Windmilling Props

[uwochris]

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.

 

[avbug]

Chris,

Minimum sink speed is not the same as best glide speed. A very general rule of thumb is that best glide speed usually is close to, or equates to Vy, while minimum sink speed more closely equates to Vx. However, These relationships don't stay the same with changes in altitude, as Vy and Vx come together with an increase in altitude, as a function of power. Best glide speed, and minimum sink speeds, do not.

To keep the ratio the same, more lift needs to be produced.

Therein lies your error. That would be the case if we were trying to maintain level flight. We are not.

Keeping the same airspeed with greater drag results in a higher rate of descent. Reduce the drag (eg, stopping the prop) results in a flatter glide; a shallower descent.

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.

Going to a lesser airspeed means flying at a greater angle of attack. Flying at a greater angle of attack means higher induced drag. Flying at the appropriate airspeed for best glide with a propeller windmilling will give the best forward glide available at the time. Flying the same speed with the propeller stopped will produce the same thing, but with increased glide performance.

Any airspeed higher or lower than this will produce reduced performance.

You're trying to make assumptions that can only be made if power is available and part of the equation.

 

[uwochris]

Avbug,

Thanks a lot; however, I do have one question in regards to your response.

Because of the greater drag produced by the windmilling prop, by going to a lower IAS, we are essentially reducing the parasite drag (parasite drag decreases with IAS); However, doesn't induced drag increase when we lower the IAS? Does this not also hurt the glide performance?

 

[A Squared]

Chris,

Here's one way of answering the question.

Draw the graph of Parasite drag and induced drag, plotted against airspeed.

Induced drag starts high at low airspeeds and has a curve that decreases with increasing airspeed.

Parasite drag starts low at low airspeed and curves up as airspeed increases.

Now total drag is going to be the sum of induced drag and parasite drag., which will plot as a curve that is above both the other two curves. The lowest spot on the total drag curve is directly over where the other two curves intersect right? Tne minumm drag position is the same airspeed for L/D max, and that's where you find your best glide, right?

So far this is basic stuff which you've already seen right? You mentioned in another post you'd been reading Kerschner, so I assume you've seen these diagrams.


OK, here's where the windmilling prop comes in. Take that graph that you just drew and assume that it represents a plane with no prop, gliding power off. Now, what happens if we add a windmilling prop?

Is the windmilling prop going to affect induced drag? No, so that curve stays the same.

The prop adds drag, but it is parasite drag. So we will have to draw a new parasite drag curve. What does it do to the parasite drag curve? well at all airspeeds, parasite drag will be greater with a windmilling prop than without. Draw the new parasite drag curve above the old parasite drag curve.

Now, look at where the "plane + Prop" parasite drag curve intersects the induced drag curve. Where is it relative to the old intersection? They should now intersect at a lower airspeed. this new intersection point at a lower airspeed represents the low spot in the new "total" drag speed, and this is where you will find your new best glide ratio airspeed.


Did you follow that? It would be much easier if we were in the same room with a chalkboard.

 

[uwochris]

A-squared,

Thanks for the response. I now understand the theory after the parasite drag curve shifts up, but I just don't see the logic in why we use a lower IAS. It makes sense, but at the same time, it seems contradictory.

I think I sometimes try to think to much

It's just that I am working on my instructor rating now, and I really want to understand the "whys" and the "hows" of evertything. But maybe I am overdoing it??

Thanks again to everyone who responds.

Chris.

 

[A Squared]

"It's just that I am working on my instructor rating now, and I really want to understand the "whys" and the "hows" of evertything. But maybe I am overdoing it??"

It's a shame everyone (especially instructors) aren't as dilligent as you in trying to really understand what is going on.

Here's what I think is tripping you up: I think that you're thinking that airspeed should increase to get the same angle of attack for L/D max with the windmilling prop. Is that right? If that is the case, with a windmilling prop, you don't want the same angle of attack, it's a different airplane with a different parasite drag curve and a different total drag curve. The angle of attack is a function of the total drag, so it stands to reason that if the total drag curve is diferent than the angle of attack for L/D max will be different.

Does that help

 

[uwochris]

"Here's what I think is tripping you up: I think that you're thinking that airspeed should increase to get the same angle of attack for L/D max with the windmilling prop. Is that right?"

That is exactly what is tripping me up! Thanks for clarifying that up- I was not considering the new drag characteristics or the fact that the parasite drag curve would shift up. It is more clear now.

Thanks again and thanks for the kind compliments.

Chris.