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Vacuum Gauge

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Since the POS membership is about up and so far today I am unable to post, I thought I'd revive the topic "Vacuum Gauge" here.


The initial question was: What is the proper reading for Vacuum during run-up/ in flight, in a 140? I have seen 5 +/- 0.1 and also 4.5 - 5.2? I mentioned that this was a general operating range and there could be errors associated with excessivly low vacuum and excessivly high vacuum.

The topic progressed in the usual fashion with major disagreement from the resident expert with (myself) one of the resident (whisper) airline pilots, that commented about excessive vacuum and the associated negative issues with same. I contacted Castleberry Instruments over the weekend and asked if my question could be directed to a knowledgeable individual to clarify issues that I am aware of but can only be heard of in some ground schools and be found in limited text references.

The issue is there can be: erroneous information displayed with high vacuum due to the instrument design and its incorporated compensating devices, and there can be increased bearing wear and associated component failures (filter issues and hoses) that can cause inaccurate information to be displayed on that most trusted instrument.

Take it for what it's worth but I don't believe anyone here at the PF will say "he doesn't know what he's talking about."

Bold added.

***

A high level of vacuum applied to a gyro will cause higher RPM of the rotor and greater rigidity as you mentioned. This actually makes the gyro more stable in the sense that it can better overcome static friction and precess less. But, since attitude gyros are corrected by a gravity mechanism, any turns can exert higher centrifugal forces on the gravity mechanism that can cause the gyro to drift. The gyro erection system (controlled by the gravity mechanism) is calibrated to the typical RPM at rated vacuum and therefore may change somewhat at different vacuum input. Higher RPM could account for a slower erection rate and thus a slower drift rate during turns as well as a slower recovery after leveling back out after a turn. The higher RPM could contribute to the spin bearings of the rotor to wear out sooner.

Other effects that could happen are to the air system. The inlet filter may not be able to handle the additional volume of airflow of increased vacuum and cause a restriction so that even with higher vacuum pressure the losses of the restriction prevents an actual increase in power to the rotor. Also flexible hoses have been known to collapse (walls suck in to reduce flow) with higher vacuum pressures also causing the same effect as filter restriction. Higher vacuum pressure can sometimes create a leak in the case of the gyro causing airflow to bypass the rotor in which case erratic behavior of the gyro could result.

Best regards,

Ronald Gipson
Technical Services
CASTLEBERRY INSTRUMENTS & AVIONICS
 

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