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can increase the weight of the wing in addition to
changing the shape and disrupting the airflow, all of
which will increase stall speed. Turbulence is another
environmental factor that can affect a glider’s stall
speed. The unpredictable nature of turbulence can
cause a glider to stall suddenly and abruptly at a higher
airspeed than it would in stable conditions. The reason
turbulence has such a strong impact on the stall speed
of a glider is that the vertical gusts change the direction
of the relative wind and abruptly increase the angle of
attack. During landing in gusty conditions, it is important
that you increase your airspeed in order to maintain
a wide margin above stall.
SPINS
A spin can be defined as an aggravated stall that results
in the glider descending in a helical, or corkscrew,
path. A spin is a complex, uncoordinated flight maneuver
in which the wings are unequally stalled. Upon
entering a spin, the wing that is more completely
stalled will drop before the other, and the nose of the
aircraft will yaw in the direction of the low wing.
The cause of a spin is exceeding the critical angle of
attack while performing an uncoordinated maneuver.
The lack of coordination is normally caused by either
too much or not enough rudder control for the amount
of aileron being used. If the stall recovery is not
promptly initiated, the glider is likely to enter a full
stall that may develop into a spin. Spins that occur as
the result of uncoordinated flight usually rotate in the
direction of the rudder being applied, regardless of the
raised wing. When you enter a slipping turn, holding
opposite aileron and rudder, the resultant spin usually
occurs in the opposite direction of the aileron already
applied. In a skidding turn where both aileron and rudder
are applied in the same direction, rotation will also
be in the direction of rudder application.
Spins are normally placed in three categories as shown
in Figure 3-26. The most common is the upright, or
erect, spin, which is characterized by a slightly nose
down rolling and yawing motion in the same direction.
An inverted spin involves the aircraft spinning upside
down with the yaw and roll occurring in opposite
directions. A third type of spin, the flat spin, is the
most hazardous of all spins. In a flat spin, the glider
yaws around the vertical axis at a pitch attitude nearly
level with the horizon. A flat spin often has a very high
rate of rotation; the recovery is difficult, and sometimes
impossible. If your glider is properly loaded
within its CG limits, entry into a flat spin should not
occur.
Since spins normally occur when a glider is flown in
an uncoordinated manner at lower airspeeds, coordinated
use of the flight controls is important. It is critical
that you learn to recognize and recover from the
first sign of a stall or spin. Entering a spin near the
ground, especially during the landing pattern, is most
often fatal. [Figure 3-27]
GROUND EFFECT
Ground effect is a reduction in induced drag for the
same amount of lift produced. Within one wingspan
3-15
above the ground, the decrease in induced drag enables
the glider to fly at a slower airspeed. In ground effect, a
lower angle of attack is required to produce the same
amount of lift. Ground effect enables the glider to fly
near the ground at a slower airspeed. It is ground effect
that causes the glider to float as you approach the
touchdown point.
During takeoff and landing, the ground alters the
three-dimensional airflow pattern around the glider.
The result is a decrease in upwash, downwash, and a
reduction in wingtip vortices. Upwash and downwash
refer to the effect an airfoil has on the free airstream.
Upwash is the deflection of the oncoming airstream
upward and over the wing. Downwash is the downward
deflection of the airstream as it passes over the
wing and past the trailing edge.
During flight, the downwash of the airstream causes
the relative wind to be inclined downward in the vicinity
of the wing. This is called the average relative
wind. The angle between the free airstream relative
wind and the average relative wind is the induced
angle of attack. In effect, the greater the downward
deflection of the airstream, the higher the induced
angle of attack and the higher the induced drag.
Ground effect restricts the downward deflection of the
airstream, decreasing both induced angle of attack and
induced drag.
3-16
4-1
Flight instruments in the glider cockpit provide
information regarding the glider’s direction, altitude,
　
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