Overview
It was decided to save some muffler weight by selecting a titanium muffler manufactured by Akrapovic. Though expensive it carved two [2] kg from the total weigh and was very compact in design so on its arrival it was found to be of a first class construction and fitted like a glove and confidence was high.
Later examination revealed a rather small perforated inlet to the exhaust side of the muffler but a internet search yielded no bad reports but it was put on the to look at list. A recent Facebook Post created a reply from Thomas at Edge Performance indicated that this was indeed a problem so it moved up my to do list.
Examination of the inlets and exhaust with a torch showed a perforated inlet of at least four times the length of the outlet with the areas begin larger than the inlet/outlet pipes based on a 50% open area. However Thomas comment concerned me so the decision was made to open up the outlet for possible modification and this is were the reason for the failure of the design became slowly apparent.
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| Exhaust outlet - No I am not kidding that's all there is |
With the access the builder decided to grab the torch and look up at the inlet and that is when another issue revealed its ugly head, that 100 mm perforated tube had only 50 mm of exposed area as 25 mm was mounted in a flared slip tube to reduce thermal stress.
At this point the logic of the system escaped me totally - OK I am not that bright but this just left me lost for why..!
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| A - the slight shadow id the end of the flared mount B - Is the 50 mm of open tube C - The end of the inlet tube |
A lot of time was spent on the internet with no comparable concept found until a clip on why a turbo muffler can flow more gas that the classic hot dog straight through. Classic turbo mufflers form a looped path with the inlets and outlets offset from each other - So why are they more efficient that a straight line of the hot dog?
The answer lied in the fact that the perforated material of the legs inside the muffler had a ever increasing boundary layer even though some gas escaped via the perforated leg until it exited to the next leg creating an increase in gas velocity.
So in a previous life I had been involved with piping systems and it just hit me - this fool has effectively coupled up both headers [inlet 1 /2 image below ]. Let me explain - fluids all have weight therefore inertia so the concept of stopping a discharge in 50 mm while turning 90 degrees before the next charge arrives is not plausible in any world.
Looking at one cycle with unequal header lengths feeding the muffler, the first charge [short header] enters the muffler with a balance of the gas accelerating past the perforated tube up the long header until the inevitable collision occurs. Now the cycle is repeated in the opposite direction with evacuation more of a result of the pressure build up in the headers.
I tend to never criticise other peoples designs but this is a total cluster stuff.
If this description is true there is only one solution and that's a 1970 hot rod muffler gutting. The edge is bead welded so I will try to grind off the bead and then the ends should be removable allowing the guts to fall out. Stubs will be welded to each inlet and the inlet cut at 30 degrees to create a smoother acceleration as it enters the exhaust wit the jury out on the inlets.
As this aircraft will fly as experimental in Australia it is in Phase 1 testing all its life so no noise restriction applies other than a social one.
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| Proposed design - bit rough but a first cut |
Comment
None - I am at a loss unless some one can explain this to me in simple terms
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