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enter image description here I have seen several YouTube videos of ultralights and even HPAs (Human Powered Aircraft) that use foam as the ribs of their wings. Are ribs not supposed to be made of rigid, strong, woody and light material i.e pine,balsa etc.?

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    $\begingroup$ Isn't wood basically a foam at the microscopic level? It's composed of once-living but now empty cells, with cell walls of cellulose & lignin. $\endgroup$ – jamesqf May 5 '18 at 18:33
  • $\begingroup$ @jamesqf; No, it isn't. Not at all. It has high anisotropy imparted by oriented cellulose fibers. $\endgroup$ – Peter Kämpf May 6 '18 at 2:21
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The answer depends on the wing loading of the aircraft. Human powered aircraft need to have a low wing loading so very little energy is needed to sustain flight. Consequently, the loads on the single structural members are low, too, so you can get away with foam-only ribs.

Next, the answer depends on the function of the rib. If it sits at some mid-span point and its only function is to keep the wing skin in the right shape, loads on the rib are indeed low. However, ribs at the wing root of a removable wing sometimes have to carry half the lift load, for example in gliders. There a foam rib would be totally inadequate.

You need to determine the local loads and then decide if foam alone will be sufficient. If not, consider strengthening the foam rib with reinforcements. Essentially, you use the foam as a scaffold and place stiffer material where the sticks on the wooden rib go.

  1. The function of the diagonal members (to carry shear loads) can be performed by ±45° glass fabric on the surface of the rib.
  2. Lengthwise reinforcements using glass rovings on the top and bottom side of the rib improve its bending strength.
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The foam is used to take small shearloads and support the primary structural element in absorbing tension and compression loads. The wood rib has the primary compression and tension members in the capstrip, with the diagonals absorbing shear loads, converted to compression and tension loads along the legs of the truss. On a foam rib, if it is a standalone rib not connecting to a stressed skin, you need to have a structural capstrip material, like wood or fibreglass because the foam only has strength when the loads are distributed over a large area. The foam itself does the job of the diagonal members in holding the caps in place and absorbing shear forces in the rib. Some ultralights have a foam rib core with an aluminum cap strip bonded to the edges to do this.

If the foam rib is inside a stressed skin wing, the stressed skin is effectively the cap strip and the foam can be bonded directly to it. Because the foam isn't very strong it requires a large number of foam ribs, closely spaced to properly support the stressed skin.

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