When testing a certain wing profile inside a wind tunnel where should the model, small scale wing be supported? Should the support be under the center of mass of the wing or under the aerodynamic center? If so, why?

My understanding is that, for similarity reasons, the Reynolds and Mach numbers for the model wing tested inside the wind tunnel must be the same as the those for the actual aircraft...


Indeed, the Reynolds and Mach numbers should be the same, or at least similar to the actual aircraft, depending on what you want to measure. For instance, the maximum lift coefficient is very sensitive w.r.t. Re and if you add a Mach number above 0.3, compressibility needs to be taken into account as well (even though quite good correction models exist for this). On the other hand, if you want to measure for instance only the zero-lift angle of attack, the Re doesn't matter much.

Anyway, where you support the airfoil doesn't change the Reynolds or mach numbers. In fact, you can theoretically support it at any point. The lift and drag will always be the same for every point; what changes is the moment that you measure. It might be convenient to choose the aerodynamic center which typically lies at around 25% of the chord, since there the pitching moment doesn't change with angle of attack. But be aware that this location can still very slightly move with AOA in practice. For more information: https://en.wikipedia.org/wiki/Aerodynamic_center.

  • $\begingroup$ Thank you. In that case the aerodynamic center becomes the fixed pivotal point.A scale connected to the support point would indicate the lift force produced by the wing (corresponding to the decrease in the force measured by scale). Manometer tubes (making holes on both the top and bottom surfaces of the wing) would measure the local pressures on the top and bottom wing surfaces. Can we still measure the aerodynamic moment about AC if we keep the AoA fixed? The wing would not be allowed to rotate if the wing is at a fixed AoA,correct? $\endgroup$ – Brett Cooper Dec 30 '18 at 23:24
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    $\begingroup$ Yes, you can still measure it. The moment about the AC is constant, but non-zero. Or you can compute it from the pressure distribution. Btw, please don't forget to accept answers on your questions once you're satisfied with the answer and the comments! $\endgroup$ – Daniel Dec 30 '18 at 23:39

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