Introduction to Glider Flying > Flight Instruments > Pitot-Static Instruments > Variometer
The variometer gives the glider pilot information
on performance of the glider while flying through the atmosphere.
The variometer operates on the same prin-ciple as the altimeter,
however, it indicates rate of climb or descent instead of vertical
distance. The vari-ometer depends upon the pressure lapse rate
in the atmosphere to derive information about rate of climb
or rate of descent. Most non-electrical variometers use a separate
insulated tank, such as a Thermos or capac-ity flask, as a reference
chamber. The tubing is plumbed from the reference chamber through
the vari-ometer 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.
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 vari-ometer 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
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 imbal-ance
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
prin-ciple. A tiny vacuum cavity on a circuit board is sealed
with a flexible membrane. Variable resistors are embed-ded in
the membrane. When pressure outside the cavitychanges, minutealterations
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 orrate 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]
Figure 4-9. When an electric variometer
is mounted in the glider a non-electric variometer is usually
installed as a backup.
Some variometers are equipped with a rotatable
rim speed scale called a MacCready ring. This scale indicates
the opti-mum 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 var-iometer 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
Figure 4-10. The MacCready ring.
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 indi-cates
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