Quest for Performance: The Evolution of Modern Aircraft
Chapter 4: Design Revolution 1926-39
Monoplanes and Biplanes
[81] The Ryan monoplane Spirit of St. Louis pictured in figure 4.1 popularized the monoplane configuration in America and marked the beginning of the decline of the biplane. Another immortal high-wing monoplane, the Ford trimotor, formed the mainstay of the infant U.S. airline industry in the late 1920's and early 1930's. The aircraft, pictured in figure 4.2, featured an internally braced wing, fixed landing gear, and three engines. The basic configuration was similar to the Fokker trimotor referred to earlier; however, the methods of construction employed in the two aircraft were totally different. The Fokker structure consisted of a mixture of wood, metal, and fabric; the Ford was of all-metal construction. The internal structure of the aircraft was entirely of metal, and the skin was a corrugated aluminum alloy. The corrugations provided stiffness in the skin panels and were aligned with the direction of air flow in order to minimize the drag. This type of construction was pioneered by Hugo Junkers in Germany.
The aircraft was produced in two versions: the model 4-AT and the model 5-AT. The aircraft were similar in appearance, but the model 5-AT was slightly larger and employed somewhat more powerful engines than the model 4-AT. Figure 4.2 depicts a model 4-AT, and the specifications given in table II are for the model 5-AT. The model 5-AT carried from 13 to 15 passengers in an enclosed cabin, had a gross weight of 13 500 pounds, and was equipped with three 420-horsepower Pratt & Whitney Wasp radial engines. By this time, the....

ground view of a Ford 4-AT
Figure 4.2 - Ford 4-AT 12-passenger trimotor transport; 1928. [NASA]
[82] ...two pilots were seated in an enclosed cockpit located ahead of the wing. Ground-handling characteristics were enhanced by the provision of differential braking on the main landing gear wheels and a swiveling tail wheel. Cockpit instrumentation was primitive by modern standards, and some of the instruments for the outboard engines were actually located on the engine nacelles, which required the pilots to look out the side windows to read them. The large, powerful engines were equipped with an inertia starter; this type of starter was often used for large engines beginning in the mid-1920's. A flywheel of large moment of inertia was brought to a high rotational speed through the use of either a geared handcrank or an electrical power source. When the proper speed had been reached, a clutch was engaged and the angular momentum of the flywheel caused the engine to rotate and start.
The Ford trimotor was especially designed to maintain flight after the loss of one engine. Under full gross weight conditions, however, the aircraft was not able to climb after takeoff following the loss of an engine, probably because of the excessive drag resulting from the windmilling propeller. Full-feathering propellers had not been developed at that time. The top speed of 150 miles per hour listed in table II for the Ford trimotor may be excessive; cruising speeds somewhat less than 100 miles per hour are indicated in reference 110 for a model 4-AT that is still flying today. The drag coefficient CD,O for the Ford is seen to be relatively high, as compared with that for the Ryan Spirit of St. Louis. The drag of the two outboard engines and the nacelles no doubt contributed significantly to the total drag of the trimotor and, to some extent, nullified the advantages of the cantilever wing. Furthermore, according to reference 72, the wetted area of an aircraft may be increased by as much as 20 to 40 percent by corrugations in the metal covering. No account was taken of this increment in calculating the drag coefficient given in table II.
The prototype of the Ford trimotor flew in 1926, and the last production aircraft rolled off the line in 1933. A total of 116 models of the 5-AT and 84 models of the 4-AT were constructed. Some of these aircraft are still flying today, and one was flying in scheduled airline service with the remarkable Island Airlines at Port Clinton, Ohio, into the 1970's. The longevity of these aircraft attests to their rugged construction and basic design soundness.
The Lockheed Vega shown in figure 4.3 was a very high-performance monoplane that first flew in 1927. The aircraft shown in the photograph is a fully developed model 5C version. Both the internal structure and the outer covering of the aircraft were wood. The wing was of....

ground view of a Vega 5C
[83] Figure 4.3 - Lockheed Vega 5C mail and passenger plane; 1929. [Peter C. Boisseau]

