Measurements have been made of the drag of circular cylinders of 3/16 to 1 in. diameter at speeds up to M=0·85 by means of the electrical balance in the Circular High‐Speed Tunnel and by pressure plotting round cylinders of ⅜ in. and ¾ in. diameter in the Rectangular High Speed Tunnel. The measurements were confined in the main to cylinders which did not show the usual critical drop in CD at a Reynolds number near 0·26 × 106. The value of CD started to rise at about M = 0·35 after which it rose at an increasing rate to a maximum at about M=0·7; this was succeeded by a minimum at M = ·75 and a sharp rise at M=0·85. The speeds at which the maxima and minima of the curves occurred for cylinders of various diameters were used to correct for tunnel interference in the circular tunnel. Photographs were taken of the shock wave system in the Rectangular Tunnel at various speeds.
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