Constant Speed Prop vs Variable Engine Timing

"AnyBody43" wrote in message
om...
(Dan Thomas) wrote in message
. com...
(Jay) wrote in message
. com...
Seems to me that some of the benefits of the constant speed prop were
based on the limitiations of timing (ignition and valve) of the
Lyco/Conti engines. If your engine was designed to have a large
dynamic range of efficient operation, you won't need the articulated
prop as much.

. . . snip . . .
A fixed-pitch prop is a compromise and is like having only second
gear in your car: lousy acceleration, lousy highway speed. Could this
be fixed with fancy engine doodads? Nope. More gears are needed, and
the constant-speed prop is the airplane's transmission.

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.

A propeller is not a gear box analog. It is more like the torque converter
in an automatic transmission. A torque converter still needs a gearbox
behind it for efficiency.

Bill Daniels