Aerodynamiska fenomen / Aerodynamic characteristics

Aerodynamic characteristics

There are a number of aerodynamic characteristics which are common to all helicopters, regardless of the type of rotor system.

Translating tendency

Translating tendency is one of these characteristics. It occurs during hover and it is a tendency for the whole helicopter to drift in the direction of the tail rotor thrust. The translating tendency is the result of the thrust of the tail rotor acting on the whole helicopter. Usually this is corrected by offsetting the main rotor mast, which will change the tip path plane of the rotor. When a helicopter has this built-in tilt of the rotor the translating tendency will not occur during hover. Another way to cancel the tendency is to rig the cyclic system to give the required tilt when the cyclic stick is level.

Ground effect

Ground effect is another characteristic that can occur, and as the name implies, when the helicopter is within one-half of the rotor diameter from the ground. This effect aids in supporting the helicopter at hover. The support is created by the much faster rate the air is displaced downwards to the ground, by the rotor, than it can escape from beneath of the helicopter. The air beneath the helicopter becomes denser and forms a cushion. This cushion supports the helicopter at hover when it is within one-half of the rotor diameter from the ground. If the helicopter travels at a speed above 8 km/h, the ground effect will be lost.

Translational lift

If the helicopter moves from a hover to forward flight, the ground effect will be lost but instead a new force appears. This force is the translational lift, it is an additional lift created by the forward flight and the increased efficiency of the rotor system. The airflow into the rotor is increased by the forward flight and also the mass of air is increased which in turn increases lift.
The effect takes place at any time the helicopter moves horizontally, but it is readily noticed at an airspeed of 25-30 km/h. This noticeable additional lift is referred to as effective translational lift. As the effect is dependent of airspeed, it is notable that it can occur at various points and even at a hover when the relative wind velocity is great enough. Also note that the additional lift will eventually be canceled by the increased drag of the fuselage.