Quest for Performance: The Evolution of Modern Aircraft
Chapter 4: Design Revolution 1926-39
Synergistic Developments
[88] The Lockheed Vega, illustrated in figure 4.3, represented the highest level of aerodynamic efficiency achieved by a high-wing monoplane with fixed landing gear by the year 1930. Reduction in drag and subsequent improvements in the performance of a monoplane such as the Lockheed Vega could obviously be achieved by retracting the landing gear. Retraction of the landing gear on a high-drag aircraft, such as the DH-4, would result in very little improvement in performance since the drag contribution of the landing gear was a relatively small percentage of the total drag coefficient. On an aircraft such as the Lockheed Vega, however, which was characterized by cantilever wings, highly streamlined fuselage, and efficiently cowled engine, the drag of the landing gear would be expected to be a significant portion of the total drag; hence, retraction of the gear would be expected to give a large increment in performance.
The Lockheed Orion, shown in figure 4.8, took this next step in improving aerodynamic efficiency. The Orion was a six-passenger, low-wing monoplane, with the pilot located in an enclosed cockpit forward of the wing. The method of construction employed in the Orion was the same as that utilized in the Vega. The low-wing configuration was particularly adaptable for the use of a retractable landing gear. The gear could be kept short and thus light, and the wing provided an ideal stowage space for the gear in the retracted position. The steerable tail wheel was also retractable in order to provide further increases in....

ground view of a Lockheed Orion 9D
[89] Figure 4.8 - Lockheed Orion 9D mail and passenger plane; 1931. [mfr]


....aerodynamic efficiency. The engine on this aircraft, as on the Vega, employed a single-speed, geared blower to provide improved engine power output at the cruise altitudes of the aircraft. The data in table II indicate that the Orion had a maximum speed of 226 miles per hour at sea level and a cruising speed of 200 miles per hour. The corresponding value of the zero-lift drag coefficient CD,O is 0.0210. The value of this coefficient is seen to be remarkably low, even when compared with values for present-day aircraft; and a comparison with corresponding values for the Lockheed Vega gives a good indication of the magnitude of the improvement in aerodynamic efficiency realized by retracting the landing gear. The retractable landing gear had been thought for many years to be too heavy for practical use in aircraft design; however, the spectacular reductions in drag associated with its use on an aerodynamically clean aircraft were found to far outweigh the relatively small increases in weight. The Orion first flew in mid-1931 and was produced in only limited quantities, perhaps because it was not really large enough for an airline transport; then too, there was a growing feeling that airline aircraft should be equipped with multiengines. Later in the 1930's Government regulations disallowed the use of single-engine aircraft for scheduled passenger-carrying operations.

The configuration and design details of the Lockheed Orion represented an extremely high level of aerodynamic efficiency, a level that [90] has seldom been exceeded in the years since 1931. Yet, the Orion lacked several features that later became an integral part of the propeller-driven aircraft in its final definitive form. An aircraft with as broad a speed range as the Lockheed Orion requires some sort of variable pitch propeller in order that the desired amount of power may be efficiently extracted from the engine over a wide range of flight conditions. The full aerodynamic potential of a low-drag high-performance aircraft cannot be realized without the use of a controllable-pitch propeller. Such propellers became generally available and were in common use on high-performance aircraft by the mid-1930's. Another feature the Orion lacked was an effective high-lift flap system for increasing the maximum lift coefficient and reducing the stalling speed. The aircraft had a rudimentary trailing-edge flap; but like most early flap systems, this was used primarily for increasing the drag in the approach and landing maneuver rather than increasing the maximum lift coefficient. Again, the use of effective high-lift flaps became standard practice on high-performance configurations later in the decade. Finally, the use of wood as a primary material for construction had many disadvantages, and some form of light, stiff, all-metal monocoque or semimonocoque structure was desired.
One of the first aircraft developed in the United States to employ an all-metal stressed-skin semimonocoque type of structure was tile Northrop Alpha, illustrated in figure 4.9. In this type of structure, the metal skin is smooth, not corrugated, and contributes significantly to the stiffness and load-carrying capability of the structure. The stability of the thin metal skin is usually enhanced by numerous internal stringers....

ground view of a Northrop Alpha
Figure 4.9 -Northrop Alpha mail and passenger plane; 1931. [Peter C. Boisseau]


[91] ...attached to the skin. The Alpha employed a low wing of cantilever construction and a full NACA-type cowling around the radial engine, but incorporated an anachronistic fixed landing gear together with an open cockpit for the pilot. The zero-lift drag coefficient for the aircraft is seen from table II to be about the same as that for the Lockheed Vega discussed earlier. The aircraft was used in limited numbers for mail and passenger operation, and the particular version shown here was employed for transportation of high-ranking military officers. Various forms of stressed-skin metal construction were destined to become the norm for propeller-driven aircraft in the years ahead.

