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Obviously, in space a vacuum powered gyro would not work because there is no air to move, but, how high do you have to go before they are unable to spin fast enough to work properly?

would a vacuum gyro work properly in a sr71 at 80,000 feet?

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    $\begingroup$ "Vacuum" gyro's are differential pressure motors, if you can provide an alternate static source that has a higher pressure than the external static source, the instrument should work fine. Maybe the question should be how much of a pressure differential is needed to run the gyro? I think somewhere around 5psi... $\endgroup$ – Ron Beyer Mar 10 '17 at 14:11
  • $\begingroup$ Vacuum powered instruments are typically driven by a pump. I imagine that any installation that will travel to very high altitudes would have one or more pumps that can support the vacuum instruments. $\endgroup$ – selectstriker2 Mar 10 '17 at 14:19
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Just like altimeters, it depends on the specifications.

The manual for this random vacuum attitude indicator says up to 40,000 feet.

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(Image source)

In an aircraft, the vacuum source is often used to power gyroscopes in the various flight instruments. To prevent the complete loss of instrumentation in the event of an electrical failure, the instrument panel is deliberately designed with certain instruments powered by electricity and other instruments powered by the vacuum source.— Wikipedia

In a piston engine airplane the vacuum pump is geared to the engine.

Historically electric motors weren't reliable, that has changed now of course.

At 80,000 feet the air density is only 14% compared to 40,000 feet. I highly doubt it would be feasible (size, weight, RPM) to create such a pump. And I'm pretty sure it's of no use to jets.

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    $\begingroup$ FYI, On most general aviation aircraft the pump is attached to the engine by a carbon gear—not a belt. That’s so that if the pump fails, which hey frequently do, the gear shears and the engine is not damaged. This image shows what the gear looks like. aircraftspruce.com/catalog/inpages/rapco_dryairpump.php $\endgroup$ – JScarry Mar 10 '17 at 16:24
  • $\begingroup$ @JScarry terrific info, thanks $\endgroup$ – ymb1 Mar 10 '17 at 16:40
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    $\begingroup$ Also of note, "vacuum" gyros work equally well whether powered by a negative pressure source (vacuum) or positive pressure source: All that's needed is a pressure differential. Beechcraft in particular produced a few aircraft with pressure-powered gyros. Theoretically at altitude (say 40,000 feet) you would not need a vacuum pump at all: Maintain cabin pressurization at 8,000 feet (22 in. Hg) and exhaust the gyros directly to the atmosphere (5.5 in. Hg: A 16.5 in Hg. vacuum.) Most gyros only need 4-6 inches of vacuum, so obviously you'd need a regulator so you don't overspeed the gyro. $\endgroup$ – voretaq7 Mar 10 '17 at 18:34
  • $\begingroup$ This answer gives a pretty good description of a powered vacuum pump system, but what about Venturi vacuum systems? Us "Old Guys" that operated aircraft with venturi systems figured "As long as I can maintain flying speed, that darn gyro should keep working." $\endgroup$ – Lloyd Easton Mar 10 '17 at 21:17
  • $\begingroup$ @LloydEaston That's a great point! For practical purposes "As long as I can maintain flying speed that gyro should keep working." holds pretty well for Venturi vacuum systems. At some altitude and airspeed combinations the venturi would no longer be able to generate a sufficient pressure differential to operate your gyros anymore simply because the air is too thin - that crossover point may be provided by the venturi manufacturers but the aircraft typically using these sytstems probably can't climb high enough for it to be a factor anyway. $\endgroup$ – voretaq7 Mar 11 '17 at 0:22

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