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I have come into possession of some of my grandfather's things including some aircraft parts that he had. He was a naval pilot who flew single-engine fighters in WW2 and then at some point transitioned to flying helicopters (some sort of Sikorsky) which he piloted in the Korean war. After he retired from the Navy he continued to fly as a civilian contractor for the Air Force until around the 1960s. It may have come from one of those aircraft but it seems too big to be in the cockpit of a WW2 fighter plane.

Unfortunately at some point the needle(?) fell off the pivot and I would like to try and repair, it but I'm worried about the potential liquid inside - which, if I had to guess, might just be mineral oil.

I have several questions about this compass:

  1. What aircraft is it from?
  2. What is the purpose of the gearing on the back of it?
  3. What liquid is inside it?

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The US Army Air Force and Navy compass type D-12 encompasses several models manufactured by Bendix/Pioneer: 1801, 1826 (yours), 1832 and 1833. The D-12:

  • Is a direct reading compass, not a fluxgate.

  • Is aperiodic, there is no dampen harmonic oscillations, the plate settles with a single rotation.

The D-12 was found on several WWII transports and bombers. As you assumed, it wasn't a pilot instrument, but an instrument for the navigator. From this site:

The US Army Air Force Type D-12 Direct Reading Aperiodic Compass was used at the navigators station across many US WWII aircraft, including heavy bombers B-17, B-24, B-29, medium bombers B-25 and B-26, and transports C-46, C-47, and C-54.

Here in a restored C-53, the navigator station is located just behind the pilot, on the left side:

enter image description here

Source.

Answers to your other questions are found in this maintenance documentation from the Royal Canadian Air Force:

  • The compass fluid is compliant with specifications 3-GP-31 (a retired CGSB standard for refined petroleum free from moisture, acidity, glue, suspended matter or other impurities).

  • The mechanism at the back is a compensator with movable magnets to cancel magnetic perturbations created by the aircraft.

The documentation provides additional internal details, the recommended disassembling and checking procedures, etc, and this cutaway drawing:

enter image description here

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    $\begingroup$ Wonderful! :) ... $\endgroup$
    – Cloud
    Dec 18, 2023 at 9:11

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