Originally Posted by
cmulder
I assume you did the same when controlling boat engines; make sure the prop pitch is chosen so that the ideal "hull speed" matches the optimum engine rpm.
Very few model pilots take this into consideration. Each model design has a optimum speed; for example a piper cub model might go about 70 to 80 km/h
So to get best performance you want a prop pitch resulting in a "prop speed" (pitch times rpm) that is a little higher then the maximum speed you want the model to fly.
For a 6.5cc for example you might end up with a 11" by 5" prop on a piper cub while a pylon racer might run a 7.4" by 7.5"
The piper would end up with much more "static trust" then the pylon racer and take off in a shorter distance.
Trust and torque are related so it is still a factor to keep in mind.
Yup... for planes that is no different than for ships, and while it is true that many planes have props that spin lower than the engine crank, and the direct crankshaft torque is unable to spin that prop at that RPM, but that is what gear reductions are for: matching engine characteristics with prop characteristics.
It TILL is the engine delivering a certain amount of power to the gearbox input, and the gearbox converting that power to the desired torque and rpm for the prop.
For simplicity reasons, in RC flying we rarely if ever use reduction gears, so yes, it IS possible that a plane performs better with a larger slower prop, meaning the engine does not deliver its optimal horsepower, but then we are talking about prop-efficiency, and that is an altogether different animal. An animal that even I have only a rudimentary understanding of...
Maybe it is more accurate to say that planes fly on the horsepower
as delivered by the prop...