For some model aircraft, as well as full size aircraft, fabric covered rib and spar construction techniques are used. Usually they are easy and cheap to build, and offer a lightweight structure. From an aerodynamic point of view, they have the drawback of interpolating from the desired airfoil shape to something we don't know, between the ribs.
As some readers of these page have pointed out, the fabric between the ribs of full scale airplanes is sucked upwards by the low pressure field on the upper wing surface. Experiments with typical model aircraft wings showed only negligible deformations, which is caused by the smaller spacing between the ribs and, mainly, by the lower flight speed of model airplanes. The local pressure on the surface is proportional to the square of the velocity. This means, that the surface pressures on a sailplane model, flying at 10 m/s, are only 10²/40² = 1/16 of the forces on a sailplane cruising at 40 m/s.
Wind tunnel tests at low Reynolds numbers have shown quite good results in terms of drag for plastic film covered rib structures [18, 30], but there seems to be no systematic investigation of the effects occurring on covered rib structures.
Fig. 1: Polars of the E 374 for a typical, high quality wind tunnel
model and a structure built up from ribs and spars, covered with plastic
film. Data was taken from [18].
Some numerical results will be presented here to shed a light on the aerodynamics of covered rib structures.
Remark:
While you might be used to terms like spanwise lift distribution, I
will talk now about spanwise sag distributions and sag factors. If you
know a better word to describe this, please let me know. Terms like these
are usually not taught in german schools.
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