Helicopters flying at low-altitude must maneuver constantly to avoid terrain contours and obstacles above the ground, hence rapid responses to control commands are critical to the safety of the vehicle and its crew. To obtain these features, the engine control system must react quickly to change engine power.; For most helicopters, the engine control bandwidth is lowered to avoid exciting rotor/engine system vibrational modes. This degrades engine control effectiveness to maintain a desired rotor speed during maneuvers. To increase the engine control bandwidth, accurate modeling of coupled rotor/engine torsional modes is essential. This research shows that the two-way coupling between rotor and engine increases the frequency and damping of rotor collective lead-lag modes, and it slows down the turbine speed lag mode. A simplified, fourth-order model expressed in multiblade coordinates is derived to represent the coupled rotor/engine system.; Even with increased fuel control bandwidth, the engine power can not be changed fast enough, because the allowable maximum fuel flow is limited to protect the engine. This research shows that controlling free-turbine inlet bleed flow is a feasible supplement to controlling fuel flow in affecting a rapid and large power change. The bleed flow control reduces speed variations and increases vehicle agility significantly.
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