Great expanation Thanks
John
PS my first post to a newsgroup (I'm a Virgin no more;-) )


Dan Thomas wrote:

(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.


Horsepower is a function of torque multiplied by RPM. A Lycoming
engine in an older Cessna 172, for example, produces 150 HP at 2700
RPM under standard conditions (sea level atmospheric pressure and
59°F). In the takeoff roll with the fixed-pitch prop, RPM will be
around 2300 RPM, which, according to the POH, would indicate a
horsepower output of about 61% of 150, or about 92 horses. Not very
good, is it?
So, we only have about 60 percent of the engine's power in the
takeoff. Worse yet, this diminished power is going into a propeller
that is largely in a stalled condition at the beginning of the takeoff
roll (because of high blade pitch angle and low forward speed) and is
producing much less than its max thrust as a result, and acceleration
is pretty dismal.
What can we gain by fooling with valve or ignition timing in a
situation like this? Not much. We add weight and failure points,
neither of which are welcome here, and gain very little in
performance.
So the constant-speed prop was invented. It is controlled by a
governor so that the engine is allowed to reach full rated RPM, which
produces full rated HP (if at sea level and standard temp), and
because the propeller's pitch is much lower in this mode, much more of
it is producing thrust instead of useless stall turbulence. In
cruising flight, the pitch increases to keep the engine RPM within
limits while still producing more thrust and a higher cruise speed
than a fixed-pitch prop can.
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.

Dan