Helicopter Flight Information
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Helicopter Flight Information |
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Retreating Blade StallRetreating blade stall is an advanced case of dissymmetry of lift, an aerodynamic phenomenon in helicopters which is automatically compensated for by blade flapping and cyclic feathering. Dissymmetry of lift occurs as the result of different airspeeds across the advancing and retreating blades during forward flight, or anytime there is airspeed through the rotor system whether by wind or flight. As airspeed increases, the advancing blade on the right side of the helicopter produces more lift; remember that lift increases at the square of the airspeed over an airfoil. Likewise, on the left side of the helicopter, due to the increase in speed, the retreating blade produces less lift because it is traveling in the opposite direction of the helicopter. Multi-bladed helicopters have a flapping hinge, while 2 bladed helicopters have a teetering hinge which allows the rotor blades to flap to compensate for this lift differential that occurs. As the airspeed increases, lift increases, and the blade flaps up on the advancing side, which in turn decreases the angle of attack of the blade, and lift is reduced. On the retreating side, the blade flaps down which increases the angle of attack on that blade, and through this flapping action, lift is equalized across the rotor disk. There is a point, which varies from one helicopter to another, where this flapping action can no longer over come the imbalance of lift across the rotor disk, and the retreating blade will stall. Awareness includes the approach of Vne (never exceed speed). It is never acceptable to fly any aircraft at an airspeed faster than Vne. During turbulent wind conditions, the airspeed should be well below Vne so that a gust will not put the aircraft at or above this speed. Further, the pilot must be aware of his/her altitude and the consequent reduction in the airspeed at which retreating blade stall will occur. The first sign of recognition will be a vibration of the main rotor which if not immediately corrected will be quickly followed by a pitch up of the nose, and then by a rolling tendency, which could (disputably) be in either direction. The corrective action must be a reduction of the collective pitch followed by an aft cyclic input to reduce the airspeed. This corrective action should include an increase of the throttle if the rpm is lower than the top of the green arc. Sooner or later you will hear some pilots brag of exceeding Vne because in a particular helicopter Vne is due to structural limitations and not retreating blade stall. Although it is true that some helicopters have a structural Vne lower than RBS, Vne must never be exceeded intentionally. Fly safe!
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