Vibration Testing

[jonesdtw]

Has anyone temporarily installed equipmnet in an aircraft for vibration testing? I need to test the vibration a piece of equipment is subjected to inside the radome comopartment. I am looking for a list of test equipment (ie. 2 single axis accelerometers, one tri-axis accelerometer , power supplies, signal conditioners, and data logger) for a temporary installation. Battery operated devices are the optimum solution, but I can tap into the aircraft power if necessary.

 

[MintJulep]

It would seem much simpler (and cheaper) to do your testing on the ground, on a suitable shaker machine.

 

[jonesdtw]

Part of the equipment is located outside the aircraft envelope into the airstream, so it has to be tested during flight.

 

[MintJulep]

What is the information goal of your test?

 

[jonesdtw]

The user wants to see the vibration that is acting ontthe equipment (which is experiencing a degradation of performance)to try determine if the mounting of the equipment is the bad actor or if the equipment itself is the bad actor.

 

[MintJulep]

Equipment vibration and equipment mounting are not independant.

I suggest that you need to breakdown your testing into smaller chunks.

1. What is the maximum vibration level that the equipment can operate at without degraded performance?

2. Is the equipment subjected to vibration in excess of that level while in flight.

I still suggest ground based testing for step 1. Aerodynamic forces can be simulated if necessary.

 

[jonesdtw]

The customer has not given me at this time any perfomance data. (I'm not sure if they bought the data from the OEM. It is hopefully qualified per MILL-STD-810) You have a valid point about the ground testing. However, the flight testing is at the customer's request. They want to know the vibration when the degradation of equipment occurs in flight .
I am just looking for test setup/equipment info, at this time.

 

[GregLocock]

We log vibration levels under operational conditions quite regularly. We have a ddepartment who spends 50% of their time doing that, and it is not a trivial learning curve.

If you are in the USA talk to Endevco, in Europe talk to B&K. They can help you select the instrumentation, and may be able to give you advice on where to stick the acceleromenters. Given that flight time is usually expensive you'll probably want some redundancy.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianR]

I have done this work using a portable analyser and have designed and used a flight test instrumentation system for experimental aircraft. The companies GregLocock proposed are good ones to talk to and there is a large range of portable devices, some of which are direct reading and others are recorders. Commonly you will power your accelerometer from a constant current source of a few mA at < 12VDC.

To identify the frequency components of the vibration then some kind of FFT system is needed in your analysis so do you carry a real time analyser on-board or record the data for ground replay. If you record the data then look closely at the requirements of the analysis system in terms of bandwidth, sample size, dynamic range and anti aliasing.

 

[MintJulep]

I still don't think that you can get to where you want to go in one shot. Certainly not easily.

You state that you are attempting to isolate the problem to either the mounts or the equipment. However they are a dependant system. Measurement of the response of the system does not allow you to draw any meaninful conclusions about the response of either component of the system without detailed knowledge of the vibration response of the other component.

Calculating transfer functions and coherence of each acclerometer against every other accelerometer and the "driving" vibrations might theroetically yield some information, but culling out the non-meaningful data is exceedingly non-trivial.

Serioulsy, ground based shaker table is the way to get started.