Non-90 degree flanges... Are they as strong as 90 deg ones?

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Chlomo

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Some of you may be designing aluminum riveted sheet metal airplanes that have graceful fuses and tapered wings. Thus the need for non-perpendicular flanges. Perhaps such projects are more suited to composites but let's assume we do it Aluminum stressed skin structure like Hummelbirds.
With appropriate thickness for given load I don't have any reasoning to believe that non perpendicular flanges are weaker than perpendicular ones but still I can't figure out why perpendicular flanges look more robust.
Are there any Hummel builders out there? Does a Hummel have any issues with flanges other than those related to forming? I see Hummelbirds have lots of circular fuse sections.
I don't know.. but boxlike sonexes sure look robust.
 
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Autodidact

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If a flange is 90° to the skin, then its "I" is large about an axis parallel to the plane of the skin. If a flange is at 0° to the skin (lays flat on it), the its "I" is small about an axis parallel to the skin. The flange is constrained against buckling in a direction parallel to the plane of the skin, so it will buckle in a direction perpendicular to the skin. The Sine of 90° is 1, and the Sine of 0° is 0, so the "I" of the flange will change from its maximum to its minimum roughly (there are other issues here and it can get complicated...) by the Sine of the angle. For slight angles, the difference isn't great, for example the Sine of 80° is 0.984 - almost 99%, but the Sine of 45° is about 71%. Z-section flanges help. On a wing spar, the flange is often constrained by both the shear web and the wing skin in two directions and is much less prone to buckling. Other people can give you a more informed answer; hopefully they will add to this.
 

WonderousMountain

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Certainly a flange can deform under load, either from pre-stresses or because the load vectors encourages buckling. Additional flanges can help, it's a matter of choosing the best sections for the build technique out of what's available. We have to make do with what is practical on a budget. Sometimes you need stiffness, sometimes area, sometimes it's better to triangulate. In this case +-15 degrees doesn't make much strength issue, but skin imprinting and creating a fracture line would be my concern. Same thing can happen if 90 deg aren't true 90 on a straight wing.

LuPi
 
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