Piper SEMINOLE PA-44-180, Maintenance Manual

Page: 498 / 881

DIRECTIONAL GYRO - AIR DRIVEN/ELECTRIC.
Both air and electrical directional gyros are used, depending on the option package installed. Both types are
displacement type gyros with "f r e e" rotors mounted in gimbal assemblies. It is important that if a magnetic
slaving system, flight director or autopilot is coupled to the DG, the A. F. C. S. manufacturer's Service Manual
should be consulted.
AIR DRIVEN DIRECTIONAL GYRO.
The air driven directional gyro is driven by the pneumatic system which is supplied by engine driven dry-
pneumatic pumps either on pressure or vacuum. It is of prime importance to realize that air VOLUME, and not
air pressure, spins the gyro rotor. The air filter can become contaminated and restrict airflow, reducing gyro rotor
speed, while the pressure regulator will automatically adjust air pressure within proper limits.
— Note —
The gyro air filter must be clean or replaced before adjusting gyro air pressure.
Airflow directed at the gyro rotor vanes, causes the rotor to spin approximately 17,000 to 22,000 RPM, thus
providing the gyroscopic ability to remain rigid in space. The instrument case moves freely about the spinning
gyro rotor in three dimensions by the use of a Gimbal Assembly and the displacement or Azimuth readings are
presented on the instrument face. This results in a positive and stable presentation.
Since the directional gyro has no reference to magnetic north, it must be set from the magnetic compass. The
directional gyro will agree only with the magnetic heading from which it was set, since an other subsequent
magnetic compass headings are subject to deviation, northerly turning, acceleration, deceleration dip and other
errors. Due to precession, inherent or apparent, the directional gyro must be caged at least every 15 minutes while
in a level attitude, even though drift may not appear to ensure rotor position is correct in relation to earth's
surface.
ELECTRICALLY DRIVEN DIRECTIONAL GYRO.
These gyros contain rotors which are electrically driven, with the gyro rotor acting as the armature of an
electric motor. To eliminate the friction of brush assemblies which would induce abnormal precession, the
rotor/armature is inductively excited. The electric directional gyro is subject to the same operational requirements
of the air driven directional gyro, except for the method of obtaining rotor rotation and the design of the erection
mechanism.
PA - 4 4 - 1 8 0 / 1 8 0 T
AIRPLANE MAINTENANCE MANUAL
3 4 - 2 2 - 0 2
Page 34-08
Revised: May 15, 1989
2J2
PIPER AIRCRAFT