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Should have known better!

Blackburn Mark

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Having waxed lyrical in another thread about focused stress points on exhausts, I thought I would prove my point AND show how retarded I can be when the mood takes me.
Having discovered the spring had broken on the pre-flight today, I chose to nip out the additional slack.... this left almost no flex but I was desperate to fly so what the hell.

Looking on the bright side, my welding on the previous fracture was stronger than the base material although looking at the break, the heat affected zone may have hardened....?
One and a half hour with not enough flex is all it took.

The silencer started clipping the prop... This is where I went full retard, I thought a redundant wire I had tied off had come undone and was clipping the prop so I thought I would attempt to limp home.
Bad move, it was clipping the exhaust which fatigued the baffle which came out.

The baffle cut one tip off and took a chunk out of the other.

I can build and repair anything on this machine dirt cheap... except the prop... maybe its time I made a copy machine and bought some wood.

It was a nice flight up until this... I had a 10mph SE over a 10mph NW sea-breeze... tail wind in any direction :)


Edited by Blackburn Mark
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3 hours ago, admin (Simon W) said:

not trying to blame the kit!

I couldn't if I tried... The cage hoop is home made, the frame has been straightened, the dog-legs have gussets welded in, the exhaust has more welds than exhaust, the throttle is homemade... if something breaks, ill guarantee iv had my mitts all over it.
Its old and heavy but I refuse to surrender :)

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Sticking with the theme:

In preparation for replacing this old battered exhaust, I had already fabricated a new down-pipe.

Here it is with an ebay-special silencer... it weighs more and is longer than the original so controlling the vibration/fatigue would be a nightmare which is why I hadn't fitted it and after a second look, its going back on the shelf.

This is option two, 1mm aluminium sheet I formed months ago. A few problems have kept me shy so far... I'm not so good at welding thin aluminium, aluminium fatigues very quickly and the original stainless exhaust has a straw tint which means the stainless steel has been getting to 340*c  / 370*c …. Aluminium loses half its strength at this temp.

But I'm desperate and I like a bit of science so here we go...
I'm not going to pretend I'm a dab hand with aluminium, you can see how often I dip the tungsten (blind in one eye is my excuse) and I ran a little cold (fear of blowing through)

My thoughts are, the old exhaust is a chambered turbo affair so suffers direct contact with exhaust gas. I'm going to try a different tactic, an old fashioned perforated, straight-through with stainless steel wool packing. This ought to insulate the aluminium skin and dampen the pulses on the skin. If I kick the exit up, it might offset some of the extra noise that innocent earthing's have to suffer.

Also, I am hoping to stave off bracket fatigue using high rigidity rather than the usual practice of floppy joints. The lightweight skin and packing lends itself to this if I attach/bracket via the straight through tube itself.
Predicted fail mode: steel wool burns out in short order (being so close to the exhaust port), skin heats up, fatigues, disintegrates, hits prop, prop launches a blob of molten aluminium into my wing, wing catches fire, I take up playing dominoes :)
Its an optimistic attempt, ill give myself a 30% chance of making it make sense in the long run.

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Not as good looking as I hoped and not as ugly as I feared :)

The perforated tube is welded to the outer two extensions (no flexible joints)... the two aluminium rings are pressed on for welding to the aluminium skin.


Its a risky strategy... no flexible joint between the flange and down pipe.

My welds might not be good for looking at but they are structurally sound :)

Stainless wire wool packing before welding end cap on.

With this bracket, the rigid flange and the lightweight canister, I am hoping it is rigid enough not to allow any flex/fatigue.

Its much quieter than I was expecting... not as quiet as the original but not miles away.
I don't know much about the dark art of muffler design so I am "assuming" the kick-up exit is causing some of the expected "snap crackle & pop" of a straight through, to propagate back into the canister....?

I will now upgrade my chances of success to 31%
I'm not looking forward to testing this... I have already ordered some 0.7mm stainless sheet in preparation for the canister failing.... but we shall see.

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Paranoia got the better of me so I made sure it couldn't get near my prop if it broke off just below the flange (where I would expect it to)


For anyone interested in such things (some of us are sad)
Two 120km+ XC flights so far and the aluminium canister is doing fine... no signs of that failing via heat damage and if it was going to, I would have expected it to have done so by now.
 The "kick up" exit can get annoying to listen to but its better than pissing Joe public off :)

Now we wait for the dreaded fatigue to try its best or the wire wool to burn out over the long term... I'm quite chuffed with it so far, ill now give it a 50% chance of lasting a year but I need to keep a beady eye... iv lost two props to exhaust failure and one to throttle cable contact... Being a bodger is getting expensive!

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40 minutes ago, AndyB said:

Have you tried heat treating post welding? Might need a big oven!

I have contemplated an oven for tempering crank-cases (if I ever get around to building some) but the exhaust is probably getting a little close to the annealing temps to make it worth bothering... it doesn't feel like it has annealed so far though so I might escape the catastrophic failure I was afraid of.


50 minutes ago, AndyB said:

so tend to stick to arc

Tig is pricey but much more civilized :) 

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Comes to mind now because last night I finished fabricating the exhaust for my racecar and for the wastegate dump pipe (dumps to atmosphere), I used a 1.5"/38mm diameter flex joint before hard mounting the end of the pipe.  I wonder if something like a flex joint would be beneficial where maybe the flex joint wears out and starts leaking, but prevents cracked pipe joints in the long run.  The braided stainless would keep the joint from failing completely because you would obviously hear it leaking before it gets that bad.



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I was attempting to escape the high flex strategy.
With such a short system, I looked like the zero flex strategy was possible "if" I could keep the weight down.

The image shows the strategy EOS has opted for on their version of the GY6 and I don't like it.
Their first few attempts failed via fatigue so they added the flex coupling and lighter canister (they also had the heavier generic muffler that I had also looked at above) 

As scruffy as mine is, its a better design to my eyes than the EOS attempt but the proof will be in how well they resist the dreaded fatigue.
If mine fails via fatigue, I will just add a perpendicular leg to prevent side on vibration (it cant vibrate lengthwise as it is)

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On my mid engined kit car I made an all stainless, 2" big bore, 3 into 2 into 1, hard mounted exhaust with no flex joints and no soft mounts. All mounts were directly to the engine. Did 60k miles with no issues.

You might argue that it didn't vibrate as much as a 2 stroke, however, it was full race engine that idled at 2800 rpm. I can assure you it vibrated like crap.

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  • 1 month later...

sorry for the late reply, the nirvana rodeo has about 150 hours now.

also, another strategy often used by Nirvana for reducing cracking of various components is the up the idle RPM...dont let your motor idle too slow or the excessive vibration at idle causes cracks everywhere.

Edited by adamjedgar
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