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Js3 jet catastrophic failure.



 
 
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  #41  
Old September 2nd 18, 10:39 PM posted to rec.aviation.soaring
WB
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Posts: 236
Default Js3 jet catastrophic failure.

On Sunday, September 2, 2018 at 12:39:58 PM UTC-5, 2G wrote:
On Thursday, August 30, 2018 at 10:44:06 PM UTC-7, wrote:
Watch this.....and consider your life.

https://youtu.be/oYpG0HDcFsA


That video claims that "the model jet engine is a scaled down version of the real thing. It is essentially the same technology that lifts enormous aircraft into the sky." This couldn't be anymore further from the truth. Watch this video and tell me that they are "essentially the same technology:"
https://www.youtube.com/watch?v=VfomloUg2Gw

What struck me on the models is that the most vital components, the fan and compressor blades, are manufactured out of aluminum. Aluminum is not a high temperature metal. One pilot who had multiple start failures of these jets told me that ONE problem was the clearance between the fan blades and the housing, which must be tight to begin with for the jet to work. The shear number of failures I have personally heard of convinced me that I would not a jet sustainer.

Tom


So these small jets are at about the same level of development and reliability as some of our gliding electronics? ;-).
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  #42  
Old September 3rd 18, 12:01 AM posted to rec.aviation.soaring
John Seaborn (A8)
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Default Js3 jet catastrophic failure.

Mike, great background and information on the small turbine world. Thank you for the in-depth and knowledgeable post. When you mention that JetCat uses an internal igniter rather than the glow plug can you explain this a bit? What is the difference and, theoretically speaking, could other jets use an igniter? Thanks again for posting.

John Seaborn
  #43  
Old September 3rd 18, 02:19 AM posted to rec.aviation.soaring
Mike Borgelt[_2_]
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Default Js3 jet catastrophic failure.

On Monday, 3 September 2018 09:01:07 UTC+10, John Seaborn (A8) wrote:
Mike, great background and information on the small turbine world. Thank you for the in-depth and knowledgeable post. When you mention that JetCat uses an internal igniter rather than the glow plug can you explain this a bit? What is the difference and, theoretically speaking, could other jets use an igniter? Thanks again for posting.

John Seaborn


Hi John,

Thanks.

The AMT and PSR (AMT derived) engines and most other small turbines used to use a model airplane glow plug with a turn or so teased out into the breeze to ignite the propane starting gas. There was a delicate balance between getting the thing to light off by making the glowplug hot enough and glowplug life. It was difficult to know if you were going to get one start or 100 starts from a glowplug. Doesn't matter when launching a model airplane or drone. From all accounts though the gas start system was pretty good. I designed a spark unit using a modified model airplane spark plug which worked well and then kero start became available. Good for military drone operators as you can't carry pressurised inflammable gas on a C-130 etc.
The AMT/M&D igniter is a silicon nitride rod with a heating coil inside it. It is a commercial part used in diesel fuel heaters etc. Fuel gets dribbled around the outside of it under no pressure so you get a vapor barrier which prevents all the fuel from heating and excess unburnt fuel falls in to the engine causing hot starts (auto shut down) or excess fuel being blown out the back of the engine. At least it isn't prone to burning out.
I've never managed to find an image of the current JetCat igniter, not for lack of trying. I suspect it is a flat heated strip with fuel sprayed on to it as droplets which would seem much better.
One thing I have thought of - the M&D system seems to use a pretty standard K2 12 volt nominal LiFePO4 battery. AMT specify a 4 cell LiPo pack which goes up to 16+ volts when fully charged. I think a 5 cell LiFePo4 would be better to get the heater hotter and spin the starter faster. I'm experimenting with this and a way of reducing the kero flow rate into the igniter while still having the main fuel pump start. There is only one pump which requires a certain minimum voltage to start but then produces too much flow rate into the igniter resulting in excess fuel.
Mike

  #44  
Old September 3rd 18, 06:47 AM posted to rec.aviation.soaring
2G
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Posts: 1,439
Default Js3 jet catastrophic failure.

