On Mon, 01 Mar 2004 12:38:15 -0800, Dan Thomas wrote:

"Bill Daniels" wrote in message news:5_- It
seems to me that the gear analogy is spot on. A variable pitch
prop has EXACTLY the same function as the gearbox on a car.

Not quite. Gears don't have preferred operating conditions, props do.

The engine has its preferred RPM and torque for optimum efficiency and
the prop blades have their optimum angle of attack. If the engine/prop
combination results in the prop operating at a higher (or lower) angle
of attack than optimum to absorb the torque of the engine (Prop
governor increases pitch to hold RPM setting.) then the combination
operates below optimum conditions.

Under some conditions, it would make sense to introduce a third
variable i.e. a gearbox between the engine and prop, to allow both the
engine and prop to operate at peak efficiency. This was the reason
that two-speed grearsets were installed in the nose case of some large
radials. This, in turn, allowed the propeller designer to optimize his
prop blades for a single AOA, thus gaining still more efficiency.

The problem, simply stated was this: How does a heavily loaded,
long-range bomber haul itself off a short runway and climb to cruise
altitude and then shift to highly efficient, long-range cruise. The
answer was just emerging from the labs as the world shifted to
turbines.
The flight engineer would shift his engines into a "hole gear" by
selecting a cam profile and engine timing optimized for the low gear
that would let the engines scream at high RPM and pump massive HP into
props set for maximum acceleration and climb. Once in cruise, the
engineer would shift his engines back to low RPM, high efficiency
settings.

First time I've ever heard of gear-shifted props in certified
engines. Which engines were these? I know that many radials (and other
engine layouts) used reduction gearing in the case nose to allow the
engine to run faster and produce more HP while keeping the prop within
safe limits, and that there were two-speed geared superchargers on many
of these engines, but two-speed props?
Jim Bede used a snowmobile-type propshaft drive in the early
BD-5s but abandoned it as unworkable. It still required a relatively
tiny prop to keep the tip speeds subsonic.
As far as the propeller pitch angles go, the constant speed prop
improves takeoff performance by more than just letting engine RPM reach
redline to produce max HP. It reduces the angle of attack so that more
of the prop is unstalled and producing thrust in the static condition,
improving acceleration and shortening takeoff distance. The inboard
sections of a fixed-pitch prop blade have a large angle so that they
still produce thrust in faster forward flight even though they don't
travel the circumferential distance that blade areas near the tips do,
but the large angle means a stalled blade, or at least a really
turbulent flow, at low forward speeds. A gear-shifted fixed-pitch prop
will still have those problems.

Dan

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.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com