-
Quest for Performance: The Evolution
of Modern Aircraft
-
-
- Part I: THE AGE OF
PROPELLERS
-
-
- 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."
-
-
-