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of the diaphragm.
TRUE AIRSPEED
The airspeed indicator is calibrated to indicate true airspeed
only under standard atmospheric conditions at
sea level (29.92 inches of mercury and 15° C or 59°F).
Because air density decreases with an increase in altitude,
the glider has to be flown faster at higher altitudes
to cause the same pressure difference between pitot
impact pressure and static pressure. Therefore, for a
given true airspeed, indicated airspeed decreases as
altitude increases or for a given indicated airspeed, true
airspeed increases with an increase in altitude.
Figure 4-2. The three types of airspeed you should know include calibrated airspeed, indicated airspeed, and
4-3
A pilot can find true airspeed by two methods. The first
method, which is more accurate, involves using a
computer. In this method, the calibrated airspeed is
corrected for temperature and pressure variation by using
the airspeed correction scale on the computer.
A second method, which is a "rule of thumb," can be
used to compute the approximate true airspeed. This is
done by adding to the indicated airspeed 2 percent of
the indicated airspeed for each 1,000 feet of altitude.
AIRSPEED INDICATOR MARKINGS
Aircraft weighing 12,500 pounds or less, manufactured
after 1945 and certificated by the FAA, are required to
have airspeed indicators that conform to a standard
color-coded marking system. This system enables the
pilot to determine at a glance certain airspeed limitations,
which are important to the safe operation of the
aircraft. For example, if during the execution of a
maneuver, the pilot notes that the airspeed needle is in
the yellow arc and is rapidly approaching the red line,
immediate corrective action to reduce the airspeed
should be taken. It is essential that the pilot use smooth
control pressure at high airspeeds to avoid severe
stresses upon the glider structure. [Figure 4-3]
The following is a description of the standard color-code
markings on airspeed indicators.
• FLAP OPERATING RANGE (the white arc).
• STALLING SPEED WITH THE WING FLAPS
AND LANDING GEAR IN THE LANDING
POSITION (the lower limit of the white arc).
• MAXIMUM FLAPS EXTENDED SPEED (the
upper limit of the white arc). This is the highest
airspeed at which the pilot should extend full
flaps. If flaps are operated at higher airspeeds,
severe strain or structural failure could result.
• NORMAL OPERATING RANGE (the green arc).
• STALLING SPEED WITH THE WING FLAPS
AND LANDING GEAR RETRACTED (the
lower limit of the green arc).
• MAXIMUM STRUCTURAL CRUISING
SPEED (the upper limit of the green arc). This is
the maximum speed for normal operation.
• CAUTION RANGE (the yellow arc). The pilot
should avoid this area unless in smooth air.
• NEVER-EXCEED SPEED (the red line). This is
the maximum speed at which the glider can be
operated in smooth air. This speed should never
be exceeded intentionally.
OTHER AIRSPEED LIMITATIONS
There are other important airspeed limitations not
marked on the face of the airspeed indicator. These
speeds are generally found on placards in view of the
pilot and in the GFM/POH.
MANEUVERING SPEED is the maximum speed at
which the limit load can be imposed (either by gusts
or full deflection of the control surfaces) without causing
structural damage. If during flight, rough air or
severe turbulence is encountered, the airspeed should
be reduced to maneuvering speed or less to minimize
the stress on the glider structure. Maneuvering speed is
not marked on the airspeed indicator.
Other important airspeeds include LANDING GEAR
OPERATING SPEED, the maximum speed for extending
or retracting the landing gear if using glider equipped
with retractable landing gear; the MINIMUM SINK
SPEED, important when thermaling; and the BEST
GLIDE SPEED, the airspeed that results in the least
amount of altitude loss over a given distance not considering
the effects of wind. MAXIMUM AEROTOW or
GROUND LAUNCH SPEED is the maximum airspeed
that the glider may safely be towed without causing structural
damage.
The following are abbreviations for performance
speeds.
VA—design maneuvering speed.
VC—design cruising speed.
VF—design flap speed.
Figure 4-3. Airspeed indicator with color markings.
4-4
VFE—maximum flap extended speed.
VLE—maximum landing gear extended speed.
VLO—maximum landing gear operating speed.
VNE—never-exceed speed.
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Glider Flying Handbook(33)
