Wills Wing Fusion 141, Owner'S Service Manual

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the wing section characteristics, etc. MCA is influenced most heavily by the tension in the sail; how
much “billow” the glider has. However, in your Wills Wing glider, as in most hang gliders, MCA and
MSA evolved towards a common value during the design and development of the glider. This is so
because if the wing is tuned so tight that minimum controllable airspeed is at a higher speed than
minimum sink speed, then effective sink rate performance can be improved by loosening the wing so as
to lower the minimum controllable airspeed. Conversely, if minimum controllable airspeed is reached at
a speed below that of minimum sink, the wing can usually be tightened so as to improve glide perfor-
mance without significant sacrifice in other areas.
Using wing tufts to find the minimum sink speed of your glider
Your Wills Wing glider has been equipped from the factory with short yarn tufts on the top surface of
each wing. The shadow of these tufts will be visible through the sail. The tufts are useful for indicating
the local reversal of the airflow which is associated with the onset of the stall in that portion of the
wing. You can use these tufts, as described below, to help determine when you are flying at minimum
sink airspeed.
On a flex wing hang glider, the wing experiences a gradual and progressive stall, and different
spanwise stations of the wing stall at different angles of attack. The tufts have been placed on your
wing at the approximate location of the first onset of stall. As the angle of attack is raised further, the
stall propagates both outward towards the tips and inward towards the root. If you wish to observe the
stall propagation across the whole wing on your glider, you can cut some more tufts from knitting yarn,
about 3-4" long, and tape these to the top surface of your sail across the rest of the span.
During normal flight the flow will be chordwise along the wing, and the tufts will point towards the
trailing edge. When the wing stalls, the tufts will reverse direction, indicating the local flow towards the
leading edge.
At the first onset of stall, the tufts will indicate the impending separation by first wiggling, and then
deflecting spanwise, before they fully reverse and point forward. The first onset of stall occurs well
before the familiar “stall break” in which the glider pitches uncontrollably nose down to recover from
the stall. By the time the stall break occurs, all tufts but those farthest outboard will have indicated
reversed flow.
The first onset of stall as indicated by the first tickling of the tufts indicates that you have reached the
angle of attack corresponding to the glider’s minimum sink airspeed. This will also be very close to the
glider’s minimum controllable airspeed. To find the glider’s minimum sink speed, fly the glider in
smooth air, early in the morning or late in the afternoon. When you are well away from the terrain, and
well clear of other aircraft, look up at the wing tufts while you very gradually reduce the speed of the
glider. Note the speed at which the first tuft first begins to wiggle just prior to blowing spanwise
toward the tip. This is your speed for minimum sink rate. Familiarize yourself with the position of the
control bar relative to your body at this speed, with the sound and feel of the wind, with the reading on
your airspeed indicator, and with the feel of the glider in terms of pitch and roll pressures. Most of the
time when you are flying it will not be practical to look up for extended periods of time at your tufts.
That is why familiarization with these other, more accessible indicators is important.
After finding your minimum sink speed, experiment with roll control response at speeds just above and
just below this speed to find the value of MCA and the corresponding bar position and other indicators
for this speed. Realize that your effective MCA is going to be higher and higher as the air becomes