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Turbine Vibration - 4/11/2012 6:38 PM   
InboundLZ



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Wondering why we don't monitor turbine vibration on our turbines. Seems to me that it would be easy to do and would be a good indication of the health of the engine. Any reason not to do this? How hard would it be to incorporate this into our engines? Adding this information to the engine data display would allow for quick health monitoring after each flight.


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RE: Turbine Vibration - 4/11/2012 6:53 PM   
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RE: Turbine Vibration - 4/11/2012 7:03 PM   
InboundLZ



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RE: Turbine Vibration - 4/11/2012 7:15 PM   
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Sound gives me more info than feel....A twin boom model often feels like it has vibration, its just the air sicking off the underside of the tailplane and transmitting along the booms!

Dw


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RE: Turbine Vibration - 4/11/2012 7:20 PM   
InboundLZ



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ORIGINAL: Dave Wilshere

Dave

Sound gives me more info than feel....A twin boom model often feels like it has vibration, its just the air sicking off the underside of the tailplane and transmitting along the booms!

Dw



Dave, are you suggesting that vibration monitoring would provide no additional benefit?

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RE: Turbine Vibration - 4/11/2012 7:40 PM   
readyturn


 

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Status: offline 		You need to show where the benefit would come in. Monitors on full scale have been more of a problem than a aid. What level would you set them to ?? The turbine manufactures would have to determine specs and then develope the sensor.... Then what ? TM the signal back to the ground so that you can shut down the engine !

Normally the sensors are set to a high a high threshold on full scale so as not to lead to an unwarrented In-flight shut down. Not sure that we need to go there with our engines and I don't think that there has been enough demonstrated occurances that warrent this monitoring system.

Rick

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RE: Turbine Vibration - 4/11/2012 7:49 PM   
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quote:

ORIGINAL: InboundLZ


quote:

ORIGINAL: Dave Wilshere

Dave

Sound gives me more info than feel....A twin boom model often feels like it has vibration, its just the air sicking off the underside of the tailplane and transmitting along the booms!

Dw



Dave, are you suggesting that vibration monitoring would provide no additional benefit?


Dave

No, just that my experience says that you can hear the things that cause vibration, sooner than you feel them (non FOD turbines take stones on take off and bend/brake compressor blades...you hear the whistle-change is sound) Bearing failures generally happen slowly and you can hear the grinding on slow down, before the vibration becomes obvious. The way turbine manufacturers balance their turbines varies, the balance changes in the first few runs as things move, how many re balance after a couple of runs?


Dave


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RE: Turbine Vibration - 4/11/2012 7:54 PM   
seflyer


 

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Status: offline 		never felt any vibration on my Jetcats. some experiments with accelerometers netted less than positive results to monitor turbine health while installed in the airframe. data was not easily monitored due to airframe vibration and movement. results on the test stand gave repeatable results. gave up after a while due to unfavorable results.

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RE: Turbine Vibration - 4/11/2012 7:54 PM   
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Status: offline 		I think the benefit would come in the form of early warning that a bearing is starting to fail. I can tell you that engine vibration is very accurate in the full scale applications, our 10 million dollar CFM turbines use vibration monitoring to determine health and can give very early indications of bearing failure long before it becomes a engine damaging event. So, I guess I disagree in your statement that engine vibration monitoring is more of a determent than a help....

Back on point, some of us just fly and as long as the engine starts we go flying. Then there are others that monitor each start, engine parameters durning flight, and monitor shutdown. These readings can give good indication of component health I.E Pump voltage increase from norm might indicate a plugged filter or possibly a pump that is failing, High EGT might indicate a internal problem with the hot section, and so on.

I like to monitor my engines closely so I can have every advantage in keeping my model in one big piece. I would love to see EPR, Vibes, fuel flow, etc....

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RE: Turbine Vibration - 4/11/2012 8:20 PM   
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Status: offline 		As above... use of in-flight accelerometers would give poor, or unreliable readings. BUT... if we were looking for bearing issues alone... I think we could "Listen" for it easier. In a modern automotive engine, there are devices called Knock sensors. They are basically an audio device, in an industrial package. The circuit involved would be listening for high pitch, or "Sharp" audio spikes. Basically, when the balls in the bearings develop flat spots... they will produce knocks.

This kind of device would not be influenced by air-frame vibration, or high-G maneuvers.


