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VS—the stalling speed or the minimum steady flight
speed at which the glider is controllable.
VS0—the stalling speed or the minimum steady flight
speed in the landing configuration.
VS1—the stalling speed or the minimum steady flight
speed obtained in a specified configuration.
ALTIMETER
The altimeter measures the height of the glider above a
given level. Since it is the only instrument that gives
altitude information, the altimeter is one of the most
important instruments in the glider. To use the altimeter
effectively, the pilot must thoroughly understand its
principle of operation and the effect of atmospheric
pressure and temperature on the altimeter. [Figure 4-4]
PRINCIPLE OF OPERATION
The pressure altimeter is simply an aneroid barometer
that measures the pressure of the atmosphere at the
level where the altimeter is located, and presents an
altitude indication in feet. The altimeter uses static
pressure as its source of operation. Air is denser at the
surface of the Earth than aloft, therefore as altitude
increases, atmospheric pressure decreases. This difference
in pressure at various levels causes the altimeter
to indicate changes in altitude.
The presentation of altitude varies considerably
between different types of altimeters. Some have one
pointer while others have more. Only the multi-pointer
type will be discussed in this handbook.
The dial of a typical altimeter is graduated with numerals
arranged clockwise from 0 to 9 inclusive as shown in
Figure 4-4. Movement of the aneroid element is transmitted
through a gear train to the three hands, which sweep the
calibrated dial to indicate altitude. The shortest hand indicates
altitude in tens of thousands of feet; the intermediate
hand in thousands of feet; and the longest hand in
hundreds of feet, subdivided into 20-foot increments.
The altitude indicated on the altimeter is correct only if
the sea level barometric pressure is standard (29.92 in.
Hg.), the sea level free air temperature is standard (+15°
C or 59° F), and the pressure and temperature decrease at
a standard rate with an increase in altitude. Since atmospheric
pressure continually changes, a means is provided
to adjust the altimeter to compensate for nonstandard
conditions. This is accomplished through a system by
which the altimeter setting (local station barometric pressure
reduced to sea level) is set to a barometric scale
located on the face of the altimeter. Only after the altimeter
is set properly will it indicate the correct altitude.
EFFECT OF NONSTANDARD PRESSURE AND
TEMPERATURE
If no means were provided for adjusting altimeters to
nonstandard pressure, flight could be hazardous. For
example, if a flight is made from a high-pressure area
to a low-pressure area without adjusting the altimeter,
the actual altitude of the glider will be LOWER than
the indicated altitude. When flying from a low-pressure
area to a high-pressure area, the actual altitude of
the glider will be HIGHER than the indicated altitude.
Fortunately, this error can be corrected by setting the
altimeter properly.
Variations in air temperature also affect the altimeter.
On a warm day, the expanded air is lighter in weight
per unit volume than on a cold day, and consequently
the pressure levels are raised. For example, the pressure
level where the altimeter indicates 10,000 feet
will be HIGHER on a warm day than under standard conditions.
On a cold day, the reverse is true, and the 10,000
foot level would be LOWER. The adjustment made by
the pilot to compensate for nonstandard pressures does
not compensate for nonstandard temperatures. Therefore,
if terrain or obstacle clearance is a factor in the selection
Figure 4-4. The altimeter indicator.
ting of a station located along the route of flight and
within 100 nautical miles (NM) of the aircraft. If there is
no such station, the current reported altimeter setting of
an appropriate available station shall be used. In an aircraft
having no radio, the altimeter shall be set to the elevation
of the departure airport or an appropriate altimeter
setting available before departure.
Many pilots confidently expect that the current altimeter
setting will compensate for irregularities in atmospheric
pressure at all altitudes. This is not always true because
the altimeter setting broadcast by ground stations is the
station pressure corrected to mean sea level. The altimeter
setting does not account for the irregularities at higher
levels, particularly the effect of nonstandard temperature.
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Glider Flying Handbook(34)
