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Unformatted text preview: ol the air after leaving the supercharger, have been generally used with turbo-superchargers, and with 2stage geared types. Such coolers are shown in figures 64 and 65. The Merlin
e ngine (fig. 64) used a water-cooled aftercoolcr w ith its own separate
r adiator and circulation system.
T he culmination of the supercharger art is represented by the Wright
T urbo-cyclone R-3350 engine shown in figure 47. This engine, introduced
a bout 1946, has three exhaust-driven turbines geared into the power
system, as well as a 2-speed centrifugal geared supercharger. It is standard
on the Douglas DC-7 and the Lockheed Super Constellation, the last
l arge piston-engine passenger-transport planes built in the United States. Vibration Control
P owerplant vibration presents two kinds of problems in aircraft. One is
e xternal vibration, or vibration of the power plant with relation to the airplane itself. T he other is internal vibration, that is vibration of parts within
t he powerplant. Considerable external vibration from engine and propeller
was accepted as normal in the early days of aviation. In my experience it
b ecame of concern first in 1920, w ith the Hispano-Suiza V-8 300-hp engine,
a l arger version of the original model. This engine, like all V-8s up to that
t ime, h ad cranks at 180°, which gave a strong second-order horizontal
v ibration. It also had an unusually large torque variation, due to its large
cylinders and high mean effective pressure. Pilots complained of discomfort with this engine.
A bout 1921 the Wright Aeronautical Corporation, which produced
t he 300-hp Hispano engine, built one with counterbalanced cranks at
9 0°, t hus eliminating the horizontal shake. Vibration-measurement at
t hat time was in a crude state, and the improvement obtained was demonstrated on the test stand by the fact that, with the 90° shaft, a penny
would remain on the crankcase, whereas with the 180° shaft the penny
w ould quickly bounce off.
T he next test was to mount two engines in similar Thomas-Morse
fighters, one with the 180° shaft and one with the 90° shaft. A number of
e ngineers ran these engines on the ground, and a number of pilots flew
t hem. The consensus was that there was no noticeable difference in vibration of the airplane. Probably, the engine torque variation was so large in
b oth cases as to obscure the improvement in sidewise shake. In any case,
73 the 90° shaft was not approved, although it soon b ecame s tandard on V - 8
engines for nonaircraft use. Such was the state of vibration analysis in 1922!
R eduction of engine vibration became essential in the e arly d ays
of commercial aviation when passenger comfort became important. In
this case, radial engines were used. Charles S. Draper and George Bentley
m ade a serious study of the shaking forces and movements of radial engines
in 1937-1938. One solution lay in flexible engine mounts to reduce the
transmission of vibration to the airplane structure. This involved a problem
of C t droop" d ue to gravity when the engine was...
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- Winter '14
- The Land