An experimental determination of the relation between the rate of sink of a sailplane and its air speed is of interest both to the designer and to the pilot. This paper describes tests made to establish this relation for the Slingsby Sky. During the tests, the aircraft was flown in a series of “ partial glides ” at constant speed, while a camera looking over the pilot's shoulder photographed the altimeter and A.S.I., together with a slip indicator and a clock. From an analysis of the photographic recordings, the mean air speed and the rate of sink during each partial glide can be determined; a large number of such results enables the relation between air speed and rate of sink to be established. The final results show that a considerable improvement in high-speed performance can be achieved if the aircraft is fitted with a long skid instead of the short skid and large fixed landing wheel; the superiority of the Sky over the Weihe for competition work is demonstrated. Sources of error in the results are discussed; it is shown that the accuracy with which the rate of sink may be determined is limited by the presence of vertical air movements. Possible future trends in performance testing technique are discussed.