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Gyro instruments are basic instruments used in all aircraft. They provide the pilot with critical attitude and directional information and are especially important when flying under IFR. Power sources for these instruments may vary. The main requirement is to rotate the gyroscope at high speed. Originally, gyroscopic instruments were strictly vacuum driven. A vacuum source drew air over the gyroscope inside the instruments to rotate the gyroscope. Later, electricity was added as a power source. The rotating armature of the electric motor doubles as a gyro rotor. In some aircraft, pressure, not vacuum, is used to turn the gyroscope. Various power systems and configurations have been developed to ensure reliable operation of gyroscopic instruments.
Aircraft Vacuum System
Vacuum systems Vacuum systems are very common for driving gyroscope instruments. In a vacuum system, a stream of air directed at the rotor blades rotates the rotor at high speed. The action is similar to a water wheel. Atmospheric pressure air is first drawn through the filter(s). It is then directed to the instrument and directed to the blades on the gyroscope rotor. The suction line leads from the instrument housing to the vacuum source. From there, the air is discharged to the sea. Either venturi or a vacuum pump can be used to provide the vacuum needed to turn the rotors of the gyro instruments.
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The vacuum value required to operate the instrument is generally between 3½ inches to 4½ inches of mercury. It is usually adjusted using a vacuum valve located in the supply line. Some roll and pitch indicators require a lower vacuum setting. This can be accomplished by using an additional control valve in the turn and coast vacuum supply line.
Venturi Tube Systems The velocity of air passing through a Venturi tube can create enough suction to turn an instrument's gyroscope. A line is run from the gyro instruments to the throat of a venturi tube mounted on the outside of the airframe. The low pressure in the venturi draws air through the instruments, rotates the gyroscope and blows the air overboard through the venturi. This gyroscopic power source was used on many simple and early aircraft.
Light, single-engine aircraft may be equipped with 2-inch venturi (2 inches of mercury vacuum capacity) to operate the roll and pitch indicator. It may also have larger 8-inch venturi to power the position and heading indicators. Simplified illustrations of these venturi vacuum systems are shown in Figure 1. Normally, the air entering the instruments is filtered.
The advantages of the venturi tube as a suction source are relatively low cost and simplicity of installation and operation. It also requires no electricity. But there are serious limitations. The venturi is designed to produce the desired vacuum at approximately 100 mph under standard sea level conditions. Large variations in air speed or density cause the suction developed to vary. Air flow can also be impeded by ice that can form on the venturi tube. In addition, since the rotor does not reach normal operating speed until after takeoff, operational checks of venturi-driven gyro instruments cannot be performed. For these reasons, alternative sources of vacuum energy have been developed.
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Motor Vacuum Pump The motor vacuum pump is the most common source of vacuum for gyroscopes installed in light general aviation aircraft. One type of motor pump is directed to the motor and connected to the lubrication system to seal, cool and lubricate the pump. Another commonly used pump is the dry vacuum pump. It works without external lubrication and installation does not require connection to the engine oil supply. It also does not require the air oil separator or check valve found in wet pump systems. In many other respects, a dry pump system and an oil-lubricated system are the same. [Figure 2]
When a vacuum pump develops a vacuum (negative pressure), it also creates a positive pressure at the outlet of the pump. This pressure is compressed air. It is sometimes used to work with pressurized gyro instruments. The components for the pressure systems are generally the same as those for the vacuum system as listed below. In other cases, the pressure developed by the vacuum pump is used to inflate deicing boots or inflatable seals or is dumped overboard.
The advantage of motorized pumps is their constant performance on the ground and in flight. Even at low engine speeds, they can produce more than enough vacuum so that a regulator is needed in the system to continuously ensure proper vacuum of the vacuum instruments. As long as the engine is running, a relatively simple vacuum system adequately rotates the instrument's gyroscope for accurate indications. However, engine failure, especially on single-engine aircraft, can leave the pilot without attitude and heading information at a critical moment. To avoid this shortcoming, the yaw and pitch indicator often works with an electrically powered gyroscope that can be briefly powered by a battery. So when combined with the aircraft's magnetic compass, sufficient position and heading information is still available.
Multi-engine aircraft usually contain independent vacuum systems for the pilot and co-pilot instruments driven by separate vacuum pumps on each of the engines. In the event of engine failure, a vacuum system driven by the still-running engine provides a complete set of gyro instruments. An interconnect valve can also be installed to connect failed instruments to a still working pump.
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Typical Pump-Driven System The following components are found in a typical gyro-driven vacuum system. A brief description of each of them is given. See pictures for detailed illustrations.
Air-oil separator - the oil and air in the vacuum pump are drained through the separator, which separates the oil from the air; the air is released from the sea and the oil is returned to the crankcase. This component is not present when a dry type vacuum pump is used. The self-lubricating nature of the pump vanes requires no oil.
Vacuum regulator or suction valve - since the system capacity is greater than required for instrument operation, the adjustable vacuum regulator is set to the desired instrument vacuum. Excess suction in the instrument lines is reduced when the spring-loaded valve is opened to atmospheric pressure. [Figure 3]
Figure 3. A vacuum regulator, also known as a suction valve, includes a foam filter. To remove the vacuum, higher pressure outside air must be drawn into the system. This air must be clean to avoid damage to the pump
Aircraft Gyroscopic Instruments
Gate Check Valve - prevents possible damage to the instrument by firing back the engine which reverses the flow of air and oil from the pump. [Figure 4]
Pressure Relief Valve - As reverse air flow from the pump closes both the check valve and the suction relief valve, the resulting pressure can break the lines. A pressure relief valve releases positive pressure to atmosphere.
Selector Valve - In twin-engine aircraft having vacuum pumps driving both engines, an alternate pump may be selected to provide vacuum in the event of engine or pump failure, with a built-in check valve to shut down the failed pump.
Limiting Valve - Since the roll pin of the yaw and pitch indicator operates at a lower vacuum than that required by other instruments, the main line vacuum must be reduced for use with this instrument. This function is performed by the line limit valve. This valve is either a needle valve or a spring-loaded check valve that maintains a constant, reduced vacuum for the roll and pitch indicator.
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Air Filter - The main air filter keeps foreign matter out of the air passing through all gyro instruments. It is an extremely import filter that needs regular maintenance. A clogged main filter reduces airflow and causes a lower reading on the intake gauge. Each instrument is also equipped with individual filters. In systems without a main filter that rely only on individual filters, filter clogging will not necessarily show up on the suction gauge.
Suction pressure gauge - a pressure gauge that shows the difference between the pressure inside the system and atmospheric or cockpit pressure. It is usually calibrated in inches of mercury. The desired vacuum and the minimum and maximum limits vary according to the design of the gyro system. If the desired vacuum is for
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