http://www.sil.si.edu/SmithsonianContributions/AnnalsofFlight/pdf_lo/SAOF-0001.4.pdf
Reduction Gears
The Wrights, with their chain
drive, were evidently aware that the optimum
speed for engines is not
usually that for propellers. Even before the Wrights,
most experimental airplanes
(Stringfellow, Maxim, Langley, and others)
had belt- or gear-driven
propellers, although the drive ratio for steam
engines was usually up rather
than down.
Direct propeller drive, with
the propeller mounted on the crankshaft,
is attractive for its
simplicity and reliability, and was used by most of the
early fliers after The
Wright's and up to the start of World War I. An
exception was the early
Renault air-cooled V-8 (fig. 17, p. 20), the propeller
shaft of which was an
extension of the camshaft (or vice-versa) and ran at
half crankshaft speed—a ratio
which has been widely used since.
Other geared engines which
appeared for use in World War I included
the RAF (a copy of the
Renault), the 8-cylinder-in-line Mercedes, and the
220-hp Hispano-Suiza. These
were soon followed by the Rolls-Royce Eagle,
with planetary gears. (emphasis
mine)
The need for propeller gearing
results from the fact that the propeller
speed for optimum propeller
efficiency is usually lower than the speed at
which the engine gives its
best performance. Without gearing, the speed
for the engine-propeller
combination is chosen as a compromise—too high
for best propeller efficiency
and too low for maximum engine power. As
improved engine design called
for higher engine speeds, this compromise
became more unsatisfactory. By
1920 most large European engines were
geared. In the United States,
however, the general use of reduction gears
came much later. For that
matter, in 1924 gearing was actually eliminated
from the Curtiss D-12 engine
in order to save 25 pounds of weight! Nevertheless,
by 1930 it was evident that
large engines should be geared to allow
of optimum performance. Pratt
& Whitney used an internal gear in 1931,
and both Wright Aeronautical
and Pratt & Whitney adopted the Farman
planetary gear for use in the DC-3 in 1933
(fig. 69). From that time on,
78
propeller reduction gears became an integral part of all large
airplane engines,
spur type planetary gears being
standard for most radials and plain
2-element spur gears for V-type engines (fig. 70). The plain spur
gears
used by the Rolls-Royce Merlin of 1945 carried 2,200 takeoff
horsepower
satisfactorily on a face 2-in. wide, a remarkable achievement in
gear design.
Now obviously since planetary drives have
been with us since the dawn of the airplane engine they must be capable of
working wouldn't you think? The PT-6 has a planetary reduction drive for the
propeller, as do most all geared turbofan engines. Spur gears have offset
loading which requires both a more substantial gear and housing for the same
load as a planetary. Any competent designer would choose the planetary over a
spur gear, unless there was some reason not to (like an offset shaft for a V
type engine)
Caveat Emptor.
Monty