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I was under the impression wing tip vortices were an inevitable result of having air deflected downward under the wing. Do wing tip props just change the behavi

I was under the impression wing tip vortices were an inevitable result of having air deflected downward under the wing. Do wing tip props just change the behavior near the center of rotation, or eliminate vortices completely?

theothermkn 8y ago
I know that, for fixed winglets, there are a couple of (equivalent) ways of looking at them. The first way to look at them is that they resist the tendency of higher pressure air under the wing to leak around the wing tip to the lower pressure air on the top. The second way to look at them is that the winglets are flying in a circulating flow, and a portion of their lift is in the direction of travel of the airplane. (That one is trickier to see, for some.) My educated guess is that tip propellers would operate counter to the vortex direction, so that their vorticity would tend to cancel the trailing vortices. Notionally, you could think of them as throwing air back under the wing, though that's just a heuristic I made up to help imagine the direction they'd spin. Tl;dr: It's complicated!
- Gibbon1 8y ago
  This might be an amusing read (from 1968) https://crgis.ndc.nasa.gov/crgis/images/5/51/Wingtip-mounted_propeller.pdf https://crgis.ndc.nasa.gov/crgis/images/5/51/Wingtip-mounted... As well this https://crgis.ndc.nasa.gov/crgis/images/e/e0/Vortex_Attenuation.pdf https://crgis.ndc.nasa.gov/crgis/images/e/e0/Vortex_Attenuat...
Gibbon1 8y ago
I think you can look at the problem two ways. One way is that the coefficient of lift drops as you approach the wing tips. The other is that energy used to generate/contained in the wing tip vortexes shows up as a decrease in efficiency. A wing tip prop that rotates in the opposite direction cancels the vortexes. And the pressure produced by the prop increases the Cl of the upwash side of the wing. Negating the loss of Cl due to spill over at the wing tips. This is from two papers I read one from 1967 and the other more recently. The result of looking at this thing. https://www.designboom.com/technology/eviation-alice-electric-aircraft-06-22-2017/ https://www.designboom.com/technology/eviation-alice-electri...

theothermkn 8y ago

I know that, for fixed winglets, there are a couple of (equivalent) ways of looking at them. The first way to look at them is that they resist the tendency of higher pressure air under the wing to leak around the wing tip to the lower pressure air on the top. The second way to look at them is that the winglets are flying in a circulating flow, and a portion of their lift is in the direction of travel of the airplane. (That one is trickier to see, for some.) My educated guess is that tip propellers would operate counter to the vortex direction, so that their vorticity would tend to cancel the trailing vortices. Notionally, you could think of them as throwing air back under the wing, though that's just a heuristic I made up to help imagine the direction they'd spin. Tl;dr: It's complicated!

Gibbon1 8y ago
This might be an amusing read (from 1968) https://crgis.ndc.nasa.gov/crgis/images/5/51/Wingtip-mounted_propeller.pdf https://crgis.ndc.nasa.gov/crgis/images/5/51/Wingtip-mounted... As well this https://crgis.ndc.nasa.gov/crgis/images/e/e0/Vortex_Attenuation.pdf https://crgis.ndc.nasa.gov/crgis/images/e/e0/Vortex_Attenuat...

Gibbon1 8y ago

I think you can look at the problem two ways. One way is that the coefficient of lift drops as you approach the wing tips. The other is that energy used to generate/contained in the wing tip vortexes shows up as a decrease in efficiency. A wing tip prop that rotates in the opposite direction cancels the vortexes. And the pressure produced by the prop increases the Cl of the upwash side of the wing. Negating the loss of Cl due to spill over at the wing tips. This is from two papers I read one from 1967 and the other more recently. The result of looking at this thing. https://www.designboom.com/technology/eviation-alice-electric-aircraft-06-22-2017/ https://www.designboom.com/technology/eviation-alice-electri...