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with small increases in angle of attack. During a stall,
the wing experiences a sizeable increase in drag.
[Figure 3-12]
WING PLANFORM
The shape, or planform, of the wings also has an
effect on the amount of lift and drag produced. The
four most common wing planforms used on gliders
are elliptical, rectangular, tapered, and swept-forward
wing. [Figure 3-13]
Elliptical wings produce the least amount of induced
drag for a given wing area. This design of wing is difficult
to manufacture. The elliptical wing is more efficient
in terms of L/D, but stall characteristics are not as
good as the rectangular wing.
The rectangular wing is similar in efficiency to the
elliptical wing but is much easier to build. Rectangular
wings have very gentle stall characteristics with a
warning buffet prior to stall and are easier to manufacture
than elliptical wings. One drawback to this wing
design is that rectangular wings create more induced
drag than an elliptical wing of comparable size.
Figure 3-12. This graph shows drag characteristics
in terms of angle of attack. As the angle of attack
becomes greater, the amount of drag increases.
Figure 3-13. Planform refers to the shape of the glider’s wing when viewed from above or below. There are
advantages and disadvantages to each planform design.
3-7
The tapered wing is the planform found most frequently
on gliders. Assuming equal wing area, the
tapered wing produces less drag than the rectangular
wing because there is less area at the tip of the tapered
wing. If speed is the prime consideration, a tapered
wing is more desirable than a rectangular wing, but a
tapered wing with no twist has undesirable stall characteristics.
Swept-forward wings are used to allow for the lifting
area of the wing to move forward while keeping the
mounting point aft of the cockpit. This wing configuration
is used on some tandem two-seat gliders to
allow for a small change in center of gravity with the
rear seat occupied or while flying solo.
Washout is built into wings by putting a slight twist
between the wing root and wing tip. When washout is
designed into the wing, the wing displays very good
stall characteristics. As you move outward along the
span of the wing, the trailing edge moves up in reference
to the leading edge. This twist causes the wing
root to have a greater angle of attack than the tip, and as
a result stall first. This provides ample warning of the
impending stall, and at the same time allows continued
aileron control.
Dihedral is the angle at which the wings are slanted
upward from the root to the tip. The stabilizing effect of
dihedral occurs when the airplane sideslips slightly as
one wing is forced down in turbulent air. This sideslip
results in a difference in the angle of attack between the
higher and lower wing with the greatest angle of attack
on the lower wing. The increased angle of attack produces
increased lift on the lower wing with a tendency
to return the airplane to wings level flight.
ASPECT RATIO
The aspect ratio is another factor that affects the lift and
drag created by a wing. Aspect ratio is determined by
dividing the wingspan (from wingtip to wingtip), by
the average wing chord. Glider wings have a high
aspect ratio as shown in Figure 3-14. High-aspect ratio
wings produce a comparably high amount of lift at low
angles of attack with less induced drag.
Figure 3-14. Aspect ratio is the relationship between the length and width of the wing and is one of the primary
factors in determining lift/drag characteristics.
3-8
WEIGHT
Weight is the third force that acts on a glider in flight.
Weight opposes lift and acts vertically through the
center of gravity of the glider. Gravitational pull
provides the force necessary to move a glider through
the air since a portion of the weight vector of a glider is
directed forward.
THRUST
Thrust is the forward force that propels a self-launch
glider through the air. Self-launch gliders have enginedriven
propellers that provide this thrust. Unpowered
gliders have an outside force, such as a tow plane,
winch, or automobile to launch the glider.
THREE AXES OF ROTATION
The glider is maneuvered around three axes of rotation.
These axes of rotation are the vertical axis, the lateral
axis, and the longitudinal axis. They rotate around one
central point in the glider called the center of gravity
(CG). This point is the center of the glider’s total weight
and varies with the loading of the glider.
When you move the rudder left or right, you cause the
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Glider Flying Handbook(26)
