Factors limiting maximal helicopter horizontal flight speed and ways to increase this speed
Answer 1. The maximal horizontal flight speed of a helicopter is limited by the engine power available. Flight at speeds higher than the maximal is not possible, since more power is required for such flight than the engine develops. Powerful gas turbine engines are installed on the new helicopters, for example, the Mi-2, Mi-6, and Mi-8, to increase the maximal flight speed.
Answer 2. With increase of the horizontal flight speed, the forward tilt of the main rotor plane of rotation must be increased. This leads to reduction
of the main rotor angle of attack. As a result of the reduction of the angle
of attack, the rotor thrust force decreases. This is then the reason for the maximal speed limitation. To increase the maximal speed we must increase the main rotor rpm, which requires the more powerful gas turbine engines.
Answer 3. The rotor thrust force must be increased in order to increase the flight speed. If the thrust force is increased by increasing the rpm, local compression shocks develop on the blades. If we increase the thrust force by increasing the main rotor pitch, then the angles of attack of the blade elements at the 270° azimuth increase. The speed is limited by the onset
of blade stall at angles of attack above the critical value. To increase the
speed the main rotor must be unloaded by installing an additional wing or thrusting propellers, i. e., compound helicopters such as the Mi-6, Kamov rotor-wing, and so on must be constructed.