guin, the inventor of the Gnome rotary aero engine,
provided as great a stimulus to aviation as any that was given anterior
to the war period, and brought about a great advance in mechanical
flight, since these well-made engines gave a high-power output for their
weight, and were extremely smooth in running. In the rotary design the
crankshaft of the engine is stationary, and the cylinders, crank
case, and all their adherent parts rotate; the working is thus exactly
opposite in principle to that of the radial type of aero engine, and
the advantage of the rotary lies in the considerable flywheel effect
produced by the revolving cylinders, with consequent evenness of torque.
Another advantage is that air-cooling, adopted in all the Gnome engines,
is rendered much more effective by the rotation of the cylinders, though
there is a tendency to distortion through the leading side of each
cylinder being more efficiently cooled than the opposite side; advocates
of other types are prone to claim that the air resistance to the
revolving cylinders absorbs some 10 per cent of the power developed by
the rotary engine, but that has not prevented the rotary from attaining
to great popularity as a prime mover.

There were, in the list of aero engines compiled in 1910, five rotary
engines included, all air-cooled. Three of these were Gnome engines, and
two of the make known as 'International.' They ranged from 21.5 to 123
horse-power, the latter being rated at only 1.8 lbs. weight per brake
horse-power, and having fourteen cylinders, 4.33 inches in diameter
by 4.7 inches stroke. By 1914 forty-three different sizes and types
of rotary engine were being constructed, and in 1913 five rotary type
engines were entered for the series of aeroplane engine trials held
in Germany. Minor defects ruled out four of these, and only the
German Bayerischer Motoren Flugzeugwerke completed the seven-hour test
prescribed for competing engines. Its large fuel consumption barred this
engine from the final trials, the consumption being some 0.95 pints
per horse-power per hour. The consumption of lubricating oil, also was
excessive, standing at 0.123 pint per horse-power per hour. The engine
gave 37.5 effective horse-power during its trial, and the loss due to
air resistance was 4.6 horse-power, about 11 per cent. The accompanying
drawing shows the construction of the engine, in which the seven
cylinders are arranged radially on the crank case; the method of