Erik Hofman writes:
> You know how fast the aircraft goes at a certain propeller RPM. > Now you want to know the propeller RPM at a certain speed.
It's not quite so simple.
1500 rpm = 125 fps = 74 kcas 2000 rpm = 167 fps = 99 kcas 2500 rpm = 208 fps = 123 kcas
That's easy enough. The problem with windmilling is that the propeller does not spin all the way up to its neutral speed, but drags somewhere behind; for example, idling at 74 kcas, you're more likely to see around 1100 or 1200 rpm (I've never shut down the engine in flight, but I imagine it would be a couple of hundred rpm lower in that case). We need to figure out the balance between engine friction and compression (slowing the prop down) and the oncoming airstream (speeding the prop up).
True. Now that you mention it.
The engine needs a certain force to drive all the mechanics *and* the propeller. While windmilling the propeller needs a certain force to drive all the mechanics *and* the engine.
So we need the amount of force needed to rotate the mechanics (crank shaft, bearings, etc.)
Erik
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