future airline passenger market, and they both jointly made the decision to pursue the project. It was an even bolder decision than that concerning the 707.
The gamble paid off. The Boeing 747 first flew in February 1969, and it entered service for the first time in January 1970 on Pan American's New York—London route. Boeing is still producing 747s.
What about commercial transportation at supersonic speeds? In the 1960s this question was addressed in Russia, the United States, England, and France. The Tupolev Design Bureau in Russia rushed a supersonic transport design into production and service. The Tu-144 supersonic transport first flew on December 31, 1968. More than a dozen of these aircraft were built, but none entered extended service, presumably due to unspecified problems. One Tu-144 was destroyed in a dramatic accident at the 1973 Paris Air Show. In the United States, the government orchestrated a design competition for a supersonic transport; the Boeing 2707 was the winner in December 1966. The design turned into a nightmare for Boeing. For two years, a variable-sweep wing supersonic transport (SST) configuration was pursued, and then the design was junked. Starting all over again in 1969, the design was caught up in an upward spiral of increased weight and development costs. When the predictions for final development costs hit about $5 billion, Congress stepped in and refused to appropriate any more funds. In May 1971, the SST development program in the United States was terminated. Only in England and France was the SST concept carried to fruition.
The first, and so far only, supersonic commercial transport to see long-term regular service was the Anglo-French Concorde (Figure 6). In 1960 both
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the British and French independently initiated design studies for a supersonic transport. It quickly became apparent that the technical complexities and financial costs were beyond the abilities of either country to shoulder alone. On November 29, 1962, England and France signed a formal agreement aimed at the design and construction of a supersonic transport. The product of this agreement was the Aerospatiale-British Aerospace Corporation's Concorde. Designed to cruise at Mach 2.2 and carry 125 passengers, the Concorde first flew on March 2, 1969. It first exceeded Mach 1 on October 1, 1969, and Mach 2 on November 4, 1970. Originally, orders for 74 Concordes were anticipated. When the airlines were expected to place orders in 1973, the world was deep in the energy crisis. The skyrocketing costs of aviation jet fuel wiped out any hope of an economic return from flying the Concorde, and no orders were placed. Only the national airlines of France and Britain, Air France and British Airways, went ahead, each signing up for seven aircraft after considerable pressure from their respective governments. After a long development program, the Concorde went into service on January 21, 1976. In the final analysis, the Concorde was a technical, if not financial, success. It was in regular service from 1976 until the entire fleet was grounded in the wake of the first concorde crash in August 2000. It represents an almost revolutionary airplane design in that no such aircraft existed before it. The Concorde designers were not operating in a vacuum. Examining Figure 6, we see a supersonic configuration which incorporates good supersonic aerodynamics—a sharp-nosed slender fuselage and a cranked delta wing with a thin airfoil. The Concorde designers had at least fifteen years of military airplane design experience with such features to draw upon. Today, we know that any future second-generation SST will have to be economical in service and environmentally acceptable. The design of such a vehicle is one of the great challenges in aeronautics.
In summary, the types of aircraft in use today cut across the flight spectrum from low-speed, propeller-driven airplanes with reciprocating engines, moderate speed turboprop airplanes (propeller driven by gas turbine engines), and high-speed jet-propelled airplanes. For low-speed flight, below about 250 mph, the reciprocating engine/propeller combination has by far the best propulsive efficiency. For moderate speeds (250–400 mph) the turboprop is superior. This is why most high-performance commuter aircraft are powered by turboprops. For high speeds (at least above 500 mph) the jet engine is the only logical powerplant choice; a propeller rapidly looses efficiency at higher flight speeds. In short, a reciprocating engine/propeller combination is a high efficiency, but comparably low thrust powerplant, and a jet engine is a lower efficiency but higher thrust powerplant. The turboprop is a middle-ground compromise between thrust and efficiency. The wide variety of airplanes in use today draw on the technology developed in both the era of the mature, propeller-driven airplane and the era of the jet-propelled airplane. In the future, airplane design will continue to be influenced by the desire to fly faster and higher, but moderated by the need for environmental effectiveness, economic viability, and energy efficiency.
John D. Anderson
Anderson, J. D., Jr. (1997). A History of Aerodynamics, and Its Impact on Flying Machines, New York: Cambridge Universtiy Press.
Anderson, J. D., Jr. (1999). Aircraft Performance and Design. Boston: McGraw-Hill.
Anderson, J. D., Jr. (2000). Introduction to Flight, 4th ed. Boston: McGraw-Hill.
Loftin, L. K. (1985) Quest for Performance: The Evolution of Modern Aircraft, Washington, DC: NASA.
Greenwood, J. T. ed. (1989). Milestones of Aviation. Washington, DC: National Air and Space Museum, Smithsonian Institution.
Gale Document Number: GALE|CX3407300016