QANTAS A380 Uncontained failure.
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WojtekSz
No. Not "fluid" drive in the sense of "Hydraulic", where the medium is contained. The Gas Path does have open ends at either end, so technically it does not qualify. In addressing this problem, the "Rigid Coupling", it isn't relevant, imo. The Vibration (causatuve of rupture) had alot of time to act on the weak portion of the design, even at that both Disc fractures occurred on climbout, and one of the Powerplants was a TRENT 700. The identified problem was not embraced by RR at any time, whether inadvertent or some thing more diabolical. They have the benefit of the doubt, as far as I am concerned. That covers the Design, Test, and Certification modes. What the firm did after the Burst/Explosion is inexcusable, keeping a client (QANTAS) in total darkness, while they casually (and cynically) completed newer Airframes for delivery. They allowed the Engines to "Time In" because they had duped EASA into relaxing the Inspex. Even in that, they almost made it.
They will correct, and move on. Hopefully, the fallout from their self sabotage will hurt deeply, suffiicient to caution themselves and others against playing Roulette with the lives of paying innocents.
No. Not "fluid" drive in the sense of "Hydraulic", where the medium is contained. The Gas Path does have open ends at either end, so technically it does not qualify. In addressing this problem, the "Rigid Coupling", it isn't relevant, imo. The Vibration (causatuve of rupture) had alot of time to act on the weak portion of the design, even at that both Disc fractures occurred on climbout, and one of the Powerplants was a TRENT 700. The identified problem was not embraced by RR at any time, whether inadvertent or some thing more diabolical. They have the benefit of the doubt, as far as I am concerned. That covers the Design, Test, and Certification modes. What the firm did after the Burst/Explosion is inexcusable, keeping a client (QANTAS) in total darkness, while they casually (and cynically) completed newer Airframes for delivery. They allowed the Engines to "Time In" because they had duped EASA into relaxing the Inspex. Even in that, they almost made it.
They will correct, and move on. Hopefully, the fallout from their self sabotage will hurt deeply, suffiicient to caution themselves and others against playing Roulette with the lives of paying innocents.
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Derg:
using the finite element method scientist/engineers can predict mechanical beahviour of any part in known conditions. Actually they can model it according to known and proven cases. Problem starts when we try to predict/calculate mechanical behaviour in the unknown/unpredictable/non-linear situations like worn bearing where actual 'worn' may have very different meanings including radial and/or axial play with different dampening properties. This calculation comes even worse if we get closer to loose spline coupling - i would bet a pint of guiness that there are no actual models for calculating behavoiur of loose spline couplings - simply because these couplings are designed to STAY and not to get LOOSE.
Once i heard a story of an experienced machine-tool designer who used to say: ok, so we have built it. Let's start it and see why it doesn't work.
using the finite element method scientist/engineers can predict mechanical beahviour of any part in known conditions. Actually they can model it according to known and proven cases. Problem starts when we try to predict/calculate mechanical behaviour in the unknown/unpredictable/non-linear situations like worn bearing where actual 'worn' may have very different meanings including radial and/or axial play with different dampening properties. This calculation comes even worse if we get closer to loose spline coupling - i would bet a pint of guiness that there are no actual models for calculating behavoiur of loose spline couplings - simply because these couplings are designed to STAY and not to get LOOSE.
Once i heard a story of an experienced machine-tool designer who used to say: ok, so we have built it. Let's start it and see why it doesn't work.
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Bearfoil:
do i get you right: technically they could have made a mistake but instead of making it clear they kept on selling the stuff hoping to get away with it?
Do you believe RR have already identified the real cause and having it fixed even before the first engine exploded, just wanted the market to stay in the dark for PR or just any other business reason.? The Greed in action?
do i get you right: technically they could have made a mistake but instead of making it clear they kept on selling the stuff hoping to get away with it?
