helicopter vibration analysis

[coldreaction]

Hi all,

I'm doing some R&D regarding an external mounted pod for some common helicopters. It would be installed on various ships like the B206/L, B407, AS350/5.

The question is, how do I obtain values for the frequency and amplitude each ship vibrates at? (eg, at the pilots seat)

I know you can deduce the basic frequency by inverting the main and tail rotor RPM quantity but I still need amplitudes and confirmation that the machine keeps the basic vibration frequency intact throughout the airframe (ie. same Hz a the front seats as it is at the mast).

Just need some basic numbers to work with.

These numbers would help in the design of an effective damping system for our sensors mounted internally to the external pod (land survey equipment).

[delta3]

Coldreaction

I am afraid you pretty much answered your own question: MR-freq plus its higher order harmonics...

Most hull resonance frequencies will be below that frequency and certainly below the TR-freq, so MR-freq is the most important design reference.

So keep resonance freqs below MR-freq I would think.


m2c, d3

[TRC]

Have a word with your local helicopter maintenance company who look after the types that your gear is for.

They will (should) have details for each helicopter that they maintain. They'll have info for vertical and lateral vibration in terms of frequency and amplitude in inches per second.

This vibration will vary to a degree with weight and airspeed, and also if they track and balance for all phases of flight - some get it spot on at cruise speeds and are less fussy for lower airspeeds. (Not saying that's the right way to do it, just speaking from experience).

You will find that there is a maintenance manual limit that you should attain by track and balance, and also an in-service value that you will encounter on a helicopter between vibration checks.

One other thing to watch for is an out of limits tail rotor balance, it will show up as a thrumming vibration felt at the extremities - so a nose mounted sensor will feel it. If it's heavy enough it might not affect it though.

[coldreaction]

Great, thanks for the information to at least DIRECT me which way to look at things.

Amazingly enough there was very little to be found on the "internets" about this, except a few studies that showcased a correlation between heli vibration and induced pain in the backs of pilots in some cases (must have been pretty bad).

I take it the MM's for most helicopters are not something that you could pull up from a PDF location somewhere. I also imagine they are probably very expensive to get (as controlled documents) so naturally, the owners dont want them out in the open.

[tcufroggie]

Tough answer. Each platform will respond differently to a rotor vibe sourcing function. (Depends upon the Xmsn used, the pylon mounts, fuselage construction material and type, etc.) Somewhere there are vibration surveys for each aircraft. Those values are used to set qualification levels of most equipment. You may have some luck using the 'internets' or your local helo Mx contacts to determine a vibe spectrum for a specific area for a specific model.

Good luck.

[Clitheroe Kid]

If you want to measure the vibration, you could try this company. Noise and Vibration Measurement, Control and Training Consultants - INVC (http://www.invc.co.uk). They can sell you transducers and a box of tricks to plug into a laptop.

[coldreaction]

^ Thanks for that. I already have a simple 2D vibe sensor package.

Was hoping to avoid having to jump into a bunch of different ships just to determine vibes as I'm sure it has been done before.

Will look at contacting a local maintenance facility next.