It may help to add a little more theory to the discussion. The points below
are based on a posting I made to the former Europa Forum on 30 March 2000:
The action of the throttle is 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, which is directly controlled by the throttle, as is
measured by a manifold absolute pressure (MAP) gauge.
With any given throttle setting, the manifold pressure goes down with
increasing r.p.m., as the engine 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 is 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.
Mike
Europa Club Safety Officer
m.j.gregory@cranfield.ac.uk
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