Exhaust Gas Reingestion During Hover In-Ground-Effect

When hovering in-ground-effect, engine exhaust gases can be contained within the main rotor downwash and recirculated to the engine reducing the power available. The amount of reingestion will vary for different helicopters and depend on engine/airframe geometry, ambient conditions, and the flow field dynamics associated with the hovering mode. Small scale model tests were conducted during the CH-53E development program to determine the significant steady state parameters influencing reingestion. Smoke studies were conducted to determine main rotor wake boundaries, and inner rotor wake temperature distributions were measured by simulating hot exhausts beneath a hovering rotor. The model results are shown to correlate well with full scale results, and the major effects of engine location, exhaust momentum, and rotor inboard geometry are discussed. Based on model results, optimum full scale engine exhaust nozzles were designed, fabricated, and tested on the prototype CH-53E. Flight results demonstrated low sensitivity to reingestion and the exhaust configurations were incorporated into the CH-53E design. It is concluded that although the reingestion phenomena is reasonably well understood, the confident prediction of reingestion levels for any arbitrary helicopter design requires a comprehensive set of design data which is not available at this time.