On Sunday, September 2, 2018 at 6:19:29 PM UTC-7, Mike Borgelt wrote:
On Monday, 3 September 2018 09:01:07 UTC+10, John Seaborn (A8) wrote:
Mike, great background and information on the small turbine world. Thank you for the in-depth and knowledgeable post. When you mention that JetCat uses an internal igniter rather than the glow plug can you explain this a bit? What is the difference and, theoretically speaking, could other jets use an igniter? Thanks again for posting.

John Seaborn


Hi John,

Thanks.

The AMT and PSR (AMT derived) engines and most other small turbines used to use a model airplane glow plug with a turn or so teased out into the breeze to ignite the propane starting gas. There was a delicate balance between getting the thing to light off by making the glowplug hot enough and glowplug life. It was difficult to know if you were going to get one start or 100 starts from a glowplug. Doesn't matter when launching a model airplane or drone. From all accounts though the gas start system was pretty good. I designed a spark unit using a modified model airplane spark plug which worked well and then kero start became available. Good for military drone operators as you can't carry pressurised inflammable gas on a C-130 etc.
The AMT/M&D igniter is a silicon nitride rod with a heating coil inside it. It is a commercial part used in diesel fuel heaters etc. Fuel gets dribbled around the outside of it under no pressure so you get a vapor barrier which prevents all the fuel from heating and excess unburnt fuel falls in to the engine causing hot starts (auto shut down) or excess fuel being blown out the back of the engine. At least it isn't prone to burning out.
I've never managed to find an image of the current JetCat igniter, not for lack of trying. I suspect it is a flat heated strip with fuel sprayed on to it as droplets which would seem much better.
One thing I have thought of - the M&D system seems to use a pretty standard K2 12 volt nominal LiFePO4 battery. AMT specify a 4 cell LiPo pack which goes up to 16+ volts when fully charged. I think a 5 cell LiFePo4 would be better to get the heater hotter and spin the starter faster. I'm experimenting with this and a way of reducing the kero flow rate into the igniter while still having the main fuel pump start. There is only one pump which requires a certain minimum voltage to start but then produces too much flow rate into the igniter resulting in excess fuel.
Mike


I noticed that you did not mention the clearance issue that one pilot reported - do you not consider this to be a problem, or are you mystified?

Tom
  #45  
Old September 4th 18, 04:32 PM posted to rec.aviation.soaring
WB
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Posts: 236
Default Js3 jet catastrophic failure.



I noticed that you did not mention the clearance issue that one pilot reported - do you not consider this to be a problem, or are you mystified?

Tom


If I recall correctly, centrifugal compressors have larger tolerances for clearance of the rotors and housing compared to axial compressors.
  #46  
Old September 4th 18, 05:11 PM posted to rec.aviation.soaring
[email protected]
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Default Js3 jet catastrophic failure.

On Tuesday, September 4, 2018 at 4:32:20 PM UTC+1, WB wrote:

I noticed that you did not mention the clearance issue that one pilot reported - do you not consider this to be a problem, or are you mystified?

Tom


If I recall correctly, centrifugal compressors have larger tolerances for clearance of the rotors and housing compared to axial compressors.


....but IIRC in the development stages of the M&D jet JS reported that they found some cases of the engine surging after starting and this was corrected by M&D tightening up the clearance tolerances for the centrifugal compressor to the inlet casing. The jet I had certainly had very close and even fit which I looked at in my pre-flight checks as I finger spun the compressor.
  #47  
Old September 5th 18, 06:20 AM posted to rec.aviation.soaring
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Default Js3 jet catastrophic failure.

I've never managed to find an image of the current JetCat igniter, not for lack of trying. I suspect it is a flat heated strip with fuel sprayed on to it as droplets which would seem much better.

The older JetCat igniter is just a rod and always seemed to work work well on both kero and diesel. I am not sure if they changed the igniter for the newer RXi with internally mounted solenoids.
  #48  
Old September 5th 18, 03:41 PM posted to rec.aviation.soaring
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Default Js3 jet catastrophic failure.

"Also the M+D jet is not a model engine and wasn't based on one. It was designed built and certified as an aircraft jet turbine by a certified manufacturer."

