How consistent is the performance or testing results of scale models of machines (airplanes, boats, etc) when compared to the actual finished product?

Question

For example, doing wind tunnel tests on scale model planes. Assuming that there are no significant differences between the scale model of a plane and the actual fully built plane, will the aerodynamics be the same, but just scaled up?

In: Engineering

Answer 1

Kinda. There’s scaling factors.

Most notably, the viscosity of air is a set value at standard conditions. So if your model is 1/10th scale, the viscosity of air isn’t 1/10th (or whatever) which means that there could be some differences in performance (this is called the Reynolds number)

Other notable differences include the effects of the tunnel walls on vortex formations, a lack of engines and internal airflow being modeled (usually), and aero elastic effects not scaling identically (so as the wing flexes under air loads, the aerodynamics of it change, and then the air loads change, and this coupled behavior is very hard to predict with either wind tunnels or with computational models, since a scale model won’t have the same stiffness and resonant frequencies as full scale).

So it’s not an exact science but it can get you decently far along.