Results from an investigation of the effects of underexpanded engine exhaust flow on the aerodynamic performance and stability of a cruise airplane at Mach 6 are presented. The influence of wing reflex angle and nozzle geometry on exhaust flow interference effects was investigated on a flat-plate model. The experiments were conducted at a free-stream Reynolds number of 17.05 X 10 (to the 6 power) based on the length of the airplane model over a model angle-of-attack range of 0(degrees) to 10(degrees) and at nozzle static-pressure ratios from 1 to approximately 4.