Hover

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An NH90 hovering

The hover flight is a flight condition of helicopters in which they remain in the air in an unchanged position and height. More power is required for hovering than for flying at cruising speed . It is aerodynamically unstable and the transition between the two flight states requires special attention from the pilot, since not only the increase in performance for ascent, but also the abruptly decreasing ground effect must be taken into account.

Requirements / differences to forward flight

The additional power for the hovering state is noticeably reduced from speeds of around 15 to 20  kt (~ 27 to 36 km / h). At higher flight speeds, it decreases to the extent that the rotor escapes the recirculation of the air passing through it. This the pilot as an additional buoyancy "translation lift" appears ( Engl . Translational lift) or "drive-lift" is called. As the speeds increase further, the torso resistance requires more power, and the total demand then increases again.

With the tail rotor configuration, there is still an additional side force ( drift ) that must be compensated for by the opposite inclination of the main rotor plane ( roll ). This in turn reduces the lift and has to be compensated for with more main rotor pitch . When flying forward, the tail rotor is relieved of power- saving power if the torque compensation is supported by the vertical stabilizer . These effects do not occur in the coaxial or tandem configuration .

A pilot needs a lot of practice with all configurations before he can achieve a calm hover. An automatic hover is therefore used in some models , such as the Russian Mil Mi-26 .

Ground effect

At a very low altitude, the rotor blades benefit from the ground effect . At the same speed of rotation, they generate more lift than in open air. As a result, hovering near the ground requires less propulsion power. This flight condition is called hover in ground effect (HIGE). From a height above the ground of about the rotor diameter, the ground effect decreases and more power is required for hovering. This is called the hover out of ground effect (HOGE).

This shows that the hovering height with ground effect is greater than in open air space and explains why pilots quickly switch to forward flight when taking off at higher altitudes. Using the NH90 as an example , which has a service ceiling of 6,000 m, it is 2,900 m or 2,355 m.

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