"Jaberoo" down (Mildura this time)

[Guest john]

The Probable Solution to Eliminate All Aircraft Failures

 

There is a lot of uncertainty & confusion occurring amongst aircraft owners & pilots as to who should accept responsibility for these ongoing engine failures in Jabirus & other aircraft of both small, medium & airline type aircraft, & furthermore how to eliminate such disasters.

 

Well the universal answer comes from the authorative scribe of an ex employee of RAA who wisely or unwisely penned the following solution in a previous edition of the RAA magazine which reads as follows:

 

"Well the answer is that all accidents , fatal or not, are preventable in some way shape or form".

 

Your mind boggles as to what the scribes unknown solution is, because the only common sense answer to this abovementioned useless statement is to ground all aircraft around the world, & there would therefore be no further accidents or incidents, & of course this suggestion is neither practical or acceptable to the majority of the worlds population.

 

 

[winsor68]

Routine maintenance. Being certificated (NOT certified) does not remove the necessity for maintenance. The fact that it's necessary suggests that the valve guide temperatures are a bit on the high side - which again says that the CHT is getting a tad higher than was demonstrated during the 50 hour certification endurance run.The temperature limits given in the TCDS are based on the temperatures that were exceeded for at least 50% of the endurance run; that's how they were established in the first place. The engine has to be run for 25 2-hour blocks, during the first hour of which the power is varied from idle to takeoff power; and the second hour is held at maximum continuous power, and red-line temperatures. CASA sits-in on all such tests. The engine is stripped and inspected afterwards, and carbon build-up in the guides would have been seen - so one must therefore deduce that if they are carboning up in service, then the CHT and/or the EGT are exceeding the TCDS limits. Hence my comments re the cold junctions.

 

I don't think you are correct re Lycomings etc; it doesn't have to be done frequently, but those air-cooled engines all run very close to limiting conditions for the exhaust valve guides. Continental has gone through about four changes in exhaust guide material over the years, and they still give trouble. The Rotax has liquid-cooled heads, whose main advantage is that they make it easier to cool the exhaust guides. Some Lycomings have oil passages in the head to try to reduce the guide temperatures; I hate to imaging what the maintenance issues are when those passages carbon up, as they must. Cars don't do it because they do not run the same duty cycle as an aero engine.

I thought they were exempted from strictly following the CASA schedule...

 

 

[alf jessup]

Port flow affecting cooling? You are so far off understanding the physics of heat transfer that it is bloody laughable.Sorry, edit - delete 'laughable', insert 'hilarious'

 

Oh, and just by the way, in regard to your claim - the heads are not 'Cast', they are CNC machined. If you had actually ever seen Jab. heads in the flesh, you'd know that. You are so much male bovine excrement, you'd overlay a Texas Rodeo paddock.

Oscar,

 

You better be careful, I didn't see anywhere he mentioned Jabba Routed heads????

 

 

[turboplanner]

So without reading though the whole 9 pages, has the actual cause of this engine failure been discovered..?

The Pilot would hardly have had time to get depressed at the loss of his aircraft, retrieve the pieces and start to pull the engine down, but his description of 9 years of reliability then the engine starting to run rough, matches a number of other stories which ended with through bolt failure - my pure conjecture.

 

For those who have drifted into the discussion on cooling, failed exhaust valves etc. I posted this thought piece on another forum this morning. I had similar combustion issues with a race engine for a couple of years, and solved the problem after understanding where the heat was and how to get rid of it.

 

Get an old exhaust valve, heat it gently with an oxy torch until the temperature is 200 degrees C measured with a probe thermometer. It should look much the same as when it was cold.

 

 

 

 

 

In a water-cooled engine this temperatures are significant because sensor usually are tapped through the head to measure the water temperature, and as we know the boiling temperature increases with the pressure we can obtain in the cooling system. If we lose pressure the head temperature goes up and the head usually warps.

 

However the exhaust flow is deeper and directly in the flame front of the combustion chamber, where operating temperature is around 1,000 to 2500 degrees C – a different environment.

 

Get the oxy torch on the old valve again and take the temperature up again – steel usually starts to get soft around 538 degrees and melts around 1370 to 1540 degrees C, so in this experiment you’ll be able to roughly find where the material for this valve would start to burn away.

 

If the valve is deforming at the low end of the scale, you can look at better valves, if not, then the flame front may not be clearing fast enough from the chamber to allow cooling before the valve starts deforming and shedding.

 

You could use the percentage difference say between 180 and 200 on the gauge, but it’s not terribly representative of what the flame front is doing.

 

 

[Oscar]

This is rather worrying Oscar, I trust you have filed a report with RAAus?

