Flying wing for high efficiency

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ThadBeier

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My idea for a flying wing design addresses a number of issues. I haven't gone so far as to actual build a large-scale model yet, but the little ones I've built fly pretty well.

The idea is to build a flying wing that is stable, controllable, and has a more-or-less elliptical lift distribution. The way I would do it would be to have the wing have a significant swept-forward stagger. The innermost section would have a substantially longer chord than the next-most-outboard section, and would fly at a lower angle of attack, so that at cruise the lift generated by both parts of the wing is about the same. An elevator at the back of the center section would have a substantial moment arm, enough to be effective. The larger chord of the center section would provide substantial depth for, say, one's shoulders.

The difference in angle of attack between the forward, outboard part of the wing and the rear inboard part will allow the use of wing sections of zero or even slightly negative moment, while still generating a stable plane overall.

Here's a picture of what I had in mind.
comp_small.jpg
 

Autodidact

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Induced drag has nothing to do with lift coefficient. - posted by pie_row
Actually, for a constant wing area and AR, induced drag goes up with Cl^2. pie_row, please calm down. Most of the conflict on internet forums is from misunderstandings. Rethink your point and try to make it from a different approach.
 

Topaz

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Yes I did say that the drag of the wing’s wetted area goes up with the square of the speed. Not the raea itself. Thank you very much for taking my words out of context.
Nothing taken out of context. Direct quotes. In context. You've just used some wrong terminology, and it's getting in the way of your meaning. Blaming me for it isn't very constructive. For example, just adding a little more bold is not "taking your words out of context":

The drag of the fuselage and the wetted area of the wing” pie_row
goes up with the square of the speed (indicated air speed).” Pie_Row
As I said I don’t think that you can read.
Okay, that's quite enough of that.

I have thought it through. I do not call someone with Orion’s experience wrong without putting a great deal of though into it.
You'll note that I'm not the only one here saying, "Hmmmm... Why don't you calm down a moment and rebuild your arguments in a more complete fashion, with more support and less drama. Right now you're not making a lot of sense." Perhaps that's worth considering.
 
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Topaz

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My idea for a flying wing design addresses a number of issues. I haven't gone so far as to actual build a large-scale model yet, but the little ones I've built fly pretty well.

The idea is to build a flying wing that is stable, controllable, and has a more-or-less elliptical lift distribution. The way I would do it would be to have the wing have a significant swept-forward stagger. The innermost section would have a substantially longer chord than the next-most-outboard section, and would fly at a lower angle of attack, so that at cruise the lift generated by both parts of the wing is about the same. An elevator at the back of the center section would have a substantial moment arm, enough to be effective. The larger chord of the center section would provide substantial depth for, say, one's shoulders.

The difference in angle of attack between the forward, outboard part of the wing and the rear inboard part will allow the use of wing sections of zero or even slightly negative moment, while still generating a stable plane overall.

Here's a picture of what I had in mind.
View attachment 13018
Interesting approach. Doesn't the quick change in effective angle of attack between the inboard and outer wing put a pretty severe "kink" in the lift distribution? I don't see a big shift in chord in the picture, and it seems the inboard and outboard sections make a pretty "sharp" transition.
 

ThadBeier

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Interesting approach. Doesn't the quick change in effective angle of attack between the inboard and outer wing put a pretty severe "kink" in the lift distribution? I don't see a big shift in chord in the picture, and it seems the inboard and outboard sections make a pretty "sharp" transition.
The point is to balance the change in chord with the change in lift coefficient. By adjusting the stagger, the difference in geometric AOA, and the relative chord, one can get more or less stability. That bit of math is easy to work out, and you would get a smooth lift distribution, at least at one speed (design cruise speed)

The big challenge with the design is aeroelastic instability. Like with a forward swept wing, the lift in the outboard sections will tend to twist the outboard wing nose-up, increasing lift, bending it more, and so on.
 

WonderousMountain

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Actually Thadbeier's approach isn't totally unfounded. A few flying wings have used chord difference as a control theory. If I remember right some of Jim Marske's planes have sported larger inner chords or park bench ailerons, and the flying pancakes had tab like control surfaces with vastly different chords. Norman might know the history better, but I've seen several concepts with this approach.

A question though; why not offset the smaller outer wing surface back instead of forward. You could use an airfoil with it's thickness far aft and then one with it's thickness far forward if you're planning on going with a main spar. Or possibly taper to the smaller outer portion would provide a smoothe and effective solution. Like delta meets plank? The offset will of course create some drag, but as long as he doesn't create a vortex in that area due to disseperate lift, the drag won't won't deviate too far from norm. In any case it's likely to be a discussion point rather a performance issue if the trim and stability are right.

I've been looking into this solution for a future project. Good drawing,

Wonderous Mountain
 

Norman

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Norman might know the history better, but I've seen several concepts with this approach.
Topaz knows the theory behind a chord extension in the elevator area and has incorporated it into his design. Generally one should avoid big cranks in the planform. Especially in high stress areas like near the root. It just leads to structural headaches that will increase the empty weight of the aircraft. But if style is more important to you than useful load then go for it
 

ThadBeier

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...

