Dissymmetry of Lift | How to Compensate Dissymmetry of Lift

Dissymmetry of Lift | How to Correct Dissymmetry of Lift

Dissymmetry of Lift | How to Compensate Dissymmetry of Lift in a Helicopter

Dissymmetry of Lift

The differential (unequal) lift between the advancing and retreating parts of the rotor disc induced by the varying wind flow velocity over each half is known as Dissymmetry of Lift, asymmetry of lift or asymmetric lift in rotorcraft/Helicopter aerodynamics. In any condition other than hovering in a calm wind, this disparity in lift would render the helicopter unstable. To achieve lift symmetry, there must be a way to compensate for, rectify, or eliminate this asymmetrical lift.

The advancing blade moves in the same direction as the aircraft, while the retreating blade moves in the opposite direction. When viewed from above, most American helicopter rotors rotate counter-clockwise, whereas French and Russian helicopters rotate clockwise.

The stability of a helicopter depends on balancing lift across the rotor disc. The amount of lift produced by an airfoil is related to its airspeed squared (velocity). The rotor blades in a hover have identical airspeeds and thus equal lift. In forward flight, however, the advancing blade travels faster than the retreating blade, resulting in unequal lift over the rotor disc.

The relative airflow through the main rotor disc is different on the advancing side than on the retreating side as the helicopter moves through the air. The forward speed of the helicopter increases the relative windspeed experienced by the advancing blade, whereas the forward airspeed of the helicopter reduces the relative windspeed met by the retreating blade. As a result of the relative windspeed, the rotor disk's approaching blade side generates greater lift than the retreating blade side.

How to Compensate Dissymmetry of Lift

Because of the difference in lift on blades, a helicopter with a counterclockwise main rotor blade rotation would roll to the left if this condition was allowed to exist. The main rotor blades automatically flap and feather to equalize lift throughout the rotor disc. A horizontal hinge (flapping hinge) is used in articulated rotor systems with three or more blades to allow the individual rotor blades to move or flap up and down as they revolve. A teetering hinge is used in a semirigid rotor system (two blades) to allow the blades to flap together. The other blade flaps down while one flaps up.

The rotor blade reaches its maximum upward flapping velocity when it approaches the advancing side of the rotor disc. The angle between the chord line and the resulting relative wind decreases as the blade flaps upward. As the AOA decreases, the amount of lift produced by the blade decreases. The rotor blade is at its maximum downward flapping velocity in position C. The angle between the chord line and the resulting relative wind increases as a result of downward flapping. This increases the blade's AOA and, as a result, the amount of lift it produces.

Dissymmetry of Lift | How to Compensate for a Dissymmetry of Lift
Counteracting Dissymmetry of Lift

The maximum forward speed of a helicopter is generally limited by the combination of blade flapping and sluggish relative wind acting on the retreating blade. Because of the high AOA and slow relative wind speed, the retreating blade stalls at a high forward speed. A nose-up pitch, vibration, and a rolling tendency—usually to the left in helicopters with counterclockwise blade rotation—are all signs of "retreating blade stall." By not reaching the never-exceed speed, pilots can avoid retreating blade stall. VNE is the designation for this speed, which is printed on a placard and highlighted by a red line on the airspeed indicator.

The advancing blade achieves maximum upward flapping displacement over the nose and maximum downward flapping displacement over the tail during aerodynamic flapping of the rotor blades as they correct for lift dissymmetry. Blowback occurs when the tip-path plane tilts to the rear as a result of this.

Higher helicopter airspeed is produced by this horizontal lift component (thrust). Blade flapping is induced by increasing airspeed to maintain lift symmetry. On the rotor system and helicopter, the combination of flapping and cyclic feathering ensures lift symmetry and the appropriate attitude.

Dissymmetry of lift is countered by reducing the advancing blade's angle of attack while increasing the retreating blade's angle of attack. Blade flapping and cyclic feathering are used to accomplish this.

Blade flapping is the most common way to correct for lift asymmetry. Rotor blades are designed to flap: as the relative wind vectors change, the advancing blade flaps up and generates a smaller angle of attack, delivering less lift than a rigid blade. The retreating blade, on the other hand, flaps down, acquires a greater angle of attack as a result of the shift in relative wind vectors, and generates more lift. Flapping causes flapback, which causes the rotor disc to tilt backwards.

Cyclic feathering, or a change in the angle of incidence of the rotor blades as they rotate around the hub, also helps to reduce dissymmetry of lift.

Angle of attack changes can only counter dissymmetry of lift to a certain extent. As a result, the helicopter's maximum forward speed is limited.

Dual rotors

The two rotor discs rotate in opposite directions in helicopters with coaxial rotors. The lift asymmetry of one rotor disc cancels out the lift asymmetry of the other rotor disc.

Because the rotors are offset from one another, tandem rotor helicopters like the CH-47 Chinook still suffer from lift dissymmetry. Automatic cyclic feathering systems are fitted on tandem-rotor helicopters. Blade flapping compensates for dissymmetry of lift at low airspeeds. These systems provide a more level fuselage attitude as airspeed increases, often beyond 70 knots, reducing strains on the rotor drive mechanisms.

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