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Most helicopter rotors spin at constant speed. However slowing the rotor in some situations can bring benefits. As forward speed increases, the advancing rotor tip speed soon approaches the speed of sound. To reduce the problem, the speed of rotation may be slowed, allowing the helicopter to fly faster.
Helicopter rotors are designed to operate at a specific rotational speed. The throttle controls the power of the engine, which is connected to the rotor by a transmission. The throttle setting must maintain enough engine power to keep the rotor speed within the limits where the rotor produces enough lift for flight.
[1] [2] Unlike fixed-wing aircraft, of which the stall occurs at relatively low flight speed, the dynamic stall on a helicopter rotor emerges at high airspeeds or/and during manoeuvres with high load factors of helicopters, when the angle of attack(AOA) of blade elements varies intensively due to time-dependent blade flapping, cyclic pitch and ...
The McDonnell XV-1 could slow its rotor from 410 to 180 RPM. The slowed rotor principle is used in the design of some helicopters.On a conventional helicopter the rotational speed of the rotor is constant; reducing it at lower flight speeds can reduce fuel consumption and enable the aircraft to fly more economically.
The tail rotor is powered by the helicopter's main power plant, and rotates at a speed proportional to that of the main rotor. In both piston and turbine powered helicopters, the main rotor and the tail rotor are mechanically connected through a freewheeling clutch system , which allows the rotors to keep turning in the event of an engine ...
Sikorsky states that the X2 has the same noise level at 200 knots that a regular helicopter has at 100 knots. Above 200 knots, the rotor speed is reduced from 446 to 360 RPM [35] [36] to keep tip speed below Mach 0.9, the rotor disc is slightly nose-up, and the lift-to-drag ratio is about twice that of a conventional helicopter. Hands-off ...
In flight, the main rotor spun at a sedate 88 revolutions per minute, less than half the speed of typical helicopter rotors. Since the rotor was driven at the tips rather than the hub, little torque compensation was required, mostly due to friction in the main rotor bearing. Thus, the XH-17 had a very small tail rotor compared to its main rotor ...
The efficiency of the hovering rotor system is greatly improved with each knot of airspeed gained by horizontal movement of the aircraft or wind speed. [2]: 2–21 As forward airspeed increases, the helicopter goes through effective translational lift (ETL) at about 16 to 24 knots. This is known as the ETL speed.