The Electric Variometer System


*A. Gene Moore

This discussion assumes that the soaring pilot is already convinced of the value of total energy compensation in his variometer system, and that he understands the basic operating principle of the diaphragm type compensator.

In order for a sailplane to fly through the air, energy must be exchanged for distance. The available energy is in two forms, potential (mass x height) and kinetic (1/2 mass x velocity**2). At his discretion, the pilot can readily exchange one form of energy for the other. By zooming, a pilot decreases his velocity and increases his height. He is trading kinetic for potential energy. Although he is momentarily higher, he has not increased his total energy, and he will hit the ground at almost the same time that he would from his lower but faster position.

During the zoom maneuver with an uncompensated variometer, the pilot suffers from an information "black out." It is impossible for him to tell whether he is in lift or sink while the air speed is changing. He is often fooled by what is called a "stick thermal," and he continues to circle in sink until the air speed becomes steady and the variometer settles down and shows that he is actually going down. A total energy compensated variometer system is required to cure this problem. One compensating scheme makes use of an operational element (diaphragm and spring) connected between the pitot and the variometer. This system is responsive to both rate of change of air speed and rate of climb (rate of kinetic and potential energy changes). These two measurements are summed by the operational element in such a way that the output indication on the variometer shows the instantaneous rate of total energy change. This makes it possible for the pilot to optimize his position in lift even while changing air speed.

Following the system concept, a new diaphragm type total energy compensator together with a dynamic bench calibrator will be described. If the static error of a particular sailplane is known, it is now possible to bench tune and test a total energy compensated variometer system with predictable flight results. The circuits and equipment to be described have resulted from a mathematical model of the sailplane - total energy system, which was worked out by Wil Schuemann. Wil hopes to give a paper on this work at the OSTIV meeting in 1970.

Historically, variomieters have been sold as component parts; the same as total energy compensators, restrictions, and even sailplanes. As glider pilots we assemble these components into a system. When the system doesn't "play" and performance is less than expected, we do the natural thing and blame it on the manufacturer whose component (the indicator) costs the most money.

The popularity of the electric variometer is due first of all to its rapid response, and second to the fact that it will drive an audio attachment. It is the speed or fast response of the electric variometer that causes most of the system problems. These problems are caused by the size and location of the restrictions used to slow up the system. We must stop thinking of our variometer as a single instrument. Instead, we should think of the whole system, which includes the sailplane, the restrictions, the total energy diaphragm, and the variometer indicator. Again, this is a system.

[Figure 1]

The variometers used in modern sailplanes are extremely sensitive instruments. For example, if you hold a brass tube 1/8" I.D. by 1" in length between your fingers, as shown in Figure 1, vertically in the gravitational field, the heat from the fingers will cause an air flow due to thermal convection through the tube that is roughly equivalent to the flow from a one pint flask when climbing 100 feet per minute. As you can imagine, the blast of air coming out of the brass tube won't blow the hat off your head. However, we expect our instruments to be able to measure signals of this level and even much less, and they do just that.

[Figure 2]

Figures 2 through 5 will provide some background on the types of variometers available today. Figure 2 shows the "Leaky Capsule" type variometer. This is the basic principle used in the standard aircraft rate of climb instrument and the Ball Electric Variometer. It operates by measuring the pressure drop across a fixed restriction (R) placed between the reference pressure capacity (C) and the static ports on the sailplane, The pressure drop across (R), which is proportional to rate of climb or descent, is measured by the sensing capsule.

[Figure 3]

Figure 3 shows a vane type variometer. The PZL uses this principle.

The principle is that the pressure drop across the restriction formed between the vane and the case is a measure of rate of climb. For any rate of climb, within the range of the instrument, there will be an equilibrium position of the vane where the torque in the hair springs balances the force on the vane. If you raise the system, air will pass from the capacity through the restriction, deflecting the vane, and give an "UP" indication.

[Figure 4]

Figure 4 shows a pellet type variometer similar to the Cosim. It has two small pellets in cross connected tapered tubes. In industry, this type is called a variable area flow meter. It operates on the principle of a fixed pressure drop across a restriction that varies in area. From the plumbing shown, you can see that the air flowing out of the reference chamber (C), if you raise the system, will raise the green ball, and the air will be exhausted through the static port of the sailplane. Now, if you lower the system the green ball will drop and back seat on the tapered tube forcing the air to go through the other branch lifting the red ball and allowing the air to flow into C.

[Figure 5]

Figure 5 shows a thermistor type variometer, and this is the one with which I am the most familiar. It operates by sensing the flow of air in and out of a fixed reference chamber (C) connected to the static source of the sailplane. The black dots are two very tiny thermistors. They are much smaller than the head of a pin, and in the Moore variometer they are located adjacent to each other, as shown, in the line connecting the reference chamber to the static source. The thermistors are configured in an electrical bridge circuit, and a voltage is applied to the bridge. The small current, about 15 ma, passing through the thermistors causes them to self heat approximately 100 degrees Centigrade above the ambient temperature of the system. There is now two hot beads, with the characteristic that their resistance changes quite drastically with temperature, operating in relatively cool air. When the flow of air is from the reference chamber to the static source, the right hand bead will be cooled and increase its resistance. There will also be a hot air bubble that will move downstream and actually warm the left hand thermistor. This is the case at very low flow. The change in temperature causes a change in resistance of the thermistors. The resistance change of the thermistors in the bridge circuit causes an output voltage at Eout. The output voltage is read in terms of climb or descent.