Do you believe RR have already identified the real cause and having it fixed even before the first engine exploded, just wanted the market to stay in the dark for PR or just any other business reason.? The Greed in action?
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WojtekSz
IMO Rolls had identified the problem prior to the AD. They negotiated a schedule of inspections meant to monitor the wear on the Splines at Abutment Face. Clearly the AD took into account the actual cause of the problem, for goodness' sake, they approved the Fix!! The "C" modification was to have mooted all concern, for the Engines under the AD's demands were to have been pulled from service so the Power was "Exonerated". The Explosive Burst put everything into limbo. It became clear Rolls was furnishing the new engine to Airbus for facilitation of delivery of new aircraft, whilst the 972 was on wing, subjecting each flight to hazards at clearly, (Demonstrably) unacceptable levels.
How could RR not have known the source of the trouble and then claimed the "C" mod was sufficient?? Magic?? Guesswork?? Chicken Guts??
The relaxation of the inspection regimen is on the EASA's shoulders solely. They put their faith in RR and their bluff, as RR cynically advantaged the gullibility of the regulator, and their Prime client for the TRENT 972. (QANTAS).
It turns out the C mod is under the gun, and QANTAS is taking their sweet time approving LAX SYD flights at max thrust (max TOW) Where does this put the TRENT 900?? Do-Overs?? How about, 'once burned, twice shy'.......
IMO Rolls had identified the problem prior to the AD. They negotiated a schedule of inspections meant to monitor the wear on the Splines at Abutment Face. Clearly the AD took into account the actual cause of the problem, for goodness' sake, they approved the Fix!! The "C" modification was to have mooted all concern, for the Engines under the AD's demands were to have been pulled from service so the Power was "Exonerated". The Explosive Burst put everything into limbo. It became clear Rolls was furnishing the new engine to Airbus for facilitation of delivery of new aircraft, whilst the 972 was on wing, subjecting each flight to hazards at clearly, (Demonstrably) unacceptable levels.
How could RR not have known the source of the trouble and then claimed the "C" mod was sufficient?? Magic?? Guesswork?? Chicken Guts??
The relaxation of the inspection regimen is on the EASA's shoulders solely. They put their faith in RR and their bluff, as RR cynically advantaged the gullibility of the regulator, and their Prime client for the TRENT 972. (QANTAS).
It turns out the C mod is under the gun, and QANTAS is taking their sweet time approving LAX SYD flights at max thrust (max TOW) Where does this put the TRENT 900?? Do-Overs?? How about, 'once burned, twice shy'.......
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We could all do with a pint of guiness right now I think .. but as yet it is somewhat premature - there is still much to be resolved.
But was there ever a pprune reunion? There's a long way to go yet, but it's a cool thought.
But was there ever a pprune reunion? There's a long way to go yet, but it's a cool thought.
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Not only counterfeit parts from bogus suppliers, but substandard parts from "approved" suppliers.
There's been a recurring problem - going back decades - with fasteners that look OK, dimensionally OK, static strength OK, but they don't meet the fatigue strength spec. It's affected all the engine mfrs. and tinknockers at one time or another. It results from cost-reduction shortcuts at the suppliers' shops.
FAA has cracked down - even the FBI was involved - and the problem has quieted down from its worst days.
There's been a recurring problem - going back decades - with fasteners that look OK, dimensionally OK, static strength OK, but they don't meet the fatigue strength spec. It's affected all the engine mfrs. and tinknockers at one time or another. It results from cost-reduction shortcuts at the suppliers' shops.
FAA has cracked down - even the FBI was involved - and the problem has quieted down from its worst days.
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DERG
Oil Consumption
For engines that use labyrinth seals, oil consumption is largely dependent on vent air-flows and oil temperatures. As the seals wear, oil consumption rates go up. Depending on the engine design, typical consumption rates would be in the range of 0.05 to 0.30 gallons (0.19 to 1.14 liters) per hour.
