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Figure 5-8. Typical placards for non-motorized and self-launch glider.
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Figure 5-10. Graphic depiction of minimum sink airspeed and maximum L/D speed.
Figure 5-9. Dual and solo performance curves for a two-seat glider.
Figure 5-11. Best speed-to-fly in a 20-knot headwind.
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repeating the procedure for different headwinds, it is
apparent that flying a faster airspeed as the headwind
increases will result in the greatest distance over the
ground. If this is done for the polar curves from many
gliders, a general rule of thumb is found, namely, add
half the headwind component to the best L/D for the
maximum distance. For tailwinds, shift the origin to
the left of the ‘0’ mark on the horizontal axis. The
speed-to-fly in a tailwind is found to lie between minimum
sink and best L/D but never slower than minimum
sink speed.
Sinking air usually exists between thermals, and it is
most efficient to fly faster than best L/D in order to
spend less time in sinking air. How much faster to fly
can be determined by the glider polar, as illustrated in
Figure 5-12 for an air mass that is sinking at 3 knots.
The polar graph in this figure has its vertical axis
extended upwards. Shift the origin vertically by 3
knots and draw a new tangent to the polar, then draw a
line vertically to read the best speed-to-fly. For this
glider, the best speed-to-fly is found to be 60 knots.
Note that the variometer will show the total sink of 5
knots as illustrated in the figure.
If the glider is equipped with water ballast, wing flaps,
or wingtip extensions, the performance characteristics
of the glider will be depicted in multiple configurations.
[Figures 5-13, 5-14, and 5-15]. Comparing the
polar with and without ballast [Figure 5-13] it is evident
that the minimum sink is higher and occurs at a
faster speed. With ballast, therefore, it would be more
difficult to work small, weak thermals. The best glide
ratio is the same, but it occurs at a higher speed. In
addition, the sink rate at higher speeds is lower with
ballast. From the polar, then, ballast should be used
under stronger thermal conditions for better speed
between thermals. Note that the stall speed is higher
with ballast as well.
Flaps with a negative setting as opposed to a ‘0’ degree
setting during cruise also reduce the sink rate at higher
speeds, as shown in the polar [Figure 5-14].
Therefore, when cruising at or above 70 knots, a –8
flap setting would be advantageous for this glider. The
polar with flaps set at –8 does not extend to speeds
slower than 70 knots since the negative flap setting
loses its advantage there.
Wing-tip extensions will also alter the polar, as shown
in [Figure 5-15]. The illustration shows that the additional
3 meters of wing span is advantageous at all
speeds. In some gliders, the low-speed performance is
better with the tip extensions, while high-speed performance
is slightly diminished by comparison.
WEIGHT AND BALANCE
INFORMATION
The GFM/POH provides information about the weight
and balance of the glider. This information is correct
when the glider is new as delivered from the factory.
Subsequent maintenance and modifications can alter
weight and balance considerably. Changes to the
glider that affect weight and balance should be noted
in the airframe logbook and on appropriate cockpit
placards. Maximum Fuselage Weight: 460 pounds
Weight is the force with which gravity attracts a body
toward the center of the earth. It is a product of the
mass of a body and the acceleration acting on the body.
Weight is a major factor in glider construction and
operation; it demands respect from all pilots. The pilot
of a glider should always be aware of the comsequences
of overloading.
LIMITATIONS
Whether the glider is very simple or very complex,
designers and manufacturers provide operating limita-
Figure 5-12. Best speed-to-fly in sink.
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Figure 5-14. Performance polar with flaps at 0° and minus 8°.
Figure 5-13. Effect of water ballast on performance polar.
Figure 5-15. Performance polar with 15 meter and 18 meter wingspan configurations.
5-10
tions which must be complied with to ensure the safety
of flight.Weight is a major factor in glider construction
and operation; it demands respect from all pilots. The
pilot of a glider should always be aware of the consequences
of overloading.The V-G diagram provides the
pilot with information on the design limitations of the
glider such as limiting airspeeds and load factors. Pilots
　
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