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RE: Turbine Vibration - 4/11/2012 8:47 PM   
InboundLZ



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Status: offline 		Seems to me, if the sensor was installed ON the engine (internally) you should be able to get a very specific reading from that engine. Every engine will be different (slightly) so it would be impossible for a manufacture to publish go/no-go specs but, if the owner/operator was to monitor vibes deviation from normal would be easy to observe. As data is collected and analyzed patterns would emerge and this could be used to determine condition. In our full scale equipment millions of hours of data are collected by the operators and sent to the manufacture for analyzation, we don't have that benefit but nonetheless, the data can be useful to a modeler that monitors such things.



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RE: Turbine Vibration - 4/11/2012 8:52 PM   
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Status: offline 		I think vibration monitoring could easily be possible but I lean more to the 'fuel and fly' side and don't spend too much time recording data for it to bother me too much.

The trick wouldn't be in the measuring but in the analysis and there would need to be a way of determining the frequency and amplitude of vibrations so that you could distinguish engine related problems from general airframe 'noise'. I guess it would be fairly easy to get a rough idea of what to look for but you might have to run an engine with a dodgy bearing to see where the spikes would be in the spectrum. This brings back memories, as I have done some research into this when I was at university by spinning a Ramtec fan and measuring the vibration signature, and then cutting off part of one of the blades and doing it again.

In my opinion though, it is a cost/effort versus benefit issue. Model turbine failures, thankfully, appear to be pretty rare. Worn bearings can often be detected by ear before failure (although I think that a properly set up accelerometer might pick this up even before you could hear it) and catastrophic failures might not be detected in time anyway.

BTW, isn't sound a form of vibration in the audible spectrum? If you say that you are listening for problems then are you not, in fact, monitoring vibration?



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RE: Turbine Vibration - 4/11/2012 8:58 PM   
InboundLZ



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quote:

ORIGINAL: siclick33

I think vibration monitoring could easily be possible but I lean more to the 'fuel and fly' side and don't spend too much time recording data for it to bother me too much.

The trick wouldn't be in the measuring but in the analysis and there would need to be a way of determining the frequency and amplitude of vibrations so that you could distinguish engine related problems from general airframe 'noise'. I guess it would be fairly easy to get a rough idea of what to look for but you might have to run an engine with a dodgy bearing to see where the spikes would be in the spectrum. This brings back memories, as I have done some research into this when I was at university by spinning a Ramtec fan and measuring the vibration signature, and then cutting off part of one of the blades and doing it again.

In my opinion though, it is a cost/effort versus benefit issue. Model turbine failures, thankfully, appear to be pretty rare. Worn bearings can often be detected by ear before failure (although I think that a properly set up accelerometer might pick this up even before you could hear it) and catastrophic failures might not be detected anyway.

BTW, isn't sound a form of vibration in the audible spectrum? If you say that you are listening for problems then are you not, in fact, monitoring vibration?




siclick33, they don't call me the "over-engineererr" (extra errr for emphasis) for nothing! Just think that pushing things like this only serves to start a dialog that improves these engines overall. Maybe the idea has no merit, but maybe it does....I imagine that the internal Kero-start idea may have met with the same opposition because "propane start works in my jet every time so why change?" attitude, but hero-start is the only way to go nowadays!

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RE: Turbine Vibration - 4/11/2012 9:03 PM   
seflyer


 

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Status: offline 		the noise from sensors can be weeded out but requires a tremendous amount of software to be written and the end results could be promising but the practicality is not super reasonable, noise of bearings is usually a good indicator that a service is required and gives adequate warning it is needed. I agree with you Dave that the info could be useful but these are 2-5k model engines not 100k full scale turbines or turboprops. the effort required to make the system light enough and be practical for install in model aircraft was not in my opinion worth it.

Kevin

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RE: Turbine Vibration - 4/11/2012 9:40 PM   
Dr Honda



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the noise from sensors can be weeded out but requires a tremendous amount of software to be written



Exactly. Even in modern balancing machines... they aren't all using acceleromerters. They use Pizo devices, and op-amps.




quote:

ORIGINAL: seflyer
noise of bearings is usually a good indicator that a service is required and gives adequate warning it is needed.



This is also a good point. Since starting into turbines, I've been absolutely amazed with how many noisy engines I hear, and how many people continue to run them saying... "It's always sounded like that"... OR... "It's not at 25hrs yet."

I know that once in a while... you may chuck a compressor... but a simple "finger on the starter pod" after you start the engine will tell you if the bearings are happy.




