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Translational lift is improved
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 ...
efficiency resulting from directional flight in a
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 attribute ...
. Translation is the conversion from the hover to forward flight. As undisturbed air enters the rotor system horizontally, turbulence and vortices created by hovering flight are left behind and the flow of air becomes more horizontal. The efficiency of the hovering rotor system is greatly improved with each
knot A knot is an intentional complication in cordage which may be practical or decorative, or both. Practical knots are classified by function, including hitches, bends, loop knots, and splices: a ''hitch'' fastens a rope to another object; a ' ...
of airspeed gained by horizontal movement of the aircraft or wind speed. As forward airspeed increases, the helicopter goes through effective translational lift (ETL) at about 16 to 24 knots. This is known as the ETL speed. Above this speed, the rotor system completely outruns the recirculation of old vortices and begins to work in undisturbed air. Efficiency continues to increase with airspeed until best climb airspeed is reached, and drag is minimised. This additional lift can enable an overloaded helicopter to climb even if it is too heavy to hover in ground effect. Liftoff can still be achieved if the helicopter has enough of a straight runway to make a "running take off", where the pilot will accelerate the helicopter across the ground on its landing gear until translational lift speed is achieved and the aircraft begins to climb. This is described in Robert Mason's book '' Chickenhawk''. During the translation from the hover to forward flight, the difference in lift across the rotor disc causes a difference in drag, resulting in a noticeable vibration between approximately 10-20 knots. As speed increases and translational lift becomes more effective, the helicopter will tend to pitch up and roll to the right or left (depending on main rotor rotation direction), due to
dissymmetry of lift Dissymmetry of lift (also known as asymmetry of lift or asymmetric lift) in rotorcraft aerodynamics refers to an unequal amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters and autogyros i ...
,
gyroscopic precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...
, and the
transverse flow effect Transverse flow effect is an aerodynamic effect encountered when a helicopter moves horizontally (typically forward) through the air, which causes the rotor disc to roll to the side. It is also known as transverse roll or inflow roll. Transverse f ...
. The pilot must anticipate and correct for these effects. The efficiency of the tail rotor is also improved with forward airspeed. This is known as translational thrust.


See also

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Ground effect (aerodynamics) For fixed-wing aircraft, ground effect is the reduced aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface.. Reduced drag when in ground effect during takeoff can cause the aircraft to "float" while below the re ...
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Transverse flow effect Transverse flow effect is an aerodynamic effect encountered when a helicopter moves horizontally (typically forward) through the air, which causes the rotor disc to roll to the side. It is also known as transverse roll or inflow roll. Transverse f ...
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Dissymmetry of lift Dissymmetry of lift (also known as asymmetry of lift or asymmetric lift) in rotorcraft aerodynamics refers to an unequal amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters and autogyros i ...
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Vortex ring A vortex ring, also called a toroidal vortex, is a torus-shaped vortex in a fluid; that is, a region where the fluid mostly spins around an imaginary axis line that forms a closed loop. The dominant flow in a vortex ring is said to be toroidal, ...
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Vortex ring state The vortex ring state (VRS) is a dangerous aerodynamic condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor, causing severe loss of lift. The vortex ring state is sometimes referred to as settling with pow ...


References

Helicopter aerodynamics {{Aviation-stub