Quest for Performance: The Evolution of Modern Aircraft
Chapter 2: Design Exploration, 1914-18
The Heritage of World War I
[64] Out of the profusion of different configuration types, structural concepts, and propulsion systems explored during the hectic days of World War I, there emerged the strut-and-wire-braced biplane, constructed of wood frame and covered with fabric, as the best overall compromise between structural strength, weight, and aerodynamic efficiency consistent with the existing state of technology. This "standard airplane" formula, with various improvements, was applied to all manner of single and multiengine civil and military aircraft for many years following the end of the war. In fact, one of the most extensively used training aircraft in the United States during World War II was the well-known Stearman PT-17 biplane. Even today, biplanes are flown for sport, aerobatic competition, and crop spraying.
Although a number of biplanes have been described above, a review of some of the salient features of the "standard airplane," the airplane design formula with which most countries entered the decade of the 1920's, may be of interest. By the end of the war, the rotary engine was obsolete, and the in-line, water-cooled type was predominant. Values of the ratio of dry weight to power had been reduced from between 3.5 and 4.0 for early Curtiss and Mercedes engines to 2.5 for the 220-horsepower Hispano-Suiza and 2.0 for the 400-horsepower Liberty. These values were lower than the typical value of 2.7 for the rotaries; however, the values given for the water-cooled engines do not include the weight of the radiator, associated plumbing, or cooling water. The propellers that transformed engine power to propulsive thrust were of fixed pitch design and laminated wooden construction. The limited speed range through which aircraft operated in that era did not warrant the use of any type of variable pitch arrangement. Large diameter propellers, consistent with the low rotational speed of most engines, were used and gave excellent takeoff and climb performance for a given amount of power. Engines were usually started by the simple expedient of having a mechanic swing the propeller by hand. [65] The callout of "off " and "contact" between the pilot operating the ignition switch in the cockpit and the mechanic turning the propeller was a familiar litany around airports for many years.
The wing loadings of aircraft in those early years were low, usually below 10 pounds per square foot, to allow operation from small fields. Most aircraft could take off and land in a few hundred feet. The typical fixed landing gear had large wheels for operation from soft unsurfaced fields and had no form of streamlining. No brakes were incorporated in the landing gear, and the tail skid was usually a fixed nonsteerable device. The action of the propeller slipstream on the rudder provided the only means of maneuvering the aircraft on the ground; accordingly, mechanics walking at the wing tips were frequently used to assist in ground handling. The tail skid served as a sort of brake on landing rollout as the aircraft moved across the soft unpaved field; it also assisted in keeping the aircraft headed in a given direction. Crosswind operations were rarely undertaken, and most airports were roughly square or circular in shape so that the pilot was always able to take off and land directly into the wind.
The control surfaces of the "standard airplane" were directly connected to the rudder bar and control stick by wires or cables; at least parts of these control lines were usually exposed to the airstream on the outside of the aircraft. Incredibly, the aileron control cables of the DH-4 ran along the leading edges of the wings. Most aircraft had no longitudinal trim system, and means for adjusting lateral and directional trim were unheard of. The relationship between the size of the control surfaces, the desired response characteristics of the aircraft, and the control forces required of the pilot were little understood in 1918. As a consequence, the flying and handling characteristics of aircraft of that day standards. A fine-handling aircraft, of which there were a few, was more a matter of luck than anything else.
Typically, the crew rode in an open, drafty cockpit exposed to the elements. In fact, pilots of that day and for many years thereafter felt that "feeling the wind in their faces" was necessary in order to fly an aircraft with skill and safety. The cockpits were, of course, unheated with no supply of supplementary oxygen, even though altitudes as high as 20000 feet could be reached by many aircraft. The extreme discomfort experienced by the flight crews at these high altitudes can readily be imagined. The well-equipped pilot's instrument panel usually consisted of oil temperature and pressure gages, water temperature gage, and tachometer. These instruments, together with some sort of fuel [66] gage, served to indicate the health of the propulsion system. In the way of flight instruments, an altimeter, airspeed indicator, and compass usually completed the instrument panel although a crude type of inclinometer was sometimes included. Radios for navigational purposes were largely unknown. Radios used for communication with ground troops were sometimes carried, and these were powered by a wind-driven generator.
Such were some of the design features of the "standard airplane" that emerged from World War I. Post-World War I aircraft development, discussed in the following chapters, began on a foundation provided by the technology and concepts of the 1918 "standard airplane."