FVA 16 - Schaumstoff-Flügel
In the mid-fifties, the FVA began to deal with the problem of “foam wings”. Experiments were carried out to develop a wing with a supporting dural skin and full foam support, which led to the following results:
The production of the test pieces by foaming a Dural-Haul lying in a hollow mold by the Bayer-Leverkusen company was satisfactory. Although the overpressure occurring during foaming necessitated very rigid molds, this also ensured good contact with the mold and thus high surface quality and profile fidelity.
The tests carried out on the specimens gave the following results:
Torsion test: only about 50% of the theoretical buckling load was achieved. This was due to an unfavorable force application (too discontinuous transition from the force application rib to the skin) and too low tensile strength between the skin and the supporting material. (Flaking of the skin from the support). Pure bending: The buckling stress calculated considering the continuous elastic support was achieved, but this stress was far below that which a real wing would have had to support. If one had wanted to achieve this stress, greater sheet thickness or stronger support would have been required, both of which would have led to intolerably high construction weights.
Further theoretical considerations showed that this construction method can only compete with other construction methods in terms of weight for parts with very low circumferential loads - i.e. possibly for rudders, etc., but never for wings. Of course, this need not apply to components that are subject to high tensile stresses, such as helicopter engines.
It has already been mentioned that in one test the skin lifted off the support material during buckling. It had been shown that such defects could not be detected from the outside by simple means, such as tapping. Since, on the other hand, it could not be guaranteed that such defects were neither present nor would form in the course of practical operation, it would have been necessary to continuously inspect the component. This inspection would not have been easy from a technical point of view and would also have had a detrimental effect on the operational capability of the aircraft in practice.
For force application, it was calculated how the skin thickness would have to increase toward the connecting bolts in order to obtain an approximately constant stress distribution and to avoid jumps in stiffness that could lead to premature buckling. With the envisaged differential design, such a transition could only be achieved by several layers of finger plates glued on top of each other, which in turn required a great deal of effort.
In summary, the development of this wing construction method was abandoned because of the costly production of a perfect force transmission and too great difficulties in quality control in practical operation and, above all, because of too high construction weights.