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When aerotowing, rate-of-climb is determined by the
power of the towplane. It is important when selecting a
towplane, to ensure that it is capable of towing the
glider considering the existing conditions and glider
weight.
Self-launching glider rate-of-climb is determined by
design, powerplant output and glider weight. The rateof-
climb of self-launch gliders may vary from as low
as 200 feet per minute to as much as 800 feet per
minute or more in others. The pilot should consult the
GFM/POH to determine rate-of-climb under the existing
conditions.
FLIGHT MANUALS AND PLACARDS
AREAS OF THE MANUAL
The GFM/POH provides the pilot with the necessary
performance information to operate the glider safely. A
GFM/POH may include the following information.
• Description of glider primary components.
• Glider Assembly.
• Weight and balance data.
• Description of glider systems.
• Glider performance.
• Operating limitations.
Figure 5-6. Effect and added weight on thermaling
turn radius.
Figure 5-7. Effect of added weight on performance airspeeds.
5-6
PLACARDS
Cockpit placards provide the pilot with readily available
information that is essential for the safe operation
of the glider. All required placards are located in the
GFM/POH.
The amount of information that placards must convey
to the pilot increases as the complexity of the glider
increases. High performance gliders may be equipped
with wing flaps, retractable landing gear, a water ballast
system, drogue chute for use in the landing
approach, and other features than are intended to
enhance performance. These gliders may require additional
placards. [Figure 5-8]
PERFORMANCE INFORMATION
The GFM/POH is the source provided by the manufacturer
for glider performance information. In the
GFM/POH, glider performance is presented as terms
of specific airspeed such as stall speed, minimum sinking
airspeed, best L/D airspeed, maneuvering speed,
rough air speed, and VNE.
Some performance airspeeds apply only to particular
types of gliders. Gliders with wing flaps, for instance,
have a maximum permitted flaps extended airspeed
(VFE).
Manuals for self-launch gliders include performance
information about powered operations. These include
rate-of-climb, engine and propeller limitations, fuel
consumption, endurance, and cruise.
GLIDER POLARS
In addition, the manufacturer provides information
about the rate of sink in terms of airspeed, which is
summarized in a graph called a polar curve, or simply a
polar. [Figures 5-9].
The vertical axis of the polar shows the sink rate in
knots (increasing sink downwards), while the horizontal
axis shows airspeed in knots. Every type of glider
has a characteristic polar derived either from theoretical
calculations by the designer or by actual in-flight
measurement of the sink rate at different speeds. The
polar of each individual glider will vary (even from
other gliders of the same type) by a few percent
depending on relative smoothness of the wing surface,
the amount of sealing around control surfaces, and
even the number of bugs on the wing’s leading edge.
The polar forms the basis for speed-to-fly and final
glide tools that will be discussed in Chapter 11–Cross-
Country Soaring.
Minimum sink rate is determined from the polar by
extending a horizontal line from the top of the polar to
the vertical axis. The minimum sink speed is found
by drawing a vertical line from the top of the polar to
the horizontal axis. [Figure 5-10]. In this example, a
minimum sink of 1.9 knots occurs at 40 knots. Note
that the sink rate increases between minimum sink
speed and the stall speed (the left-hand end point of
the polar). The best glide speed (best L/D) is found by
drawing a tangent to the polar from the origin. The
best L/D speed is 50 knots. The glide ratio at best L/D
speed is determined by dividing the best L/D speed by
the sink rate at that speed, or 50/1.9, which is approximately
26. Thus, this glider has a best glide ratio in
calm air (no lift or sink and no headwind or tailwind)
of 26:1 at 50 knots.
The best speed-to-fly in a headwind is easily determined
from the polar. To do this, shift the origin to the
right along the horizontal axis by the speed of the
headwind and draw a new tangent line to the polar.
From the new tangent point, draw a vertical line to
read the best speed-to-fly. An example for a 20 knot
headwind is shown in Figure 5-11. The speed-to-fly
in a 20 knot headwind is found to be 60 knots. By
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Glider Flying Handbook(47)
