The graph to the left shows that after the engine failure the Power required has increased significantly. Most of this is due to the increased parasite drag from the windmilling propeller.

In the situation to the left the power available has been reduced to half and the power required has been increased significantly. The net result is that the power available is less than the power required and this aircraft will not be able to maintain it's current altitude (remember climb rate depends upon Px. Therefore, this aircraft will descend since Px is negative.)

The propeller produces much more drag as it windmills than it would if it simply stopped turning.

The diagram to the right is one we have already seen. It shows how the propeller produces thrust by being at a positive angle of attack to the relative wind. You should review this material if you have forgotten it.

As we know, the angle of attack on the propeller blade decreases when the rpm decreases.

Windmilling Propeller

As soon as the engine stops producing power it will of course slow down, due to the drag, both aerodynamic and frictional (within the engine.)

As the propeller slows down there will be an rpm at which the angle of attack on the propeller blade becomes negative. This is shown in the diagram to the left.

As the air flows over the propeller it produces lift, as always. But, now the Lift, or "Thrust" is in the wrong direction. As, we can see in the diagram the "Thrust" is backwards. IE, it is actually drag.

You will also note, that anther component of the propeller lift vector is now keeping the propeller turning. This is why the propeller does not stop turning after the engine failure. (Note: the faster you fly, the faster the prop will windmill.)

The faster the aircraft flies the faster the prop will windmill and the more drag it will produce. What we need is a way to stop the propeller from turning.