[Figure 6]

Figure 6 shows a complete variometer circuit. The two thermistors shown in Figure 5 are T1 and T2, and the rest of the schematic is a differential amplifier used to drive the meter and to tailor the characteristics of the circuit.

[Figure 7]

Figure 7 is the complete pneumatic circuit of a total energy variometer system. The important considerations in tuning this system are:

1. The sailplane should have an error free static source. If this is not the case, the static error must be determined in order to adjust the diaphragm compensator.

2. The altitude at which you wish the best compensation must be selected. The diaphragm type compensator (C1) can only be adjusted for one altitude. If it is on the nose at 3,000 feet, it will overcompensate below and undercompensate above this level.

3. The diaphragm compensator (C1) must be adjusted to exactly match the volume change required by C2 at the altitude where you want exact compensation. in practice a system tuned for 3,000 feet would work satisfactorily from ground level to 6,000 feet, but not at 10,000 feet (like over Marfa).

4. It will be necessary to add some restrictions to the system. R1 is normally added to cut down the effect of gusts hitting the pitot. But R1 works with C1 to form a time constant R1C1, and this makes it necessary to add R2 to the circuit so that R1C1 = R2C2. In fact, adding R2 to any electric variometer system that already has R1 will greatly improve the system. R3 should be left out if possible by making both R1 and R2 longer to slow down the system. Some starting values for R1 and R2 are I" x 0.020" capillary. If the system is still too fast, make them 2" x 0.020". This would be better than adding R3. In any event don't make R3 longer than one inch.

5. Put 3 Chore-Girl pot cleaners in C2 to act as heat sink material. The reason will be explained later.

[Figure 8]

Figure 8 shows the dynamic bench calibrator connected to a variometer system under test. With this calibrator it is very easy to tune or match the time constants R1Cl and R2C2 mentioned earlier.

The calibrator has a variable transformer for controlling the output air pressure (pitot) from the pump. The pitot pressure is divided by two precision restrictions R4 and R5. A virtual static point is formed at the junction between R4 and R5, and a standard rate of climb indicator is connected to the static point. An air speed indicator is connected between the pitot line and the static point. The variometer system as shown in Figure 7 is connected to the bench calibrator.

In operation, we can vary the airspeed on the calibrator and watch the rate of climb indicator showing what a sailplane would actually be doing if undergoing similar air speed changes. At the same time, the variometer indicator shows how well we are doing in tuning the pneumatic portion of the variometer system. We start by fixing R2C2 and adjusting R1C1 until the variometer reads a continuous zero for zooms and dives between 35 and 140 knots .

The only question left unanswered when the variometer system leaves the calibrator is the static error of the sailplane. Once this is pinned down for a particular model, a compensating correction can be made to R5. R5 also serves as the altitude adjustment. By increasing the pneumatic resistance of R5, we can raise the altitude at which compensation is optimum.

When tuning a variometer system with response in the one half to two second range, it is helpful to put heat sink material in the thermos.

[Figure 9]

Figure 9 shows the results of putting three Chore Girl pot cleaners in a one pint thermos used as the reference chamber for a variometer. The test shown is this. If you are climbing at 1,500 ft/min and level off, it will take the normal system nearly ten seconds before the indicator is below 100 ft/min. The air has expanded and cooled, but the glass wall of the thermos hasn't cooled as rapidly as the air, and as the air inside the flask warms back up to the glass temperature it continues to expand and flows through the variometer giving a false "up" reading.

With the heat sink material the expansion process is changed from near adiabatic to near isothermal. The variometer will now recover in two seconds instead of ten, an improvement of five to one.

[Figure 10]

Figure 10 shows the performance measurement of three total energy type diaphragms. This is the change in volume that is necessary with pressure or air speed to compensate a one pint reference bottle. The diagonal line represents the volumetric change with pressure needed from a diaphragm to exactly match a one pint bottle at sea level. The Burton was over-compensating in the low speed ranges, crossed at 50 or 60 miles per hour, and above 90 miles per hour was very inadequate. The next One is a PZL. You can see from the slope of the line that this particular compensator will not compensate a one pint bottle, but it does have a linear response. To get adequate compensation out of the PZL, you would need to go to a smaller bottle. The squares are the output plots from a Moore-Schuemann compensator. This test shows that it is possible to get a linear response from a diaphragm type calibrator over a wide speed range.

[Figure 11]

Figure 11 is the Moore-Schuemann total energy compensator. It has all of the features necessary for good total energy compensation. The thermos is filled with heat sink material. R2 is potted in place. C1 is adjusted for the selected altitude and the static error of the particular sailplane. R1 is cut to length so that R1C1 = R2C2 as determined on the beach calibrator. When the Moore-Schuemann compensator is connected between the pitot and the vario it is ready to fly.

The outcome of all this effort is summed up on 16 mm data film (running time six minutes). This film shows the actual testing of several variometer systems on the dynamic bench calibrator. The highlights are the tremendous improvement obtained by adding R2 to the garden variety total energy system and the near perfect performance of the Moore-Schuemann system when zooming from 120 to 35 knots air speed.

*-Mr. A. Gene Moore is Project Engineer for Electronic Products with Hercules Incorporated at Cumberland, Maryland. He is currently working full time on a recently patented fluidic angular rate sensor that he invented while working with electric variometers in his basement.

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