Controlling Roll/Yaw Oscillations on Flex Wing Hang Gliders

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All high performance flex wing hang gliders have some degree of susceptibility to high speed roll / yaw oscillations. This can be a problem when flying fast, especially in turbulence, and when aerotowing. In aerotowing, it is usually a greater problem towing behind a trike, as opposed to a Dragonfly type of tug, since the tow speeds with a trike are usually higher. The faster one is flying, and the more turbulent the air, the more skill is required to control or avoid these oscillations.

Minimizing or avoiding oscillations is a matter of using proper flying technique. The oscillations are not “pilot induced” as they are sometimes called, but to some degree they can be “pilot controlled.” Specifically, what is required is that the pilot fly “ahead of the glider” instead of “behind the glider.” Flying ahead of the glider means being able to sense, by feel, what the glider is about to do in the future rather than observing what the glider has already done. The pilot must be sensitive enough to the glider that he can feel through the pressures on the control bar, and by sensing very small attitude changes, that the glider is about to begin veering to one side. He must also be sensitive enough to be able to feel when the glider is beginning to respond to a correcting control input. Pilots without this sensitivity will instead only be able to respond to their observation that the glider has actually changed heading. By the time the glider has actually changed heading, the control input to correct is too late.

For example, if the glider gives an indication through the pilot’s feel of the control bar that it is about to veer towards the right, the pilot should immediately respond by inputting a weight shift correction to the left. The glider at this point hasn’t changed heading. By applying the proper left control input, the pilot will prevent the change in heading. Then, however, the pilot must be able to feel the glider’s diminishing tendency to veer right (experienced as a reduction of roll bar pressure as the glider begins to respond to the left control input), and then immediately re-center on the control bar.

What pilots often tend to do instead is to input the left correction only after the glider has actually veered to the right, and hold the correction until the glider’s heading has returned to the original desired heading. The glider in this case will way overshoot the desired heading, and the oscillation process has begun, and will continue to get worse as long as the pilot continues to input corrections in response to what the glider has already done instead of what it is about to do. The other thing pilots often tend to do is to respond to any perception of loss of control by doing two things: gripping the bar more tightly and pulling in for more speed. Both of these will only aggravate a roll/yaw oscillation, as the glider becomes more subject to this the faster one flies, and the pilot loses all feel for the glider by tightening his grip.

There is no way to develop the sensitivity required to execute these techniques properly, except by lots of experience. There is a technique that pilots can use, however, to achieve some measure of the same results.

First, if you experience severe oscillations in free flight (off tow) do not continue trying to fly fast. Immediately bring the bar smoothly to the normal trim position in pitch (slow down to trim speed) and center yourself on the bar. The glider will recover to normal flight right away. If the glider ends up in a turn, it will be a simple matter to correct it once you have slowed down. (When on tow, you are constrained to fly at the speed of the tug, so this remedy isn’t available.)

If while flying fast you notice that the glider has begun to veer to one side, make a quick, sharp and deliberate weight shift in the opposite direction, and then immediately return to the center of the bar (without waiting for the glider to respond). If the first correction of this type isn’t enough, do another one, but don’t hold the correction longer. This technique approximates the proper technique of flying ahead of the glider by feel, without requiring the same degree of sensitivity from the pilot.

The use of an optional vertical stabilizer significantly increases the glider’s damping and stability in yaw, and reduces the tendency to enter into oscillations. On gliders equipped with variable geometry, using a tighter VG setting will measurably reduce the glider’s roll sensitivity, and reduce the tendency to oscillate. With regard to aerotowing, at the speeds normally flown on tow, you should have adequate control with some degree of VG on to be able to follow the tug.