Dissymmetry of lift (also known as asymmetry of lift or asymmetric lift) in

rotorcraft A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Internat ...

aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dy ...

refers to an unequal amount of

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...

on opposite sides of the

rotor Rotor may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...

disc. It is a phenomenon that affects single-rotor

helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...

s and

autogyro An autogyro (from Greek and , "self-turning"), also known as a ''gyroplane'', is a type of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. Forward thrust is provided independently, by an engine-driven propeller. Whi ...

s 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.'' When viewed from above, most American helicopter rotors turn counter-clockwise; French and Russian helicopters turn clockwise. 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 (velocity). In a hover, the rotor blades have equal airspeeds and therefore equal lift. However, in forward flight the advancing blade has a higher airspeed than the retreating blade, creating uneven lift across the rotor disc.

Analysis

Consider a single-rotor helicopter in still air. For a stationary (hovering) helicopter, whose blades of length of ''r'' metres are rotating at ω

radian The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before tha ...

s per second, the blade tip is moving at a speed ''r''ω meters per second. 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: ''r''ω+''v''. But the speed of the blade-tip at point B, relative to the air, is the ''difference'' of its rotational speed and the forward-flight speed: ''r''ω-''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 (for an American-style helicopter). This imbalance is "dissymmetry of lift".

Effects

Dissymmetry of lift causes a pitch backwards, not a roll. At very high forward speeds, the retreating blade has insufficient airspeed to maintain lift, a condition called

retreating blade stall 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 ...

. This causes the helicopter to roll to the retreating side and pitch up. This situation, when not immediately recognized can cause a severe loss of aircraft controllability. Retreating blade stall is a factor determining the maximum forward speed of the helicopter, the never-exceed speed (V_NE). At very high forward speeds, the advancing blade may go supersonic. This is another factor determining the never-exceed speed.

Counter-measures

Dissymmetry of lift is countered by reducing the angle of attack of the advancing blade and increasing the angle of attack of the retreating blade. This is done by blade flapping and cycling feathering. Blade flapping is the primary means of countering dissymmetry of lift. Rotor blades are designed to flap: the advancing blade flaps up and develops a smaller angle of attack due to a change in relative wind vectors, thus producing less lift than a rigid blade would. Conversely, the retreating blade flaps down, develops a higher angle of attack due to a change in relative wind vectors, and generates more lift. Flapping results in

flapback Flapback or blowback is the tilting of a helicopter rotor disc, usually aft (backwards), which occurs in several circumstances. In normal operating circumstances, forward flight results in flapback caused by dissymmetry of lift and the transverse ...

, a tilting backwards of the rotor disc. Dissymmetry of lift is also countered by cyclic feathering, i.e. a change to the angle of incidence of the rotor blades as they rotate around the hub. There is a limit to which angle of attack changes can counter dissymmetry of lift. This imposes a limit on the maximum forward speed of the helicopter.

Dual rotors

In helicopters with

coaxial rotors Coaxial rotors or coax rotors are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions ( contra-rotating). This rotor configuration is a feature of helicop ...

, the two rotor discs rotate in opposite senses. The dissymmetry of lift of one rotor disc is cancelled by the dissymmetry of lift of the other rotor disc.

Tandem rotor Tandem, or in tandem, is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction. The original use of the term in English was in ''tandem harness'', which is used for two ...

helicopters such as the

CH-47 Chinook The Boeing CH-47 Chinook is a tandem rotor helicopter developed by American rotorcraft company Vertol and manufactured by Boeing Vertol. The Chinook is a heavy-lift helicopter that is among the heaviest lifting Western helicopters. Its name, C ...

still suffer from dissymmetry of lift, because the rotors are offset from one another. Tandem-rotor helicopters are installed with automatic cyclic feathering systems. At low airspeeds, blade flapping compensates for dissymmetry of lift. As airspeed increases, typically above 70 knots, these systems allow a more level fuselage attitude which reduces stresses on the rotor driving mechanisms.

References

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