Ground Vibration Testing
Thread Starter
Ground Vibration Testing
Has anyone been involved in this before.
How long does it take to set up an aircraft (say a wide bodied)?
How long does it take to carry out the testing?
I imagine that the aircraft has to be isolated but can any work go on around the aircraft, like an adjacent aircraft in the hangar. The frequencies used are very small (1Hz?) which doesn't seem too sensitive.
It sounds like a facinating subject, so if anyone has any tales to tell please share them.
Thanks,
Saintsman
How long does it take to set up an aircraft (say a wide bodied)?
How long does it take to carry out the testing?
I imagine that the aircraft has to be isolated but can any work go on around the aircraft, like an adjacent aircraft in the hangar. The frequencies used are very small (1Hz?) which doesn't seem too sensitive.
It sounds like a facinating subject, so if anyone has any tales to tell please share them.
Thanks,
Saintsman
Do a Hover - it avoids G
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My only association with ground resonance testing (which is I suspect what you are interested in) has been as somebody who was waiting for the job to be finished and the results analysed, so that I could then go and do the airborne follow on stuff – or flutter testing as it then becomes.
So while I cannot really answer your question, I would say that I do not see why anybody should not be passing through the hangar as long as they stay clear of the aircraft under test – which is likely to be on jacks anyway.
As to how long the ground test would take I suppose rather depends on how much of the aircraft is being investigated (the whole structure or just a recently modified component) and also which particular vibration modes are being investigated.
Depending on the scope of the investigation I would suggest that to set it up and do it could involve anything from a day to perhaps a week or more. But you really need a reply from a guy whose day job is such testing.
So while I cannot really answer your question, I would say that I do not see why anybody should not be passing through the hangar as long as they stay clear of the aircraft under test – which is likely to be on jacks anyway.
As to how long the ground test would take I suppose rather depends on how much of the aircraft is being investigated (the whole structure or just a recently modified component) and also which particular vibration modes are being investigated.
Depending on the scope of the investigation I would suggest that to set it up and do it could involve anything from a day to perhaps a week or more. But you really need a reply from a guy whose day job is such testing.
I'm not somebody who has ever had a deep involvement with the subject, but I have had a few skirmishes.
One was when I was running the approval testing of a modified Tucano T1 for flight test use. We'd fitted a big instrumentation boom at about half span on the port wing, which was there to provide slipstream-free AoA and Beta. Initial tests (on which I hadn't flown) showed that the data was completely useless - all over the shop. In an effort to try and understand what was going on, I ran the AoA data through a digital FFT (Fast Fourier Transform) analyser, and clearly showed a continuous resonance which was queering the results - from memory I think that it was somewhere between 10 and 1 Hz. I then went rear seat for a sortie (looking at other stuff as well) and noticed visually that there was some tendency of the boom (which was easily visible from the rear seat). Putting two and two together, I ordered a "bonk test" to be carried out on the boom on the ground.
At which point one should explain what a bonk test is. This is a method by which the resonant frequencies of discrete parts of an aeroplane can be determined. The part is fitted with a very lightweight high rate accelerometer feeding an equally high rate data-logger. You then hit the item with a calibrated soft faced hammer (actually it's usually a device firing a little rubber encapsulated bullet nowadays). So, when you hit the aircraft part, it resonates (one hopes briefly!), this is recorded by the data logger, and you can then run that data through an FFT analyser - this tells you what the resonant frequencies are of the part. You obviously don't want this to, say, co-incide with any of the normal cruising RPM values of the engine.
In this case, to my unsurprise, the main resonant frequency of the boom/head combination was co-incident with the frequency that I'd dropped out from my FFT analysis of the AoA data.
So, I then went away and did a simple SHM (Simple Harmonic Motion) analysis of the boom as a mass-spring system, from this I could drop out some assumptions about the spring constant (mass was easy, I just took it off the design drawings, but I wasn't in the mood to spend days doing a probably inaccurate estimate of the spring characteristics of a reasonably complex internal shape). A bit of playing around with numbers showed that if we shortened the boom by 6" it should be fine. So, we did that, and went and tested it - the AoA trace was steady as a rock, and the design got signed off. To the best of my knowledge, it's still flying and providing ETPS students with much material for writing up longstab and lat-dir reports.
