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An example of the effect of rotor blade number is the UH-72 (EC145 variant); the A model had four blades, but the UH-72B was changed to five blades which reduced vibration. [24] Other blade numbers are possible, for example, the CH-53K, a large military transport helicopter has a seven blade main rotor. [25]
Most helicopter designs compensate for this by incorporating a certain degree of vertical "flap" movement of the rotor blades. When flapping, a rotor blade will travel upward during its advance, creating a lesser angle of attack (AOA) and therefore lesser lift. When the blade retreats, the blade falls downward again, increasing the AOA and ...
Collective angle of attack for the rotor main blades via the swashplate: Increase/decrease pitch angle of all main rotor blades equally, causing the aircraft to ascend/descend Increase/decrease torque. In some helicopters the throttle control(s) is a part of the collective stick. Rotor speed is kept basically constant throughout the 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 ...
Below, the helicopter rotor has lost power, and the craft is making an emergency landing. Autorotation is a state of flight in which the main rotor system of a helicopter or other rotary-wing aircraft turns by the action of air moving up through the rotor, as with an autogyro, rather than engine power driving the rotor.
When a helicopter flies low to the ground in sandy environments, sand can strike the metal abrasion strip and cause erosion, which produces a visible corona or halo around the rotor blades. The effect is caused by the pyrophoric oxidation of the ablated metal particles. [6] [7]
Dissymmetry of lift in an American-style helicopter. Consider a single-rotor helicopter in still air. For a stationary (hovering) helicopter, whose blades of length of r metres are rotating at ω radians per second, the blade tip is moving at a speed rω meters per second. As the blades rotate, the speed of the blade-tips relative to the air ...
A rotor blade produces more lift in the advancing half. As a blade moves toward the direction of flight, the forward motion of the aircraft increases the speed of the air flowing around the blade until it reaches a maximum when the blade is perpendicular to the relative wind. At the same time, a rotor blade in the retreating half produces less ...