Discussion Thread: The design of a tailless flying wing

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Aerowerx

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I have been doing some virtual experiments, with changing sweep angle.

This is a tandem configuration with the pilot at 5 feet from the nose and the passenger at 8 feet. A picture of the configuration is in the previous post in this thread.

Here is a tabulation of the results, from an Excel spreadsheet...

Capture.jpg
Any one of these sweep angles would probably work, but 23 degrees puts the CG closest to the passenger.

What I did not record here was the "efficiency". I do not know how XFLR5 calculates this but I would guess that higher is better.

What I did was to change the sweep angle, and then adjust the wing position to get a zero pitching moment with both the pilot and passenger at 200 pounds each. Next removed the passenger and tried various pilot weights. It might be possible to have a lower pilot weight for some sweep angles, but I used the same weights for consistency. A 160 pound pilot works for all sweeps. Anything less (like 150 pounds) and the analysis model gives up with the wing in stall (model is tail heavy). This was all done with no trim control, by the way.

You can see how the sweep angle affects the NP position and the static margin.

Don't take these results as design guidance, since I will have to do this all over again after I get a better number for the structure weight. At this time I only have a reasonable guesstimate of the pod and wing weights.

Ooops! Somehow got that lift distribution attachment. Didn't mean to.
 

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Aerowerx

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Visualizing vortex and downwash

Interesting.

Here is a screen snip from XFLR5, of my tailless project.

The arrows are the downwash and the curly lines the "stream", which shows the vortices coming off the wing.

Note that the downwash transitions to an upwash near the tip, due to the washout.

Although it is hard to tell because of the perspective (this is a right rear quarter view), it appears that the vortex is more violent a few feet inboard of the tip. Just what Al Bowers predicted.

Capture.JPG

A few specs: airspeed 66 mph, twist at tip -1.67, twist at root +8.83 degrees, crank -30 degrees, sweep 24 degrees.
 

fly2kads

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Re: Visualizing vortex and downwash

I haven't delved into flying wings enough to know if that's normal, but 10.5 degrees of twist sure sounds like a lot! At a single wing AOA, it would seem that the local Cl along the span would be covering most of the useful range of your airfoils. If true, that sounds the wing would have to operate in a relatively narrow AOA band. Is that expected behavior, or am I weak on my understanding of flying wings?
 

Topaz

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Re: Visualizing vortex and downwash

I haven't delved into flying wings enough to know if that's normal, but 10.5 degrees of twist sure sounds like a lot! At a single wing AOA, it would seem that the local Cl along the span would be covering most of the useful range of your airfoils. If true, that sounds the wing would have to operate in a relatively narrow AOA band. Is that expected behavior, or am I weak on my understanding of flying wings?
It is a lot of twist, but that's one of the implications of the bell-shaped lift distribution (BSLD), especially as applied to swept flying wings. Your concerns are valid, but then that's one of the many tradeoffs in designing any airplane, and flying wings in particular. Whether the alleged induced-drag benefits of the BSLD are enough to offset the inherent induced-drag penalties of tailless aircraft for a given application, plus the likely parasite drag penalties of such severe twists, plus possible additional-structure weight penalties (more induced drag) because of the extra span allowed by the BSLD, is as yet unexplored. I had hoped that the recent paper by Al Bowers and others would address these questions but, now that I've finally read it all in detail, it does not. We're still waiting on an answer to that question.
 

fly2kads

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Re: Visualizing vortex and downwash

Tradeoffs and compromises! Sounds like you would have to do a lot of detailed trade studies to figure out the best combination of design variables. This configuration also seems to be one that would benefit from a fair amount of airfoil tailoring.
 

Aerowerx

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Re: Visualizing vortex and downwash

....airfoil tailoring.
That is one of the questions I have...Is there a more optimum airfoil, or airfoil distribution? By "airfoil distribution" I am referring to the PRANDTL-D having a Horten-ish root and a symmetrical tip, and smoothly transitioning between the two.

My impression of Al Bowers most recent paper was that he was trying eliminate adverse yaw without the use of a rudder, and to explain why birds don't have rudders.

And, yes, you have to modify your thinking when dealing with tailless designs.
 

TFF

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Re: Visualizing vortex and downwash

What is interesting is Al Bowers wants you to build a wing. He wants the data, and he wants to know if you can do it better.
It looks to me on the paper that the twist is pretty mild until it gets to the control surface part of the wing; at that point the washout is extreme; wing taper too. The tip is very narrow with the washout, pretty much making sure the control surfaces are always in unstalled condition, but the meat of the lift is in the center 2/3s. I see the wing really divided into two parts, lift and control, and the control part is where they are minimizing the drag.
 

