Loss of Momentum and Drag: A Method of Measuring the Drag by Exploration of the Wake Behind a Wing

IT has for a long time been realised that if a complete exploration is made of the wake behind a body and the loss of momentum suffered by the air flowing past it measured the drag on that body can be deduced. In 1925, Betz suggested a method whereby this principle could be conveniently applied to the measurement of the drag of wings in flight and a little later, in 1926, Schrenk published results obtained by the method. At that time interest in wing profile drag was not great and the results Schrenk obtained for wings covered with surfaces of different roughnesses were mainly of academic interest. At the time when aircraft had overall drag coefficients (CD based on wing area) of 0·040 to 0·050 a change of wing profile drag coefficient of 0·002 meant only 4 per cent or 5 per cent change in total drag or 1·3 per cent to 1·6 per cent change in the top speed of the aircraft. With the rapid improvement of performance which has taken place in the last 5 to 7 years due to a general “cleaning up” of fuselage shapes, elimination of strut and undercarriage drag, overall drag coefficients of 0·015 have been achieved and a saving of 0·002 in wing profile drag now means a 5 per cent increase in top speed. For this reason designers are now much more interested in accurate data regarding the effect of wing thickness and surface roughness on profile drag.