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RE: Turbine Vibration - 4/11/2012 10:14 PM   
afterburner



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ORIGINAL: InboundLZ

I think the benefit would come in the form of early warning that a bearing is starting to fail. I can tell you that engine vibration is very accurate in the full scale applications, our 10 million dollar CFM turbines use vibration monitoring to determine health and can give very early indications of bearing failure long before it becomes a engine damaging event. So, I guess I disagree in your statement that engine vibration monitoring is more of a determent than a help....

Back on point, some of us just fly and as long as the engine starts we go flying. Then there are others that monitor each start, engine parameters durning flight, and monitor shutdown. These readings can give good indication of component health I.E Pump voltage increase from norm might indicate a plugged filter or possibly a pump that is failing, High EGT might indicate a internal problem with the hot section, and so on.

I like to monitor my engines closely so I can have every advantage in keeping my model in one big piece. I would love to see EPR, Vibes, fuel flow, etc....


Dave,
I'm not going to agree or disagree on the benefit of having a vibration sensor but I think a more important question might be to look at the percentage of jets lost due to engine failure compared to radio, servo, battery failure or pilot error. Like DW said, we start to hear things and notice irregularities and what's when we need to get it checked out. Maybe I'm not watching enough youtube crash videos but I'm thinking in air engine failure is a very small percentage with today's name brand turbines. JMO.

Marty

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RE: Turbine Vibration - 4/11/2012 10:32 PM   
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Status: offline 		quick and dirty vibration check is to lay a screw driver on the turbine case, you'll hear any vibration as it rattles the screw driver on the metal. or you can feel like Tony said by laying your fingernail on the case too.

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RE: Turbine Vibration - 4/12/2012 2:43 AM   
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As above... use of in-flight accelerometers would give poor, or unreliable readings. BUT... if we were looking for bearing issues alone... I think we could ''Listen'' for it easier. In a modern automotive engine, there are devices called Knock sensors. They are basically an audio device, in an industrial package. The circuit involved would be listening for high pitch, or ''Sharp'' audio spikes. Basically, when the balls in the bearings develop flat spots... they will produce knocks.

This kind of device would not be influenced by air-frame vibration, or high-G maneuvers.



Automotive Knock Sensors detect spark knock or "pinging" due to a variety of reason, most commonly is low octane fuel. When knock is detected the PCM retards the ignition timing to stop the knock. I guess what you are suggesting could be designed but it would have to know what to listen for...

I think these guys on this video clip have it all figured out !!!!


http://www.poetv.com/video.php?vid=100243



Danno

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RE: Turbine Vibration - 4/12/2012 7:07 AM   
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Status: offline 		I have done some work with a guy who was doing seeded fault testing of helicopter gearboxes and measuring the "noise" signature this created with a series of sensors including accelerometers and piezos. The idea here is you stick a bad bearing, gear, etc into the gearbox and then run it under normal loaded conditions and data log the traces as it fails. You can imagine the complexity and cost of such an endeavour as you try to create every possible failure case and do destructive testing for each one. You can burn thru countless test stand time, hardware and $ and in the end the whole thing would rely on some really sophisticated software to do anything meaningful with the results. The same testing was done for locomotives.

It's way easier to set maintenance intervals and go from there.

PaulD


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RE: Turbine Vibration - 4/12/2012 8:53 AM   
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RE: Turbine Vibration - 4/12/2012 10:54 AM   
Taz_Hobbies


 

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quote:

ORIGINAL: rcjetsaok


quote:

ORIGINAL: Dr Honda

As above... use of in-flight accelerometers would give poor, or unreliable readings. BUT... if we were looking for bearing issues alone... I think we could ''Listen'' for it easier. In a modern automotive engine, there are devices called Knock sensors. They are basically an audio device, in an industrial package. The circuit involved would be listening for high pitch, or ''Sharp'' audio spikes. Basically, when the balls in the bearings develop flat spots... they will produce knocks.

This kind of device would not be influenced by air-frame vibration, or high-G maneuvers.



Automotive Knock Sensors detect spark knock or "pinging" due to a variety of reason, most commonly is low octane fuel. When knock is detected the PCM retards the ignition timing to stop the knock. I guess what you are suggesting could be designed but it would have to know what to listen for...

I think these guys on this video clip have it all figured out !!!!


http://www.poetv.com/video.php?vid=100243



Danno


And those knock sensors are pretty primitive. They are "listening" for pre-ignition, which in many instances, can be heard by the human ear 2 blocks away in most cases. "knock" noise in an internal combustion engine is akin to whacking a hammer against a steel anvil as hard as you can possibly swing.



