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I just saw the naked wing (it didn't have its wing fabric) of a Weedhopper ultralight and wondered the types of forces that the wing lift struts must transfer to the fuselage, apart from the apparent lift force?

Secondly, in a wire braced wing like the Weedhopper, do lift struts carry the lift load on the span between their attach point and the fuselage or do they carry the lift on the span from the attachment point to the wing tip?

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  • $\begingroup$ I made some assumptions for your second question, please have a look at it in case that is not what you meant. $\endgroup$ – AEhere Dec 21 '18 at 12:57
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I've made an insanely oversimplified sketch to show the general loads on something like a weedhopper. All of the attachments are pin joints that can't resist rotation about the pin, and therefore take shear loads only.

It's a simple pin jointed triangle, complicated by the fact that the main element is a long beam extended beyond the triangle, loaded across its axis. Most of the weight of the airplane is on the main lift struts, with mostly a compression force on the root fittings but with some up or down shear as well, depending.

The shear loads on the root fittings depend a lot on where the main lift strut picks up the wing, having an upward/inboard component, downward/inboard component, or neutral/inboard component depending on where the lift strut is attached along the spar beam.

Drag and anti-drag loads on the wing, and pitching moments also add or subtract from the shear loads caused by the lift force on the root fittings. Working it all out gets pretty complicated.

The root fitting loads are usually lower than the strut attach loads out along the wing and you'll often see surprisingly small bolts attaching the wing to the fuselage on high wing aircraft. On little puddle jumper planes like Aeroncas, you'll see fairly beefy 3/16" or 1/2" bolts at the wing to lift strut attach, but itty bitty quarter inch bolts joining the wing to the fuselage.

On a Twin Otter, with strut attachments way inboard at the nacelles, the root shear loads are more substantial and are inboard and down.

On the Weedhopper my guess is the lifting force transmits an upward and inboard shear load to the root bolts and this force varies with drag loads and pitching moments.

Spar Loads

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For your first question, it depends on the way they are attached, as is often the case in structures. I haven't seen this model up close to know their fitting type, but:

  • If they are fixed by a joint that allows them to rotate (the easiest and cheapest solution) then they can only carry one kind of load, that being tensile (or compressive) stresses. this would mean they can only transmit force along their own axis.

  • If they are rigidly jointed (cannot rotate at the joint) then they also carry some torque, although how much is a matter for the structural engineers. My guess is not much as the attachment points already offer a large moment arm.

For your second question, I believe you mean to ask for what part of the overall lift are the lift struts responsible for. That I cannot readily answer without some calculations I don't have time for right now, but it is not as straightforward as you imply. The Wing-Fuselage-Strut structure is hyperstatic (statically indeterminate or over-constrained) so it is a bit too long to napkin-math.

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