Main Rotor RPM Control

The main rotor rpm will change both with change of the power supplied,

i. e., the engine power, and with change of the power required, i. e., with change of the main rotor reactive torque. The magnitude of the thrust devel­oped by the main rotor changes with change of the rpm.

We need to know the optimal rotor rpm, i. e., is it better to increase the thrust by increasing rpm or pitch? Moreover, we need to know how to maintain the optimal main rotor rpm with variation of the magnitude of the thrust.

The answer to the first of these questions can be obtained by examining the characteristic termed specific thrust. Main rotor specific thrust is a quantity equal to the ratio of the thrust developed by the rotor to the power required to turn the rotor

T

q = N • (12)

req

The specific thrust shows the number of units of thrust per unit of /24

power expended by the engine in turning the rotor. The larger the specific thrust, the more efficient the main rotor.

We substitute the values of the thrust from (8) and the power required from (10) and (11) into (

Consequently, to increase the thrust we should reduce the main rotor tip speed. This means that it is better to increase the thrust by increasing the main rotor pitch at minimal rpm. Here, it must be emphasized that there is a minimal permissible rpm for every rotor. Reduction of the rpm below the minimal acceptable value leads to flight safety problems, deterioration of helicopter controllability and stability.

This conclusion is very important, as it provides an answer to the question of why heavy and complex main rotor reduction gearboxes are installed in helicopters. These reducers make it possible to connect the main rotor shaft, which rotates at a low angular velocity, with the engine shaft, which rotates with an angular velocity 10-15 times that of the rotor.

Thus, we have established that it is advisable to turn the main rotor at low speed and increase thrust by increasing the pitch. But increase of the pitch leads to increase of the reactive torque and, therefore, increase of the power required. This means that in order to maintain constant rotor rpm

the power supplied to the rotor must be changed at the same time the pitch is changed. The main rotor and the engine must be controlled simultaneously. Simultaneous control is accomplished with the aid of a special lever, termed the "collective-throttle" lever. This lever is installed in an inclined position to the left of the pilot’s seat. If the collective-throttle lever is displaced upward, both the main rotor pitch and the engine power are increased simultaneously, and the main rotor rpm remains approximately constant. The throttle twist grip is located on the end of this lever. The engine power alone, and therefore the main rotor rpm, can be altered by rotating this grip.