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I guess for small aircrafts ,hangliders etc root bending moment is not important at all,because it is relativly easy to achieve sufficient strength ,so their ideal solution will be eliptic..?

Does that mean,small aircrafts dont have any benefits of using bell distribution?

I am also interested what is root bending moment/wing weight ratio between small aircrafts and big ones,airliners etc?

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Root bending isn't an issue for braced structures using wires or struts. The root fitting is just a pin joint on most wire and strut braced aircraft, and even if there is a one piece beam going across, like some hang gliders, it's not under significant bending, except at the point where the wires/struts attach outboard.

Root bending only matters on cantilever wings, and the structural considerations and calculations will be the same for any cantilever wing aircraft, depending on the design G limits, and the dimensions and loads involved. It's a beam with a tension member and a compression member and you work out how strong the top and bottom caps need to be as with any cantilever beam whether it's an RV-6 or a 747.

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The root bending issue has been well answered.

Ultralights and hang gliders often have tailless swept wings. Such a wing with an elliptical distribution can be unstable in pitch, especially if the aerofoil is cambered. The bell distribution effectively puts a "tail at the tips of the wings", making the plane stable.

The rule of thumb is that doubling the dimensions of an aircraft will give it four times the wing area and hence four times lifting capacity. It gives double the spar depth, which works out at twice the stress, so four times the bending moment. The spar is twice as long so that's also four times the weight, proportional to lift. Drag scales with frontal area plus wing area, i.e. also times four. So L/D is constant at all scales. Secondary effects such as materials usage efficiency and control authority provide challenges which the ingenious designer can often overcome.

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