ORIGINAL: JPMacG
I thought a lower pitch prop provided more thrust at low airspeed (airspeed of the plane) because it moved more air at slow airspeed than a higher pitch prop. Thrust is created by the action of accelerating air after all.
I thought high pitch props were not effective at low airframe airspeeds because the prop airfoil became ineffective due to the high angle of attack of the blade (stalled?). At higher airspeed the higher pitch prop starts to work better because the angle of attack decreases as airspeed increases.
So if the goal is to cool the engine on a test stand I would think you would want a low pitch prop.... or am I wrong?
JP,
Any prop spinning in air creates a vortex, in which air moves through it at nearly pitch speed.
The prop blades may initially be partially stalled, but after the vortex has formed, they no longer are.
A lower pitch,
larger diameter prop provides more thrust, not because it does move more air, but because it has a lower disk loading.
Use a smaller diameter, higher pitch prop, spun by the same engine at the same RPM and you will get less thrust, because the disk loading is higher.
Cooling an engine doesn't require a large amount of air moving relatively slowly. It require a smaller amount of air moving at higher speed
in closer vicinity to the engine... Air moving 2" above the head fins doesn't cool the head at all. Neither does air moving 3", or 4" above it...
A small, higher pitch prop causes air to move quickly and close to the engine, while loading the engine the same as does a larger, low pitch prop. The effect - better cooling, even if Brian insists the head temperature (which is controlled, among other things, by the mixture strength) is the same.
A piston engine powered helicopter has the most thrust (lift, actually) and moves the largest amount of air...
But does its engine get good cooling? Not from the rotor, it doesn't.
Instead, there is a separate, small fan blowing air directly on the engine cooling fins.
Cooling is the only other function of the break-in prop; besides providing a load for the engine.
So the answer is yes, you are wrong.