A question though; why not offset the smaller outer wing surface back instead of forward. You could use an airfoil with it's thickness far aft and then one with it's thickness far forward if you're planning on going with a main spar. Or possibly taper to the smaller outer portion would provide a smoothe and effective solution. Like delta meets plank? The offset will of course create some drag, but as long as he doesn't create a vortex in that area due to disseperate lift, the drag won't won't deviate too far from norm. In any case it's likely to be a discussion point rather a performance issue if the trim and stability are right.

I've been looking into this solution for a future project. Good drawing,

Wonderous Mountain
I've built those before too. They fly very well, as well. The challenge is that you want the inboard section to have a longer chord (to fit stuff into it, and for structural reasons), but it has to fly at a higher angle of attack, so it's generating a huge amount of lift compared to the outboard sections. The forward stagger works better. I'm building a big model, and will post flight results by the end of July.
 

Topaz

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Topaz knows the theory behind a chord extension in the elevator area and has incorporated it into his design. Generally one should avoid big cranks in the planform. Especially in high stress areas like near the root. It just leads to structural headaches that will increase the empty weight of the aircraft. But if style is more important to you than useful load then go for it
Yeah, I'm scratching my head a bit as to how this solves any of the fundamental problems. I mean, I get what he's trying to accomplish, but the structural ramifications are messy, and I have a hard time seeing how the lift distribution isn't kinked over the abrupt transition as well. Visually it's an interesting look, though.
 

Topaz

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The point is to balance the change in chord with the change in lift coefficient. By adjusting the stagger, the difference in geometric AOA, and the relative chord, one can get more or less stability. That bit of math is easy to work out, and you would get a smooth lift distribution, at least at one speed (design cruise speed)
I'd be more concerned with having a lift distribution as close to elliptical as possible when the airplane is flying at higher lift coefficients: climb. Induced drag is a much larger factor then, and the shape of the lift distribution affects induced drag, primarily. A low induced drag at cruise is great if you've got a very high wing loading and so a high cruise CL, but that's rarely going to be the case with a tailless airplane. At cruise, parasite drag dominate.

The big challenge with the design is aeroelastic instability. Like with a forward swept wing, the lift in the outboard sections will tend to twist the outboard wing nose-up, increasing lift, bending it more, and so on.
That it will, and there will be a rather nasty stress riser in the planform transition, as Norman has described. Like I mentioned in my previous post - I see what you're trying to do, but I'm not sure that I agree that this is the way to get there.
 

WonderousMountain

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Yes, it's a bit silly structurally;
However, the AR is fairly moderate and the offset-sweep isn't too sever either. My opinion on the vortex sheet is it works both far forward and far rearward of the wing. So that drag won't be significantly affected by the plane-form. Although, I think we're all curious as to how it trims out. Have you chosen airfoils yet? Done the weights and balance? Would you send me the info on the ones you've flown like the one I described? What's the wing area and loading? Design cruise Cl? Stall predictions? Can I think of any more question before the computer eats this post. Oh yeah, what's the construction method?

Intrigued,

Wonderous Mountain
 

BBerson

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The innermost section would have a substantially longer chord than the next-most-outboard section, and would fly at a lower angle of attack, so that at cruise the lift generated by both parts of the wing is about the same.
Lower angle inner wing.
Would this cause dangerous tip stall?
BB
 

ThadBeier

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Lower angle inner wing.
Would this cause dangerous tip stall?
BB

There would be substantial washout at the tips so they shouldn't stall, but yes, the first stall would be on the middle of each wing, and would yield a substantial rolling and yawing moment. Some kind of stall strips at the inner part of the outer wing would be appropriate. Maybe slots on the outer part to keep the flow attached would be good too.
 

fly2kads

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Aircar

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Just to confuse things further -the swept forward inner wing and swept back outers is of course the standard bird planform but whether it has any aerodynamic virtues or is just needed to fold them is not so clear --several aircraft built with this sort of planform didn't seem to be successful (the SZD Flying wing with gulled shape, the Sth african Exulans, the Manx and even some proposed supersonic designs where this was called the "M" wing .

Tailless aircraft still seem to be problematical as regards trim drag and to some extent stability and control --the high aspect ratio versions need sweep to get enough pitch authority while the low aspect ratio types can be more tolerant on CG position and structure with more usable volume .

The 'tailed' flying wings, like the Genesis, are a compromise that might be better than trying to be 'pure' --several papers in OSTIV notes proposed the more birdlike layout with a discrete elevator but basically stable wing and no real fuselage (but the Stuttgart Uni FS 26 built to this sort of formula was not a raging success either --the higher inertia in roll and yaw of a high aspect ratio cantilever wing works against the control power of a short moment arm for rudder for one thing.
 
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