The timescale of this was fairly short - a few weeks within another (bigger) task. I don't think it's unrepresentative of most ground resonant testing that gets done, which is usually alongside other tasks, and supported by analysis and flight test results - basically looking for and solving problems that you know are there already (or at the very least, anticipate).
Watch this space, and I'll follow up when I've time with some tales of vibrating beaver tails and spontaneously combusting propellers, both of which were related to resonance and required ground testing and aircraft design changes.
G
One was when I was running the approval testing of a modified Tucano T1 for flight test use. We'd fitted a big instrumentation boom at about half span on the port wing, which was there to provide slipstream-free AoA and Beta. Initial tests (on which I hadn't flown) showed that the data was completely useless - all over the shop. In an effort to try and understand what was going on, I ran the AoA data through a digital FFT (Fast Fourier Transform) analyser, and clearly showed a continuous resonance which was queering the results - from memory I think that it was somewhere between 10 and 1 Hz. I then went rear seat for a sortie (looking at other stuff as well) and noticed visually that there was some tendency of the boom (which was easily visible from the rear seat). Putting two and two together, I ordered a "bonk test" to be carried out on the boom on the ground.
At which point one should explain what a bonk test is. This is a method by which the resonant frequencies of discrete parts of an aeroplane can be determined. The part is fitted with a very lightweight high rate accelerometer feeding an equally high rate data-logger. You then hit the item with a calibrated soft faced hammer (actually it's usually a device firing a little rubber encapsulated bullet nowadays). So, when you hit the aircraft part, it resonates (one hopes briefly!), this is recorded by the data logger, and you can then run that data through an FFT analyser - this tells you what the resonant frequencies are of the part. You obviously don't want this to, say, co-incide with any of the normal cruising RPM values of the engine.
In this case, to my unsurprise, the main resonant frequency of the boom/head combination was co-incident with the frequency that I'd dropped out from my FFT analysis of the AoA data.
So, I then went away and did a simple SHM (Simple Harmonic Motion) analysis of the boom as a mass-spring system, from this I could drop out some assumptions about the spring constant (mass was easy, I just took it off the design drawings, but I wasn't in the mood to spend days doing a probably inaccurate estimate of the spring characteristics of a reasonably complex internal shape). A bit of playing around with numbers showed that if we shortened the boom by 6" it should be fine. So, we did that, and went and tested it - the AoA trace was steady as a rock, and the design got signed off. To the best of my knowledge, it's still flying and providing ETPS students with much material for writing up longstab and lat-dir reports.
The timescale of this was fairly short - a few weeks within another (bigger) task. I don't think it's unrepresentative of most ground resonant testing that gets done, which is usually alongside other tasks, and supported by analysis and flight test results - basically looking for and solving problems that you know are there already (or at the very least, anticipate).
Watch this space, and I'll follow up when I've time with some tales of vibrating beaver tails and spontaneously combusting propellers, both of which were related to resonance and required ground testing and aircraft design changes.
G
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Saintsman
Did this in a previous life as an aeroelastic engineer with Canadair for the RJ and the CL-415 Waterbomber. From memory it took a few days for the technicians to instrument-up the aircaft and about 2 days for us to take the measurements. I guess a bit longer for a widebody.
No problem with nearby activity - we just did it in a corner of the hangar near the Challenger production line. I don;t suppose you'd want someone nearby with a jack hammer though.
Did this in a previous life as an aeroelastic engineer with Canadair for the RJ and the CL-415 Waterbomber. From memory it took a few days for the technicians to instrument-up the aircaft and about 2 days for us to take the measurements. I guess a bit longer for a widebody.
No problem with nearby activity - we just did it in a corner of the hangar near the Challenger production line. I don;t suppose you'd want someone nearby with a jack hammer though.
