Quote:
Originally Posted by Mich48041
A flight instructor told me that a plane such as the RV-12 will glide farther with the prop windmilling. Is that correct?
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Only if the engine is producing power. Otherwise, a prop that is turning, when your engine is not running, will create drag that will shorten your gliding distance.
The best case glide scenario after an engine failure is a catastrophic failure where the prop stops turning, and is jammed ("dead stick").
Somewhere in between is any situation where your engine isn't running, but your prop is still turning... the compression strokes slow the prop rotation on each revolution, creating some drag that will depend on prop pitch and whether you have a fixed pitch prop or a constant speed (and whether your constant speed goes coarse, or fine, when it fails).
Worst case is a failure of the crank that allows the prop to truly spin free, ie. massive failures of multiple cylinders, or a shaft failure that disconnects the front of the shaft from the rest of the engine. When the propellor is free to truly "windmill", the drag that results is about equivalent to mounting a disc of plywood the diameter of your prop to the front of your aircraft.
I have done some testing in a 100hp/100mph cruise airplane with both a wooden prop and a metal prop appropriate for that power/cruise speed. I found that the wooden prop would stop within a few turns even at 100mph when the mixture was pulled to idle cut-off. The metal prop would not stop turning until the airplane was slowed to about 70mph, but doing so gained me 300-400 feet of altitude which could be useful in an emergency.
I haven't repeated these tests in my RV with it's metal prop, but others here have reported that it takes about the same speed to get the prop to stop turning, and that getting there requires a rather impressive AOA when you start from cruise flight. It's also been reported that the prop will start turning again if you get over 90mph, so maximizing the benefit of the stopped prop also means having good speed control.