SFC - Trent verses GP
One of the things that strikes me on the SFC situation is the difference between a well designed two spool engine verses a three spool design. IMO, a bare two spool engine in a test cell will have better SFC than a three spool RR engine every time. The reason has to do with the more efficent HPT of a two spool engine. It is a different story on wing. The shorter three spool engine in a shorter nacelle makes up the difference of SFC shortfall in the test cell. The reason is less aircraft drag. However, looking at the Trent 900 & the GP7200 on the A-380 wing, a longer nacelle is used for both engines and the shorter engine advantage of a three spool engine is lost. I do believe, at this moment, the GP7200 does have a SFC advantage over the Trent 900 and RR is working the issue. This may also be an issue on the Trent 1000 verses the GEnx on the B787.
Just some thoughts,,,
Oil Consumption
For engines that use labyrinth seals, oil consumption is largely dependent on vent air-flows and oil temperatures. As the seals wear, oil consumption rates go up. Depending on the engine design, typical consumption rates would be in the range of 0.05 to 0.30 gallons (0.19 to 1.14 liters) per hour.
SFC - Trent verses GP
One of the things that strikes me on the SFC situation is the difference between a well designed two spool engine verses a three spool design. IMO, a bare two spool engine in a test cell will have better SFC than a three spool RR engine every time. The reason has to do with the more efficent HPT of a two spool engine. It is a different story on wing. The shorter three spool engine in a shorter nacelle makes up the difference of SFC shortfall in the test cell. The reason is less aircraft drag. However, looking at the Trent 900 & the GP7200 on the A-380 wing, a longer nacelle is used for both engines and the shorter engine advantage of a three spool engine is lost. I do believe, at this moment, the GP7200 does have a SFC advantage over the Trent 900 and RR is working the issue. This may also be an issue on the Trent 1000 verses the GEnx on the B787.
Just some thoughts,,,
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DERG:
I honestly do not recall any oil consumption figures, other than it might be comparable to a Continental A-65 in my old Taylorcraft.
The vent system design has a lot to do with overboard oil loss. A good centrifugal air-oil separator (sometimes built right into the main shafts) can make a big difference.
Certainly excess oil consumption can indicate internal problems that need addressing, and not neglected! But do we know for sure the sequence of events in the present case?
I(t) seems to me that if oil was slopping around in that T-972 then maybe a few more tins than ususual would have been used. What you do you think?
The vent system design has a lot to do with overboard oil loss. A good centrifugal air-oil separator (sometimes built right into the main shafts) can make a big difference.
Certainly excess oil consumption can indicate internal problems that need addressing, and not neglected! But do we know for sure the sequence of events in the present case?
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FAA has cracked down - even the FBI was involved - and the problem has quieted down from its worst days.
I personally watched the FBI raid an aircraft repair station, haul away many records, two helicopters...and three persons...in cuffs.
The FAA and FBI appeared to mean business, as in...lock 'em up.
Oil consumption is a maintenance issue and rarely a safety issue beyond being an indicator needing other parameters to be considered (vibration, Oil temp, EGT)
Oil leakage can be a flight safety issue if you don't know where the oil is going. I've seen some operators who added quarts of oil at every major turn, figuring that next week they could get the engine changed out. Only they never made it to next week before they blew a turbine out the side of the engine. On the other hand when I was aboard one of these operators on a long haul over water, the pilot got a low oil light and turned back. Sure enough the ground crew pushed a stand up to the engine as we, the passengers watched, and there on top of the stairs was a full case of oil.
Ah but it was good news, they poured in the oil and motored the engine and presto out came the oil from a blown CSD gasket (external from the engine). That confirmed where the oil had gone! So they replaced the gasket and we were off again for our flight. Had they simply replaced the oil, I would have gotten off (I did chat with the capt about this
Most of the oil consumption is in fumes out the breather and thankfully the pressure balances do a good job of keeping the oil out of rotor-disk cavities. The problem is when wet oil finds it's way into a rotor disk cavity where it is often hot enough to light it off.
