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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.
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.
A constant rotor rotational speed is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forces from the driven and stall regions. By controlling the size of the driving region, the pilot can adjust autorotative rotational speed.
The H/V curve also contains a take-off profile, indicating how a pilot can start from 0 height and 0 speed, and safely traverse to cruise. At low heights with low airspeed, such as a hover taxi, the pilot can simply cushion the landing with collective by converting rotational inertia into lift. Conversely, a complete power loss, and resultant ...
The gearbox at the end of the tail boom provides an angled drive for the tail rotor and may also include gearing to adjust the output to the optimum rotational speed for the tail rotor, measured in rotations per minute (RPM). On larger helicopters with a tail pylon, intermediate gearboxes are used to transition the tail rotor drive shaft from ...
But the speed of the blade-tip at point B, relative to the air, is the difference of its rotational speed and the forward-flight speed: rω-v. Since the lift generated by an aerofoil increases as its relative airspeed increases, on a forward-moving helicopter the blade-tip at position A produces more lift than that at point B.
When the helicopter is hovering, the outer tips of the rotor travel at a speed determined by the length of the blade and the rotational speed. In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational speed.
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.