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by 60°F causes an eightfold increase and so on.
By itself, humidity usually is not considered an important
factor in calculating density altitude and glider performance.
Nevertheless, high humidity does cause a
slight decrease in glider takeoff and climb performance.
At relatively low temperatures, the effect of
humidity is very slight because the total amount of
water vapor the airmass can hold is relatively small. At
relatively high temperatures, on the other hand, the
effect of humidity is more significant because the total
amount of water vapor the airmass can hold is many
times larger. There are no rules-of-thumb or charts used
to compute the effects of humidity on density altitude.
Expect a minor decrease in takeoff performance when
humidity is high.
HIGH AND LOW DENSITY ALTITUDE
CONDITIONS
Every pilot must understand the terms “high density
altitude” and “low density altitude.” In general, high
density altitude refers to thin air, while low density altitude
refers to dense air. Those conditions that result in
a high density altitude (thin air) are high elevations,
low atmospheric pressure, high temperatures, high
humidity, or some combination thereof. Lower elevations,
high atmospheric pressure, low temperatures and
low humidity are more indicative of low density altitude
(dense air). However, high density altitudes may
be present at lower elevations on hot days, so it is
important to calculate the density altitude and determine
performance before a flight.
One way to determine density altitude is to use charts
designed for that purpose. [Figure 5-1] For example,
assume you are planning to depart an airport where the
field elevation is 1,165 feet MSL, the altimeter setting
is 30.10, and the temperature is 70°F. What is the density
altitude? First, correct for nonstandard pressure
(30.10) by referring to the right side of the chart and
subtracting 165 feet from the field elevation. The result
is a pressure altitude of 1,000 feet. Then, enter the chart
at the bottom, just above the temperature of 70°F
(21°C). Proceed up the chart vertically until you intercept
the diagonal 1,000-foot pressure altitude line, then
move horizontally to the left and read the density alti-
Figure 5-1. Density Altitude Chart.
5-3
nose of the glider pointed somewhat upwind of the
target on the ground. For instance, if the crosswind is
from the right, during final glide the nose of the
glider is pointed a bit to the right of the target on the
ground. The glider’s heading will be upwind (to the
right, in this case) of the target, but if the angle of
crab is correct, the glider’s track will be straight
toward the target on the ground. [Figure 5-3]
tude of approximately 2,000 feet. This means your selflaunching
glider or towplane will perform as if it were
at 2,000 feet MSL on a standard day.
Most performance charts do not require you to compute
density altitude. Instead, the computation is built
into the performance chart itself. All you have to do is
enter the chart with the correct pressure altitude and
the temperature. Some charts, however, may require
you to compute density altitude before entering them.
Density altitude may be computed using a density altitude
chart or by using a flight computer.
WINDS
Wind affects glider performance in many ways.
Headwind during launch results in shorter ground roll,
while tailwind causes longer ground roll before takeoff.
Crosswinds during launch require proper crosswind
procedures. [Figure 5-2]
During cruising flight, headwinds reduce the groundspeed
of the glider. A glider flying at 60 knots true airspeed
into a headwind of 25 knots has a groundspeed
of only 35 knots. Tailwinds, on the other hand, increase
the groundspeed of the glider. A glider flying at 60
knots true airspeed with a tailwind of 25 knots has a
groundspeed of 85 knots.
Crosswinds during cruising flight cause glider heading
(where the glider nose is pointed) and glider track (the
path of the glider over the ground) to diverge. When
gliding toward an object on the ground in the presence
of crosswind, such as on final glide at the end of a
cross-country flight, the glider pilot should keep the
Figure 5-2. Wind effect on takeoff distance and
climb-out angle.
Figure 5-3. Crosswind effect on final glide.
5-4
Headwind during landing results in a shortened ground
roll and tailwind results in a longer ground roll.
Crosswind landings require the pilot to compensate for
drift with either a sideslip or a crab. [Figure 5-4]
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Glider Flying Handbook(45)
