Wingtip vortices and tip mounted props?

[CAFplanekid]

My friend had an interesting thought today: Wingtip vortices spin in a certain direction for each wingtip. What would be the effect of mounting a motor and prop to each wingtip, with correct counter-rotating props to spin with the vortices? I can see a number of benefits, but have not seen anything done like that before, or any reason why not to. What do you think?

[Majortomski]

But then the prop generates two more vorticies. And single engine out is a bit of a mess.

It is easier to re design the tip to reduce the original vortex.

[Ollie]

Be careful. The vortex is how the wing produces lift. If you reduce the vortex you reduce lift distribution along the wing span. If you cut the vortex, it makes the span infinite.


"Big whorls have little whorls,
Which feed on their velocity,
And little whorls have lesser whorls,
And so on to viscosity."
By Lewis F. Richardson

[Brandano]

Well, if you look at the wings of the V22 osprey, and of this feller here:
http://aerofiles.com/vot-xf5u1.jpg
you'll see that both have their props/rotors at the wingtip, and both have a small wingspan. when building something with propellers as big as these the sensible thing seems to have them set up so that the tips of the prop closer to the ground move outwards from the fuselage, so that they won't hit the fuselage if they detach on a hard landing and won't send any debris on the runway against the same in normal operation. However, in both these cases the props move in the opposite direction. This doesn't really make the wing more efficient, there's no payoff in reduced induced drag, but it increases the amount of lift you can obtain for such a small wing, and makes for better control at higher AOA's. Check the landing gear on that thing!

[Brandano]

Be careful. The vortex is how the wing produces lift.
Not quite, but it's a direct consequence of how the wing produces lift. Less pressure on the upper side, more pressure on the lower side. Air spills from the lower to the upper side of the wing at the wingtip, and you have a vortex. Since the vortex uses up some energy, it is obvious that this energy must be taken from somewhere, and so it must be (and it is!) a source of drag. Even with carefully studied wingtips you can just reduce this effect, there'll always be a pressure differential behind the wing that hasn't been completely used to produce lift, so at some point you'll always get some sort of vortex forming behind the plane.

[Ollie]

I agree with Brandano.

In the case of the V22, the wing aspect ratio is about one. The wing has a very high induced angle of attack. Perhaps with an AOA more than 30 degrees before stalling. The weight is supported by thrust and wing lift. Both props wash helps the air velocity thru lift. The idea of the V22 was to reduce landing speed. If the thrust is enough to support the weight and drag, then it doesn't need wing lift. With counter-rotating props control is simple for very small landing speed.

[Brandano]

Oh, the one in the pic isn't the V22, the v22 Osprey is a convertiplane and has a higher aspect ratio, it's easy to find pics of it around. The "Flying Pancake" in the pic is the Chance-Vought XF5U
here's the aerofiles page: http://aerofiles.com/_vot.html
it was proof-of-concept for the F5U plane, another prototype, the XF5U-1 was more representative of the production plane. Here's a site with some pics of this last one: http://www.airbornegrafix.com/HistoricAircraft/FlyingWings/XF5U.htm
It could fly at extreme AOA's and with a speed range between 40 and 425 mph, but with a turboprop it could have hovered and flown horizontally at more than 500 mph. It's not totally clear why the project was abandoned, but probably the coming of the jet age was one of the main factors. Notice how the engines are inboard of the props, and probably the engines are linked by a shaft to keep the props syncronized like in the V22 osprey. Differential thrust from the props would make it impossible to control the plane even in the more conventional portions of th flight, and it must have been a real handful to glide as well :)