# Cause of Drag From Windmilling

I know that when a propeller is windmilling it creates drag, I assume this is from the propeller being flat against the air causes drag, but propellers are designed to essentially pull an airplane forwards with rotation so wouldn't the incoming air spin the propeller and create thrust? Does the wind not cause the propeller to spin fast enough for thrust and only just wastes energy by using it up spinning a propeller too slow to actually help with propulsion?

a windmilling prop is extracting work from the air moving through the prop disc, and using that work to rotate the dead engine. the power extracted is approximately equal to the speed of the air times the drag force exerted on the prop.

This is the reverse of the situation in which a running engine is spinning a propeller which performs work on the air, generating a reaction force which is expressed as thrust.

• So it's only the friction in the engine? Commented Aug 11, 2023 at 13:02
• @Bergi: plus the aerodynamic drag of the blades Commented Aug 11, 2023 at 14:07
• @Bergi, "friction" is probably the wrong word. Most of the work goes into compressive heating of the air in the cylinders. Commented Aug 11, 2023 at 18:59
• @BowlOfRed Virtually all of the work that goes into compressing the air should be recovered back during the power stroke (even when there is no combustion); so there shouldn't be a net energy loss. What actually absorb the energy (from the windmilling prop) are the pumping losses, i.e. Induction and exhaust stroke. Induction stroke: This is because the engine is essentially pulling a vaccum through the induction manifold (a volumetric efficiency of less than 10% would be typical with the throttle fully closed). This effect can be greatly reduced by fully opening the throttle valve. Commented Aug 12, 2023 at 8:55
• Secondly, exhaust stroke: This loss results from the back pressure in the exhaust manifold, and the piston trying to force the exhaust gases against it. For example, a muffler alone can cause around 10 kPa (1.5psi) of back pressure. Apart from the pumping losses, the second biggest power absorber is the internal friction of the engine. Commented Aug 12, 2023 at 8:55

Does the wind not cause the propeller to spin fast enough for thrust...?

Think of it in terms of cause and effect. If you had a windmill but no wind, nothing is moving, there is no cause or effect.

• If the wind starts to blow, the windmill will turn. The wind is the cause, the windmill turning is the effect.
• If you instead hooked a motor up to the windmill and forced it to turn, the air would start to move. In this case, the windmill turning is the cause, and the air moving is the effect.

It doesn't make sense to think that in the first case the wind causing the windmill to turn is what causes the wind. That just isn't how things work, energy must be transferred from one form into another - it isn't just created.

An airplane propeller is either being powered by the engine, moving air and producing thrust, or it is unpowered, and being rotated by the force of the air. It can't do both. When it is unpowered it is producing drag, not thrust. The speed at which the wind rotates an unpowered propeller is irrelevant to whether or not it is producing thrust.

I would encourage you to do some research into Newton's laws of motion for more background on this.

Normally, an airplane converts thermal power (from combustion) into thrust (mechanical power). When the propeller is windmilling, drag (also mechanical power) is converted to thermal power.

As mentioned in another answer, windmilling the propeller rotates the engine. The engine moves gasses around and it has internal friction. Therefore the engine consumes power which contributes to the power of drag.