Micky
The performance is about how much of that power at the crankshaft you can turn into thrust, as a prop is an aero foil it will only give the best performance at the most efficient angle of attack, as A of A decreases with TAS most fixed pitch aircraft start the takeoff run with the prop fully stalled and at about 30 Kt the prop un-stalls and starts to work properly until the A of A decreases at the higher TAS and the aircraft can't accelerate any more.
The short answer is that a fixed pitch prop is a compromise.
There are very few multi role light aircraft but the DR400 is a good example the tourer will cruise at 135 Kts with reasonable field and climb performance with a course fixed pitch prop, the glider tug variant with a very fine fixed pitch prop will (without the glider attached) take off in a very short distance and climb at 70 Kt very well but don't ask it to fly faster than about 100 Kt because you will overspeed the engine because the prop can't turn the power into thrust due to the low A of A of the prop blades at higher TAS. So in the glider tug example cruise performance is sacrificed for performance in the 65-80 Kt speed range that the glider tug mission requires.
The C150 with the VP prop can turn all of the power into thrust all of the time (within reason!) as the A of A of the prop is always close to the optimum for that TAS due to the variable pitch of the prop blades.
Last edited by A and C; 28th Apr 2013 at 08:58.