Tailless Aircraft - Reflex and other design issues

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Aesquire

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If span is not constrained, but stall speed is, then increased AR allows lower area.

Correct me on that, please.

Or.... Increased L/D allows lower area?

Either assertion fits the facts if you look at wing area on modern hang gliders. http://www.moyesusa.com/products.html

These are all roughly the same planform, a flying wing. The Malibu has a thinner airfoil and much more exposed structure than the other 3, more twist, so if you prefer to keep it as oranges to oranges, just consider the 3 models with the buried cross spar.

None of these is as efficient as an ideal wing. Laminar flow is very limited, as the first seam in the upper surface trips the flow, if near imperceptible span wise ripples haven't already. The exposed pilot can be more than half the total parasitic drag. However, as a side by side comparison, those factors are about equal.
 

Himat

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If span is not constrained, but stall speed is, then increased AR allows lower area.

Correct me on that, please.
No, area and coefficient of lift do set the stall speed.

Or.... Increased L/D allows lower area?
In some cases, yes. An increased L/D, or rather Cl/Cd make it possible to operate at a higher Cl for the same Cd. If power constrained, the airplane can then have a smaller wing
 

Himat

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Everything above is correct, however you said constant span, but not constant speed.
What I said (maybe not clearly enough) is that at the same span the high A/R wing will reach it's maximum L/D at a higher speed with (possibly) less induced drag than the low A/R wing. The exact speed of max L/D depends on the parasite drag, but if the fuselage is very clean (as in: sailplanes) you get not only lower overall drag, but also lower induced drag at max L/D for the high A/R wing. As i said before at any given speed the low A/R wing will have less induced drag. That is trivial, of course.

In your initial post you didn't contrain airspeed. If your requirements are indeed span and (low) airspeed, then at some point you'll have to increase area which means lowering A/R. No surprise either.
Evaluated at constant span and equal speed the formula for induced drag can be rearranged to:

Cdi = k*Cl^2*AR

If the airplanes are built to equal standards the only drag part that reduces with aspect ratio is the skin friction. This will change with the area that follow:

Area = constant / AR

If the high aspect ratio airplane does achieve a higher Cl/Cd than a low aspect ratio airplane then depend on if the skin friction at some point get larger than the induced drag. This at least at speeds below where wave drag gets significant.

My conclusion is that increasing the aspect ratio work by reducing span loading. In some ways the dimensionless aspect ratio number mask this.
 

BJC

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Evaluated at constant span and equal speed the formula for induced drag can be rearranged to:

Cdi = k*Cl^2*AR
Not exactly. That is a formula for a non-dimensionalized induced drag coefficient, not the induced drag. If you work with actual induced drag, you will see that it depends on span, not aspect ratio.


BJC

edit. I’m a big proponent of always carrying / showing units with formulas and calculations. Doing so helps avoid confusing a coefficient with a physical quantity.
 

Himat

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Not exactly. That is a formula for a non-dimensionalized induced drag coefficient, not the induced drag. If you work with actual induced drag, you will see that it depends on span, not aspect ratio.


BJC

edit. I’m a big proponent of always carrying / showing units with formulas and calculations. Doing so helps avoid confusing a coefficient with a physical quantity.
Yes, imprecise of me, I should have said induced drag coefficient.

Edit:
Dimensionless numbers are fine when scaling designs, but in the case of the induced drag coefficient I am not sure. The trouble to me is it is not really scaling a design or comparing two similar designs. Used for design it alter the design and mask the change in physics between the designs. The difference in span loading.
 
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Aesquire

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Digging through the intraweb, I found a glider I mentioned much earlier in this thread.

a19780263000cp2.jpg

valkyrieDRAWNG.jpg

Short coupled little beast, and marginal in pitch stability. I wonder how I would have reacted to it if I'd had much more experience with short chord wings, as I later had. I might have kinder words to say. But performance wasn't all that impressive, no matter how much my adrenals were squirting at the time.
 

