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There are two important airspeeds with which all hang glider pilots should be intimately
familiar; minimum sink airspeed (hereinafter referred to as MSA) and minimum controllable
airspeed (MCA).
The most important of these two is MCA.
Minimum sink airspeed is that
speed at which your descent rate is the slowest possible. It is the speed to fly when you
want to maximize your climb rate in lift, or slow your rate of descent to a minimum in non
lifting air. (You would normally not fly at MSA in sinking air; the strategy there is normally to
speed up and fly quickly out of the sink. By minimizing your time spent in the sinking air you
minimize altitude lost, even though you have momentarily increased your sink rate by speed-
ing up.)
Minimum controllable airspeed
is that speed below which you begin to rapidly lose effec-
tive lateral control of the glider. Recognition of this speed and its implications is a more
subtle problem than many pilots realize. We have seen several instances of pilots who were
having a lot of trouble flying their gliders simply because they were unknowingly trying to fly
them too slowly; below the speed at which the glider responded effectively to lateral control
inputs. It is our opinion that a great percentage of hang gliding accidents are caused by
inadvertent flight below MCA, and subsequent loss of control of the glider with impact pre-
ceding recovery. Such incidents are usually attributed to “stalls,” but it is not the stall per se
that causes the problem, indeed the glider need not even be “stalled” in the traditional sense.
There is no necessary cause and effect relationship between minimum sink speed and
minimum controllable airspeed. MSA 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 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
On a flex wing hang glider, the wing experiences a gradual and progressive stall, and differ-
ent spanwise stations of the wing stall at different angles of attack. Contrary to popular
belief, a hang glider wing 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 factors influence the stall propagation on the wing. Specifically,
a flex wing hang glider stalls first somewhere outboard of the root on each wing, approxi-
mately one fifth to one third of the way out from the root to the tip, 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.
