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when airspeed decreases, reduced suction from the
venturi moderates the pressure at the static outlet of the
variometer. If the venturi is properly designed and
installed, the net effect is to reduce climb and dive indications
caused by airspeed changes.
Another type of total energy system is designed with a
diaphragm-type compensator placed in line from the
pitot tube to the line coming from the capacity flask.
Deflection of the diaphragm is proportional to the
effect the airspeed change has on pitot pressure. In
effect, the diaphragm modulates pressure changes in
the capacity flask. When properly adjusted, the
diaphragm compensator does an adequate job of masking
stick thermals. [Figure 4-11]
NETTO
A variometer that indicates the vertical movement of
the airmass, regardless of the sailplane’s climb or
descent rate, is called a NETTO variometer system.
Some NETTO variometer systems employ a calibrated
capillary tube that functions as a tiny valve. Pitot pressure
pushes minute quantities of air through the valve
and into the reference chamber tubing. The effect is to
remove the glider’s sink rate at various airspeeds from
the variometer indication (polar sink rate). [Figure 4-
12]
Electronic, computerized NETTO variometers employ
a different method to remove the glider performance
polar sink rate from the variometer indication. In this
type of system, sensors for both pitot pressure and
static pressure provide airspeed information to the
computer. The sink rate of the glider at every airspeed
is stored in the computer memory. At any given airspeed,
the sink rate of the glider is mathematically
removed, and the variometer displays the rate of ascent
or descent of the airmass itself.
Figure 4-11. A total energy variometer system.
4-10
ELECTRONIC FLIGHT COMPUTERS
Electronic flight computers are found in the cockpits of
gliders that are flown in competition and cross-country
soaring. Since non-powered gliders lack a generator or
alternator, electrical components, such as the flight
computer and VHF transceiver, draw power from the
glider battery or batteries. The battery is usually a 12 or
14 volt sealed battery. Solar cells are sometimes
arrayed behind the pilot or on top of the instrument
panel cover to supply additional power to the electrical
system during flight in sunny conditions.
The primary components of most flight computer
systems are an electric variometer, a coupled
Global Positioning Satellite (GPS) receiver, and a
microprocessor. The variometer measures rate of climb
and descent. The GPS provides position information. The
microprocessor interprets altitude, speed, and position
information. The microprocessor output aids the pilot
in cross-country decision-making. [Figure 4-13]
The GPS-coupled flight computer can provide you with
the following information.
• Where you are.
• Where you have been.
• Where you are going.
• How fast you are going there.
• How high you need to be to glide there.
• How fast you are climbing or descending.
• The optimum airspeed to fly to the next area of
anticipated lift.
• The optimum airspeed to fly to a location on the
ground, such as the finish line in a race, or the
airport of intended landing at the end of a
cross-country flight.
The primary benefits of the flight computer can easily
be divided into two areas: navigation assistance and
performance (speed) enhancement.
Fundamental to the use of the flight computer is the
concept of waypoint. A waypoint is simply a point in
space. The three coordinates of the point are latitude,
longitude, and altitude. Glider races and cross-country
glider flights frequently involve flight around a series
of waypoints called turnpoints. The course may be an
out-and-return course, a triangle, a quadrilateral or
other type of polygon, or a series of waypoints laid out
more or less in a straight line. The glider pilot must
navigate from point to point, using available lift
sources to climb periodically so that that flight can
continue to the intended goal. The GPS-enabled flight
computer aids in navigation and in summarizing how
the flight is going. When strong lift is encountered, and
if the pilot believes it is likely that the strong lift source
may be worth returning to after rounding a turnpoint,
the flight computer can "mark" the location of the thermal.
Then the glider pilot can round a nearby turnpoint
and use the flight computer to guide the return to the
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Glider Flying Handbook(38)
