7 - Performance

Summary

The calculation of rotorcraft performance is largely a matter of determining the power required and power available over a range of flight conditions. The power information can then be translated into quantities such as payload, range, ceiling, speed, and climb rate, which define the operational capabilities of the aircraft. The rotor power required is divided into four parts: the induced power, required to produce the rotor thrust; the profile power, required to turn the rotor through the air; the parasite power, required to move the aircraft through the air; and the climb power, required to change the gravitational potential energy. The aircraft has additional contributions to power required, including accessory and transmission losses and perhaps anti-torque power. In hover there is no parasite power, and the induced power is 65% to 75% of the total. As the forward speed increases, the induced power decreases, the profile power increases slightly, and the parasite power increases until it is dominant at high speed. Thus the total power required is high at hover, because of the induced power with a low but reasonable disk loading. At first the total power decreases significantly with increasing speed, as the induced power decreases; then it increases again at high speed, because of the parasite power. Minimum power required occurs roughly in the middle of the helicopter speed range.

The task in rotorcraft performance analysis is the calculation of the rotor forces and power. Procedures to perform these calculations have been developed in the preceding chapters.