Main Rotor Thrust as a Function of Flight Speed

The thrust of a particular rotor at constant air density depends on the flight speed and the induced velocity. With increase of the flight speed there is an increase of the resultant velocity, which leads to increase of

the mass flowrate of the air deflected by the main rotor. Consequently, the higher the velocity in the forward flight regime, the larger the air mass flowrate and the greater the thrust developed by the rotor. But thrust increase is possible only up to some limit. This is associated with the change of the induced velocity which, in turn, depends on the flight speed. However, this relation is complicated by the variation of the main rotor angle of attack (Figure 28a). This figure makes it possible to draw some important conclusions:

the induced velocity decreases with increase of the flight speed;

with increase of the main rotor angle of attack the induced velocity increases and vice versa;

for negative angles of attack the induced velocity decreases with increase of the flight speed;

for A > 0° the induced velocity first increases with increase of the flight speed up to 15-20 km/hr and then decreases;

for flight speeds up to 50-60 km/hr the induced velocity depends to a considerable degree on the main rotor angle of attack, while at higher

flight speeds this dependence becomes less significant;

the induced velocity decreases very rapidly with flight speed in the range from 0 to 60-70 km/hr.

With further increase of the flight speed, the reduction of the induced velocity becomes more gradual.

These conclusions are necessary for understanding the nature of main rotor thrust variation in the forward flight regime, and also for under­standing the nature of helicopter motion in horizontal flight, climb, and descent along an inclined trajectory. If we take into account the nature /43 of the induced velocity variation, then the variation of main rotor thrust with change of the flight speed becomes clear (Figure 28b). This figure shows that main rotor thrust increases with increase of the flight speed and reaches the maximal value for a speed of about 100 km/hr. All the conclu­sions drawn on the variation of the induced velocity and thrust relate to operation of a main rotor with constant power expended in turning the rotor.

The thrust increase with increase of the flight speed is explained by the fact that, as the flight speed increases, a larger amount of air approaches the rotor, i. e., the mass flowrate of the air interacting with the rotor in­creases. The rotor deflects the large air mass downward and, thus, force impulse increases, i. e., the main rotor thrust increases.

Upon further increase of the flight speed, the time of interaction of the rotor with the air diminishes. The rotor "fails to" deflect the air markedly downward, which means a decrease of the induced velocity and, therefore, of the force impulse. Moreover, the energy received by the rotor from the shaft is expended not only in creating the induced velocity, but also in overcoming frictional drag forces, and with increase of the flight velocity these forces increase.