Lendo

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Henryk, Not being familiar with tailless Aircraft, how do they manage the lack of the Elevator?
George
 

Riggerrob

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Henryk, Not being familiar with tailless Aircraft, how do they manage the lack of the Elevator?
George
Most flying wings still have elevators in their wing trailing edges. If the elevators and combined with ailerons, they are called elevons.

Like more conventional airplanes, flying wings still keep their center-of-gravity forward of their center-of-lift, creating a nose-down pitching moment. To balance, crew are seated well forward. Most piston-powered, small flying wings mount their tractor engines in the extreme nose. This is countered by reflex airfoils or elevators (e.g. Kasper) always providing a bit of nose-up trim. Flying wings create stability by forcing these two different to work against each other.
Straight "Plank" type flying wings (Backstrom and Fauvel) are comparatively rare because the very short distance between C. of G. and C. of makes them very pitch sensitive. If a plank is not balanced properly, it can be impossible to control.
That is why most flying wings have aft swept wings (Horten, Northrop, etc.). The sweep allows a longer moment arm between the elevators (or elevons) and the C. of L. A disadvantage is that span-wise air flow can create several problems that are countered by wing fences or dogs' teeth.

A few flying wings are forward swept (Marske) to produce the same longer moment arm between the elevators and C. of L. Controls can be simpler on forward-swept wings with almost conventional elevators in the center trailing edge and conventional ailerons near the wing tips. A disadvantage is structural divergence during high G maneuvers. Divergence can be controlled by stiffer structures, but stiffer structures are heavier. That is part of the reason that Marske invented carbon pull-truded rods to stiffen wings without adding too much weight.

Aerodynamically, flying wings are similar to tail-less deltas (Convair, Mirage, Verhees, etc.) . The greatest difference is that deltas fill in the gap in swept trailing edges. The filler only provides a little more lift, but it is structurally much simpler and lighter. Rear spars can run straight from wing-tip to wing-tip. Deltas also have more internal volume for fuel.
 

Bill-Higdon

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Digging through the intraweb, I found a glider I mentioned much earlier in this thread.

View attachment 79139

View attachment 79140

Short coupled little beast, and marginal in pitch stability. I wonder how I would have reacted to it if I'd had much more experience with short chord wings, as I later had. I might have kinder words to say. But performance wasn't all that impressive, no matter how much my adrenals were squirting at the time.
Do you know how did it handle compared to the Icarus wings?
 

Norman

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Not being familiar with tailless Aircraft, how do they manage the lack of the Elevator?
There are basically two sets of rules for pitch control:
Control the airfoil Cm by camber change (typical of planks) or treat the tips as horizontal stabilizers with combined ailerons and elevators (elevons (typical of swept wings)).
 

Aesquire

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Do you know how did it handle compared to the Icarus wings?
My opinion is a little bit "Apples to oranges" there. The Valkyrie used a triangle bar & most Icarus II & V gliders ( and the Easy Riser kit ) used parallel ( under armpit ) cages. But I have flown an Icarus V with triangle bar,( and the Manta Fledgling ) so... The Valkyrie plank was twitchy fast in pitch and less directionally stable in yaw, than the swept Icarus V or Fledgling. Performance was imho slightly less than the original Quicksilver, and noticeably lower L/D & higher sink than either swept monoplanes.

Both Icarus V ( with triangle bar ) and Fledgling are quick in pitch compared to the old Rogallos of their day, but not too different than a modern competition glider. Not bad at all.

I was heavy for the Valkyrie so it's not really fair, ( I'd have preferred at least 160 vs. 138 sq. Ft. ) but pitch stability was very marginal, I didn't dare let go. I never got into really unstable pitch behavior on my few short flights, but it felt like it was near zero. OTOH, the Icarus II & V are solid feeling, especially in hang cage configuration, not unresponsive, but when you trim for, say, max L/D speed, you can hit heavy turbulence, hang on tight, and it's right back after the bumps.