That is not true in the slightest. The engine was designed by what can best be described as an 'enthusiastic amateur'. M&D entered an arrangement to use his design and continues to buy certain critical parts off him, per their contractual agreement. The fact that some of the installed fleet are 'certified' shouldn't aid your sleep in any way. The requirements for performance, reliability and failure containment that apply to the jets power commercial aircraft don't apply.

High altitude starting

I haven't experienced the same problems others are referring to, though my starts are most often around 2,000ft AMSL. My lowest temp start has been 10deg OAT and highest 40deg OAT. I use Jet A1 and Aeroshell Turbine Oil 560 exclusively. My highest altitude start was at the factory, perhaps 6,000ft AMSL without issues.

Reliability

I know of a number of fellow owners of the same type that have had huge problems with their jets. I had escaped such troubles until the 2 year mark when I had a series failed starts during pre-XC tests (I have an inflight test-start requirement of the first flight and then once per week after that, on aerotow). The culprit was a compressor housing O-ring that had broken in several places. Snicked during installation due to a weakness in the design and failed over time. Post replacement, performance returned to is regular, exemplary level until 'the failure'.

"Anyone know the cause of the turbine failure first mentioned in this thread?"

A failure of the main bearing due insufficient lubrication. The reason for this was not determined. The tube to lubricate the bearing is remarkably thin, it doesn't take much to block it. There is no fuel filter downstream of the aircraft-engine connections in the fuel line, another design weakness

"How much damage was done to the airframe when the engine failed?"

With successful operation of the jet established, the pilot departed the chosen outlanding field. 'Hopping' from landable backup to laudable backup field, at the point of failure the pilot entered the circuit for a pre-selected field and landed safely.

Overall, I'm cautiously happy with the jet (so long as I don't experience a repeat of this failure!)

CJ



  #49  
Old September 5th 18, 04:29 PM posted to rec.aviation.soaring
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Default Js3 jet catastrophic failure.

There is only one pump which requires a certain minimum voltage to start but then produces too much flow rate into the igniter resulting in excess fuel.

I'm glad you mention the pump. Turbine is one thing but the pump and its settings are the key to successful starts(of course together with a good battery). The pump voltages(sometimes referred to as pump power) supplied at different start stages are critical to getting a reliable start. Too much and you get a flaming hot start, too little and it wont stay lit and restarts thereafter result in residual fuel and another hot start. Getting it just right is a tuning process(through various params in the ecu and also depends on fuel type) but is also complicated with breakin of the pump over time in the case of new pumps. So as an owner, monitoring the pump power during operation so as to monitor "pump health" is fairly important. Think we all agree clean filtered fuel is must but they can and do wear out. I've had one with 200 hundred runs overshoot on rapid throttle advance and never expected to see 6 foot of flame out the back which actually extinguished the engine. After a new pump, all was good again after a retune.

  #50  
Old September 6th 18, 05:42 AM posted to rec.aviation.soaring
Mike Borgelt[_2_]
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Default Js3 jet catastrophic failure.

On Thursday, 6 September 2018 01:29:45 UTC+10, wrote:
There is only one pump which requires a certain minimum voltage to start but then produces too much flow rate into the igniter resulting in excess fuel.


I'm glad you mention the pump.....


Thanks for that. Good information on break in of the pump.

As for close clearances just contemplate the clearances in the crankshaft and bearings of a two stroke or the clearances in the cylinders. BTW your piston engine ought to melt. The fuel burns hotter than the melting point of the metal but there is a boundary layer in the gases at the surface of the metal.

The origins of the M&D engine mentioned are in accordance with what I was told by someone who ought to know.
Last time I looked the certification on jets for sustainer use was that it wasn't allowed to blow up or catch fire. M&D seem to handle the blow up bit by limiting the number of cycles before overhaul. HpH in the Sharkjet went the ballistic shielding route.
The turbine blade of a Titan weighs about the same and moves at the same speed as a standard velocity 0.22 rifle bullet. It was fun doing ballistic shielding tests. 1 mm of 4130 dents but does not break. 8 layers of 170 gm Kevlar catches the bullet. Interesting.


 




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