Gyroscopic instruments are used on all aircraft where they provide the pilot with critical attitude and directional information, especially when flying under instrument flight rules. The power sources of these instruments can vary, as the end goal is simply to spin the gyroscopes at a high speed. This blog will cover the three most common sources of power for gyroscopic systems: vacuum, pressure, and electrical.
Vacuum systems are by far the most common type of system used to drive gyro instruments. In a vacuum system, a stream of air directed against the rotor vanes causes the rotor to turn at high speeds, similar to a water wheel. Air at atmospheric pressure is drawn in through one or more filters before being routed into the instrument and directed at the gyro’s rotor vanes. From here, a suction line leads the air from the instrument case to the vacuum source where it is vented overboard.
Because vacuum systems are so widely used, there are many different types of them. The most common are venturi tube systems, engine-driven vacuum pumps, and twin-engine aircraft systems. In a venturi tube system, the instrument gyros are spun by suction created by the velocity of the air rushing through the venturi. A line is run from the gyro instruments to the opening of the venturi located on the outside of the airframe. The low pressure in the venturi tube pulls air through the instruments, thereby spinning the gyros, expelling the air overboard through the venturi. The venturi tube system is used on many simple, older aircraft.
The engine-driven vacuum pump is the most common source of vacuum for gyros in light, general aviation aircraft. In this system, a vacuum pump develops negative pressure inside the pump, which results in positive pressure at the outlet of the pump. This pressure is compressed air, which is then used to operate pressure gyro instruments. The advantage of engine-driven pumps is their consistent performance both on the ground and in flight. Even at low engine power, they can produce more than enough vacuum so that a regulator in the system is needed to continuously provide accurate suction to the vacuum instruments.
Twin-engine aircraft vacuum systems are the most complicated of all. These feature an engine-driven vacuum pump on each engine. The associated lines and components for each pump are kept isolated from each other and act as two independent systems. The vacuum lines are routed from each pump through a relief valve, a check valve, and finally to the four-way selector valve. The four-way valve allows either of the two pumps to supply a vacuum manifold. Full vacuum power flows to both the lines to the artificial horizons and the directional gyro. From here, lines are routed to the vacuum gauge via a turn and bank selector valve with three positions: main, left turn and bank , and right turn and bank. In the main position, the vacuum gauge denotes the vacuum in the lines of the artificial horizons and directional gyro. In the other positions, the lower value of vacuum for the turn and bank indicators is indicated.
Gyroscopic instruments are finely balanced devices that feature jeweled bearings which must be kept clean in order to function properly. When early vacuum systems were developed, only oil lubricated pumps were widely available. Even while using air-oil separators, the pressure outputs of these pumps still contain traces of oil, dirt, and other contaminants. As such, the method of drawing clean air through the gyro instruments with a vacuum system as opposed to using pump output pressure was preferred, as the latter risked contamination. Pressure gyro systems were not made possible until the development of self-lubricated dry pumps.
Pressure-driven gyros are favorable for use in high altitudes, where they are most efficient. They are similar to vacuum systems and utilize the same components, but are designed for pressure rather than vacuum. Therefore, instead of a suction relief valve, they feature a pressure regulator. Filters are still critical in preventing damage to the gyros. In a normal pressure-driven system, air is filtered at the inlet and outlet of the pump.
The working principle for electrically driven gyroscopic instruments in which the gyro rotor is spun by an electric motor is relatively simple: A spinning motor armature acts as a gyroscope. Gyros of the electronic type have the distinct advantage of having backup battery power. This allows the gyro to continue to work for a limited time in the event a generator fails or an engine is lost. Because there is no air being sent through the gyro to spin the rotor, contamination worries are also virtually eliminated. Furthermore, the lack of a need for vacuum pumps, plumbing, and vacuum system components decreases weight.
On small, single-engine aircraft, electric turn and bank or turn coordinators are frequently paired with vacuum-powered attitude and directional gyro instruments to provide an additional system. The reverse of this configuration can also be done. By combining the two types of instruments in the instrument panel, the pilot has a wider range of options. On more complex multi engine aircraft, the reliability offered by redundant electrical systems allows pilots to make use of all electric-powered gyro instruments possible. It is important to note that electric gyro instruments will feature some type of indicator on the dial face to denote when the instrument is not receiving power. In most cases, this indicator comes in the form of a red flag or another mark of some sort.
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