User Guide

Sideways, Backwards and Crosswinds

If you have absorbed and understood the basic principles of helicopter flight it should be

obvious that you can fly the helicopter in any direction from the hover, without turning, by

tilting the cyclic the way you want to go. You can also hover on the spot in a wind blowing

from any direction by tilting the cyclic into the wind. There are, however, a few pitfalls which

should be pointed out.

Weather-Cocking

The helicopter’s tailboom is there for two main reasons. We’ve already mentioned that it

provides a convenient mounting point for the tail rotor, outside the worst of the main rotor

downwash, and at the end of a long lever arm. Those same factors also make it the best

place to mount a vertical fin (or fins) very like what you find at the tail end of most fixed-

wing aircraft, and serving exactly the same purpose; to provide automatic directional

stability in fast forward flight, just like the fletching of a dart or an arrow.

The tail fin works against you when you try to fly sideways or hover in a cross-wind,

generating a force which tends to turn the helicopter’s nose into the relative wind. The

whole helicopter acts like a weather-vane. To counter this effect you must use pedal inputs

– and the faster you fly (or the stronger the crosswind) the larger the input needed to

maintain heading. Eventually you’ll reach a point where the tail rotor simply cannot provide

any more thrust, and the nose will inexorably turn into the relative wind. This is one of the

main reasons why a helicopter’s maximum sideways speed is much lower than the

maximum forward speed – and a wise pilot will always try to avoid flight regimes which can

only be maintained by jamming any control hard against its stops.

Stability in Backwards Flight

Backwards flight is something that obviously needs to be done carefully – you can’t see

where you’re going, and you’re flying tail rotor first. There are less obvious problems which

affect you even if you have unlimited space for maneuver, or if you’re simply trying to hover

in a strong tail-wind. The explanation involves some basic physics, but the diagram should

help you grasp what’s going on.

The strength of the turning force generated by the tail fin depends mainly on three factors:

1) The speed of the relative wind.

2) How large a surface the tail fin presents to the relative wind.

3) The effective length of the tail fin’s lever-arm with respect to the relative wind and the

helicopter’s center of mass, which we can assume will be more or less directly under the

main rotor hub.

Taking these factors in order; 1) more airspeed means more force. In fact, because the

force is proportional to the square of the airspeed, a little more speed means a lot more

force. 2) When the tail is pointing straight into the wind, it is exposing the least possible area

and generating the least possible force. As the tail swings out across the wind, it presents

more and more surface area, generating more and more force. 3) When the tail is pointing

straight into the wind, it is in line with the center-of-mass and there is no leverage. As the

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