Improved rotor efficiency resulting from directional flight in a helicopter is called translational lift. The efficiency of the hovering rotor system is greatly improved with each knot of incoming wind gained by horizontal movement of the aircraft or surface wind. As the incoming wind produced by aircraft movement or surface wind enters the rotor system, turbulence and vortices experienced in hovering flight are left behind and the flow of air becomes more horizontal. While transitioning to forward flight at about 16 to 24 knots, the helicopter goes through effective translational lift (ETL). The rotor blades of the helicopter become more efficient as forward airspeed increases. Between 16 and 24 knots, the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air.
An example of how beneficial this additional lift can be is: a helicopter might be slightly overloaded for take off so that it cannot 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 slowly accelerate the helicopter across the ground on its landing gear until translational lift speed is achieved, extra rotor-disc lift is produced, and the aircraft will begin to climb.
In Robert Mason's book Chickenhawk, Mason describes a situation where an aircraft, loaded with 2500 pounds of high explosives, was above max gross weight and should have been unable to achieve flight, but was actually "coaxed" into flight via the "running take off".
- Ground effect (aerodynamics)
- Transverse flow effect
- Dissymmetry of lift
- Vortex ring
- Vortex ring state
- Helicopter Flying Handbook, FAA-H-8083-21A, pp 2-19, 2-20
- Chickenhawk, Robert Mason, published 1983
- Principles of Helicopter Flight, W.J. Wagtendon-k, pp 64-65