....the internally braced, cantilever type, and the fuselage was of semimonocoque construction. A new feature, which appeared on this aircraft, was a circular cowling surrounding the 450-horsepower Pratt & Whitney Wasp air-cooled engine. This cowling concept was one of NACA's early contributions and provided substantial increases in the speed of aircraft employing radial engines, but, at the same time, directed the cooling air through the engine in such a way as to provide adequate cooling. The maximum speed of the Lockheed Vega was increased from 165 miles per hour to 190 miles per hour by the addition of the NACA cowling. Fairings, called pants, around the wheels of the landing gear also reduced the drag and resulted in an increase in the speed of the aircraft. The Lockheed Vega had a very low zero-lift drag coefficient of 0.0278, as shown by the data in table II. The low zero-lift drag coefficient was obtained through careful attention to detailed aerodynamic design of the aircraft and by the absence of drag-producing struts, wires, and other external drag-producing elements. The fixed landing gear, however, remained as a significant drag-producing feature of the airplane. The maximum lift-drag ratio of the Vega was 11.4, which was unusually high for that time period. The Lockheed Vega was used in airline service (six passengers) and was also employed in many record-breaking flights. The aircraft shown in figure 4.3 is painted to represent the famous Winnie Mae, which Wiley Post flew solo around [84] the world in about 7 1/2 days in the summer of 1933. The actual aircraft Post flew on this remarkable flight is in the National Air and Space Museum In Washington, D.C. The Lockheed Vega was a highly advanced and refined design for its day, and, even now, the performance is very good for an aircraft with fixed landing gear.

The demise of the Jenny and its contemporaries opened the way for a new generation of general aviation aircraft for fixed-base operators and barnstormers. Most of these new aircraft employed a welded steel-tube fuselage and wooden wing structure and incorporated a fabric covering over the entire structure. The aircraft depicted in figures 4.4 and 4.5 are typical of the classes of aircraft produced during the latter part of the 1920's. The Curtiss Robin shown in figure 4.4 was designed along the lines of the strut-braced monoplane formula popularized by Lindbergh's Spirit of St. Louis. The aircraft was ruggedly built with a view toward operation from poorly prepared airfields or pastures. The enclosed cabin provided seating for a pilot in the front and two passengers in the rear seat. The aircraft was usually equipped with either a Curtiss Challenger six-cylinder radial engine or a Wright J6-5 five-cylinder radial engine. The specifications given in table II are for the Challenger-powered Robin, which had 185 horsepower and was capable of a maximum speed of 115 miles per hour. The aircraft was fitted with wheel brakes and a steerable tail wheel or skid. The drag coefficient of the Robin was a very high 0.0585, which probably resulted from the very large cylinders of the exposed radial engine, the many sharp corners of the forward-facing windshield, and the relatively unfaired junctures between the multitude of struts supporting the wings....

ground view of a Curtiss Robin
Figure 4.4 - Curtiss Robin three-place-cabin monoplane; 1929. [Peter C. Boisseau]


Ground view of a Travelair 4000
[85] Figure 4.5 - Travelair 4000 three-place open-cockpit biplane; 1928. [Peter C. Boisseau]


....and landing gear. The zero-lift drag coefficient of the Robin is seen to be more than 0.020 greater than that of the Ryan Spirit of St. Louis.

