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From: "Gary Casey" <glcasey@adelphia.net>
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Subject: Re:  Vacuum supply for backup gyros
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<<How about the old tested method using an outside venturi?>>

I like this idea, and I once designed a "modern" configuration that had a
heated inlet lip to prevent icing and was highly streamlined for minimum
drag.  This is an external component to build and will add a certain amount
of drag.  The vacuum is not available on the ground so one has to have faith
that the backup gyros will spin up shortly after takeoff.  Unfortunately
some electrical failures happen then as well.  One could design the venturi
into existing drag-producing shapes, such as gear leg fairings, etc.
Existing low pressure areas don't generate the required vacuum without
adding something - like a secondary venturi.  The good thing about an
external venturi is that it doesn't rely on the engine in any way.  It does
require a pressure regulator if accurate gyro indications are expected at
all airspeeds.

Gary Casey