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Oh dear, we are off at a tangent. My take on 'ground resonance' is the sound my feet are making on another Friday afternoon as I walk to the QC's office and ask 'why the f****n jet' isn't on the line yet, and I'm fed-up of supporting 'Shop's over-time'. You know the sketch, John! Let's keep this discussion on-course !!
Partz
Partz
Oi Partz, where does a tangent start????
Your ( and previously my) ground resonance experiences chasing jets at 4pm on a Friday (or any afternoon for that matter) are not the sole preserve of the Lancashire aerospace industry you might be please to know. It appears to be a universal malaise of the shop floor fitter which has realised that if "I can get paid time and a half or double time for working on this jet out of normal hours then why don't I ponce around until management insist on overtime!!"
Ask William the Canadian probe jouster when you see him at SETP....
Having a Guy Fawkes party this year? Hope to be there if you are.
Tarnished
Your ( and previously my) ground resonance experiences chasing jets at 4pm on a Friday (or any afternoon for that matter) are not the sole preserve of the Lancashire aerospace industry you might be please to know. It appears to be a universal malaise of the shop floor fitter which has realised that if "I can get paid time and a half or double time for working on this jet out of normal hours then why don't I ponce around until management insist on overtime!!"
Ask William the Canadian probe jouster when you see him at SETP....
Having a Guy Fawkes party this year? Hope to be there if you are.
Tarnished
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Partz
I hope your SETP presentation follows the subject matter more than your efforts at contributing to this 'ground resonance' thread. Is this SETP paper not the same presentation that Heinz Spolgen gave a few years ago to SETP?
Hope the ground doesn't resonate too much when you fall over at the Sky bar!!
SG
I hope your SETP presentation follows the subject matter more than your efforts at contributing to this 'ground resonance' thread. Is this SETP paper not the same presentation that Heinz Spolgen gave a few years ago to SETP?
Hope the ground doesn't resonate too much when you fall over at the Sky bar!!
SG
A second tale of resonance testing, with sadly no appropriate SETP banter.
A few years ago, a TP (not me, but working for me) was minding his own business conducting what passed for high speed testing (somewhere around 80kn I think) in a low speed STOL bird. He may or may not have oversped the prop slightly (the debrief was slightly uncertain on the point), but anyhow was rather disconcerted to find the cockpit filling with fumes that smelled rather like burnt plastic.
He took the normal actions you'd expect of a grown up TP, but nonetheless managed to get it back onto the runway without shutting down or diverting. On inspection, the propeller spinning and the root of one blade had combusted in flight. For those who missed it THE PROPELLER SPONTANEOUSLY COMBUSTED IN FLIGHT.
It's worth understanding the characteristics of the powerplant on the machine. It consisted of a direct drive 4-stroke petrol engine with max.RPM of 3,300 (there, that's identified it, but never mind) driving a 3-blade ground-adjustable high inertia composite prop. Unusually for any piston-prop combination, the engine incorporated no torsional shock absorber.
Anyhow, after a great deal of peering through microscopes, doing sums, interrogating operators of anybody who'd ever had to replace a prop fitted to the same engine type, and just plain sitting down over a mug of tea and saying "what the ***** happened ????", we reached the conclusion that an engine resonance had co-incided with some resonance of the prop which had caused microscopic level heating.
So, in the time honoured practice of bewildered and confused engineers, we decided to do some experiments, thrash around with the data, and see what we got. My "lab equipment" (actually it was all done out in the middle of a field where I could minimise noise nuisance to anybody else) was...
- Front end of a ground-test airframe, on tyres, but nonetheless securely bolted down.
- Serviceable engine
- Half a dozen different prop types.
- Ready supplies of AVGAS 100LL, unleaded MOGAS, leaded MOGAS (the latter was still in use at the time).
- A couple of incredibly sensitive, high-rate, low-mass accelerometers bonded to bits of the engine with superglue.
- A very high rate data-logger, feeding a PC.
