Gyroscopes 3
Principle of Operation
With a very rigid gyro, control systems can be cut out and the gyro left on its own for long periods
The pendulous vanes of the air driven unit are replaced by mercury tilt switches mounted on the rotor case, the inner gimbal. Any displacement of the gyro axis from earth vertical is sensed by the tilt switches which make and break electrical circuits connected to torque motors on the gimbals, that re-erect the system at about 5° a minute.
Electric AHs usually have complete freedom in roll, but are restricted to about ±85° in pitch. A fast erection button is provided which supplies a higher voltage to the torque motors, and bypasses the cut-outs, to erect the gyro at up to 180° a minute. This facility should only be used on the ground or in straight and level flight.
The pitch switch, here on top of the rotor case, senses pitch errors, and drives the pitch torque motor, on the roll axis. This motor tries to roll the gyro, but precession takes over, and the effect is transferred through 90° to correct pitch errors. Roll errors are detected by a second mercury switch lying at right angles to the pitch switch, and corrective signals are fed to a roll torque motor on the pitch axis. Errors due to false erection during acceleration are accepted, as acceleration regimes are relatively short, as in a take-off.
Servo Driven Attitude Indicators Modern aircraft take their attitude information from the intertial reference systems (IRS), which are free of turn and acceleration errors, and the instrument is only a remote indicator. This system has the advantage that the attitude signals going to the Captain’s and first officer’s instruments from IRS 1 and IRS 2 can be compared, errors can be detected and, if required, a failed attitude source can be deselected. IRS 3 is the back-up source. In older systems with just two inertial navigation systems (INS) a master attitude reference gyro, which is just a big independent AH, is used as a back up. An independent and self-contained standby AH is always retained, powered directly from the aircraft batteries.
However, aircraft spend a long time in turns. To prevent the gyro erecting to a false datum in an extended turn there is a roll cut-out switch fitted on the roll axis, to disconnect the roll torque motor at bank angles in excess of 10°. A development of this system dispenses with the roll cut-out switch, but uses a pair of modified mercury switches which provide power to the pitch and roll erection systems under small accelerations, but then disconnect the power supply under larger accelerations or bank. Figure 3.14 elaborates.
Remote artificial horizons, or attitude indicators, can also be provided with expanded scales for pitch movements near the horizontal. This means that very precise attitudes can be set. Bank indications can be at the bottom of the instrument, in which case it is referred to as an earth pointer, or at the top, a sky pointer. Information from other aircraft systems can also be displayed. This includes ILS information, radio height, airspeed and flight directors. Figure 3.15 shows a relatively modern sky pointing attitude indicator.
Figure 3.14
Instrumentation
v6.1.5
3.11