Oil leakage can be a flight safety issue if you don't know where the oil is going. I've seen some operators who added quarts of oil at every major turn, figuring that next week they could get the engine changed out. Only they never made it to next week before they blew a turbine out the side of the engine. On the other hand when I was aboard one of these operators on a long haul over water, the pilot got a low oil light and turned back. Sure enough the ground crew pushed a stand up to the engine as we, the passengers watched, and there on top of the stairs was a full case of oil.
Ah but it was good news, they poured in the oil and motored the engine and presto out came the oil from a blown CSD gasket (external from the engine). That confirmed where the oil had gone! So they replaced the gasket and we were off again for our flight. Had they simply replaced the oil, I would have gotten off (I did chat with the capt about this
Most of the oil consumption is in fumes out the breather and thankfully the pressure balances do a good job of keeping the oil out of rotor-disk cavities. The problem is when wet oil finds it's way into a rotor disk cavity where it is often hot enough to light it off.
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barit1 DERG
The Centrifugal Breather is located in the external gearbox. It is on the rear face of the gearbox between the oil pump and the Fuel Pump (HP). Aerated Oil, returning from the bearing boxes via the vent pipes and scavenge system, enters the oil tank, where it de-aerates (partially). The oil is sent then to the Centrifugal Breather where it attempts to pass through the rotating retimet, but is centrifuged out, to return to the Oil Tank. The air is dumped overboard. The Oil returns through the Oil Scavenge Pump Elements.
The Centrifugal Breather is located in the external gearbox. It is on the rear face of the gearbox between the oil pump and the Fuel Pump (HP). Aerated Oil, returning from the bearing boxes via the vent pipes and scavenge system, enters the oil tank, where it de-aerates (partially). The oil is sent then to the Centrifugal Breather where it attempts to pass through the rotating retimet, but is centrifuged out, to return to the Oil Tank. The air is dumped overboard. The Oil returns through the Oil Scavenge Pump Elements.
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There is one piece of evidence to what is the root cause of the oil pipe cracking which has not been debated here. In the court filing that Qantas filed there is the following statement (from flightglobal http://www.flightglobal.com/blogs/wings-down-under/2010/12/fixing-rolls-royces-disposable-a380-engines.html)
This seems to indicate that the pressure causes movement of the structure where the stub oil pressure pipe enters into the bearing box. My conclusion is that on the A and B engines the support structure between HPT and IPT is simply not strong enough to stop the bearing box from moving at max thrust to such a level that it affects the pipes fatigue life. The solution is a strenghtend support structure. It is a big operation and explains why in the A version the problem was dectected but to late to fix it for production of the engine, the B mod is a reinforcement of the original structure and the C mod is a redesigned support structure that fixes the problem (but had to be extensively tested before applying it). The flight blogger concludes that the oil pipe needed strengthening, this is an easy fix and should not have required 3 different versions of the engine therefore I don't think this is the fix in the C mod. Further the crack is where the stub oil pipe meets the bearing box, if the box is moving a stronger pipe is not the fix, you need a more flexible and durable pipe but primarly a box that is not moving.
So what is the communities thought about this piece of evidence?
However, the extra thrust exposes the engine to 540 psi at P30, which causes the engine to experience "high severity", the affidavit says. Rolls-Royce's interim suggestion to Qantas has to been to derate the engines in order to "reduce the engine pressure ratio in the 'P30' area of the engine and therefore increase the life of the oil transfer tubes within the HP/IP support structure", the affidavit says.
So what is the communities thought about this piece of evidence
?
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Well, I am glad to see that after that excursion into oil related items we have come back to technical questions. Oil supply, that is my strongest believe, has never been a problem in this engine failure. The monitoring readings published by ATSB clearly show that there was sufficient / normal oil pressure on the feeds untill the very last seconds of the usable life of that engine.
