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compass heading should be done only when the
glider is in wings-level unaccelerated flight; otherwise
erroneous magnetic compass readings may be
obtained. [Figure 4-26]
G-METER
Another instrument that can be mounted in the instrument
panel of a glider is a G-meter. The G-meter measures and
displays the load imposed on the glider during flight.
During straight, unaccelerated flight in calm air, a glider
experiences a load factor of 1G (1.0 times the force of
gravity). During aerobatics or during flight in turbulent
air, the glider and pilot experience G-loads greater than
1G. These additional loads result from accelerations
imposed on the glider. Some of these accelerations result
from external sources, such as flying into updrafts or
downdrafts. Other accelerations arise from pilot input on
the controls, such as pulling back or pushing forward on
the control stick. G-loads are classified as positive or negative.
Positive G is felt when increasing pitch rapidly for
a climb. Negative G is felt when pushing over into a dive
or during sustained inverted flight. Each glider type is
designed to withstand a specified maximum positive Gload
and a specified maximum negative G-load. The
GFM/POH is the definitive source for this information.
Exceeding the allowable limit loads may result in deformation
of the glider structure. In extreme cases,
exceeding permissible limit loads may cause structural
failure of the glider. The G-meter allows the pilot to monitor
G-loads from moment to moment. This is useful in
aerobatic flight and during flight in rough air. Most Gmeters
also record and display the maximum positive Gload
and the maximum negative G-load encountered
during flight. The recorded maximum positive and negative
G-loads can be reset by adjusting the control knob of
the G-meter. [Figure 4-27]
Figure 4-27. The G-meter. Figure 4-28. A typical outside air temperature (OAT)
gauge.
4-18
OUTSIDE AIR TEMPERATURE
GAUGE
The outside air temperature gauge (OAT) is a simple and
effective device mounted so that the sensing element is
exposed to the outside air. The sensing element consists
of a bimetallic-type thermometer in which two dissimilar
metals are welded together into a single strip and twisted
into a helix. One end is anchored into a protective tube,
and the other end is affixed to the pointer, which reads
against the calibration on a circular face. OAT gauges are
calibrated in degrees Celsius, degrees Fahrenheit, or both.
An accurate air temperature will provide the glider pilot
with useful information about temperature lapse rate with
altitude change.
When flying a glider loaded with water ballast, knowledge
of the height of the freezing level is important to safety of
flight. Extended operation of a glider loaded with water
ballast in below-freezing temperatures may result in frozen
drain valves, ruptured ballast tanks, and structural damage
to the glider. [Figure 4-28]
4-19
4-20
5-1
Glider performance during launch phase and during
free flight phase depends on many factors. The design
of the glider itself is one factor. Weather, wind, and
other atmospheric phenomena also affect glider performance.
FACTORS AFFECTING
PERFORMANCE
Glider performance during launch depends on the
power output of the launch mechanism and on the
aerodynamic efficiency of the glider itself. The three
major factors that affect performance are density altitude,
weight, and wind.
DENSITY ALTITUDE
Air density directly affects the launch performance of
the glider. As the density of the air increases, the
engines power output of the launching vehicle (towplane,
ground tow, or self-launch glider) and the aerodynamic
lift of the glider’s wings increase. When air
density is less dense, the launch performance
decreases. Density altitude is the altitude above mean
sea level (MSL) at which a given atmospheric density
occurs in the standard atmosphere. It can also be
interpreted as pressure altitude corrected for nonstandard
temperature differences.
PRESSURE ALTITUDE
Pressure altitude is displayed as the height above a
standard datum plane, which, in this case, is a theoretical
plane where air pressure is equal to 29.92 inches
of mercury (in. Hg). Pressure altitude is the indicated
height value on the altimeter when the altimeter setting
is adjusted to 29.92 in. Hg. Pressure altitude, as
opposed to true altitude, is an important value for calculating
performance as it more accurately represents
the air content at a particular level.
　
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