Aerowerx

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Re: Visualizing vortex and downwash

The stall occurs first at about 30% of the semispan at an AoA of about 6 degrees. So, as has been mentioned elsewhere, it would land faster than a typical small plane. That is why I want to use pitch-neutral flaps.

Up to an AoA of 10 degrees the elevons at the tips are still effective. At this point the rest of the wing would be in deep stall.

From my virtual tests, you get a higher CL/CD with a higher aspect ratio but the same twist seems to work with just about any span/taper. This leads me to think that there is more optimization that can be done.

Al Bower's PRANDTL-D is a pretty close copy of some of the early Horten sailplanes, with it's extreme aspect ratio. I am shooting for a span of about 36 feet, the same span as a c182, because of construction/hanger/runway/taxiway limitations. One of my concerns is how small and delicate that tip looks, so (at this time) I am using a taper ratio of 0.5, which would also make Karl Nickel happy.

Although I think I will take a look at some extreme spans and aspect ratios and see what they do with respect to CL/CD.
 

timberwolf8199

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Re: Visualizing vortex and downwash

My impression of Al Bowers most recent paper was that he was trying eliminate adverse yaw without the use of a rudder, and to explain why birds don't have rudders.

And, yes, you have to modify your thinking when dealing with tailless designs.
I have found this theory/topic interesting and yet somewhat frustrating as well. The observation and theory are both centered on the elimination of a tail. That is, a vertical tail...and yet the immediate thrust has been towards empennage-less designs, not just tail-less. This seems like an over complication when attempting to prove a concept. Birds still have a horizontal tail so why constrain everything to the stability range and flight dynamics of a flying wing? Keeping a stabilizer would result in a more traditional looking model (audience familiarity/comfort) and less stringent design constraints. Birds' tails tend towards being close coupled and/or undersized so it may very well prove out that the best approach is something like a FW with an extended operational envelope, but to start at a FW and work back seems like the hard way to prove a theory.
 

Topaz

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Re: Visualizing vortex and downwash

The closest thing to what you're describing is the Genesis II sailplane. Despite appearances, this is a "tailless" design (the wing performs all or nearly all pitch stabilization), with the elevator alone moved to the top of the vertical tail. Doing this resolves a really remarkable number of issues that crop up with tailless designs, and retains most of their benefits. However, it's not "pure" tailless, and the tailless community has a penchant for "pure" designs (this is not a criticism), to the point of wanting to eliminate all "tail" surfaces, both horizontal and vertical.

gen_2.jpg

From a strict aerodynamic standpoint, tip-mounted vertical tails (as winglets) are, IMHO, the best solution, in that they provide a definite reduction in induced drag and positive yaw control. A logical extrapolation would be to move the pitch controls to the top of the winglets, creating a "C" wing, but I think that's beyond us, structurally, at this point. If it worked, you'd get the benefits of tailless with a much-reduced version of the induced drag penalty of having pitch control surfaces embedded in the wing.

CWing1.jpg

IF (and I do mean, IF) the bell-shaped-lift-distribution (and by this I mean Prandtl's 1933 version, not the Horten's, which latter is apparently not as good) proves to result in an measurable decrease in induced drag in real-world aerostructures (and without any significant increase in parasite drag due to the extreme twist), there's a possibility of using the proverse-yaw characteristics of that distribution to reduce the amount of time yaw controls are deployed, and getting a reduction in overall parasite drag during a flight as a result, but we're still waiting on someone to do that fundamental research on the 1933 Prandtl distribution. Bower's paper showed that it does generate proverse yaw moments in turns, which is great, but not a plus if there's a net increase in induced drag as a result of the basic distribution itself. Long way to go yet.

Aerowerx, are you okay with this bit of diversion from your original topic? I can move it off to its own discussion if you prefer, or leave it here if you like. Up to you; say the word.
 

Aerowerx

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Re: Visualizing vortex and downwash

Aerowerx, are you okay with this bit of diversion from your original topic? I can move it off to its own discussion if you prefer, or leave it here if you like. Up to you; say the word.
This is fine, Topaz, although I had created a thread specific to tailless design and was hoping that all this would end up there. Don't feel bad. I'm just as guilty as everyone else in that respect.

As far as bird tails are concerned... In the "The Experts" thread, hanger talk forum, I mentioned the Turkey Vulture twisting its tail. I said that as an example that birds do use their tails as a flight control surface, and have wondered at times if it would be possible to build an aircraft on the same principle. I have no idea how many different "modes" they have for their tails. In the case of the Vulture, it was turning tight to stay within the thermal, but I have seen Starlings do the same thing on take off.

