Many aircraft use air-driven gyro
instruments as either the primary gyro instruments or as backup instruments
when the primary gyros are electrically driven.
For many years all of the
air-driven gyro instruments used an engine-driven vacuum pump to evacuate the
instrument case, and filtered air was pulled into the instrument to spin the
gyro. The reason for this was that it was much easier to filter air being
pulled into the instrument than it was to filter the air after it had been
pumped by an engine-driven pump lubricated by engine oil. The output of these
pumps always contained some particles of the oil.
Pressurized aircraft created
extra problems for suction-operated instruments, and the latest generations of
air-driven gyros now almost all use pressure. Turbine-powered aircraft bleed
some of the pressure from the engine compressor, regulate and filter it, and
then direct it over the gyros. Aircraft with reciprocating engines use engine driven
air pumps to provide the airflow for the gyros. This air is regulated and
filtered before it is ready for the instrument.
Figure 1.9: Vane-type air pump
There
are two types of air pumps used to provide instrument airflow, and both are
vane-type pumps. Sliding vanes are rotated by the driveshaft and as the shaft
turns, the chambers located at positions A and B become larger, while those at
positions C and D decrease in size (Figure
1.9). Air is pulled into the pump at the position the chambers enlarge, and
it is moved out as they decrease. "Wet" vacuum pumps use steel vanes
moving in a cast-iron housing and are sealed and lubricated by engine oil
metered into the inlet air port. This oil is discharged with the air and is
removed with an oil separator before the air is either used for inflating
de-icer boots or is pumped overboard. See Figure
1.10.
Figure 1.10:
Vacuum system using a wet-type vacuum pump
The more modern instrument air systems use "dry"
pumps (Figure 1.11) that have carbon
vanes and rotors and require no external lubrication. These pumps may be used
to drive the instruments by producing a vacuum and pulling air through them,
as we see in Figure 1.12, or buying
using the output of the pump to force the air through the instruments, Figure 1.13.
Figure 1.11: Dry-type air pump
Figure 1.12: A vacuum system
for the instruments of an aircraft using a dry-type air pump
Figure 1.13: A pressure system
for the instruments of an aircraft using a dry-type air pump
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