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airplane motor differs from the automobile motor in shape. The Liberty
type of engine is V-shaped, with both sets of cylinders driving toward
a common center, the crankshaft. Most airplane motors have special
carbureters, and their oiling systems are extremely finely adjusted to
take up any friction at their high speed. They will be found to be
lighter in weight, with pistons, piston heads and other parts made of
aluminium. They are, as a rule, more carefully made than most
automobile motors, with especial attention to the fitting of all
working parts.

One advantage which an airplane mechanic has is standardization, which
has reached a high point with Liberty, Hispano-Suiza, and Curtiss
engines. Once a mechanic has learned his type he has learned
practically every engine of that type. For a long time to come the
18,000 Liberty engines which this country had at the time the
armistice was signed will be carrying commercial airplanes across
broad stretches of the United States. If it had not been for the
pressure of the war this engine might have been developed slowly, as
the automobile engines were, with changes from year to year. The
Liberty engine has reached a high standard of efficiency, and is
likely to be the standard airplane engine in this country for several
years to come. An airplane mechanic who knows his Liberty engine will
be able to look after most of the airplanes with which he will come
into contact.

An engine which was not developed to the same high point in this
country as the Liberty motor is the rotary engine, of which the Gnome
Monosoupape or Clerget are perhaps the best-known types. These were
favorites with airmen flying fighting scout-planes. They weighed
practically nothing, for an engine. A one-hundred-horse-power motor
weighed only two hundred and sixty pounds, and it was a splendid type
for fast work. Briefly, the power generated by the explosions in the
cylinders, operating against two centers of pressure, gave a rotary
motion to the cylinders and crankcase, revolving around a stationary,
hollow crankshaft. Cylinders and crankcase were bolted together, and
the cylinders looked like the blades of an electric fan. There was
always an odd number of cylinders, so that there would be no
dead-centers, no point at which two opposing strains would be
balanced, causing the engine to stop. The propeller was bolted on a
nose cap which revolved with the engine. This type of engine is not
likely to