Nickel says something about winglets, but I don't remember and don't have time to look it up right now.
 

Topaz

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Re: Visualizing vortex and downwash

Birds use their tails for both pitch and yaw control - in both cases for gross control movements when their normal, wing-only, control movements aren't enough. It's fun to watch sea gulls ridge soaring off the beach cliffs near here. You can see everything they're doing, as they glide back and forth along the cliffs.

This is fine, Topaz, although I had created a thread specific to tailless design and was hoping that all this would end up there.
I can merge this material into that other thread, if you like. The posts will end up in chronological order in the destination thread.
 

autoreply

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Re: Visualizing vortex and downwash

[...] plus possible additional-structure weight penalties (more induced drag) because of the extra span allowed by the BSLD, is as yet unexplored.
Structurally it causes less bending moment for given induced drag and wing lift... at positive G's. At negative G's though, it causes much higher bending loads. Gusts don't differentiate between up and down.

From a strict aerodynamic standpoint, tip-mounted vertical tails (as winglets) are, IMHO, the best solution, in that they provide a definite reduction in induced drag and positive yaw control. A logical extrapolation would be to move the pitch controls to the top of the winglets, creating a "C" wing, but I think that's beyond us, structurally, at this point. If it worked, you'd get the benefits of tailless with a much-reduced version of the induced drag penalty of having pitch control surfaces embedded in the wing.

View attachment 54406
That's the holy grail of best overall performance. (BWB with C-wings and pitch and yaw control on the tips). With composites that's do-able, even for an amateur very well versed in composites, but it's a pretty tough job to pull off in terms of flutter. I know it's been considered by at least one Akaflieg, but buried because the FEM on the vibrational modes was still deemed to complex to successfully pull off. With the rapid advance in FEM, I think it's do-able nowadays. The Winston Opal configuration, but with twice the span and significant sweep might be a good starting point.
 

Topaz

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Re: Visualizing vortex and downwash

Structurally it causes less bending moment for given induced drag and wing lift... at positive G's. At negative G's though, it causes much higher bending loads. Gusts don't differentiate between up and down....
Seems like everyone confined their thinking to just bending moment. I have no argument that a good BSLD can create equal or even a little less bending moment for a given induced drag and wing area, or less induced drag for a given bending moment, as it's usually expressed.

But there's more to it than just that. More span and higher aspect ratio means heavier control runs (they're just longer), more and longer control surface hinges, more material added (assuming equally optimized wings) for equal stiffness (in consideration of aeroelastic effects, flutter, etc.), and so on. You mention gust loads, and that's a great point, too. The weight penalty is there, however large or small, and it either increases the wing area for same wing loading, or the induced drag directly through a small increase in wing loading. I'm not saying the penalties are "killer" or even that they overwhelm the reduction in induced drag offered by the BSLD, but they might be, and nobody has done the work to say how big a factor this really is.

Then there's the parasite drag increase from all that twist, and trying to keep the airfoils in their drag buckets on a wing with that much twist, over a useful range of lift coefficients. Heck of a challenge all on its own.

Bowers is saying Prandtl's 1933 BSLD offers an 11% reduction in induced drag over an "equal" wing with an elliptical distribution (equal areas and wing loadings), mostly through a 22% increase in span. In terms of a purely mathematical exercise, I have no reason to doubt his analysis. His claim, however, is that the two wings will have exactly the same weight ("exactly the same amount of material") because they have equal root bending moments. I'm extremely skeptical of that part, and I think the longer-span wing will have some penalties in weight that reduce the advantage in induced drag given by the BSLD. How much that is, I have no idea, and neither does anyone else, since I've yet to see someone sit down and do the work to prove it. All Bower's research and paper shows is that proverse yaw can really be obtained by a real-world wing using BSLD. No more, no less. Nobody's done the research to show that BSLD has an real world advantage in induced drag at all, let alone quantify what it might be when you go beyond the gross oversimplifications that have been used to date. Somebody needs to do the math for some example aspect ratios and see what the real-world results really are. I'd really like to see those numbers, before I accept the "grand faith of the BSLD."

Sorry, pet rant.
 
