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At the first onset of stall, the tufts will sometimes indicate the impending separation by first
wiggling, and then deflecting spanwise, before they fully reverse and point forward. The first
onset of stall in the midspan 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 oc-
curs, 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, 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. (If the tufts
contain static electricity, they may not show this lateral wiggle prior to reversal. However,
you may get other clues to the beginning of separation, such as slight flutter or rumble in the
top surface of the sail.) 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 more and more turbulent; control response that is perfectly ad-
equate in smooth air will not be good enough in rougher air. Try flying the glider with the
midspan tufts fully reversed; you will probably find that the glider is somewhat controllable,
but only with a lot of physical effort. Note that both MCA and MSA come well before the
glider actually “stalls” in the traditional sense, i.e. pitches uncontrollably nose down. You may
also be able to sense, or your vario may tell you that although the glider has not “stalled”
(pitched nose down) your sink rate has increased significantly. In this mode the glider is
“mushing.”
Once you have familiarized yourself with the glider’s characteristics in this range of speeds,
you will not need to look at the tufts very often. You will know from bar position and bar
pressure, and from the sound and feel of the relative wind when you are at your minimum
sink / minimum controllable airspeed. In general, you should not fly your glider below this
speed. Be aware, however, that when you are flying at minimum sink in thermal gusts and
turbulence, you will experience gust induced separation of the airflow which will periodically
cause the tufts on your sail to reverse.
Of course in a turn, your minimum sink SPEED goes up because you are banked, and the
bank effectively increases your wing loading which increases your flying SPEED for any
angle of attack. But note this: THE TUFTS INDICATE ANGLE OF ATTACK, WITHOUT
REGARD TO AIRSPEED! Therefore, if you practice flying various bank angles in smooth air
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