A condition report (about six pages, with photos) when transferring the rego and a defect report, yes. However, we intend to meet with Darren Barnfield and go through the logbooks and our hundreds of photos and probably more than a half-an-hour of videos of repair work. Our interest is getting the thing ticked-off as OK to fly again as a 55-rego aircraft; where RAA may want to go with our documentation, is their concern.

 

 

[bexrbetter]

Oscar,You better be careful,

Meh, the guys playing childish semantics to try to score some sort of points for reasons unknown to me.

 

The heads belonged to VFR Pilot, a member of this forum. He wanted to do some stuff so I offered to have the heads ported by one of Australia's best so he sent them but my guy didn't think there was any great gains to be made due to not enough metal in the right places hence the inability to reshape them so back they went to VFR. I paid for everything.

 

 

[Dafydd Llewellyn]

I thought they were exempted from strictly following the CASA schedule...

Which "they" are you referring to? And which CASA schedule?

 

 

[Oscar]

The Pilot would hardly have had time to get depressed at the loss of his aircraft, retrieve the pieces and start to pull the engine down, but his description of 9 years of reliability then the engine starting to run rough, matches a number of other stories which ended with through bolt failure - my pure conjecture.For those who have drifted into the discussion on cooling, failed exhaust valves etc. I posted this thought piece on another forum this morning. I had similar combustion issues with a race engine for a couple of years, and solved the problem after understanding where the heat was and how to get rid of it.

 

Get an old exhaust valve, heat it gently with an oxy torch until the temperature is 200 degrees C measured with a probe thermometer. It should look much the same as when it was cold.

 

 

 

 

 

In a water-cooled engine this temperatures are significant because sensor usually are tapped through the head to measure the water temperature, and as we know the boiling temperature increases with the pressure we can obtain in the cooling system. If we lose pressure the head temperature goes up and the head usually warps.

 

However the exhaust flow is deeper and directly in the flame front of the combustion chamber, where operating temperature is around 1,000 to 2500 degrees C – a different environment.

 

Get the oxy torch on the old valve again and take the temperature up again – steel usually starts to get soft around 538 degrees and melts around 1370 to 1540 degrees C, so in this experiment you’ll be able to roughly find where the material for this valve would start to burn away.

 

If the valve is deforming at the low end of the scale, you can look at better valves, if not, then the flame front may not be clearing fast enough from the chamber to allow cooling before the valve starts deforming and shedding.

 

You could use the percentage difference say between 180 and 200 on the gauge, but it’s not terribly representative of what the flame front is doing.

As anybody with a basic understanding of four-stroke engine thermodynamics knows, there is bugger-all heat transfer through port flow in the exhaust port. The valve stem carries some of it, the valve guide and the seat carry the rest. The head and the cylinder ultimately dissipate the heat to the cooling medium (water, oil or air). Titanium valves, sodium-filled valve stems, beryllium-alloy valve seats and various types of valve guide materials have all been used to improve the heat transfer / heat tolerance regime, with titanium valves being also useful for valve-train inertia reduction . No race engine engineer I have ever met/worked with has considered port flow from the POV of heat transfer from the valve/ seat/guide - though as you say, flame front/ velocity erosion is a matter of concern. Our wee mate Bex appears to be suffering from a spatial dislocation of his mouth to about two feet south of its conventional location. Not a good look in a supposed engine designer.

 

 

[deadstick]

A condition report (about six pages, with photos) when transferring the rego and a defect report, yes. However, we intend to meet with Darren Barnfield and go through the logbooks and our hundreds of photos and probably more than a half-an-hour of videos of repair work. Our interest is getting the thing ticked-off as OK to fly again as a 55-rego aircraft; where RAA may want to go with our documentation, is their concern.

Wow, And I'm sure Darren is looking forward to the condescension I mean meeting! LOL. Special meetings with the Tech manager to go through your photos, videos and repair logs! Your faint grip on reality is as you put it in a previous post, laughable or hilarious!

 

I'm not sure why you would want to waste the tech managers time to go through your logs, etc unless its for some self gratification or confirmation that your repairs are legit! If its the latter I thought you were the 'bomb digity' when it comes to aircraft! Especially Jabiru's....

 

Not much Darren could tell you that you don't already know, right?

 

Cant wait to see this jabiru of yours in the flesh! must be special!!!

 

sigh...

 

 

[alf jessup]

Meh, the guys playing childish semantics to try to score some sort of points for reasons unknown to me.The heads belonged to VFR Pilot, a member of this forum. He wanted to do some stuff so I offered to have the heads ported by one of Australia's best so he sent them but my guy didn't think there was any great gains to be made due to not enough metal in the right places hence the inability to reshape them so back they went to VFR. I paid for everything.

It's ok Bex I was just stating that you never mentioned what heads they were so you could have been dishonest and said they weren't that cheap brand they were something else.

