Dynamics of Aircraft Flight

3Wind Tunnel

3.1. Description of wind tunnel tests

The lift of an aircraft depends on its speed. To understand this, just think of take-off. In order to obtain sufficient lift for take-off, the aircraft must reach a minimum lift speed.

We can thus say that, lift increases with speed. To verify this experimentally, we can place a model in a wind tunnel and use a balance to measure the lift (Figure 3.1).

Figure 3.1. Wind tunnel test

The model and its support are placed in the test section. The support is based on a sensitive scale (at 0.01 g, for example) in order to record the variations in the apparent weight. When the wind tunnel is stopped, the scale is subjected to the weight of the model and the indicated mass m corresponds to that of the model. By taring the scale, the indicated mass changes to 0 g.

When the wind tunnel is started, a lift is created on the wings of the model and the scale indicates a negative mass. In fact, the scale is no longer subjected to the weight P of the model, but only to its apparent weight P−R_z. The mass indicated by the balance then translates the norm of the lift:

The lift is given by the equation:

R_z = m ∗ g

Experience shows that m decreases (increasingly negative value) as the flow velocity increases. We can therefore deduce that the lift ...