In order to obtain full knowledge of the performance of an aircraft, one of the important quantities to be determined is the drag. The usual procedure is to measure the level speed of the aeroplane and to calculate the drag under these conditions from the engine thrust, as obtained from the engine makers’ specification. To do this, accurate thrust data are essential and in certain cases these may not be readily available.

In order to eliminate the possibility of systematic errors in the determination of engine thrust, a method has been developed whereby the drag at low Mach number can be measured directly,under such conditions that accurate knowledge of the engine performance is of minor importance. This method is satisfactory only on jetpropelled aircraft, since on conventional aircraft there are the added complications of propellers, slipstream, radiators and so on.

The method has been used to determine the drag of the de Havilland D.H. 100 Vampire and has given good results. The drag determined from earlier tests by the usual procedure is found to be in good agreement with that given by the new method.