 

No biggie to me I don't need to tinker with what I fly behind and I don't need to defend it to the hilt either as they stop on a not so regular basis and usually don't need much more than oil & plugs in their life time.

 

Mines not bullet proof I know but I have more trust in it than the well talked about one.

 

Alf

 

 

[deadstick]

As anybody with a basic understanding of four-stroke engine thermodynamics knows, there is bugger-all heat transfer through port flow in the exhaust port. The valve stem carries some of it, the valve guide and the seat carry the rest. The head and the cylinder ultimately dissipate the heat to the cooling medium (water, oil or air). Titanium valves, sodium-filled valve stems, beryllium-alloy valve seats and various types of valve guide materials have all been used to improve the heat transfer / heat tolerance regime, with titanium valves being also useful for valve-train inertia reduction . No race engine engineer I have ever met/worked with has considered port flow from the POV of heat transfer from the valve/ seat/guide - though as you say, flame front/ velocity erosion is a matter of concern. Our wee mate Bex appears to be suffering from a spatial dislocation of his mouth to about two feet south of its conventional location. Not a good look in a supposed engine designer.[/QUOTE]"our wee mate" lol I bet he'd love to meet you in person! I know I do!

[Guest Andys@coffs]

Im afraid Im not very knowledgable when it comes to engine, hence the reason I don't do major work myself but rather give it to a L2 who is....

 

Anyway I read the Wiki article on porting http://en.wikipedia.org/wiki/Cylinder_head_porting and specifically the bit about the length and size of the inlet manifolds....I was thinking that on cars etc going back to Carbys, not EFI, that the Carb was pretty much in the middle of the engine and as such the variability in the inlet manifold lengths wasn't that large (at least as I recall it from my uneducated perspective) however on my 3300 the carb is right at the far end on the engine meaning that the #5 and #6 cylinders (the ones traditionally called hottest....but not on mine) have short inlet manifold lengths and #1 and #2 have an inlet manifold length that is at least twice probably closer to 3 times the length of #5 and #6. I wonder if that is an issue. I know that getting balanced mixture into all cylinders is a problem but always thought that was a mixing/swirling air issue and didn't ever think about the inlet manifold lengths and the tuning impact it would have.......

 

Anyway bottom line interesting reading but the chances of being able to re-port a jab head.......There isn't too many spare grams of aluminium available to change the port as I see it......Isnt that what Oscar was saying? whether cast head or machined, whether steel or aluminium (presuming you have a way of depositing and removing material) to change the port shape is likely not something you do cause you have a few spare hours....seems lotsa hours and Dollars would be the order of the day

 

Back to my submarine cables......

 

 

[Oscar]

Wow, And I'm sure Darren is looking forward to the condescension I mean meeting! LOL. Special meetings with the Tech manager to go through your photos, videos and repair logs! Your faint grip on reality is as you put it in a previous post, laughable or hilarious!I'm not sure why you would want to waste the tech managers time to go through your logs, etc unless its for some self gratification or confirmation that your repairs are legit! If its the latter I thought you were the 'bomb digity' when it comes to aircraft! Especially Jabiru's....

Not much Darren could tell you that you don't already know, right?

 

Cant wait to see this jabiru of yours in the flesh! must be special!!!

 

sigh...

Ah, Darren will already have had the EO and the L2 and L4 tick-offs. And the structural proof-tests reports, and the flight tests under VH-exp category. And he might just wish to review the performance of certain L2's, he seems to be a decently meticulous person by nature. Or perhaps that is not something that you'd wish to see happen in the RAA?

 

 

[bexrbetter]

on porting.

See where the hottest part of this exhaust system is, a bit of thought will have you understanding where and why while I'm off to dinner ..

 

 

 

[deadstick]

Ah, Darren will already have had the EO and the L2 and L4 tick-offs. And the structural proof-tests reports, and the flight tests under VH-exp category. And he might just wish to review the performance of certain L2's, he seems to be a decently meticulous person by nature. Or perhaps that is not something that you'd wish to see happen in the RAA?

L4 tickoff? wow the dizzy heights! ( heaven help the poor unsuspecting punter).

 

LOL its not a simple tickoff pal! cause as you said Darren is meticulous. I suspect he wont be impressed with your pages of documentation or testing but will ask himself, why?.

 

I've got nothing to fear 'D.B'!

 

Darren recently reviewed my L2 and L4 privileges as well as my ongoing 'in conjunction' part 66! but whatever! I've met your type plenty. sigh again!

 

 

[deadstick]

Oscar,

 

Its funny that being the Jabiru aficionado that you are, you attack Bex's statement on port heat as Jabiru themselves have contradicted your statement! sigh! again again!!!!

