Wills Wing Alpha 180 Owner's Service Manual page 22

There is no necessary cause and effect relationship between minimum sink speed and minimum
controllable airspeed. VMS is determined primarily by the wing loading and span loading, the wing
planform, 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 VMS 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 performance without significant sacrifice in other areas.
Using wing tufts to find the minimum sink speed of your glider
On a flex wing hang glider, the wing experiences a gradual and progressive stall, and different span-
wise stations of the wing stall at different angles of attack. Contrary to popular belief, a hang glider
wing usually does not stall first in the root or center section. It is true that because of wing twist the
root section is at the highest angle of attack relative to the remote free stream airflow, but other fac-
tors influence the stall propagation on the wing. Specifically, a flex wing hang glider usually stalls
first somewhere outboard of the root on each wing, approximately one fifth to one third of the way
out from the root to the tip, in the area where your tufts are located. 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 and those farthest
inboard will have indicated reversed flow.
The first onset of midspan 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,
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