Kevin Horton wrote in message ...
Some of the supercharged recips had a gear box with two different gear
ratios to drive the supercharger. They needed to spin the supercharger at
high rpm at high altitude in order to get enough manifold pressure. But
if they used the same supercharger gear ratio at low altitude it would
produce more manifold pressure than the engine could handle at full
throttle. The engine would then have to be run very throttled, and there
would be a lot of wasted power used to spin that supercharger at a
needlessly high rpm. So, they used a different gear ratio to spin the
supercharger at a lower rpm for take-off and low altitude flight.


Yes, I knew that about the supercharger gearing to allow different
settings at altitude, but the poster I was questioning had discussed
(or seemed to hint at) a two-speed propeller drive; in other words, a
transmission. I had never heard of it, outside of Jim Bede's
belt-driven variable-ratio system in the early BD-5.
The only propeller gearing I've ever seen is a fixed reduction as
used in many larger radials, all the V-12s except very early ones, and
many opposed engines such as the Continental GO-300 (Cessna 175),
Lyc's GO-480 (Helio), and GTSIO-540 (Cessna 414 or 421?), and
Continental's Tiara engine that never reached significant production.
And, of course, all turboprop, turbofan and turboshaft engines. All
with a fixed ratio, single reduction. And all to allow the engine to
develop high RPM and therefore higher HP, while allowing a larger,
slower prop to operate in an efficient range.
It seems to me that turning a prop faster in cruise flight is
self-defeating, since drag rises as tip speeds rise with forward speed
factored into the operation. It's why airplanes with big props like
the Dash 8 turn at around 1300 for takeoff and 850 or so in cruise,
and why variable pitch of the blade is absolutely necessary.
Dan