Engine vibration units

[Genghis the Engineer]

I'm going through some reports related to the FDR on a BAe-146.

There's a parameter for engine vibration - fine. It measures it in inches per second, with test points at 0 and 2 in/sec.

Can anybody explain that to me? I'm used to vibration being expressed in terms of frequency, amplitude and damping ratio. Distance over time doesn't make a lot of sense to me.

G

[Janner200]

Genghis - velocity (inches per second) is the normal parameter for engine vibration. I'm no expert on vibration, but the velocity is the product of frequency (number of cycles per second) and amplitude (displacement). It is normally measured by an accelerometer mounted on or close to the engine case or cases where the main shaft bearings are located. It is aimed at monitoring the level of vibration generated by out of balance on the engine shafts.

As it is a function of both frequency and amplitude, it is a good indication of the level of "damage" being caused to the structure - low amplitude x high frequency can be equivalent to high amplitude x low frequency in terms of damage mechanisms like fatigue.

The latest engines have much more sophisticated analysis carried out either on board or in a ground station.

[lomapaseo]

the mounting location of the pickup itself is critical to the interpretation, also the health of the wiring and pickup often leads to invalid readings.

Vibration is like smell, in an engine, it is only a single sense and often not valuable by itself.

unmeasured vibration is what breaks tubing and chaffs wiring.

All info is helpful to the engineers on the ground, in the air follow the guidelines

[Genghis the Engineer]

the velocity is the product of frequency (number of cycles per second) and amplitude (displacement).

Aha, thank you, that was what I was missing.

G