I use to measure vibration on jet engines with jet cal analyzers Depending on which section of the engine (fan inlet, diffuser section, or the turbine section) ranges from 2.5 mils to 4.5 mils. Limits. A mil is .001 thousands of movement. The analyzer had frequencies dials to dial certain rotating parts which each vibrated at different frequency.
This was common problem on older cable throttle controlled aircraft the vibration could be felt through throttles. On newer fly by wire models have and others have vibration monitors in the cockpit. Those monitors were very expense. I’m sure that some what close to same analyzers available with improvements form my day.
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