The wind channel and the whirling arm have been devised and perfected over a period of a number of years with a view to providing reliable aerodynamic data for aircraft designers. Of late years the wind channel has been used considerably more than the whirling arm.

In the two pieces of apparatus distinct methods are used. In the whirling arm the model is carried through the air, which is stationary, in a circular path by a long arm. In the wind channel, on the other hand, the model is stationary and a current of air is caused to flow past it.

Various types of wind channel have been evolved, but I intend to describe fully the English wind channel in its present form, as perfected by the National Physical Laboratory and the Royal Aircraft Establishment, Farnborough.