Variometer instruments measure the vertical ascent or descent of the local air mass and glider combined and displays that information as speed. The principles of operation are similar to the altimeter. The variometer is viewed as a critical part of the glider’s instrument display, giving the pilot information on performance of the glider while flying through the atmosphere. The variometer depends upon the pressure lapse rate in the atmosphere to derive information about rate of climb or rate of descent. A non-electric variometer uses a separate insulated tank (thermos or capacity flask) as a reference chamber. The tubing is plumbed from the reference chamber through the variometer instrument to an outside static port. [Figure 4-20]
A variometer with tubing is connected to an outside static port is uncompensated. The internal hairspring mechanism determines sensitivity of the variometer. The variometer has a very rapid response due to the small mass and lightweight construction of the moving parts. [Figure 4-21]
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. 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 vertical descending air mass or sink which is falling air that forces the glider to lose height. Figures 4-22 and 4-23 illustrate how the variometer works in level flight and while the glider is ascending. In addition, Figure 4-24 illustrates certain flight maneuvers that cause the variometer to display changes in altitude.
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 4-25]
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 (STF) between thermals. If expected rate of climb is low, optimum interthermal cruise airspeed is relatively low. When expected lift is strong, however, optimum interthermal cruise airspeed is much faster. [Figure 4-26] A MacCready ring has single pilot values in front and dual pilot values for the second seat instrument. Marked for specific aircraft, some may have empty ballast and full ballast values.
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 air mass is rising or descending despite changes in airspeed.
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 or simply as a slot or pair of holes on the back side of a quarter inch vertical tube. When airspeed increases, more suction from the venturi moderates (offsets) the pressure at the static outlet of the variometer. Similarly, when airspeed decreases, reduced suction from the venturi moderates (offsets) 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. To maximize the precision of this compensation effect, the total energy probe needs to be in undisturbed airflow ahead of the aircraft nose or tail fin (the “Braunschweig tube”, the long cantilevered tube with a kink in the end that can be seen projecting from the leading edge of the tail fin on most modern sailplanes.) [Figure 4-27]
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 reference chamber (thermos or 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 4-28]
A variometer that indicates the vertical movement of the air mass, regardless of the glider’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 4-28]
Computerized (electronic) 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 air mass itself.