We then spent a thoroughly entertaining day in our field, wearing ear defenders, running the engine throughout it's available power range (often briefly, since it didn't cool all that well static, and we hadn't the facility to put extra wind through it). This was done at pretty much every combination of propeller type/pitch/fuel that seemed likely.
After a day of testing, was then some weeks of analysis, passing each data set through the ubiquitous FFT analyser, comparing plots, and trying to make sense of it.
The result was that we (I !) identified a combination of conditions (unleaded MOGAS, high tip Mach No., high inertia propeller) that could lead to severe HF spikes in the frequency spot. This led to some RPM restrictions on the engine, which would avoid the problem, and some advice on prop type and fuel selection for maximising engine life.
A different approach and problem to the Tucano bonk test / boom issue. The underlying issue are however.
- Much vibration testing is done after problems have been found.
- Analysis and reporting takes much longer than testing.
- Anybody undertaking the testing MUST be very hot on their analysis tools, and up to speed on the relevant maths.
- Whatever the Bode plots may indicate, real resonances (as measured rather than predicted) tend to occur at disrete frequencies.
G
A few years ago, a TP (not me, but working for me) was minding his own business conducting what passed for high speed testing (somewhere around 80kn I think) in a low speed STOL bird. He may or may not have oversped the prop slightly (the debrief was slightly uncertain on the point), but anyhow was rather disconcerted to find the cockpit filling with fumes that smelled rather like burnt plastic.
He took the normal actions you'd expect of a grown up TP, but nonetheless managed to get it back onto the runway without shutting down or diverting. On inspection, the propeller spinning and the root of one blade had combusted in flight. For those who missed it THE PROPELLER SPONTANEOUSLY COMBUSTED IN FLIGHT.
It's worth understanding the characteristics of the powerplant on the machine. It consisted of a direct drive 4-stroke petrol engine with max.RPM of 3,300 (there, that's identified it, but never mind) driving a 3-blade ground-adjustable high inertia composite prop. Unusually for any piston-prop combination, the engine incorporated no torsional shock absorber.
Anyhow, after a great deal of peering through microscopes, doing sums, interrogating operators of anybody who'd ever had to replace a prop fitted to the same engine type, and just plain sitting down over a mug of tea and saying "what the ***** happened ????", we reached the conclusion that an engine resonance had co-incided with some resonance of the prop which had caused microscopic level heating.
So, in the time honoured practice of bewildered and confused engineers, we decided to do some experiments, thrash around with the data, and see what we got. My "lab equipment" (actually it was all done out in the middle of a field where I could minimise noise nuisance to anybody else) was...
- Front end of a ground-test airframe, on tyres, but nonetheless securely bolted down.
- Serviceable engine
- Half a dozen different prop types.
- Ready supplies of AVGAS 100LL, unleaded MOGAS, leaded MOGAS (the latter was still in use at the time).
- A couple of incredibly sensitive, high-rate, low-mass accelerometers bonded to bits of the engine with superglue.
- A very high rate data-logger, feeding a PC.
We then spent a thoroughly entertaining day in our field, wearing ear defenders, running the engine throughout it's available power range (often briefly, since it didn't cool all that well static, and we hadn't the facility to put extra wind through it). This was done at pretty much every combination of propeller type/pitch/fuel that seemed likely.
After a day of testing, was then some weeks of analysis, passing each data set through the ubiquitous FFT analyser, comparing plots, and trying to make sense of it.
The result was that we (I !) identified a combination of conditions (unleaded MOGAS, high tip Mach No., high inertia propeller) that could lead to severe HF spikes in the frequency spot. This led to some RPM restrictions on the engine, which would avoid the problem, and some advice on prop type and fuel selection for maximising engine life.
A different approach and problem to the Tucano bonk test / boom issue. The underlying issue are however.
- Much vibration testing is done after problems have been found.
- Analysis and reporting takes much longer than testing.
- Anybody undertaking the testing MUST be very hot on their analysis tools, and up to speed on the relevant maths.
