Originally Posted by
B2N2
At idle the engine is still running and developing some power.
Windmilling the airflow keeps the propeller rotating. This means rotating the crankshaft and moving the pistons up and down against the compression stroke.
That takes a lot of energy.
That's not what causes the additional drag.
If you draw out the forces for an undriven (windmilling) propeller, you can see that the aerodynamic force that for a driven propeller would be thrust now acts aft of the propeller disc due to the different AoA. This presents as additional drag.
Originally Posted by Pilotfriend
The angle of attack of a fixed pitch propeller, and thus its thrust, depends on the forward speed of the aircraft and the rotational velocity. Following a non catastrophic engine failure the pilot tends to lower the nose so that forward airspeed is maintained while at the same time the rotational velocity of the engine/propeller is winding down. As the forward velocity remains more or less unchanged while the rotational velocity is decreasing the angle of attack must be continually decreasing and at some particular rpm the angle of attack will become negative to the point where the lift component becomes negative ([reverses) and the propeller autorotates, driving the engine. This acts as greatly increased aerodynamic drag which seriously affects the aircraft's L/D ratio and thus glide angles. The drag (including the negative lift) is much greater than that of a stationary propeller, also the engine rotation may cause additional mechanical problems if oil supply is affected.
If the forward speed is increased windmilling will increase, if forward speed is decreased windmilling will decrease, thus the windmilling might be stopped by temporarily reducing airspeed, probably to near stall, so that the negative lift is decreased to the point where internal engine friction will stop rotation. This is not something which should be attempted without ample height.
In the diagram the upper figure shows the forces associated with a section of a propeller blade operating normally. The lower figure shows the forces and the negative angle of attack (aoa) associated with the propeller now windmilling at the same forward velocity.
A variable pitch propeller may have a feathering facility which turns the blades to the minimum drag position (i.e. the blades are more or less aligned fore and aft) and halts windmilling when the engine is stopped.
(Source)
Edit: Looking at this thread again, B2N2 will rightly point out that his quote I chose above relates to an engine at idle, not to engine out and a truly windmilling prop. I'll leave this post here, though, since it's not always a point well understood.