 

 

[deadstick]

Nice Bike Bex

 

 

[deadstick]

Oscar,

 

Frankly champ, I don't see the Ian Bent that I know having anything to do with a nasty piece of work like yourself, but by all means name drop till the cows come home little buddy or should that read wee mate?

 

 

[gandalph]

Gents,

 

How 'bout a bit less of playing the man and a bit more of playing the ball? The personal nastiness being posted here does none of the posters credit and adds nothing to the debate.

 

Time for the Moderators to do a bit of moderating?

 

 

[deadstick]

Gandalph, a spade is a spade is a spade! LOL it would be bad enough for someone to have a serious lack of any credibility, the ability to tag on other qualified coat tails and a superiority complex! but then we have Oscar! gimme a break his personal attacks on people deserve a little bit of comment!

 

 

[gandalph]

Gandalph, a spade is a spade is a spade! LOL it would be bad enough for someone to have a serious lack of any credibility, the ability to tag on other qualified coat tails and a superiority complex! but then we have Oscar! gimme a break his personal attacks on people deserve a little bit of comment!

I'd much rather get back to topic. If people want to throw Sh*t let them start a separate thread that we can all ignore. This isn't the place for juvenile rants or bullying

 

 

[turboplanner]

As anybody with a basic understanding of four-stroke engine thermodynamics knows, there is bugger-all heat transfer through port flow in the exhaust port.

I didn't mention anything about heat transfer through port flow, nothing, and nothing to do with it.

 

 

[Dafydd Llewellyn]

So without reading though the whole 9 pages, has the actual cause of this engine failure been discovered..?cheers

 

JimG

Not to my knowledge

 

 

[bexrbetter]

See where the hottest part of this exhaust system is,

Anyone involved with older bikes knows where the exhausts "blue up", on the outer edge of the first bend. The reason is simple, anything with mass that is moving has inertia and doesn't want to change direction, be it a ball you throw, a car you want to turn a corner or in this case, a rapidly flowing gas. It's one of the fundamental rules of physics.

 

Very simply the gas, bloody hot gas in this case, goes straight to the outer wall of the bend in the pipe, the sharper the bend the worse it is.

 

Here is another example of a different Norton with even tighter pipe bends and hence even greater blue'ing of the pipes along with quick MS Paint of why including a 90 degree bend which is naturally worse..

 

 

Now I don't think that's so hard to understand, so now onto the relativity of that to the Jab exhaust port.

 

Although I understand the reasoning behind the Jab exhaust port, packaging mostly, at the end of the day it's pretty awful. It's basically a quite sharp aprox 70 degree bend that as I have laid out in the above MSPaint picture, directs an enormous amount of heat to the port's roof, of course that's where the exhaust guide and valve stem are, but far worse is the amount of port space that is taken up by the exhaust guide boss, have a look at a casting made of the ports to see what I mean and my depiction (on an actual Jab port blueprint) of where the exhaust gas are doing the worst, the blue 'X' is the guide boss protruding into the port ..

 

 

Just for comparative purposes, here is a port casting from a Falcon 6, notice the port bulge around the (much smaller) guide boss to help flow past that point and look again at the Jab port casting

 

 

and ultimately, a Formula 1 port casting. f1 don't run much in the way of a guide boss port intrusion as they are relatively short life. Because they have ports that are as straight as possible for best flow, the valve ends up very long ...

 

 

IMO, and you can see how I come to the conclusion, is part of Jab's problems might be attributed because they went a bit far with the packaging to offer a narrower engine, which is understandable.

 

Some of you may now better understand why I wanted to port the heads to ease the restriction around the guide boss area so not so much heat was retained there as well as straighten out the port as much as possible to help it not condense on the roof as much - but not much can be done with out starting from a scratch redesign.

 

 

[bexrbetter]

In my opinion there is another choice, respective to price and hassles etc. of going to watercooled.

 

Porcupine heads stated to become known and popular in motocross racing around the mid 70's and took off with many aftermarket heads available before the onset of water cooling. Here is a standard 125 mid 70's air cooled MX bike head and an aftermarket porcupine to illustrate a bit ..

 

 

And, here is a picture of the 1974 World Championship winning CZ Falta 250 along with the original 1973 engine, where the works team not only porcupin'ed the head, but also took every second fin from the barrel as well for better cooling and less weight...

 

 

It may seem counter-tuitive to remove finning, but you actually increase extractable radiant area which is more important than finning area. Lots of fins are useless if you can't get the heat away and even worse, too much finning acts as a solid block.

 

So you would end up with a VW head, because that's what I was discussing elsewhere, that would look roughly (and I mean roughly!) like below, you merely make cuts with a hacksaw and break out every second fin tab with a pair of pliers ...

 

 

And of course, a Jab would look something like this ...

 

 

After seeing today the test cooling cell posted above by Dafydd, I now understand why Jab did the finning as tight as they did.