The first aircraft that assembled most of the desirable features discussed above in a single configuration was the Boeing 247 shown in figures 4.10 and 4.11. The first flight of the aircraft was in February 1933, and airline operations were begun later that year. The enclosed cabin accommodated 10 passengers, 2 pilots, and 1steward. Two 525-horsepower Pratt & Whitney Wasp engines were employed, and the aircraft could maintain an altitude of 6000 feet on one engine at full gross weight. The earlier models of the aircraft, such as the one shown in figure 4.10, had Townend rings on the engines and employed fixed-pitch propellers. The definitive version of the aircraft, the model 247D (fig. 4.11), had both controllable-pitch propellers and full NACA-type engine cowlings. All aircraft were later converted or retrofitted to the model 247D configuration. The synergistic design features of this aircraft are as follows:
(1) Cantilever wings
(2) Retractable landing gear
(3) Efficiently cowled, light radial engine
(4) Controllable-pitch propellers
(5) Single-speed geared supercharger
(6) All metal, stressed-skin construction
The Boeing 247D did not employ wing flaps and had a relatively low wing loading of 16.3 pounds per square foot. A contemporary and very similar aircraft, the Douglas DC-2, employed all the features mentioned for the Boeing machine and, in addition, had a higher wing loading and split-type landing flaps. The model 247D was one of the first transport aircraft to employ rubber deicer boots and a significant amount of instrumentation for blind flying. The aircraft is seen from table II to have a low zero-lift drag coefficient and a value of the maximum lift-drag ratio of 13.5, which compares favorably with the values...

ground view of a Boeing 247
[92] Figure 4.10 - Early version of Boeing 247 10-passenger twin-engine transport; 1933. [Peter C. Boisseau]

ground view of a Boeing 247D

Figure 4.11 - Fully developed Boeing 247D. [mfr]

...of this parameter for the previously discussed aircraft. About 75 Boeing 247's were built but the type was not developed further, perhaps because of Boeing's preoccupation with bomber aircraft development during that period of time.

[93] The Douglas DC-3 was developed from the DC-2 and is, by any measure, one of the best-known aircraft ever produced anywhere in the world. The aircraft first flew in December 1935 and was in airline operation by the summer of 1936. It incorporated all the advanced technical features of the Boeing 247 and the Douglas DC-2 but, in addition, was sufficiently large to carry 21 passengers. With this number of passengers and a cruising speed at 10 000 feet of 185 miles per hour, the airlines for the first time had an aircraft with operating costs sufficiently low so that money could be made from carrying passengers without complete dependence on revenue from airmail contracts.
A DC-3 in flight is shown in figure 4.12. A distinctive identification feature of the aircraft is the sweptback wing, which was inherited from the DC-2 and was used to position the aerodynamic center of the aircraft in the proper relationship to the center of gravity. The design of the DC-2 wing did not initially employ sweepback but had a highly tapered straight wing. As the design of the aircraft progressed, however, it became evident that the center of gravity was farther aft than had been anticipated. Mounting the outer panels with sweepback offered a simple means for moving the aerodynamic center into the correct position. The Douglas DC-3 was powered either with two Wright Cyclone radial air-cooled engines of 1000 horsepower each or two Pratt & Whitney R-1830 engines of 1200 horsepower each. Both the Wright and Pratt & Whitney engines had 14 cylinders arranged in 2....

aerial view of a DC-3
Figure 4.12 - Douglas DC-3 21-passenger twin-engine transport; 1935. [mfr]

[94] rows of 7, one behind the other. [note for the Web version: the Wright engine actually only has a single row of nine cylinders, while the Pratt and Whitney engine does indeed have 2 rows of 7 cylinders each. Thanks to Tom Emmert for pointing out this error that appears in the printed version of this book.] The double-row radial engine was extensively used throughout the subsequent development of large high-performance piston-engine aircraft. A comparison of the aerodynamic parameters for the Douglas DC-3 and the Boeing 247D, given in table II, indicates that the zero-lift drag coefficient of the DC-3 is about 17 percent higher than that of the Boeing aircraft; the larger zero-lift drag coefficient of the DC-3 results from the larger ratio of wetted area to wing area caused by the larger fuselage of the DC-3, which was designed to accommodate three-abreast seating as compared with two abreast for the Boeing aircraft. The value of the maximum lift-drag ratio for the Douglas DC-3, however, is 14.7 as compared with 13.5 for the Boeing machine; the higher aspect ratio of the DC-3 is responsible for the larger value of maximum lift-drag ratio. The wing loading of 25 pounds per square foot for the DC-3, as compared with the 16 pounds per square foot for the Boeing 247, reflects the use of split trailing-edge flaps on the DC-3 aircraft.

