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Pic 1 is the top and lower wing of a biplane called Bloop Ultralight,what is the function of the cross bracing between the interplane struts as seen in pic 1(I'm not talking about landing and flying wire)?

Is it there to resist wing twist? If yes, why is it not in between the interplane struts closer to the wing tip since wing twist and torsion is greatest at the wing tip?....as seen in pic 2


2 Answers 2


from Youtube

The cross bracing is to prevent the upper wing collapsing onto the lower one, the Bloop is an ultralight without the fuselage that the wings are normally attached to.

The two wing struts are in parallel and together with the parallel wings form a shape which must be cross-linked.. The bracing is for defining and retaining the structure and shape of the wings relative to each other, not to compensate for wing twist.

In the OP picture I cannot see any evidence of wing twist, only a constant wing incidence. The bracing wires need to be inboard only where normally the fuselage provides shape definition: in a classic biplane, there is no cross wiring required at the outer struts.


The bracing wires prevent their surrounding rectangle from "racking" into a quadrilateral. The forces that would cause that are each wing's pitching moment (that may be what you mean by wing twist, rather than washout). But why the wires are inboard rather than outboard, I'm not sure. Inboard, they reduce the airplane's roll inertia slightly. As this is a slow ultralight, the pitching moment may be gentle enough that inboard and outboard bracing isn't needed.


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