KELPkid
Topic Author
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

Windmilling Prop Drag?

Sat Jul 08, 2006 6:57 am

Hi,

One of the things in aviation I have always accepted the pat answer for is that a windmilling prop produces more drag than a stopped prop, and that the amount of drag being produced by a windmilling prop in a piston-powered aircraft is roughly equal to that of a flat disk approximately the size of the propeller arc.

Obviously, in a single, when engine failure is encountered, you want the prop to windmill (for air starts and keeping systems going). I've never had an instructor advise me that there's a time to stop the prop from windmilling (it seems like you could do it by bringing the nose up and bleeding off your airspeed until you're near the stalling speed, if need be). The only place I could see this being of value is if you had to stretch a glide, for instance, engine failure over mountanous terrain with no suitable landing site within gliding range.

I won't even question feathering in a twin with a single engine inoperative!

But seriously, any insight into where the drag figures we've all learned in ground school for a windmilling prop come from?

Thanks,

Brent
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MrChips
Posts: 933
Joined: Wed Mar 30, 2005 2:56 pm

RE: Windmilling Prop Drag?

Sat Jul 08, 2006 8:24 am

1) Simple frontal area - propellers are like big boards, to put things in grossly simplified terms.

2) Think of this in terms of engine braking with a manual transmission. If you are in gear and you release the gas pedal, you will slow down considerably (especially if you are in a low gear). This is because you are using the kinetic energy of the car to rotate the engine but to continue pumping air through it as well. The principle here is largely the same - the airflow over the propeller is continuing to rotate the engine. Now this energy has to come from somewhere, and the kinetic energy of the vehicle becomes the source of energy doing all this. Unless you are adding more kinetic energy to the system (by gravity in all cases), you will lose speed.

[Edited 2006-07-08 01:26:24]
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roseflyer
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Joined: Fri Feb 13, 2004 9:34 am

RE: Windmilling Prop Drag?

Sat Jul 08, 2006 8:31 am

It is complicated. I never really understood it in my fluid systems class during college very well. But in general a rotating prop has two forms of drag on it. First of all is the drag of the face of the propeller hitting the air. This is the same regardless of whether the propeller is spinning or not. It is caused by the (roughly) flat surface hitting the air. But then on on a windmilling prop there is another form of drag. The prop is spinning, so the edge of the blade is also contacting the air because it is spinning. There is drag that resists the spinning.

Overrall there is something that has to give the energy to make the propeller. Energy is always conserved, so it has to come from somewhere, and the air is the only force that can give it this energy, so without something the propeller from spinning, it takes more energy from the air and thus more drag.

[Edited 2006-07-08 01:40:16]
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liedetectors
Posts: 323
Joined: Thu Jul 21, 2005 10:44 am

RE: Windmilling Prop Drag?

Sat Jul 08, 2006 10:26 am

Quoting RoseFlyer (Reply 2):
The prop is spinning, so the edge of the blade is also contacting the air because it is spinning. There is drag that resists the spinning.

Isnt this case on any prop? Only difference in the case of windmilling, there is no mechanical force from the engine to push against the force of drag.
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roseflyer
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Joined: Fri Feb 13, 2004 9:34 am

RE: Windmilling Prop Drag?

Sat Jul 08, 2006 10:46 am

Quoting Liedetectors (Reply 3):
Isnt this case on any prop? Only difference in the case of windmilling, there is no mechanical force from the engine to push against the force of drag

This is true for any prop that is spinning. A stationary prop does not have as high of a relative velocity to the air and does not have the second component of drag.
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