ORIGINAL: highflyinguy
A thought I have been pondering is , at 5500ft. altitude where I fly, how the thinner air affects prop pitch. Should I be using a prop with more pitch when flying at this altitude and expect to use less pitch at a lower altitude, such as sea level?
The opposite. For R/C use a lower pitch at higher altitude for propellor efficiency. R/C spends most time climbing, turning, manoeuvreing & descending. Our cruise content of the flight envelope is minute unless we are racing pylon.
Increased altitude = decreased air density.
A propellor is simply a spinning aerofoil. For practical purposes you can ignore the effect of density upon either the prop or the wing in terms of density within the lift or drag equations. In both cases V² has by far the higher impact upon resulant performance, and that is restricted more by reduced hp output from the engine itself than prop or wing efficiences due to reduced density which both achieve a balancing tradeoff due to resultant diminished drag.
In simple terms, the engine will be significantly down on hp due to the reduction in air density. To accomodate this diminshed hp, prop efficiency needs to be increased. You need to reduce the pitch for the same diameter or reduce the diameter for the same pitch to accomodate this hp loss. Which you choose will be role requirement dependent. Low pitch offers superior acceleration, braking and climb performance. High pitch top higher top end speed and (pitch chosen dependent) lower rpm at full throttle.