Riblet drag at flight conditions

This paper describes perforated and nonperforated riblet tests on the fuselage of a modified Learjet model 28/29 twin-engine business jet at Reynolds numbers of 1.0-2.75 x 10/ft and Mach numbers of 0.3-0.7. Drag reductions of the order of 6% at nondimensional wall spacings of 12 were obtained using boundary-layer rakes and direct drag balances. At the measurement locations, the Reynolds number based on distance was 12-46 xlO. The nondimensional wall spacing for maximum drag reduction was well predicted by low-speed wind-tunnel data, but the maximum drag reduction was lower. The low drag is tentatively ascribed to various instrumentation difficulties and the flowfield on the aircraft. Riblets with 0.010-in. perforations at center spacings of 0.25 in. were found to give the same drag reduction as nonperforated riblets.