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RE: Turbine Vibration - 4/12/2012 11:20 AM   
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Status: offline 		Integrating vibration monitors into a model jet engine seems an awful lot of work for very little return. An easier way of health monitoring is simply to log EGTs at idle and full power and watch the trend. ( I will soon be doing this automatically and logging the values on my Weatronics radio with the new temp sensors). ANY deterioration of the engine is ALWAYS indicated by an increase in EGT and bearing health is also easily monitored by timing period from fuel cutoff from idle to stopped. (source: RR 757 RB 211 field engineer )

Noisy(ish) bearings can continue in service but should be monitored, according to the AMT manual !

Readyturn may be a little misplaced in denigrating full-size vibration monitors, the one time the I have seen a significant increase in vibration in flight was followed rapidly by engine failure (RR Conway on BA VC10 losing most of the HP turbine blades). At least it gave us warning of an impending failure and we were ready for it. Most engine manufacturers and the operators, still use them to monitor trends, but for model engines what's the real gain ?

I never cease to be amazed at just how reliable the current crop of model gas turbines are, particularly when you see how some people (mis)treat them !

Regards,

David.

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RE: Turbine Vibration - 4/12/2012 12:29 PM   
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Status: offline 		Knowing what to listen for is the best means. Turbines that are out of balance can be heard to whine in flight. This leads to wear on the bearing races which in turn is easily recognised during shut down or startup as a "siren" type rough sounding noise.
It is also possible for a well balanced rotor to develop wear for various reasons and exibit the same "siren" sound. This is not neccesarily so serious, depending on the bearing seating and preload. Which is what AMT is refering to probably.
However poor balancing coupled with poor bearing seating and preload on some engines will lead to a very likely catastrophic failure. This can go 2 ways. A seizure with rotor rub, or extreme stress on compressor plade tips coupled with near stall surge will cause tips to fracture. This is often mistakenly attributed to compressor failure, hence the promoting of billit machined compressors.
Primary cause of this is the failure to properly balance the compressor dynamically in 2 planes. Many do this for the rotor as a whole only. The resulting compressor and overhang combo then start to "whirl". Early indications are the front blades rubbing the intake.
Rotor and shaft should never be able to develop any play that you can feel with finger pressure alone.
Hope this helps.
Andre



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RE: Turbine Vibration - 4/12/2012 2:01 PM   
Dr Honda



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Automotive Knock Sensors detect spark knock or ''pinging'' due to a variety of reason, most commonly is low octane fuel. When knock is detected the PCM retards the ignition timing to stop the knock. I guess what you are suggesting could be designed but it would have to know what to listen for...

....

Danno



Sort of. The sensor itself is basically a microphone that is designed around amplitude spikes. The supporting electronics determines what it's listening for. Since we dont' have to worry about "Combustion Knock"... we could make the electronics listen for bearing knock. The only thing that may be hard to do is to get the frame rate of the electronics into the RPM range of the engine. Realistically... it may only be functional at idle since the tiny little balls may add a factor of 10 (or more) to the RPM's.


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RE: Turbine Vibration - 4/12/2012 2:10 PM   
Dr Honda



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And those knock sensors are pretty primitive. They are ''listening'' for pre-ignition, which in many instances, can be heard by the human ear 2 blocks away in most cases. ''knock'' noise in an internal combustion engine is akin to whacking a hammer against a steel anvil as hard as you can possibly swing.





Well... yes and no.

YES... they are primitive. As I said... they are basically a microphone in an industrial case. But this would work to our advantage since we would be listening for a "Primitive" noise.


AND....


NO... you will never hear "Knock" in a modern engine. This is because, the electronics have gotten better, and the sensor itself is very sensitive. Besides... if you can hear engine knock from 2 blocks away... that engine isn't going to live long. My favorite quote was from Bernie Bergman. (a VW builder back in the day) "By the time you can hear ignition knock... it's the equivalent of hitting the tops of your pistons with a 10# sledge hammer, as hard as you can."


The problem with using accelerometers is that there are too many outside influences, (Engine balance, air-frame vibration, high-G maneuvers, etc) and most of the time... They bearings are just "Noise" to them. Remember... they are looking for an acceleration, in some direction, that is measurable. Noisy bearings offer very little directional acceleration. It's simply the wrong sensor for the job.



OK... and to clear up any uncertainty... here's my Wren 44 with bad bearings. (you can really hear them as it's cooling down) This engine ran fine, but I'm sure it would have eaten itself in sort order.


http://youtu.be/oCVKIkYhfow


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