ROTORCRAFT TURBOSHAFT ENGINE IDLE POWER SCHEDULING AIR4121

The purpose of this AIR (Aerospace Information Report) is to provide aircraft and engine designers with a better understanding of helicopter turboshaft engine idle power characteristics and objectives to be considered in the design process. Idle is the lowest steady state power setting. At this setting, the engine typically does not produce enough power to obtain governed output shaft speed (i.e. the shaft speed is determined by the load imposed by the aircraft). In the aircraft, the engine is typically stabilized at this power setting after starting, prior to taxi and for some period of time after rotor shutdown for cool down prior to engine shutoff. Traditionally, the aircraft designer wants idle power scheduled as low as possible and of course, does not want any resulting aircraft operational difficulties such as overcoming the rotor brake. The engine designer, however, desires a higher scheduled power because of the reduced probability of engine operational problems. The attributes of conflicting idle desires are discussed. Other inputs are also considered to establish the engine idle power settings. The information contained herein will assist in the early design phase and ultimately will result in a more optimum match between engine and aircraft requirements.