# How must the engine power be varied with change of the flight speed?

Answer 1. With increase of the flight speed there is an increase of the power required for motion and the power required for overcoming profile drag. The induced power decreases. Therefore, the engine power remains constant.

Answer 2. The power required and the power developed must be equal for

any flight speed. With increase of the speed from 0 to Vec>the power required

decreases; therefore, in this range the engine power developed must be reduced.

With increase of the speed from V to V the power required increases,

вс max

therefore, the power developed must also be increased.

Answer 3. With increase of the flight speed the helicopter parasite drag increases as the speed squared, while the power required for motion increases as the speed cubed. The profile drag power also increases with increase of the speed, while the induced power decreases only slightly. Con­sequently, the power developed by the engine must be increased with increase of the flight speed.

Question 3. How does helicopter performance vary with change of the flight weight?

Answer 1. With increase of the flight weight, the power required to develop the lift force Y = G increases. However, to increase the lift force, the thrust force vector tilt must be reduced. As a result of this, there is a reduction of the helicopter parasite drag, which in turn leads to increase of the maximal horizontal flight speed and increase of the speed range.

Answer 2. Increase of the flight weight leads to increase of the thrust force required. But since the thrust developed by the main rotor increases faster than the required thrust with increase of the speed, the engine power developed must be reduced. Therefore, the maximal horizontal flight speed, speed range, and excess power increase with helicopter weight increase.

Answer 3. With increase of the flight weight the power required for helicopter horizontal flight increases as a result of increase of the induced power = GV^. The power available is independent of helicopter weight.

The power required curve is shifted upward. As a result, the maximal horizon­tal flight speed is reduced, and the minimal horizontal flight speed is increased. The speed range and the excess power decrease.