Some earlier investigations on the flows in S-shaped diffusing ducts suggested that the first bend was the primary source of pressure loss and engine-face flow distortion, e.g., Goldsmith. By increasing the curvature ratio (ratio of the duct curvature to the duct inlet cross-sectional diameter) of the first bend and by incorporation of a straight diffuser joining the end of the first bend and the inlet of the second bend, a significant improvement in the total pressure recovery at the engine face could be achieved (Fig. 1). A large proportion of the gain in the total pressure was attributed to the central straight diffuser. Results at a lower inlet speed (incompressible; inlet Reynolds number about 60 000) in the same ducts by Ho, Myring and Livesey(2) revealed the same trend as in the original high inlet speed reported by Goldsmith(1) (compressible; inlet Mach number from 0·4 to 0·8; inlet Reynolds number over one million). However, changing the curvature ratio in the first bend also changes the direction of the axis of the inlet plane, which should be aligned in the same direction as the axis of the exit plane (cf Fig. 1). Thus, the previous experiments were actually comparing ducts at different angles of attack relative to the axis of the exit plane; in relation to a typical jet aircraft intake installation. Hence, the effectiveness of the straight diffuser at zero angle of attack (relative to the axes of the inlet and exit planes) on the overall total pressure recovery in an S-shaped duct had not been assessed.