How to interpret Vibration Logs?

[PaoloFCantoni]

I'm now on 5 weeks holiday overseas and so the only flying of the Protos MAX I'll be doing is on simulators.

However, I've got my logs with me and I thought I'd give interpreting them a go. I couldn't find any documentation on how to interpret them. iS there any MSH published documentation on how to analyse what the logs are telling us?

If not, then perhaps this Topic can be a place for people to come to.

TIA,
Paolo

[jhamel]

Virtually all vibration logs work the same way. The X-axis is frequency and the Y-axis is magnitude. The X-axis frequency is either in Hertz (cycles per second) or RPM (revolutions per minute). 1 Hertz = 60 RPM. Using the example of a typical 700 size helicopter, the head speed is around 2,000rpm (33.33 Hz). The tail usually spins around 4.5 times faster than the head, so 9,000rpm (150 Hz). The motor spins around 10 times faster than the head, so 20,000rpm (333.33 Hz).

When you look at the frequency-magnitude graph, there should be three main peaks along the X-axis (frequency). From left to right these peaks should correspond to head, tail, and motor frequencies. You are looking for peak magnitude (Y-axis) for head, tail, and motor frequencies. These peaks are known as amplitude. The higher the peak/amplitude, the higher the level of vibrations. If the peak/amplitude is higher than a predetermined "safe" limit, then it means that something is vibrating too much. If the high amplitude vibration corresponds to the the head frequency on the X-axis, it means there is some vibrating component from the main blades down to the lower main shaft bearing. Same concept applies to the tail and the motor frequencies.

[TheBum]

There’s also what Mikado refers to as “sea grass”. It’s a relatively low amplitude vibration across a large part of the spectrum. It can be caused by bearings about to go out.

[jhamel]

I've not come across this "sea grass" concept. Can you provide a link to it? I am interested in learning more about it. Thanks.

[PaoloFCantoni]

Over in the Brain forum, there's a similar thread to this and it mentions peaks at 2x head frequency.

It also mentions that these may be relayed to blades as we have two per head rotation. It further mentions that if you get peaks in that range, they should correspond to changes in either cyclic or collective or both.

I seem to have these and they do correspond to "executing manoeuvres".

How high should they get before I need to worry?

Paolo

[TheBum]

Makes sense. Every time you input cyclic, it necessarily throws the blades out of track.

[PaoloFCantoni]

Seems to me it would be a good idea to add another reference point for the blades. The Brain knows how many blades we are using so it shouldn't be too hard.

Thoughts?

Paolo

[flint723]

Originally Posted by PaoloFCantoni

....

I seem to have these and they do correspond to "executing manoeuvres".

How high should they get before I need to worry?

Paolo

Spikes up to 80-90 (occasionally 100) for the 2x head speed range are pretty normal.

The attached log is perfectly healthy.

[PaoloFCantoni]

Originally Posted by flint723

Spikes up to 80-90 (occasionally 100) for the 2x head speed range are pretty normal.

The attached log is perfectly healthy.

Thanks, flint723,

That was very useful! Mine appear to be similar, so that puts my mind at rest.

As I said earlier, I think it would be useful to have another reference line for the blades (for the appropriate configuration). Perhaps a note indicating that momentary high values during manoeuvres on the blade frequency are acceptable.

Paolo