Controlling Roll/Yaw Oscillations on Flex Wing Hang Gliders by Mike Meier
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
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.