Dear Thomas
You are forgetting the action of the throttle, which is there to reduce the
manifold pressure from ambient (or higher if you have a turbo-charged
engine) to a working value for the power required.
The power is directly proportional to the rate of mass flow of air through
the engine, which is proportional to the r.p.m. multiplied by the inlet air
density. For a given inlet temperature, density is proportional to the
absolute pressure (as measured by your gauge) which is directly controlled
by the throttle.
With any given throttle setting, you are quite correct in expecting manifold
pressure to go down with increasing r.p.m., as your "big air pump" is
sucking more air past the restriction of the throttle aperture, causing a
pressure drop.
There are, however, other effects that alter the efficiency of the engine so
that the theoretical power output is never reached. These account also for
the shape of the power output curve, which normally rises with r.p.m. but
may reach a peak and even reduce at very high r.p.m.
With a constant-speed prop, a combination of r.p.m. and manifold pressure is
the usual way of knowing how much power is set. An alternative would be fuel
flow: given a correct mixture setting, this is a good indicator of mass flow
and hence power.
With a fixed-pitch prop, at a known setting calibrated for your aircraft
(following for example trials measuring fuel flow at a range of altitudes,
corrected for ambient temperature), you should be able simply to use r.p.m.
to set your cruise to normal, fast or economical and relax.
Happy flying!
Mike
-----Original Message-----
From: 113-23@auswaertiges-amt.de
Subject: AW: Manifold Pressure Gauge
Sorry to once more come up with this matter - I have a Mani-pressure
gauge installed (with a fix-pitch Warp Drive prop), however, I cannot
understand the function of the meter. How by all means does the
pressure in the carburator affect the power setting ?
Isn't the pressure there simply a function of the ambient air pressure ?
How does the rpm and the torque excerted by the prop pitch feedback
to the pressure in the carb ? The engine is a big air-pump. And
with high rpm it should be sucking a lot of air and that should
make the mani-pressure drop, isn't it ?
Could someone please bring light into this miracle ?
Sincerely,
<Thomas Scherer, N81EU>
PS: I was called by the factory - they're shipping speed kits for the
Trigear now !
|