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

There are various conflicting factors when scaling size, velocity, media, etc. This applies for boat models, whether ordinary surface ships, or submarines. Also for aircraft, especially at speeds near Mach 1. You will encounter the terms Froude Number, Reynolds Number, Mach Number, and a few others.

Reynolds Number relates the viscous and inertial forces of the medium (air, water).

Froude Number is about the wake/wave making aspect of surface ships.

Mach Number is about the relation of a a model’s speed to the speed of sound in the medium.

Generally, the problem is that It is not possible to have the same “Numbers” both at full-size and at scale-size..

I highly recommend the book “On Size And Life” by McMahon and Bonner, from the Scientific American Library collection. It is very accessible to the layperson.