The biplane type was still popular and is illustrated by the Travelair 4000 of 1928 shown in figure 4.5. The aircraft was typical of a large number of three-place open biplanes in which the pilot sat alone in the rear cockpit and two passengers were placed forward under the wing near the center of gravity in an open front cockpit. The aircraft is seen to employ struts and wires for bracing the wings, but they are far fewer in number than those used on the typical World War I biplane represented by the DH-4 pictured in figures 2.26 and 2.27. Many different power in the late 1920's. The venerable Curtiss OX-5 water-cooled engine of World War I fame was still available in large numbers and formed a cheap source of power plants for new aircraft. Engines of higher power and greater reliability, plants were used in the various open cockpit biplanes produced such as the Wright Whirlwind, were also available, but these engines were considerably more costly than the surplus World War I engines. The Travelair 4000 shown in figure 4.5 has the Wright Whirlwind nine-cylinder radial engine. The large horn-balanced ailerons and rudder on the Travelair are particularly noteworthy. [86] Balanced controls of this type were used on the World War I German Fokker D-7, figure 2.14, and formed a distinctive identifying feature of the aircraft. For this reason, the Travelair 4000, which was manufactured in Wichita, Kansas, is often referred to as the Wichita Fokker. Aircraft of the vintage of the Curtiss Robin and the Travelair 4000 are highly prized antiques today and are the subject of painstaking restoration. The Robin was used in the 1920's and 1930's in several recordbreaking endurance flights, and in the late 1930's it was flown nonstop across the Atlantic by Douglas Corrigan.
Meanwhile, the military services remained wedded to the biplane concept for their fighters, observation planes, bombers, and other classes of aircraft. One of the last biplane fighters developed for the U.S. Army Air Corps, and one of distinctly elegant design, was the Curtiss Hawk P-6E shown in figure 4.6. This aircraft traces its lineage back to the Curtiss Hawk P-1 of 1925, which in turn was derived, at least in part, from the Curtiss racing aircraft of that period. The P-6E was the last of the biplane line of Hawk fighters built for the U.S. Army Air Corps. Various versions of the Hawk were also procured by the U.S. Navy and a number of foreign countries. The entire Hawk series employed tapered wings, and the model P-6E featured a low drag, single-strut landing gear together with a carefully streamlined installation of....

view of a pilot in the cockpit of a Curtiss Hawk P-6E
Figure 4.6 - Curtiss Hawk P-6E fighter; 1931. [Peter C. Boisseau]

[87]....the 650-horsepower Curtiss conquerer engine. The construction of the aircraft was conventional; the fuselage was of the welded steel-tube type, and the wings were constructed of a wood framework. The entire aircraft except for the engine cowling, wing leading edges, and other special portions was covered with fabric. The P-6E was one of the first fighters to employ a droppable auxiliary fuel tank mounted under the fuselage and was equipped with wheel brakes and onboard oxygen equipment. The engine was liquid cooled and employed a chemical known as ethylene glycol rather than water as the coolant. This chemical is essentially the same as the antifreeze used in modern automobile engines. The drag coefficient of the Hawk was a relatively low 0.0371. A comparison of this coefficient with the corresponding value for the Ryan Spirit of St. Louis indicates that a well-designed biplane could be as efficient from the point of view of friction drag as a multistrutted monoplane. The lower effective aspect ratio of the biplane wing cell, however, gives a substantially lower maximum lift-drag ratio for the Hawk than for the Spirit of St. Louis.
The Hawk model P-6E made its first flight in 1931. A transitional monoplane fighter designed by Boeing was first flown in 1932. This aircraft, known as the P-26 or Pea Shooter, is shown in figure 4.7. The aircraft was a wire-braced monoplane design that incorporated a fixed landing gear and open cockpit but was of all-metal construction, including the skin. The cowling around the engine, known as a Townend ring, reduced the drag of the radial engine but was not as effective as...

Photo of a man at the engine of a Boeing P-26A
Figure 4.7 - Boeing P-26A fighter; 1932. [NASA]


[88]....the full NACA type of cowling discussed in connection with the Lockheed Vega. The aircraft in its original form had a relatively high landing speed; consequently, all production versions were equipped with simple trailing-edge flaps to reduce the landing speed. This was the first fighter aircraft developed in the United States to employ landing flaps. Thus, the P-26 represented a strange collection of the old and the new in airplane design and was an anachronism when it went into production in 1934. The zero-lift drag coefficient of the Boeing P-26A given in table II is seen to be higher than that of the Curtiss Hawk biplane; however, the drag area of the P-26 is only about 60 percent of that of the Hawk. The P-26 was a transitional type of fighter and had a relatively short service life. Most of the P-26's had been recalled from first-line service by the beginning of World War II, although at least one P-26 flown by a Philippine pilot is thought to have engaged a Japanese fighter in the early days of World War II.