A total of 10 926 DC-3-type aircraft were built in the United States between 1936 and 1945. Of this total, about 10 000 aircraft were procured by the military services for their use, and many of these were later converted for various commercial activities following the end of World War II; today, over 45 years after its first flight, there are still many hundreds of DC-3 aircraft in service throughout the world. The DC-3 has been used for every conceivable purpose to which an airplane can be put and surely must be considered as one of the truly outstanding aircraft developments of all time.
The Boeing B-17 bomber was a highly significant military aircraft that first flew in prototype form during July 1935. A fully developed version of the aircraft, a Boeing B-17G utilized during World War II, is illustrated in figure 4.13. The aircraft incorporated the same significant structural and aerodynamic design features discussed in connection with the Boeing 247D and the Douglas DC-3 but was equipped with four engines instead of two. The aircraft had a gross weight of 55 000 pounds, which was considered very heavy at the time of its introduction. The four engines developed 1200 horsepower each and were equipped with turbo superchargers. In contrast to the gear-driven, single-speed supercharger previously discussed, the turbosupercharger makes use of the energy in the exhaust gases from the engine. The supercharger blower is connected to a turbine that is driven by the exhaust gases. The fraction of the total exhaust gas that passes through the turbine can be varied by a valve in accordance with the altitude at which the aircraft is flying. Thus, the maximum rated power of the....

ground view of a B-17G
[95] Figure 4.13 - Boeing B-17G World War II four-engine heavy bomber; prototype first flown in 1935. [Peter C. Boisseau]
...engine can be maintained up to an altitude at which all the exhaust gases pass through the turbine; at higher altitudes, the power drops off with altitude in very much the same way as an unsupercharged engine at low altitude. The critical altitude for the engines on the B-17, that is, the maximum altitude at which rated power could be maintained, was 25 000 feet. Experiments with turbo superchargers had been underway for many years, but the B-17 was the first aircraft in large-scale production to employ such a device. The turbosupercharged engines together with the relatively good aerodynamic parameters shown in table II for the B-17 gave the aircraft outstanding speed and range capability. The B-17 was used by the U.S. Army Air Forces throughout World War II as a heavy bomber. Nearly 13 000 of these aircraft were constructed, and a number of them are still employed today for various purposes.
The transformation of the military fighter aircraft into a thoroughly modern form had also taken place by 1936. The Seversky XP-35 shown in figure 4.14 was typical of the modern fighter aircraft developed in the middle to late 1930's. The XP-35 was a low-wing cantilever monoplane with a retractable landing gear, a fully cowled radial engine equipped with a geared single-speed supercharger, and a controllable-pitch propeller; the enclosed cockpit was, at that time, quite an innovation in fighter design. The aircraft was of stressed-skin metal construction and employed trailing-edge landing flaps. Wheel brakes...

ground view of a Seversky XP-35
[96] Figure 4.14 - Seversky XP-35 fight; 1937. [NASA]
....and a tail wheel were also fitted. In 1939, the Seversky Aircraft Company changed its name to Republic Aviation Incorporated; thus, the XP-35 may be considered the progenitor of the famous P-47 Thunderbolt fighter of World War II. Only about 75 P-35 fighters were built, between July 1937 and August 1938, at which time the aircraft was probably obsolete or obsolescent because of its relatively low horsepower. Refinements in fighter aircraft development were taking place at a rapid pace during this time, although the basic configuration concept of the propeller-driven fighter aircraft changed very little from that of the P-35.
The Douglas DC-3, the Boeing B-17, and the Seversky XP-35 are representative of the definitive and final configuration of the propeller-driven aircraft concept as applied to transport aircraft, bombers, and fighters. Many aerodynamic and structural refinements lay in the future, and both radial and in-line engines of ever-increasing horsepower were employed, but the basic configuration of these aircraft may be thought of as something of an upper plateau in propeller-driven aircraft design.