Valkyrie with 40' span or a pilot 50 pounds lighter would be better matched. The Fledgling is better in every way except ground handling, being tail heavy on the ground. The Icarus V, ditto, is also tail heavy with the triangle bar, and a bigger hassle to transport. Darn pretty, too.

 
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Aesquire

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The difference in ease of transport between a folded flex wing, the "padded flagpole" you can strap to a roof rack, and the fragile to ground handling damage Rigid wings, which often use big boxes of plywood is huge. There's a reason the rigid wings are a tiny minority in hang gliders. Weight and skill requirements a bit, too, but that's also a difference between modern beginner wings and competition wings.
 

Fanhead

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A few years ago, I built a Klingberg wing RC model. It was by assembling and studying this design that it occurred to me that a pitch-stable flying wing could be built without a reflexed airfoil if the wing had a substantial amount of wash-out, or twist -- tip angle of attack lower than root angle of attack. This makes the lift coefficient at the tip of the wing less than the lift coefficient of the root. So if the angle of attach increases from trim, the % of lift increase at the tip is greater than the % of lift increase at the root. This difference provides a restoring, pitch-down moment. A swept wing makes this affect more pronounced since the Cl vs alpha curve is more curved for a swept wing. A swept wing also gives more lever arm to outboard elevons for pitch control. The ME-163 is a cool design because it uses slots in the outboard portion of the wing to recover some of the lift lost due to the washout, maintain good roll control at higher AOA, and lower the landing speed.
 

Bill-Higdon

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My opinion is a little bit "Apples to oranges" there. The Valkyrie used a triangle bar & most Icarus II & V gliders ( and the Easy Riser kit ) used parallel ( under armpit ) cages. But I have flown an Icarus V with triangle bar,( and the Manta Fledgling ) so... The Valkyrie plank was twitchy fast in pitch and less directionally stable in yaw, than the swept Icarus V or Fledgling. Performance was imho slightly less than the original Quicksilver, and noticeably lower L/D & higher sink than either swept monoplanes.

Both Icarus V ( with triangle bar ) and Fledgling are quick in pitch compared to the old Rogallos of their day, but not too different than a modern competition glider. Not bad at all.

I was heavy for the Valkyrie so it's not really fair, ( I'd have preferred at least 160 vs. 138 sq. Ft. ) but pitch stability was very marginal, I didn't dare let go. I never got into really unstable pitch behavior on my few short flights, but it felt like it was near zero. OTOH, the Icarus II & V are solid feeling, especially in hang cage configuration, not unresponsive, but when you trim for, say, max L/D speed, you can hit heavy turbulence, hang on tight, and it's right back after the bumps.

Valkyrie with 40' span or a pilot 50 pounds lighter would be better matched. The Fledgling is better in every way except ground handling, being tail heavy on the ground. The Icarus V, ditto, is also tail heavy with the triangle bar, and a bigger hassle to transport. Darn pretty, too.

Thank You
 

Aesquire

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You're welcome. :)

Fanhead, that's pretty much how every current flex wing hang glider is stabilized.

Roll and yaw are coupled with sweep and anhedral. ( swept wings have a dihedral effect, look at a C-5, or other swept wing jet, that often have "drooping" wings to counteract ) Pitch control is by weight shift, and roll by weight shift & wing warping.

I chose this page, because the twist is most visible in photos. Good glider, too. Check out the other models for more views. The Falcon is a "single surface" glider for beginners through advanced. The cross spar is exposed, and creates increasing drag as speed increases, but the light weight and easy handling and low sink rate make it an excellent soaring machine if you aren't trying to go somewhere in a hurry. The T3 & others are "double surface" wings that hide the cross spar in the wing, and have higher L/D at the expense of complexity and weight.
 

Aerowerx

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....look at a C-5, or other swept wing jet, that often have "drooping" wings to counteract )....
I thought that this was because the wings sag when on the ground, but level out when in flight due to the lift. If you made a wing stiff enough to not sag when on the ground then it would be too heavy.
 
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