Windmilling Props

[chris]

Hey guys,

In regards to the effects of a windmilling prop....

Theoretically, you should be using the same AOA (and hence, IAS) to achieve a minimum sink glide, as compared to flying for max endurance. In both cases, you are looking for the AOA where the lowest power is required.

The same thing applies to a best distance glide and flying for max range- in this case, you fly at the AOA where the lift/drag ratio is maximized (also considering prop efficiency and BSFC).

Now, the IAS to use is lower in the case of the glides because these are power-off scenarios and your prop will be windmilling, producing a lot of drag.

If the drag is higher with your prop windmilling, and the goal is to maintain the correct AOA, why do we use a lower IAS and not a higher one?

For example, in the best distance glide, we fly at the best L/D AOA. If our prop is windmilling, there is more drag. To keep the ratio the same, more lift needs to be produced. The only way to achieve this is by increasing IAS.

What am I misunderstanding? I just don't understand why we use a lower IAS in the power off situations, where drag is very high.

Thanks for any responses.

 

[E_Dawg]

When you add parasite drag (windmilling prop) that line moves up, making the point of L/D max at a lower IAS than before.

Also, lift does not increase when you speed up, lift stays the same at all airspeeds (Lift = Weight, or tries to), it is AOA that varies.

 

[Ophir]

Yeah, and don't forget that each prop blade is producing lift while in a power-on situation. This too would create induced drag on the prop and therefore and net increase in drag with power-on.

One consideration when you talk about this; are you in a fix or constant speed prop? If you move the prop to full pitch (flat pitch) in a power-off descent you will have considerable more drag. Therefore, it is good to teach students that pulling the prop back all the way, in an engine out scenario, will decrease drag and therefore decrease your Vy, or best glide. As Ed said, you will need to produce only the amount of lift of the plane and no more, this will result in a given AOA for the density altitude and weight. The poor man's AOA indicator is airspeed. Just remember, when computing the best L/D, you have to take into account the total drag on the airplane. If you reduce drag, you reduce your best glide speed.