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I’ve got a problem with understanding propeller windmilling. What I know is that when rpm are low and tas is high the blade aoa goes „over” the blade and is some kind of negative now, producing thrust in opposite direction than usual. But I don’t understand what really happens there. Is prop rotating in opposite direction or sth? I also read somewhere in the atpl book that when prop is windmilling the prop is driving the engine. How is that possible? Can somebody clarify that to me?

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    $\begingroup$ Do you know what a windmill is and how it works? Really, I'm not trying to be a smart alec or anything, but go to a toy store and buy a little pinwheel and blow against it, or swing your arm through the air. The force of the air makes it spin. That is all that is happening with a windmilling prop. $\endgroup$ – Michael Hall Feb 15 at 21:44
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    $\begingroup$ Think of a windmilling propellor as like downshifting your car. If you downshift (and back off the throttle), your car slows down because the wheels transmit force back to the engine, which drives the engine, causing it to speed up (that is, rotate faster) without producing more power. $\endgroup$ – jamesqf Feb 16 at 2:28
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You can think of the propeller as a screw. Take a screw, a screwdriver, and starts screwing. The threads of the screw drive the screw into the block of wood.

The propeller works in a same way: the engine drives the propeller that "screws" through the air.

It also works the other way around. Well kind of, with the screw, because of the high friction btw the block of wood and the threads of the screw, but forget about the friction for a while, if you can.

If you stop twisting the handle of the screwdriver, and instead push the block of wood against the screw + screwdriver combo, what should happen? The screw should rotate just as if you were screwing it with the screwdriver, as the threads are still sinking deeper into the block of wood. The screwdriver would of course rotate with it.

Again, it's the same with propellers (and engines). If you have speed, and you shut down the engine, the airflow will push against the propeller just as the (magically frictionless) wood against the screw. The propeller will keep rotating in the same direction, and as it does so, it keeps the engine turning also.

Now, in the case of you no longer twisting the screwdriver handle or the engine driving the propeller, there is no longer a force driving the screw or the propeller through the medium they are engulfed in. Instead, the medium (wood or air) is imposing a force on the screw & the screwdriver, or the propeller & engine to keep them turning.

If you do a google image search with words "propeller tip contrail" you'll find pictures displaying nicely how the propellers are actually screwing their path through the air.

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  • $\begingroup$ Okey, I got it, thank you! One more question. Is there any „logic” explanation, apart from rpm&tas giving negative aoa why there is negative trust generated? Because „logicly” if the prop is rotating still in the same direction it is still repelling air backwards. This negative trust is generated because the acceleration of air is lower than the tas of aircraft and in fact the prop is stopping the aircraft or I’m wrong? $\endgroup$ – Konrad Feb 15 at 22:50
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    $\begingroup$ Negative thrust is just a fancy word for drag. It is the force that is imposed on the propeller by the air. When engine is turning the propeller, and it is pushing the air bacwards = thrust. When the air is pushing the propeller to keep it turning = negative thrust = drag. $\endgroup$ – Jpe61 Feb 15 at 23:02
  • $\begingroup$ Note that the fact that it is windmilling as opposed to being stationary, it's generating lift to rearward, which is much higher than a stationary blade just making turbulence. If you stop a windmilling prop when gliding (which you can if you slow down enough) the drag of the stopped prop is way lower and the glide ratio improves quite a lot. $\endgroup$ – John K Feb 15 at 23:44
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When windmilling, the propeller rotates in the same direction as when propelling the aircraft.

enter image description here

Picture above was also included in this answer, and shows the local Angle of Attack at the blade profile: a vectorial summation of rotational speed of the prop, and airspeed. As long as the AoA is positive, spinning the propeller by applying engine torque will create thrust. This is essential: torque rotates propellers.

enter image description here

Now stop applying engine torque to the propeller, and its rotational speed will decrease. Airspeed decreases much slower, so the AoA of the prop blade will decrease. At negative AoA, the incoming air flow will apply the necessary torque for turning the propeller, as depicted above right hand side. The windmilling propeller turns the same way.

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