Aerodynamic drag of a two-dimensional external compression inlet at supersonic speed

Abstract

This study forms the basis from which the aerodynamic drag of a practical supersonic inlet can be predicted. In air-breathing propulsion systems, as used in high performance flight vehicles, the fuel is carried onboard and the oxygen required for combustion is ingested from the ambient atmosphere. The main function of the inlet is to compress the air from supersonic to subsonic conditions with as little flow distortion as possible.

When the velocity of the vehicle approaches or exceeds sonic velocity (M = 1,0) a number of considerations apply to the induction system. The reason for this is that the velocity of the ingested air has to be reduced to appreciably less than sonic velocity, typically to M = 0,3. Failure to do so will cause the propulsion system to be inoperative and cause damage.

In the process of compressing the air from supersonic to subsonic conditions a drag penalty is paid. The drag characteristics are a function of the external geometry and internal flow control system of the inlet. The problem which was investigated dealt with drag of a specific type of inlet, namely a two-dimensional external compression inlet.

This study is directed at formulating definitive relationships which can be used to design functional inlet systems. To this effect the project was carried out over three phases, a theoretical investigation where a fluid-flow analysis was done of the factors influencing drag. The second phase covered a comprehensive experimental study where intensive wind-tunnel tests were conducted for flight Mach numbers of M = 1,8; M = 2,0; M = 2,2; M = 2,3 and M = 2,4. During the third phase a comparison, between the theoretical values and experimental data was done, for validating the predicted aerodynamic drag figures. The following findings are worth recording: • the increase in total drag below the full flow conditions is more severe than predicted due to the contribution of spillage drag; • the range for subcritical mode of operation is smaller than expected due to boundary layer effects. The study has shown that reasonably good correlation could be achieved between the theoretical analysis and empirical test at low subcritical modes of operation. This suggests that the study has achieved its primary objective.