So now the question should concentrate on what has, respectively what will RR change to make these Trents as healthy as planned ?? A lot of approaches to the problems identified in this thread are thinkable, to start from cheap to expensive:
usage of sturdier oil feed tubes? re-desighn of bearingchamber and use of stronger bearings ? re-desighn of supporting structures ? re-desighn of shafts ? or the finite decision, a re- desighn of the entire core engine ?
Would be interesting to read if anywhere a leak has opened in that wall of silence RR is hiding behind ?
I think the basics of that engine are delicate and the craftsmenship with which the modules are assembled and put together are out of doubts. So what is left ?? A real engineering fault ? Or just a too early stop in development and testing, ordered by the beanscounters ?? Anyway, as it looks something very substantial must have slipped through the controlling systems! Probably just an internal communications break / interruption ?? Probably a set of wrong priorities ?? Looking foreward to more and substantial information!!
Jo
So now the question should concentrate on what has, respectively what will RR change to make these Trents as healthy as planned ?? A lot of approaches to the problems identified in this thread are thinkable, to start from cheap to expensive:
usage of sturdier oil feed tubes? re-desighn of bearingchamber and use of stronger bearings ? re-desighn of supporting structures ? re-desighn of shafts ? or the finite decision, a re- desighn of the entire core engine ?
Would be interesting to read if anywhere a leak has opened in that wall of silence RR is hiding behind ?
I think the basics of that engine are delicate and the craftsmenship with which the modules are assembled and put together are out of doubts. So what is left ?? A real engineering fault ? Or just a too early stop in development and testing, ordered by the beanscounters ?? Anyway, as it looks something very substantial must have slipped through the controlling systems! Probably just an internal communications break / interruption ?? Probably a set of wrong priorities ?? Looking foreward to more and substantial information!!
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This is off the main line direction in this thread as to the whys and wherefores this engine seems to be suffering from some "built-in" shortcoming(s), but I was wondering if one of you could address a few questions as to the failure mode of the LPT disk itself. As a non-engineer, I've been musing about the "normal" operating speed of these turbines vis-a-vis the rotational speed necessary for self-destruction. (I'm retired... and I can afford to play golf and muse)
My questions start with a very basic one, which is, in the quest for efficiency and low mass, are these units designed to spin in a near supersonic regime, or do they experience tip velocity excursions approaching SS, say at the highest power settings, like N1 does?
Its moot to the cause of this engines demise, but, regardless of whether the LPT may or may not ever operate near the "shock" regime, at some point after the output shaft separation, and prior to disk impingement with the stators, could the free wheeling disk (blades) have been almost instantly accelerated past Mach 1 prior to the disk burst point? If so, could the blades have failed first from "swallowed shock," their sequential departure thus initiating intolerable out of balance, and, therefor, premature disk rupture?
If management of near supersonic blade tip velocity is, in fact, a parameter in this engines software, its actions in this regime and any harmonic issues impinging on the bearing box would have been completely addressed... No surprises there, right?
My, oh, My! You all have an extremely interesting discussion on-going here.
My questions start with a very basic one, which is, in the quest for efficiency and low mass, are these units designed to spin in a near supersonic regime, or do they experience tip velocity excursions approaching SS, say at the highest power settings, like N1 does?
Its moot to the cause of this engines demise, but, regardless of whether the LPT may or may not ever operate near the "shock" regime, at some point after the output shaft separation, and prior to disk impingement with the stators, could the free wheeling disk (blades) have been almost instantly accelerated past Mach 1 prior to the disk burst point? If so, could the blades have failed first from "swallowed shock," their sequential departure thus initiating intolerable out of balance, and, therefor, premature disk rupture?
If management of near supersonic blade tip velocity is, in fact, a parameter in this engines software, its actions in this regime and any harmonic issues impinging on the bearing box would have been completely addressed... No surprises there, right?
