Originally Posted by
Above The Clouds
Big Pistons Forever
What controls power output is density in a normally aspirated engine not RPM. And cabin noise has no bearing on optimum altitude.
With a fixed pitch prop the
control of the power of the engine is achieved by changing the RPM. What power any RPM setting will give you is mostly but not totally dictated by the density of the surrounding air. (Carb temp and mixture and airspeed also effect power output). It is up to the pilot to go to the POH and find out what RPM he/she must set to achieve the desired power output at any given altitude and temperature.
IMO cabin noise has a significant effect on pilot and especially pax comfort. For instance many Cessna's have an annoying buzz at max RPM but a more pleasant sound at, and much less vibration, in the 2400-2500 RPM band. This may lead to choosing a lower altitude in order to get the desired power output at a lower and hence quieter RPM particularly if you are flying a long trip against a headwind.
Selecting a real world "optimum" altitude should consider all factors effecting the flight. For example on a sunny summer day I will always choose a higher altitude for a mid afternoon flight as compared to an early morning flight, because of the likely hood of low level turbulence in the afternoon.