Dissymmetry of lift

Dissymmetry of lift in rotorcraft aerodynamics refers to an uneven amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters in forward flight.

A rotor blade that is moving in the same direction as the aircraft is called the advancing blade and the blade moving in the opposite direction is called the retreating blade.

Balancing lift across the rotor disc is important to a helicopter's stability. The amount of lift generated by an airfoil is proportional to the square of its airspeed. In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefore equal lift. In forward flight the advancing blade has a higher airspeed than the retreating blade, creating unequal lift across the rotor disc.

When dissymmetry causes the retreating blade to experience less airflow than required to maintain lift, a condition called retreating blade stall can occur. This causes the helicopter to roll to the retreating side and pitch up (due to gyroscopic precession). This situation, when not immediately recognized can cause a severe loss of aircraft controllability.

Envisage a viewpoint above 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. At all points around the disc mapped out by the blade-tips, the speed of the blade-tip relative to the air is the same: everything is balanced.

Now imagine the helicopter in forward flight at, say, 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 blade-tip speed at point B, relative to the air, is the difference of its rotational speed and the forward-flight speed: rω-v.

Since the lift generated by an aerofoil increases as its relative airspeed increases, on a forward-moving helicopter the blade-tip at position A produces more lift than that at point B. So the rotor disc produces more lift on the right hand side than on the left hand side. This imbalance is the "dissymetry of lift".

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