- Whatever the Bode plots may indicate, real resonances (as measured rather than predicted) tend to occur at disrete frequencies.
G
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Taa-nished and Soiled Glove(Puppet ?)
Sorry for the late reply - just been off lookin' for one of those tangent's again !
Agree on the old Friday pm sketch and it aint getting any better. For all the attempts by T10 to nail times and jets, the Programme just won't allow us to walk away from it (well not me, anyway - nudge, nudge!). Anyway, it's a Bank Hol tomorrow and guess what, there's a 1400 promise on BT013 !!
Regards SETP - yep, HS did a brief a couple of years ago (ask him if the FTEs at Lancs Control were p****d off about that !), although since no flying on the ALSR had `taken place at that stage, it was all a little hypothetical. Also, his chat covered 'assymetric stiffness' therefore was thin on the detail. Since CW and I thought we did ok in Rome it appeared 'churlish' not to go for a re-attack in LA. I have promised to wear socks for this one !!
Good banter on the Sky Bar - I thought Tony Curtis was the only one to have witnessed the moving staircase !!
Partz
Sorry for the late reply - just been off lookin' for one of those tangent's again !
Agree on the old Friday pm sketch and it aint getting any better. For all the attempts by T10 to nail times and jets, the Programme just won't allow us to walk away from it (well not me, anyway - nudge, nudge!). Anyway, it's a Bank Hol tomorrow and guess what, there's a 1400 promise on BT013 !!
Regards SETP - yep, HS did a brief a couple of years ago (ask him if the FTEs at Lancs Control were p****d off about that !), although since no flying on the ALSR had `taken place at that stage, it was all a little hypothetical. Also, his chat covered 'assymetric stiffness' therefore was thin on the detail. Since CW and I thought we did ok in Rome it appeared 'churlish' not to go for a re-attack in LA. I have promised to wear socks for this one !!
Good banter on the Sky Bar - I thought Tony Curtis was the only one to have witnessed the moving staircase !!
Partz
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Guys,
I used to (And still do from time to time) do this on several aircraft, ranging from Helicopters of all shapes and sizes, Robins, BAe ATP's and even one rather lovely Dessoutter from the Shuttleworth Collection... Obviously I didn't shake that one very hard!!!
The method of vibe testing, as Genghis has already mentioned can involve a bonk test, although I personally use a shaker. Basically we Jack the aircraft into a level attitude, gear up, and attach the shaker. The frequency and amplitude of the induced vibration is controlled via and amplifier. I use real time FFT analysis, although I have been known to rig up oscilloscopes and function generators etc.... too much of a kerfuffle in my opinion.
You didn't really say why you are doing the vibe survey? If its a design function then your approach will be very different to mine, my methods are used purely in tracking down vibration related defects on in-service aircraft.
DE
I used to (And still do from time to time) do this on several aircraft, ranging from Helicopters of all shapes and sizes, Robins, BAe ATP's and even one rather lovely Dessoutter from the Shuttleworth Collection... Obviously I didn't shake that one very hard!!!
The method of vibe testing, as Genghis has already mentioned can involve a bonk test, although I personally use a shaker. Basically we Jack the aircraft into a level attitude, gear up, and attach the shaker. The frequency and amplitude of the induced vibration is controlled via and amplifier. I use real time FFT analysis, although I have been known to rig up oscilloscopes and function generators etc.... too much of a kerfuffle in my opinion.
You didn't really say why you are doing the vibe survey? If its a design function then your approach will be very different to mine, my methods are used purely in tracking down vibration related defects on in-service aircraft.
DE
Thread Starter
DE,
Thanks for your reply. We will be doing two a/c types following major conversion (into tankers), a DC-10 and A330, both a precurser to flight test and flutter trials.
I wondered how long it took to set up and how long to carry out the GVT?
Saintsman
Thanks for your reply. We will be doing two a/c types following major conversion (into tankers), a DC-10 and A330, both a precurser to flight test and flutter trials.
I wondered how long it took to set up and how long to carry out the GVT?
Saintsman
