Take a good look at the small dots • on the bays of this two spar wing.

My question is:

  • Are the drag and anti drag wire as seen in bay 1 supposed to connect to the root of the compression tube
  • Or to the intersection between the wing spars and compression tube as seen in bay 2 or
  • are they supposed to connect to the wing spars as seen in bay 3?

In other words, which bay is set up correctly? 1, 2, or 3?

This is the point I'm at in my project, your answer will be much appreciated. enter image description here


Neither. The best one would something between 2 and 3. The continuation of the wire should cross the intersection between the middle lines of the spar and compression tube.

enter image description here

In structures with two-force members, such as trusses and combinations of rods and beams with wires, the members are intended to work primarily in tension/compression. To achieve this, the middle lines of the cross sections of all members at a joint should intersect in a single point. If they don't, some of the members will have to carry additional loads such as bending or torsion moments. enter image description here Problems due to eccentricity at joints often show up as fatigue problems: even if the aircraft structure passes all static tests, the stresses due to the additional bending moment make the structure fail much earlier than it would have if correctly assembled/designed.

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  • 2
    $\begingroup$ 45 degree braces, especially formed as a cantilever structure, actually replaced cross wires in aircraft wing design many years ago. But for ultra lights, this design still is around. I would worry about wires stretching and losing tension, and yes, how to attach them. $\endgroup$ – Robert DiGiovanni Feb 4 '19 at 0:22

I would have to go with 3. It is important to determine the weakest point. Cross brace wires work in tension. In other words, any force trying to collapse the square side ways (into a parallelogram) is resisted in tension with the bracing wire. In a wing viewed top down these would be drag (wind folding backwards) and antidrag (wing folding forwards).

Notice this is exactly the same as bracing wires for wings folding up or down (for a biplane configuration).

Although 45 degree cross members are extremely good to hold square shape in compression (especially in wood), I would not attach a tension wire to it as it may simply rip free. Some type of reinforced grommet in the stronger forward and rear spars may prove to be a better design.

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