Vacuum-Gyro Instruments; Pitot-Static Instruments - Piper PA-24 Comanche Service Manual

PIPER COMANCHE SERVICE MANl:AL
SECTIO\ XI
I!'STRUME!'TS
•
II-I. Description. The instrumentation in the Comanche is designed to give a quick and actual indication
of the attitude. performance and condition of the airplane. The instrument panel has been arranged to
accomodate all the advanced flight instruments on the left side in front of the pilot and required engine and
miscellaneous instruments on the right side. The instruments are divided into four groups: Vacuum.
Pitot-Static, Electrical and Miscellaneous. Some of the instruments are components of indicating systems
that indicate conditions at remote parts of the airplane. A few of the instruments. however. are self-contained
and merely have to be correctly installed to give an indication. Instruments requiring power from the
electrical system are provided with circuit breakers to isolate the individual systems in the event of trouble,
11-2. Instruments.
11-3.
Vacuum-Gyro
Instruments. The directional gyro is a night instrument incorporating an air
driven
gyro stabilized in the vertical plane. The gyro is rotated at a high rate of speed by lowering the pressure in
the air tight case and simultaneously allowing atmospheric air pressure to enter the instrument against the
gyro buckets. Due to gyroscopic inertia. the spin axis continues to point in the same direction even though
the aircraft yaws to the right or left. This relative motion between the gyro and the instrument case is
shown on the instrument dial which is similar to a compass card. The dial. when set to agree with the airplane
magnetic compass. provides a positive indication free from swing and turning error. •
The gyro horizon is essentially an air driven gyroscope rotating in a horizontal plane and is operated
the same principal as the directional gyro. Due to the gyroscopic inertia. the spin axis continues to point i
the vertical direction. providing a constant visual reference to the attitude of the airplane relative to pitch
and roll axis. A bar across the face of the indicator represents the horizon. A miniature adjustable airplane
is mounted to the case and aligning the miniature airplane to the horizon bar simulates the alignment of the
airplane to the actual horizon. Any deviation stimulates the deviation of the airplane from the true horizon.
The gyro horizon is marked for different degrees of bank.
The suction gauge. a pan of the airplanes vacuum system. is mounted in the right side of the instrument
panel. This gauge is calibrated in inches of mercury and indicates the amount of vacuum created by the engine
driven vacuum pump.
11-4. Pltot-Static Instruments. The airspeed indicator provides a means of indicating the speed of the
airplane passing through the air. The airspeed indication is the differential pressure reading between pitot
air pressure and static air pressure. This instrument has the diaphragm vented to the pitot air source and the
case is vented to the static air system, As the airplane increases speed. the pitot air pressure increases.
causing the diaphragm to expand. A mechanical linkage picks up this motion and moves the instrument
pointer to the indicator speed. The instrument dial is calibrated in knots and miles per hour. and also has
the necessary operating range markings for safe operation of the airplane.
The altimeter indicates pressure altitude in feet above sea level, The indicator has three pointers and a
dial scale. The long pointer is read in hundreds of feet. The middle pointer is read in thousands of feet and
ISSl'ED: 8/18/72
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