Shake, rattle and roll

[Richard Mole]

Are there any affordable hand held instruments or even self-build devices that can give a quantitative measure of cockpit vibration? Perhaps there are devices for hire?

I ask because people talk of how much 'smoother' their engine runs after its top overhaul, or propeller change or whatever. After spending all that money on upgrading your precious aircraft you would obviously want to feel that way. But is there a way to measure a change in 'ride quality' that doesn't break the bank?

If money is no object then one can turn to dynamic propeller balancing or those fiendishly clever monitoring systems for assessing the continued airworthiness of helicopter gearboxes or whatever.

Does anyone know of a cheapish digital instrument that might be what I'm looking for?

Richard

[Nick Allen]

Richard, How about this: http://www.analog.com/en/prod/0,2877,ADXL202,00.html
plus a soldering iron!?
This also has sundry links: http://www.sensorland.com/HowPage003.html

[Richard Mole]

Nick,

thanks for the links.

I tried measuring aceleration last year in another context (drop-tests) using an accelerometer that was sold for robot hobbyists - it took a lot of programing and then the accelerometer failed just as the tests started in earnest! So once bitten twice shy. But the link to the impressive range of hire equipment demonstrates what has become available although the cost is still perhaps a little too high.

I was really hoping that someone who had tried measuring cockpit vibration would be able to pass on their experience and offer advice. It may be one of those areas where a lot of background knowledge in sound and vibration is required to make a good fist of it.

[PB]

This post has set me thinking. I have access to some good quality 6-axis accelerometers with integral data loggers. I'll set one up in the Nipper and see what the output looks like.

[Nick Allen]

Six axis...? Are these measuring ripples in space-time as well?

[Mark A]

Richard,

It does beg the question 'What are you trying to measure?' and 'What do you hope to achieve by measuring it?'.

A single point accelerometer will give you a vibration amplitude and spectrum for the point it is located, though it will modify that response by the addition of the instrument's own inertance. Optical vibration measurement is less invasive, but is really only a lab technique.
An accelerometer located somewhere quite rigid, such as on the main spar will tell you about the forcing function but not about the structures modal response.

If you think that you have a particular modal vibration problem, then probably the easiest way to survey the structure is with strain gauges rather than accelerometers, as they weigh less and cost considerably less.
But you will need to do some spectrum analysis to interpret the results, which will be stress/strain amplitudes rather than vibration amplitudes.

p.s. The six axis devices, if they are the same type that we use for crash testing, are three orthogonal accelerometers plus three angular rate FOG type gyros used for capturing the full rigid body motion of their location point - not cheap.

[Richard Mole]

Mark,

yes indeed.

As I think I said I am interested in whether it is possible to cheaply establish some objective measurement of 'ride quality' in order to establish whether the subjective assessment of an improvement (or otherwise) following some change to the engine/propeller can be substantiated and to what degree.

My aircraft does not have a vibration problem I am glad to say.

I only too willing to acknowledge that my query was ill-posited. But it seemed worth the asking because digital technology is resulting in relatively affordable devices with very impressive capabilities.

For example, I have just obtained a cheapish 57mm instrument weighing about 100g that will provide rpm, oil temperature, oil pressure, two Cylinder head temperatures and two exhaust gas temperatures, oh and keep track of flight times and the flight time remaining between scheduled inspections.

I am vaguely aware - having vanishingly little specialist knowledge of sound and vibration - that some hand held devices are making their appearance for the purpose of establishing whether work-place tools fall within Health and Safety limits and just thought that perhaps the same type of technology might have got to the point where 'ride quality' in an aircraft might somehow (!) be measureable.

Richard

[Mike Mold]

OK Richard, I'll bite first. How cheap, what & from where?

[PB]

Nick - Actually it’s a six/nine axis device three linear accelerometers, three rate-gyros and three-axis magnetometer which keeps the rate gyro drift under control.

It is based on this: http://www.xsens.com/en/products/machin ... ti_oem.php
The only problem is that the sampling rate (30Hz) may be too low to be useful.

We have it connected to a self-powered data-logger that also has high precision GPS integrated. It has been developed from motor racing technology. Sadly, I am unable to say what we use it for. The same company supplies similar kit to the Red Bull Air Race organisers.

We also have some pretty sophisticated software for looking at the results. Getting the data is usually the easy bit. Working out what it all means is often not so simple.

[Mark A]

Richard,

At the cheap and cheerful end of the scale, this might be of interest:
http://www.kionix.com/accelerometers/ac ... KXPS5.html
I've seen them on E-bay for about £5 and dynamic ranges upto about 3g/1000Hz. No other experience though.

[Richard Mole]

Mike,

its the Stratomaster Infinity E3 at £140 (includes pre-wired D connectors).
I got mine from Parts for Aircraft http://shop1.actinicexpress.co.uk/shops ... /index.php

[Richard Mole]

PB,

that sounds quite fascinating. It will be interesting to see what you can learn from taking it along for a Nipper flight.

Perhaps one realistic test would be to gather data in the cruise, then a second set with the left mag off and a third set with the right mag off.

As you say, trying to arrive at a few summative graphs, indices or whatever when safely back on the ground may be the hardest part.

[Richard Mole]

Mark,

its probably a bit too much of a challenge for little me although they do say that anythingthat hurts is good for you!

Lets hope PB is blazing the trail

[PB]

It's one thing to get the accelerometer and altogether another to make it into a useful device. They suffer from drift and zero error for a start... Rate gyros are worse.

Even when you have a useful device, the real hard bit is turning the data into something meaningful...

Believe me, I know.

I'll let you know how I get on.

[Pete]

Hi Mike, getting the sexy stuff such as vibration frequency and all the other sorts of data that gets described in big words is quite an expensive job.

Getting a simple measurement of the vibration amplitude is relatively easy, all you need is a small loudspeaker the sort found in old transistor radios, a diode, a resistor and a sensitive voltmeter. The larger the vibration, the larger the voltage. an an alternative, you could use a suitably adapted record player pickup - a pea shaped blob of blue tac should provide sufficient inertia to the pickup needle.

The wee loadspeaker will also pick up exhaust noise etc, but the amplitude of that noise will be quite small compared with the mechanically coupled vibration from the engine or prop

Crude, but it should give you a good before/after comparison.