Unlike fixed-wing aircraft, helicopters must rely on the rotation of rotors to obtain the lift and thrust needed to fly in the air.
However, according to the traditional flight principle of rotor aircraft, when the rotor speed of helicopter is too fast, the forward blade often produces shock waves, and the rear blade will stall.
As a result, traditional helicopters have long been unable to break the maximum forward speed limit of 340 km/h.
In order to achieve a breakthrough in speed, foreign helicopter designers have long proposed whether to change the traditional structure design of the rotor as both lift and thrust, to the helicopter alone with propulsion device to obtain thrust, which has produced the concept of composite helicopter.
This kind of high speed helicopter works in helicopter mode when hovering and low speed. When the speed increases to a certain degree, select to enter the rotation state.
When the speed is increased again, the rotor speed is reduced and gradually unloaded, and the wing is gradually loaded.
In this way, the reduction of blade profile Angle of attack and rotor speed delay the flow separation of the rear blade and the shock stall of the front blade, respectively.
Offloading the wing for the rotor is the core idea of the compound helicopter, whose typical models include the American X-2 and X-49A, the European X-3 and the Russian Ka-90.
But this design also just delay the shock waves and winds, but can’t completely eliminate the phenomenon, therefore, the largest of the helicopter flight speed is much lower than the fixed wing aircraft, the X – 2 high-speed helicopter of the United States is the world’s fastest helicopter, cruising speed of 463 km/h, active service in the United States twice the “black hawk” helicopter speed,
But not nearly as fast as a supersonic fixed-wing aircraft.
The V-22 Osprey tiltrotor is a special helicopter designed to operate in helicopter mode while hovering and at low speeds.
After the speed increases to a certain point, the tiltrotor becomes a propeller and becomes a fixed-wing aircraft mode.
Even so, its maximum speed is only 556 kilometers per hour.
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