Load, Shear and Moment along the Wing

by Kenny Andersen

Load Distribution

Shear

Moment

On the top is the distributed load on the wing. Nothing is to scale. The middle diagram is the shear diagram. The shear is the maximum at the root, and is equal to the total vertical load coming from the wing. Because there is an equal load being reacted at the fuselage, those 2 loads balance (wing load up = reaction down at the fuselage). Therefore, we see that there is no new shear being introduced between these two attach points so the shear in the middle is zero (implication is that you don’t need much of a shear web (so you can tie just the upper and lower caps together).

The lower diagram is the Moment diagram. It’s is also maximum at the wing root, but note it doesn’t drop, but is constant between the fuselage attach points.

So, what does this tell us, and how does one go about optimizing the structure? Generally I think we can say that the fewer the breaks the lighter the structure will be. Obviously the lightest and most efficient way to do it is with no splice. Of course that leaves you with a 44 foot object to tote around.

The next option would be to break it at the fuselage. This has a couple of advantages: 1) only one joint, 2) no shear-web in-between the wings, 3) wing fore/aft loads are easily reacted at the fuselage. Disadvantages are that the wing sections are longer. Also, flap is not separate from the ailerons. To be honest, I’m not sure how the CD works, but I think it uses flaperons. It would seem like there would be an advantage to having them separate.

Next would be to make two joints. The more I look at this the less inclined I am to believe that this is actually lighter than the single break, but there are some advantages: 1) you can have a smaller trailer (down from 21 ft to 16 ft (internal), 2) parts are lighter and easier to handle, 3) the flap can operate independently and is self-contained in the center section. The disadvantage is that I think it’s actually heavier, and the outboard wing attachment is actually a little trickier than it first look. Ostensibly there is a fairly lightweight spar, so how are the fore/aft loads reacted? There is no aft spar to speak of, only the D-tube… I’m quite sure it can be done, but I’m wondering if it is worth it, or is there something else I’m not seeing that is driving the Arc in this direction?

Personally, if there is no problem with the flaperons, then I would prefer the simplicity of the single joint at the fuselage. I think it is actually stronger, and more robust to do it that way. How many sailplanes are configured this way as opposed to a 3-piece wing? I do like the fact that you get separate flaps with the 3-piece, but other than that, I don’t see the length reduction from 21 to 16 as a show stopper, though sure, it would be nicer to have it shorter…