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How true is it that the deflection (due to large aspect ratio, flexible wing ) of a wing has a certain effect on the stall characteristics? I read about this in the case of large wingspan gliders. I was considering using an aerodynamic or geometric twist in the worst case but I read about this and wanted to see if for gliders this could be used to their advantage.

General Aviation Aircraft Design, by Snorri Gudmundsson:

Sailplane wingtips at a high AOA will flex substantially due to aeroelastic effects. This effectively unloads the wingtip and loads up the center portion of the wing, which then stalls before the wingtip. For this reason, sailplane usually have good stall characteristics.

Camille Goudeseune
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andy
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    Welcome to Aviation SE, Andy! Let's narrow down what you're asking. What does "deflection of a wing" mean? Angle of attack, twist, camber due to ailerons or flaps? Also, is "stall characteristics" the behavior of the airplane near stall, or just the wing's L/D curve? (Please edit your question, instead of replying in another comment.) – Camille Goudeseune Oct 31 '23 at 15:30
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    Snorri's authoritative quote seems to answer your own question. "Yes, a wing that twists spanwise changes the AoA along the span, thus changing which parts stall sooner, thus changing the airplane's stall characteristics." – Camille Goudeseune Oct 31 '23 at 18:06

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Wing flex is an important consideration for most airplanes. Even the wing of a modern airliner will flex substantially, both in bending (tips up) and in torsion (tips twisted). Wing flex will also reduce aileron effectiveness at high speed; that is why Boeing used to add high-speed ailerons to their airliners.

Wing flex comes in two varieties: Bending means that the lift force will curve the wing tips up while the moment caused by the chordwise distance between the lift force and the elastic line of the wing will twist the wing. The high camber of glider airfoils causes large variations in the chordwise position of the center of lift, and since the effect of this torsion adds up over span, long aspect ratio wings are especially prone to torsion.

However, the largest torsion happens to occur at small angles of attack, going down as angle of attack is increased. Since the weight of the glider is constant, the same goes for the lift force, so torsion is inversely proportional to angle of attack. Stalling happens at high angle of attack when torsion is low, so it contributes only a little to the stall characteristics of gliders. However, in an accelerated stall or in a tight circle, when the wing has to produce more lift in proportion to the load factor, torsion can become significant and will unload the tips, reducing bending and increasing the stall margin on the outer wing.

Flaps have a larger effect on stall characteristics: For low speed flying they are lowered more on the inner wing, so it will contribute more to total lift and unload the tips. In the ASW-22 or the ASH-25 with flaps set to "L", the ailerons will actually move trailing edge up. If you fly those gliders at slightly more than approach speed, the wing tips will bend down from the negative lift on the outer wing.

See here for an answer demonstrating the effect of bending on stall characteristics.

Mr. Gudmundsson isn't directly wrong, but I think he exaggerates the effects of wing flex if he attributes the good stall characteristics of gliders to them. More important is careful airfoil selection and planform design: Glider designers know that their products will spend a lot of time near stall so they use airfoils with docile stall behavior and select taper and twist such that the tips will stall much later than the inner wing.

Peter Kämpf
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    Why is it that "the largest torsion happens to occur at small angles of attack, going down as angle of attack is increased"? My intuition would be, more AoA = more twisting, no AoA, no twist. Where is my thinking wrong there? – Ralph J Oct 31 '23 at 18:29
  • Thank you. I was looking at the possibility of applying an aerodynamic twist, however I could hardly find another airfoil that performed equal to the design lift coefficient and well it doesn't have flaps either, that's why I considered that possibility. There is the other thing about vortex generators but I can't support it well, so I was looking for "clearer" ways. The glider we are looking at has a 20 m wingspan and for the moment we plan to use a single profile. Do you think it would be appropriate to use other methods to treat the loss effect? – andy Oct 31 '23 at 18:33
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    @andyv: what do you have in mind exactly? Maybe a better answer can be given if you explain better what you want to create – sophit Oct 31 '23 at 20:33
  • Well. It is a university project and it is to create a training sailplane however we have little time. I already selected the profile that best suited the lift coefficient and offered a good maximum lift coefficient and everything that comes with the commitment between phases of the mission. – andy Oct 31 '23 at 21:53
  • However, when we are working on the Schuemman platform (leading edge with taper and straight trailing edge) which is the classic one used in sailplane, due to the taper we have the detail of the stall at the tip where The aileron is going to be placed and that is why we were looking at how to do it (we had the option of using the straight leading edge and the trailing edge with the tapers, which does offer good stall characteristics) however the section where the aileron would go It could cost work due to the taper. – andy Oct 31 '23 at 21:54
  • Well, at first we thought about using two profiles but it will take time to analyze and support the uses. So our last option was to use a geometric twist of approximately 1.5° but we would adjust the spar then, hence the question of how feasible it is to use the wing's own deflection to treat the stall. There are other devices but they entail a certain complexity that we cannot afford. – andy Oct 31 '23 at 21:54
  • @RalphJ Your mistake is to assume the same dynamic pressure. I assume the same lift, so dynamic pressure has to compensate what is missing in AoA. Just as in real life. – Peter Kämpf Nov 01 '23 at 02:18
  • @PeterKämpf Ah! So, for a given lift, the higher AoA means less airspeed, and less torsion, than getting the same lift at a lower AoA and higher airspeed, yes? That makes good sense, thanks! – Ralph J Nov 01 '23 at 02:24
  • @andyv: being an university project don't you have a professor following you? Or advising you where to find the needed information (i.e. not from random people on the internet)? – sophit Nov 01 '23 at 11:24