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What are aerodynamics benefits of using unloaded wingtips (zero lift)? does unloaded wingtip make inner parts of wing more efficient?

(I know that unloaded wingtip reduces bending moment,so structural weight can be lower, thus reduction in L/D and higher net wing lift (lift minus wing weight), but you can connect wingtip with wire and use elliptical distribution..so..)

enter image description here

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  • $\begingroup$ What is the diagram showing, that is not already in the question? $\endgroup$ Feb 1 at 17:26
  • $\begingroup$ What do you mean about the wire? External bracing? That is aerodynamically inefficient, it really cuts the L/D. $\endgroup$ Feb 1 at 20:43
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Twofold (at least): firstly it results in less energy going into the tip vortices, secondly, is the reduction in lift is gradual towards the wingtip then it tends to reduce the spin tendency - the angle of attack of the tip is typically lower than at the root and so it will stall later than the root. In a slow-flying aircraft such as a paraglider the airspeed may be significantly lower at the tip when turning, and so the angle of attack for a given sink rate will be higher, increasing the risk of a spin.

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  • $\begingroup$ yes but if you load the tip and use eliptic distribution,then L/D must be better.. $\endgroup$
    – user53913
    Feb 1 at 11:38
  • $\begingroup$ In reality you hopefully wouldn’t have a step between lifting and non-lifting, but a smooth transition. In situations where there is an abrupt step (e.g. hang glider tip extension) then the performance is hopefully better than without the extension but agreed not as good as a smoothly graduated change. Although I’d be interested to see the mathematical basis for an elliptical distribution being optimal. $\endgroup$
    – Frog
    Feb 1 at 18:41
  • $\begingroup$ @EBV It depend on what you are looking for. Yes for a given span but variable stressing the elliptical distribution is most efficient, but for a given bending stress but variable span the bell distribution with zero-lift tip is most efficient. $\endgroup$ Feb 1 at 20:38
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    $\begingroup$ @Frog The ellpitical distribution as the most efficient solution for a given span was calculated by one of Ludwig Prandtl's students around 1922 and became famous. $\endgroup$ Feb 1 at 20:41
  • $\begingroup$ Thanks @Guy Inchbald I’ll check that out $\endgroup$
    – Frog
    Feb 1 at 23:46
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I don't think any kinds of external wires are going to affect the aerodynamic properties of the zero-lift tip, they will just add their own independent effects.

For a straight wing the benefits are as already described. You note minimal root stress, hence light weight, hence minimal induced drag. A previous answer notes improved stall characteristics at slow speeds or in tight turns.

For a swept wing the benefits are more profound. The most visible difference is that it acts like a "tail at the end of the wings" to confer pitch stability and control, so the wing can be tailless. It also improves directional stability so that for a pure flying wing (an all-wing), no tail fin is needed either (yaw control is then effected by "drag rudders" located outboard near each wing tip). Then, the lift over the tip section is mostly a little above zero. Due to its washout and the upflow induced by the inboard wing sections, its lift vector is tilted forwards, which can lead to negative induced drag, i.e. a small amount of thrust. This not only makes the wing more efficient, but also in the turn it creates proverse aileron yaw, which is a tendency for the plane to swing into the turn. Adverse yaw, the tendency to swing the nose out of the turn, is a problem with constant-angle (zero washout) wings and, especially when swept, can lead to Dutch roll and other unpleasantnesses.

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  • $\begingroup$ form this paper eliptical and bell wing have same root bending moment so i dont see why would bell wing will be lighter? aviation.stackexchange.com/questions/84015/… $\endgroup$
    – user53913
    Feb 2 at 19:04
  • $\begingroup$ @EBV821 It all depends on which parameters you hold constant and which you allow to vary. Jones held the bending moment constant between the two, so it is absurd to expect it to vary in his analysis. A more realistic design consideration is to hold the wing loading constant instead, in which case the bending moment will vary. $\endgroup$ Feb 2 at 19:29

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