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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]
The effect of dynamic stall limits the helicopter performance in several ways such as: The maximum forward flight velocity and thrust; High blade structural loads, which may result in excessive vibrations and blade structural damage; Control system loads, manoeuvre capability, and handling qualities; Helicopter dynamic performance.
At the top of the mast is the attachment point (colloquially called a Jesus nut) for the rotor blades called the hub. The rotor blades are then attached to the hub, and the hub can have 10-20 times the drag of the blade. [1] Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor ...
The adjacent blade stalls as a result of the incidence spike, thus causing stall cell "rotation" around the rotor. Stable local stalls can also occur which are axi-symmetric, covering the complete circumference of the compressor disc, but only a portion of its radial plane, with the remainder of the face of the compressor continuing to pass ...
In a stationary hover, each rotor blade will experience the same airspeed at a constant RPM. In forward flight conditions, one rotor blade will be moving into the oncoming air stream while the other moves away from it. At certain airspeeds, this can create a dangerous condition in which the receding rotor blade stalls, causing unstable flight. [5]
The inner 25 percent of the rotor blade is referred to as the stall region and operates above its maximum angle of attack (stall angle) causing drag, which slows rotation of the blade. A constant rotor rotational speed is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the ...
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Typical rotorcraft blades have a Lock number between 3 and 12, [4] usually approximately 8. [5] The Lock number is typically 8 to 10 for articulated rotors and 5 to 7 for hingeless rotors. [3]: 186 High-stiffness blades may have a Lock number up to 14. [4] Larger blades have a higher mass and more inertia, so tend to have a lower Lock number.