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Introduction to Glider FlyingFlight 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 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 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 4-10]

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 indication.

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