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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]
[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 ...
As the blades rotate, the speed of the blade-tips relative to the air remains constant. Now imagine the helicopter in forward flight, at v meters per second. The speed of the blade-tip at point A in the diagram relative to the air is the sum of the blade-tip speed and the helicopter forward-flight speed: rω+v. But the speed of the blade-tip at ...
The blades of a helicopter are long, narrow airfoils with a high aspect ratio, a shape that minimizes drag from tip vortices (see the wings of a glider for comparison). They generally contain a degree of washout that reduces the lift generated at the tips, where the airflow is fastest and vortex generation would be a significant problem.
[12] [37] At mu=1, V is equal to u and the tip airspeed is twice the aircraft speed. At the same position on the opposite side (retreating blade), the tip airspeed is the aircraft speed minus relative blade tip speed, or V t =V-u. At mu=1, the tip airspeed is zero. [30] [38] At a mu between 0.7 and 1.0, most of the retreating side has reverse ...
Washout is a geometric twist in the blade, such that the blade root near the hub has a higher angle-of-attack, thus higher lift. [3]: 2–9 [4]: 1–20 Varying the airfoil cross-section, such as flattening the airfoil towards the tip, or tapering the blade towards the tip, which reduces its surface area thus reducing lift.
Disk area can be found by using the span of one rotor blade as the radius of a circle and then determining the area the blades encompass during a complete rotation. When a helicopter is being maneuvered, its disk loading changes. The higher the loading, the more power needed to maintain rotor speed. [3]
The advance ratio is a useful non-dimensional quantity in helicopter and propeller theory, since propellers and rotors will experience the same angle of attack on every blade airfoil section at the same advance ratio regardless of actual forward speed. It is the inverse of the tip speed ratio used for wind turbines.