Main Rotor Power Available

The power required to turn the main rotor is supplied to the rotor from the engine through the transmission. But the rotor does not receive all the power the engine develops, since part of this power is expended for other purposes and does not reach the rotor. The overall power losses are made up of the losses in:

Turning the tail rotor;

Turning the engine cooling fan;

Overcoming friction in the transmission components;

Driving the accessories;

Overcoming air drag on fuselage and other parts of the helicopter.

Let us examine the magnitudes of these losses, or the energy balance of the helicopter.

On the average, 8% of the engine power is expended in turning the tail

rotor (N ) ;

t. r ’

The fan absorbs 5% (N_ );

tan

The accessories absorb 1% (N );

acc

Helicopter parasite drag absorbs 2% (N.

That portion of the engine power which is supplied to the main rotor is called the power available. It is defined as the difference between the effective engine power and the sum of the losses

N = N – (N + + N + N + N ).

avail e t. r fan trans acc par

The ratio of the power available to the effective engine power is termed the Bower utilization coefficient

hence

Navail = N £ .

e

The difference 1 – t, = is called the power loss coefficient.

For single-rotor helicopters, the average power utilization coefficient is 0.75-0.80, and the average power loss coefficient is 0.25-0.20. The power utilization coefficient, and consequently the power available, vary with variation of the helicopter flight speed. The speed dependence of the power /30 available is shown in Figure 22.

The following conclusions can be drawn from this figure:

1) The effective engine power is independent of the flight speed;

2) The overall power loss decreases with speed up to 80-100 km/hr and then increases with further increase of the flight speed;

3) The power available increases with increase of the flight speed to 80-100 km/hr and then decreases;

J

4) The maximal power available is obtained at a flight speed from 80 to 100 km/hr for most helicopters.

V

Figure 22. Main rotor power available versus speed.