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Sep 15, 2019, 08:16 PM
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Wing Tip Drag/Wing Tip Propellers

The Vought V-173 (flying pancake) had a low A/R and used counter rotating propellers to counteract wing tip drag.

The reports say that this work, but that the gearing of two counter rotating propellers to a single motor was a problem.

Currently all human powered airplanes have very high A/R to keep wing tip drag to a minimum. This creates problems with construction, weight, and maneuverability.

Would the counter rotating wing tip propellers work to reduce drag on a low Reynolds number human powered airplane? Enough to use a much lower A/R?

I am thinking the power system would be a pedal driven brushless generator powering two brushless motors on the wing tips
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Sep 16, 2019, 12:49 AM
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So far as I know, the wings of all successful human powered aircraft were long specifically to win the Kramer prizes which required certain minimum heights along the flight (in specific places, like turn-arounds). This was achieved by using Olympic-level cyclists (or similar) and ESPECIALLY using long wings to allow the minimum heights to fall within ground effect. Got it? Ground effect.

Further, re: the OP suggestion, long wires produce electric "drag" and add weight which is a major obstacle and design requirement to minimize (no overweight cyclists, for ex.). Otherwise, a clever notion.
Sep 16, 2019, 06:39 AM
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I think the interesting point of the question for me is, if the wingspan is 20m for conventional single prop human-powered craft, and you reduced it, then put props out on the tips so the prop now is less than 20m from each other (IE you have reduced the wingspan but not replaced it with a prop. so now the whole aircrafts could fit between two posts without hitting)

It is plausible that the air off the prop may have a spiral flow so the outer most part may help to reduce lateral flow inward from the top. and outward from the bottom. But...

to put the power source out there, there needs to be a greater structural setup which is a weight penalty you can not afford.

Getting the wings that long is a burden in itself. and if it were possible. they would make them even longer.

Lee mentioned the extra loss due to wires and there is also the loss due to motor-generator in efficiencies. you are losing too much converting to electricity and back to kinetic.
Sep 16, 2019, 08:46 AM
An itch?. Scratch build.
eflightray's Avatar
If the wing tips create that much drag for a man powered aircraft that it is noticeable, don't have wing tips.

Simple solution.

Sep 16, 2019, 10:25 AM
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Good point, Ray!

On the tip-drag issue, in the FF forum I posted a re-done room circling copy-paper glider I made for my Budgies: took far longer to trim than design and build.

more details:

I further improved its L/D by reducing induced drag in a very simple way: increased the conical camber at the tips. I first tried this with other small low-Re designs and my heuristic explanation to myself is that many things which work for such tiny planes are similar to what works for supersonic aircraft (I exclude afterburners and supercruise, etc.). And somehow, for whatever reason, this demonstrably works! I know that some full-size aerodynamicists will "beg to differ" on this, but I like "simple" and to use what works, for whatever reason. But I suspect the answer lies in viscosity effects at low Re.



Sep 16, 2019, 11:27 AM
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BMatthews's Avatar
I'm not so sure that going to as low an aspect ratio as the original "flying pancake" would work. But perhaps a medium aspect ratio with wing tip props? I think at that point you'd need to run some pretty fancy CFD program. Or perhaps more likely since you're working with stuff that the CFD software may not fully model you'd need to run some physical wind tunnel tests to get some proper numbers.
Sep 17, 2019, 05:56 PM
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re: L/D improvement in small paper glider by increasing conical camber (Post #5)

I could easily be totally wrong about this, but I may now have a hint as to why the conical camber works so well in small low-Re deltas and similar swept wing models. Flow studies of a full-size F-106B show the conical camber somehow flattens the tip vortex and moves it inboard. In that way the vortex is more open (less drag) and as in the Prandtl twist and high AR soaring water birds, "pushes" the wing fwd (actually, just less "rear push" drag and I think a more fwd-tilted lift vector at the tip).

The low-Re connection with supersonic flow is by my observation (smoke and thread-on-wire-probe flow studies) the result of very low-Re flows wanting to stick together and "keep going" rather than complexly curving, like the water past the stern of a planing speedboat stays flat. So as in the F-106B studies, in a small delta, etc., without conical cambered L.E. the air would get thrown outward (rather than down: is that nice to say anymore in this forum without antagonizing certain parties?) and the vortex would then be tightly wound at the tip, but conical camber could/should/might have the same effect in the small swept paper model as in supersonic flight in which the flows tend to "keep on going" (like the speedboat analogy). I am sure I've got the above all backwards and the conical camber just prevents multiple vortices forming over the wing.

Still, with the swept paper glider wing, if you look hard, the down-wrapped L.E. does have a Prandtl twist-ish aspect, and similar to Rogallo wings.

Better yet, it might be something as simple as the down-curved L.E. adding a kind of "vector curl" in the opposite direction of the vortex which opens it, allowing the multiple vortices to join in the F-106B. And simply opening the vortex is the main result of most or all tip vortex-improving concepts which applies to the swept paper glider.

Just SWAG, of course (but it DOES work!)
Last edited by xlcrlee; Sep 17, 2019 at 06:33 PM.
Sep 19, 2019, 03:37 AM
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If you could make the generator and motor VERY efficient, if the motors could spin really large props, and everything was incredibly light, maybe it would work. But I suspect even modern brushless motors aren't light enough, particularly with low kv values. The wiring will weigh something, and everything will snowball. Also, props on human powered planes are already quite large. For something like this, they should probably be larger! That would require longer, heavier landing gear.....

On the other hand, the performance of the V-173 was good enough that the idea might be useful for general aviation or STOL aircraft. Many problems to be worked out. I had an idea for an ultralight using two tip mounted props driven by a long belt which also functioned as the speed reduction. I doubt if I'll ever do it, but I think the idea has promise. However, it's not a cure for everything.

I think conical camber is more about low aspect ratio and sweep than it is about Reynolds number. There are definitely strong vortices. Maybe these vortices stir up the boundary layer and prevent separation, which would be even more important at low Reynolds numbers than at high ones. For high angles of attack, sharp leading edges may help.

It may be that the conical camber in the model shown acts like washout, which would allow the rest of the wing to develop more lift before the tips stalled. Swept and tapered wings may need quite a bit of washout. I think the Hortens would use as much as 8 degrees on some of their aircraft, though I suppose some of that was for stability and structural considerations.

I tried to find some flow visualization on line for conical camber wings, but I couldn't. Hoerner mentions that a bit of camber may help with the L/D at very high speeds, but the Cl is quite low when going that fast, so it's a whole other set of considerations. He doesn't go into much detail in his Fluid Dynamic Lift.
Sep 19, 2019, 12:54 PM
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I have in other models and more recently in this considered the washout and the conical camber as two totally independent variables! With foam, but especially this paper, it is easy to do that. Above I am specifically referring to increasing the conical camber and tried various combinations of the two, finally deciding from test results that the conical camber gave the greatest L/D improvement throughout the flt. regime, from fast hand-launch to slow hand plop, noting stability and where the changes occurred along the flight. This kind of stuff comes from my old side-arm "javelin" throw HLG days.

I now tend to think this downturned L.E. effect in swept wings makes it somehow like the Prandtl twist,

Look at the downturned (looking towards orthogonally inward toward the root from the tips) rear outer section in small light Rogallo wings:

Kiki's low-Re Rogallo front

Rogallo vortex

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