Translational lift

Translational lift is the extra lift created/received when a helicopter's forward speed increases from the hover. It should be noted that if there is a steady wind (above 10 knots) a helicopter in the hover may already be in translational lift.

It is created when the moving helicopter creates an increased airflow over the advancing blade and increasing its lift efficiency.

Usually, when a helicopter is getting ready for take off there will be some wind flowing around the main rotor which will decrease the lift power of the main rotor. Translational lift is the point where the helicopter (and the main rotor) is moving with enough speed to cancel out the effects of the wind around it.

The most common example of the effect is this: a helicopter might be slightly overloaded for take off so that it cannot hover. Liftoff can still be achieved if the helicopter has enough of a straight runway to make a "Running take off". The maneuver is quite simple: the pilot will line up the runway by pulling almost maximum collective to make the ship light on its wheels or skids. Having lined up the runway or road he/she would then simply accelerate the helicopter across the ground until translational lift speed is achieved (depending on weather conditions-wind and humidity have an adverse effect on rotor effectiveness - this is almost never the same speed) at which time the helicopter starts flying through undisturbed air and gains extra lift power.

One example of this happening is in Robert Mason’s book, "Chickenhawk". Mason describes a situation where he took off with a load of 2500 pounds of high explosives where he should never have been able to take off at all. This was achieved by a "Running takeoff".