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QUESTION: Why are aviation gyro instruments powered by the vacum pumps, as opposed to
a pressure pump? No one I know seems to have an idea.
Thanks
ANSWER: I'm no expert by any means, but I'll take a wild (though hopefully
somewhat logical) guess at why airplanes have vacuum pumps instead of
pressure pumps.
As you know by walking around the flightline where older aircraft are
parked, a number of them have external venturis that are placed in either
the aircract slipstream or the propeller wash or both. Air passing through
the narrowed portion of the venturi tube accellerates and its pressure
drops (partial vacuum relative to ambient air). This vacuum can then draw
air through gyro intruments to operate them. I guess the insturment
doesn't know or care whether the airflow past its vanes is due to pressure
IN or a vaccum OUT -- actually, from a standpoint of physics, its exaclty
the same thing.
Why current instruments retain the vacuum operation, I don't know. Perhaps
the early designs for venturi tube operations were kept for economic or
other reasons, with the external venturi being replaced by a vacuum pump.
Maybe (again, just a guess), the idea was to have a powered vacuum pump
with a "passive" venturi back-up in case of failure.
Then again, why couldn't early instruments have been driven by PRESSURE,
possibly drawn from some ram-air source on the aircraft. I'd hazard a
guess that ram-air sources mught be more variable due to angle of attack,
whereas venturi tubes might provide more constant vacuum in all attitudes.
Of course, this still begs the question of why modern aircraft don't have
a pressure pump. Maybe (and here I go out on a limb again), vacuum is
better for PRESSURIZED aircraft. Seems a pressure pump might have to be
more powerful to take thin ambient air, then boost it to a relative
pressure OVER that in the pressurized cabin, whereas a vacuum pump might
not have to be as powerful since part of its work is already done becuase
the ambient air is at a lower pressure than the cabin air -- the vacuum
pump in this case would just lower the ambient pressure a little more (or
maybe not sat all.)
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