My, oh, My! You all have an extremely interesting discussion on-going here.
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Re DERG's post # 296 Spline Fretting and stuff
Hopefully some of you gentlemen can advise me, re, on page 3 of the PDF file
http://workspace.imperial.ac.uk/medy...ng_Trb2008.pdf
the illustration shows an " External spline run-out " on the external component. Is this the " 2.65 mm spline crown " dimension as mentioned in the original AWD, re the spline wear problem and is this a result of uncontrolled/unexpected wear of the thrust washers from vibration/harmonics ? Is the disintegrated turbine disc the equivalent of the " Internal component " ? I appreciate that this drawing is not type specific, but I have been confused as to what the 2.65 mm referred and there were no mentions of which axial faces were wearing.
Also, is the helix on the splines designed to continually apply the thrust loading towards the thrust washers ? Thank you..
http://workspace.imperial.ac.uk/medy...ng_Trb2008.pdf
the illustration shows an " External spline run-out " on the external component. Is this the " 2.65 mm spline crown " dimension as mentioned in the original AWD, re the spline wear problem and is this a result of uncontrolled/unexpected wear of the thrust washers from vibration/harmonics ? Is the disintegrated turbine disc the equivalent of the " Internal component " ? I appreciate that this drawing is not type specific, but I have been confused as to what the 2.65 mm referred and there were no mentions of which axial faces were wearing.
Also, is the helix on the splines designed to continually apply the thrust loading towards the thrust washers ? Thank you..
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The "Tube" is not the problem . It is the result. No matter the cause of the damage to the "Stub". Misbore or Vibration, Box movement (!), etc. The Support Structure is insufficient. In other words, the Core has mass issues, as DERG points out (as have I).
The Splines too, are not the problem. They are also a result.Oil problem? Certainly, but again, not the cause.
If the Splines were the only issue, There would be no AD, there would have been a revocation of the certificate. "Inspections only" certify that both Rolls and EASA were cognizant of the procuring cause. Again, Simply Certifying the "C" Mod as curative means that the source of the problem was KNOWN, and from the beginning. We do not know if the "C" is the complete answer, simply because QANTAS is not flying it, nor have the EASA certified the Engine as yet, UNDER THE ORIGINAL CERTIFICATE. Inspections are still required.
Think about that. This Engine has had two rebuilds since approval, and still the Engine is not released for its predicted service life. The Engine is still in TEST, can one see it any other way?? Either the engine itself with the C is complete, and needs no re-cert, or it is not, and its reliability is subject to doubt.
10 hours over water. That's the "good news". The bad? Ditch, Crash, loss of life.
Vibration, Resonance, lack of sufficient mass, destructive rapid wear. This engine had an oil fire on the stand, Remember? What happens when the Stub (IP) shaft is severed next time? At some point, the word Burst will have to be retired. It is insufficient to describe what may as yet prove out.
. It is the result. No matter the cause of the damage to the "Stub". Misbore or Vibration, Box movement (!), etc. The Support Structure is insufficient. In other words, the Core has mass issues, as DERG points out (as have I).The Splines too, are not the problem. They are also a result.Oil problem? Certainly, but again, not the cause.
If the Splines were the only issue, There would be no AD, there would have been a revocation of the certificate. "Inspections only" certify that both Rolls and EASA were cognizant of the procuring cause. Again, Simply Certifying the "C" Mod as curative means that the source of the problem was KNOWN, and from the beginning. We do not know if the "C" is the complete answer, simply because QANTAS is not flying it, nor have the EASA certified the Engine as yet, UNDER THE ORIGINAL CERTIFICATE. Inspections are still required.
Think about that. This Engine has had two rebuilds since approval, and still the Engine is not released for its predicted service life. The Engine is still in TEST, can one see it any other way?? Either the engine itself with the C is complete, and needs no re-cert, or it is not, and its reliability is subject to doubt.