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Norman

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Re: Visualizing vortex and downwash

Up to a point birds can fly during the molt. Obviously they can't lose too many wing feathers but people have seen birds flying with most of the tail feathers gone. This shows that a tail is at best a convenience and at worst a big flashy dangerous sexual accessory. The poor peacock probably feels a great relief when his tail feathers fall out.
 

autoreply

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Re: Visualizing vortex and downwash

[...]
Bowers is saying Prandtl's 1933 BSLD offers an 11% reduction in induced drag over an "equal" wing with an elliptical distribution (equal areas and wing loadings), mostly through a 22% increase in span. In terms of a purely mathematical exercise, I have no reason to doubt his analysis. His claim, however, is that the two wings will have exactly the same weight ("exactly the same amount of material") because they have equal root bending moments. I'm extremely skeptical of that part, and I think the longer-span wing will have some penalties in weight that reduce the advantage in induced drag given by the BSLD. How much that is, I have no idea, and neither does anyone else, since I've yet to see someone sit down and do the work to prove it.
It's pretty easy actually. For a typical short-winged sailplane, with an AR of 25, spar weight is about 15% of the total wing mass. Of that spar mass, about 2/3rd of it scales with the bending moment, the rest is minimum gauges for the shear web, glue joints and the shear web foam core. The rest of the wing weight scales almost perfectly with wing area, the number of wing panels and the complexity of the controls (notably Schempp-Hirth drag brakes).
So for that range of AR, the influence of the bending moment on total wing weight is almost an order of magnitude smaller as the effects from increasing the wing area.

Admittedly, that's all for a carbon spar. For glass fiber, especially with high AR's where stiffness constrains the spar it is an entirely different matter.
All Bower's research and paper shows is that proverse yaw can really be obtained by a real-world wing using BSLD. No more, no less. Nobody's done the research to show that BSLD has an real world advantage in induced drag at all, let alone quantify what it might be when you go beyond the gross oversimplifications that have been used to date. Somebody needs to do the math for some example aspect ratios and see what the real-world results really are. I'd really like to see those numbers, before I accept the "grand faith of the BSLD."
I've done some of that and seen plenty by others. BSLD loses every single time to a wing with winglets. Note that at least two of the people involved with today's nec-plus-ultra ships with an AR over 50 are well aware of BSLD and dediced to use winglets. That's as hard a datapoint as we'll likely get. For if there were any advantage, or even the chance of an advantage, believe me, they would've at least tried it.
Sorry, pet rant.
Same here ;)

@ Norman,

If you change the equation and allow active stability as birds can, tails are a stupid addition to an airframe. But we're still a long way from full FBW airplanes that carry paying passengers of private pilots.
 

Topaz

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Re: Visualizing vortex and downwash

Yeah, I've been considering buckling down and doing the analysis myself, since nobody else seems to have published anything. You and I are of a single mind regarding BSLD versus winglets or other non-planar solutions. The only "plus" I'm seeing from BSLD is proverse yaw where, perversely, the impact on parasite drag from having to use discrete yaw controls less often during the flight might, in isolation, convey a very slight advantage to a high-performance sailplane. But the other costs would overwhelm it, IMHO.

At any rate, BSLD loses all advantage when span is constrained (Bowers concedes this in his paper), and sailplane racing is all about span constraints. Only the Open Class might have any use for it, at all, and as you say, they've looked and gone other directions.
 

Aerowerx

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Re: Visualizing vortex and downwash

Topaz said:
Nobody's done the research to show that BSLD has an real world advantage in induced drag at all, let alone quantify what it might be when you go beyond the gross oversimplifications that have been used to date. Somebody needs to do the math for some example aspect ratios and see what the real-world results really are. I'd really like to see those numbers, before I accept the "grand faith of the BSLD.
I've done some of that and seen plenty by others. BSLD loses every single time to a wing with winglets. Note that at least two of the people involved with today's nec-plus-ultra ships with an AR over 50 are well aware of BSLD and dediced to use winglets. That's as hard a datapoint as we'll likely get. For if there were any advantage, or even the chance of an advantage, believe me, they would've at least tried it.
Maybe it is because we don't know enough about BSLD?

And do these "nec-plus-ultra ships" have a tail, or are they tailless?
 

Topaz

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Re: Visualizing vortex and downwash

Maybe it is because we don't know enough about BSLD?
What's not to know? It's a lift distribution. One of many possible. There's no magic here. Nobody's sat down and done the math. Or, if they have, they haven't published the outcome.

And do these "nec-plus-ultra ships" have a tail, or are they tailless?
They all have tails.

591_b_aircraft_0.jpg
 

Aerowerx

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Re: Visualizing vortex and downwash

... Nobody's sat down and done the math. Or, if they have, they haven't published the outcome.
Which is why I asked my question!
Maybe it is because we don't know enough about BSLD?
Besides, this topic was originally about visuallizing downwash and vortices, and has drifted to a debate about BSLD.

As I see it, the advantage of BSLD is that it eliminates adverse yaw, which makes tailless aircraft more viable. Maybe it has less drag. Maybe not. But it does have proverse yaw, so vertical control surfaces are not needed.
 
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