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The effect of airfoil geometry on dynamic stall is quite intricate. As is shown in the figure, for a cambered airfoil, the lift stall is delayed and the maximum nose-down pitch moment is significantly reduced. On the other hand, the inception of stall is more abrupt for a sharp leading-edge airfoil. [8] More information is available here. [13]
Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack, causing a stall and loss of lift. Retreating blade stall is the primary limiting factor of a helicopter's never exceed speed, V NE. [1]
In aviation, and in particular in helicopters, servo transparency (also called servo reversibility or jack stall), [1] is a phenomenon affecting the servomechanisms (or servos) that assist a helicopter's flight controls, which, in certain flight conditions, can result in a significant stiffening of the controls handled by the pilot.
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 ...
The angle of attack can be increased to compensate, however beyond a certain point the rotor blades stall and lift rapidly decreases. Rotor stalls are not recoverable because the descending helicopter has airflow moving upwards through the rotor disc, so even full down collective will not restore normal airflow. When the helicopter rotor stalls ...
The helicopter will exhibit a tendency to make a sudden, uncommanded yaw that will develop into a high turn rate if not corrected. Recovery from this condition may be difficult if no airspeed is available, and will require entry into an autorotation (thus removing the torque of the engine and transmission).
It is a common emergency procedure taught to helicopter pilots as part of their training. In normal powered helicopter flight, air is drawn into the main rotor system from above and forced downward, but during autorotation, air moves into the rotor system from below as the helicopter descends.
Dynamic rollover begins when the helicopter starts to pivot around its skid or wheel. This can occur for a variety of reasons, including the failure to remove a tiedown or skid securing device, or if the skid or wheel contacts a fixed object while hovering sideward, or if the gear is stuck in ice, soft asphalt, or mud.