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Regarding airplane wings, are compression struts needed in a mono spar wing? Secondly, are anti drag and drag wires needed in a mono spar wings?


1 Answer 1


The wing in your picture has a tubular spar. In addition, smaller tubes are used at the leading and trailing edge, and one is running diagonally through the inner half to add in-plane bending stiffness.

Tubes have the same stiffness in all directions, so using a tube gives already some in-plane and torsional stiffness. For the outer part of the wing in your picture, this stiffness seems to be sufficient to not use drag or anti-drag wires. Since loads accumulate from tip to root, the root has the highest loading, and here a bracing tube is added in the inner wing so in-plane stiffness and strength are sufficient. I expect that this wing also uses bracing for the out-of-plane loads, and I checked with the manufacturer's home page: Yes, it does.

Kolb Fire Star II side view

Kolb Fire Star II side view, from the manufacturer's home page.

The ribs are supplemented by half ribs placed between the full ribs, so the forward part of the airfoil, where more aero loads and more curvature require more support, is adequately shaped and strengthened. The ribs themselves seem also to be made from rather thin tubes, and this being a rectangular wing, need only a single jig to be manufactured. An N-type truss allows the ribs to transmit shear to the spar.

In all, this is a very efficient and pragmatic wing design. There is nothing missing (well, except for the fabric covering, of course)

  • $\begingroup$ Will there be any consequence or needed modification if I should replace the tubular spar with an I beam ?@Peter Kämpf $\endgroup$ Jul 22, 2018 at 5:55
  • $\begingroup$ @DavidTeahay: Yes, an I-beam has high stiffness in one and low stiffness in the orthogonal direction. You cannot simply replace one with the other. What is normally done when the spar is an I-beam is to replace the fabric cover with a strong skin over the forward part of the airfoil. This improves torsional and in-plane stiffness, basically turning the forward part of the wing into a tube. $\endgroup$ Jul 22, 2018 at 7:45
  • $\begingroup$ ...."to replace the fabric cover with a (strong skin) over the forward part of the airfoil"......will a light slim aluminum sheet do the job of replacing the fabric cover ? $\endgroup$ Jul 22, 2018 at 15:48
  • $\begingroup$ @DavidTeahay: Yes, an thin aluminum sheet is ideal if it has enough support against buckling. In one direction the airfoil curvature helps, but in the other you would be advised to add internal stiffeners (also for the bottom part where I expect to be less curvature). Spanwise L- or Z-stringers plus a few ribs would give the best support. $\endgroup$ Jul 22, 2018 at 17:53
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    $\begingroup$ @DavidTeahay: Interesting picture! I have my doubts about the torsional stiffness of this wing (= aileron flutter already at low speed). Unfortunately, I could not find out what aircraft belongs to this wing. That should be a very light aircraft with low power - just what you want to build.This one at least has a D-shaped nose, which could be made from aluminium sheet or plywood. $\endgroup$ Jul 24, 2018 at 18:56

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