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plumbed from the reference chamber through the variometer
to an outside static port. By using different
hairsprings, the sensitivity of the variometer can be
controlled. The variometer has a very rapid response
due to the small mass and lightweight construction of
the moving parts.
Figure 4 -7. Types of altitude.
4-8
Pressure differences between the air inside the
variometer/reference chamber system and the air
outside of the system tend to equalize as air flows from
high pressure areas to low pressure areas. When pressure
inside the reference chamber is greater than the pressure
outside, air flows out of the reference chamber through
the mechanical variometer to the outside environment,
displacing a vane inside the variometer. The vane, in
turn, drives the needle to display a climb indication.
When air pressure outside the reference chamber is
greater than pressure inside, air flows through the variometer
and into the reference chamber until pressure is
equalized. The variometer needle indicates a descent.
[Figure 4-8]
Electric powered variometers offer several advantages
over the non-electric variety. These advantages include
more rapid response rates and separate audible signals
for climb and descent.
Some electric variometers operate by the cooling effect of
airflow on an element called a thermistor, a heat-sensitive
electrical resistor. The electrical resistance of the thermistor
changes when temperature changes. As air flows into or out
of the reference chamber, it flows across two thermistors
in a bridge circuit. An electrical meter measures the imbalance
across the bridge circuit and calculates the rate of
climb or descent. It then displays the information on the
variometer.
Newer electric variometers operate on the transducer principle.
A tiny vacuum cavity on a circuit board is sealed
with a flexible membrane. Variable resistors are embedded
in the membrane. When pressure outside the cavity
changes, minute alterations in the shape of the membrane
occur. As a result, electrical resistance in the embedded
resistors changes. These changes in electrical resistance
are interpreted by a circuit board and indicated on the
variometer dial as climb or descent.
Many electrical variometers provide audible tones or
beeps that indicate the rate of climb or rate of descent of
the glider. Audio variometers enhance safety of flight
because they make it unnecessary for the glider pilot to
look at the variometer to discern the rate of climb or rate
of descent. Instead, the pilot can hear the rate of climb or
rate of descent. This allows the pilot to minimize time
spent looking at the flight instruments and maximize time
spent looking outside for other air traffic. [Figure 4-9]
Some variometers are equipped with a rotatable rim speed
scale called a MacCready ring. This scale indicates the optimum
airspeed to fly when traveling between thermals for
maximum cross-country performance. During the glide
between thermals, the index arrow is set at the rate of climb
expected in the next thermal. On the speed ring, the variometer
needle points to the optimum speed to fly
between thermals. If expected rate of climb is slow,
optimum inter-thermal cruise airspeed will be relatively
slow. When expected lift is strong, however, optimum
inter-thermal cruise airspeed will be much faster.
[Figure 4-10]
Variometers are sensitive to changes in pressure altitude
caused by airspeed. In still air, when the glider dives, the
variometer indicates a descent. When the glider pulls out
of the dive and begins a rapid climb, the variometer indicates
an ascent. This indication is sometimes called a "stick
thermal." A glider lacking a compensated variometer must
be flown at a constant airspeed to receive an
accurate variometer indication.
TOTAL ENERGY SYSTEM
A variometer with a total energy system senses
changes in airspeed and tends to cancel out the resulting
climb and dive indications (stick thermals). This is
desirable because the glider pilot wants to know how
rapidly the airmass is rising or descending despite
changes in airspeed.
Figure 4-9. When an electric variometer is mounted
in the glider a non-electric variometer is usually
installed as a backup.
Figure 4-10. The MacCready ring.
4-9
A popular type of total energy system consists of a
small venturi mounted in the air stream and connected
to the static outlet of the variometer. When airspeed
increases, more suction from the venturi moderates the
pressure at the static outlet of the variometer. Similarly,
　
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