In the present study, the influence of the engine unit (air-intake, nacelle and nozzle) to the aerodynamic characteristics of the airframe was experimentally investigated at a free stream Mach number of 2.75. A series of wind tunnel tests was performed in NAL's M4 supersonic wind tunnel for the airbreathing engine unit/airframe integrated models of which the engine unit was installed on the bottom surface of the fuselage. These models were designed for Mach 3 flight. To identify the flow field (e. g., boundary layer and vortices) upstream of the engine unit, pitot pressure survey and flow visualizations by schlieren and oil-flow techniques were applied. The streamwise vortices due to down wash from upper side to lower side of forebody were observed at the low angle of attack, but these vortices could be suppressed by the modification of the forebody configuration. The diverter worked effectively so that the upstream boundary layer was removed. With the measurements of aerodynamic forces and schlieren visualization, it was found that the flow state, i. e., start, buzz or unstart condition, of air-intake affects the upstream flow field and the aerodynamic characteristics of the airframe. In the case that the shock spilled out from the air-intake, the aerodynamic force normal to the airframe axis decreased as the angle of attack increasing, because the force in the nozzle decreased due to higher loss of the total pressure in the engine unit.