10 hours over water. That's the "good news". The bad? Ditch, Crash, loss of life.
Vibration, Resonance, lack of sufficient mass, destructive rapid wear. This engine had an oil fire on the stand, Remember? What happens when the Stub (IP) shaft is severed next time? At some point, the word Burst will have to be retired. It is insufficient to describe what may as yet prove out.
radken
\
Happy to see that you are at least prone to asking questions before forming ill advised opinions
You questions about speed have two parts imbedded in them. One part has to do with aerodynamics which of course has something to do with shock waves. The secoind part is a structural question which has a great deal to do with stress and strain.
Talking about speed of sound at the largest diameter (of a rotor) in the fan is easier to comprehend since it operates at a more familiar ambient temperature and pressure. On the other hand deep inside the engine the pressures and temperatures are quite extreme so the speed of sound would be far different. Simplistically the engine revolves around a defined cycle and as such the designer goes for the most effeciency per pound of thrust per pound of fuel. Thus one designs the compressor to operate at the highest pressure that can be obtained without tip stall or any other perturbation you might want to call it, including shock waves etc. The turbine driving the compressor had about the same tip diameter so its surface speed at its tips are about the same.
The bottom line after this is to ensure that centrifugal stresses are kept within an acceptable envelop with substantial margins against combinations of overspeeds and vibratory modes that might combine and produce rapid fatigue cracking. This is basic regulatory stuff and codified in the design regulations.
The problem comes in when the operation of the engine is outside of the certifcated conditions. Either from mis-operation of the engine (including maintainence) or mis-manufacturerd parts.
Vibratory modes are predictable and calibrated via experience including the hundreds of hours in development testing before the engine is certified. So obviously no mis-designed or undertested severe failure conditions are expected in service before any recommended maintainence cycles. assuming of course no mis-operation or mis-manufacturerd parts.
I've tried to keep this brief and cover some nuggets of your questions.
Just one editorial comment; don't confuse low mass with low cost or low safety margins. Margins of safety are regulated and it's stiffness and dampening that make it possible to design for light weight and still meet safety margins, else all the plumbing visible around the outside of engines wouldn't be called tubing but would be made out of thick steel pipe
This is off the main line direction in this thread as to the whys and wherefores this engine seems to be suffering from some "built-in" shortcoming(s), but I was wondering if one of you could address a few questions as to the failure mode of the LPT disk itself. As a non-engineer, I've been musing about the "normal" operating speed of these turbines vis-a-vis the rotational speed necessary for self-destruction. (I'm retired... and I can afford to play golf and muse)
My questions start with a very basic one, which is, in the quest for efficiency and low mass, are these units designed to spin in a near supersonic regime, or do they experience tip velocity excursions approaching SS, say at the highest power settings, like N1 does?
Its moot to the cause of this engines demise, but, regardless of whether the LPT may or may not ever operate near the "shock" regime, at some point after the output shaft separation, and prior to disk impingement with the stators, could the free wheeling disk (blades) have been almost instantly accelerated past Mach 1 prior to the disk burst point? If so, could the blades have failed first from "swallowed shock," their sequential departure thus initiating intolerable out of balance, and, therefor, premature disk rupture?
If management of near supersonic blade tip velocity is, in fact, a parameter in this engines software, its actions in this regime and any harmonic issues impinging on the bearing box would have been completely addressed... No surprises there, right?
My questions start with a very basic one, which is, in the quest for efficiency and low mass, are these units designed to spin in a near supersonic regime, or do they experience tip velocity excursions approaching SS, say at the highest power settings, like N1 does?
Its moot to the cause of this engines demise, but, regardless of whether the LPT may or may not ever operate near the "shock" regime, at some point after the output shaft separation, and prior to disk impingement with the stators, could the free wheeling disk (blades) have been almost instantly accelerated past Mach 1 prior to the disk burst point? If so, could the blades have failed first from "swallowed shock," their sequential departure thus initiating intolerable out of balance, and, therefor, premature disk rupture?
If management of near supersonic blade tip velocity is, in fact, a parameter in this engines software, its actions in this regime and any harmonic issues impinging on the bearing box would have been completely addressed... No surprises there, right?
Happy to see that you are at least prone to asking questions before forming ill advised opinions
You questions about speed have two parts imbedded in them. One part has to do with aerodynamics which of course has something to do with shock waves. The secoind part is a structural question which has a great deal to do with stress and strain.
Talking about speed of sound at the largest diameter (of a rotor) in the fan is easier to comprehend since it operates at a more familiar ambient temperature and pressure. On the other hand deep inside the engine the pressures and temperatures are quite extreme so the speed of sound would be far different. Simplistically the engine revolves around a defined cycle and as such the designer goes for the most effeciency per pound of thrust per pound of fuel. Thus one designs the compressor to operate at the highest pressure that can be obtained without tip stall or any other perturbation you might want to call it, including shock waves etc. The turbine driving the compressor had about the same tip diameter so its surface speed at its tips are about the same.
The bottom line after this is to ensure that centrifugal stresses are kept within an acceptable envelop with substantial margins against combinations of overspeeds and vibratory modes that might combine and produce rapid fatigue cracking. This is basic regulatory stuff and codified in the design regulations.
The problem comes in when the operation of the engine is outside of the certifcated conditions. Either from mis-operation of the engine (including maintainence) or mis-manufacturerd parts.
Vibratory modes are predictable and calibrated via experience including the hundreds of hours in development testing before the engine is certified. So obviously no mis-designed or undertested severe failure conditions are expected in service before any recommended maintainence cycles. assuming of course no mis-operation or mis-manufacturerd parts.
I've tried to keep this brief and cover some nuggets of your questions.
Just one editorial comment; don't confuse low mass with low cost or low safety margins. Margins of safety are regulated and it's stiffness and dampening that make it possible to design for light weight and still meet safety margins, else all the plumbing visible around the outside of engines wouldn't be called tubing but would be made out of thick steel pipe
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lomapaseo
"Vibratory modes are predictable and calibrated via experience including the hundreds of hours in development testing before the engine is certified. So obviously no mis-designed or undertested severe failure conditions are expected in service before any recommended maintainence cycles. assuming of course no mis-operation or mis-manufacturerd parts."
Problem. That is not ill-informed, but it is wrong. It is the casual expectation of results that gets engineers into trouble. Most vibratory modes are known, especially when building a "replica" of a known model (TRENT). The 900 is quite different from the others, in RPM, discrepant RPM, and bearings. I predict it is exactly what you casually discard that is the problem, just as you have predicted that all vibratory modes are predictable.
"Vibratory modes are predictable and calibrated via experience including the hundreds of hours in development testing before the engine is certified. So obviously no mis-designed or undertested severe failure conditions are expected in service before any recommended maintainence cycles. assuming of course no mis-operation or mis-manufacturerd parts."
Problem. That is not ill-informed, but it is wrong. It is the casual expectation of results that gets engineers into trouble. Most vibratory modes are known, especially when building a "replica" of a known model (TRENT). The 900 is quite different from the others, in RPM, discrepant RPM, and bearings. I predict it is exactly what you casually discard that is the problem, just as you have predicted that all vibratory modes are predictable.
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Trent rpm limits
Originally Posted by bearfoil
The 900 is quite different from the others, in RPM, discrepant RPM, and bearings.
(Based on the limits stated in FAA TCDS for RB211 TRENT 500-series and 900-series, % converted to RPM)
.........Trent 500 ... Trent 900
HP ...... 12954 ....... 11932
IP ......... 9045 ......... 8117
LP ........ 3608 ......... 2787
Are these the differences you are referring to?
Last edited by HazelNuts39; 27th Jan 2011 at 10:01.