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Repairing ram air ducts

gas man

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Hello peoples,


I am loosing my posting virginity having been strickly voyeur up until now. I have a J430 only 140 hours old and the ram air ducts wont stay where they should because the fibreglass of the ducts has pulled through where the rocker cover nuts tighten.


Is this a common affliction?


Various remedies spring to mind such as:


a fibreglass repair job ( which will probably pull through even faster than the orininal)


larger diameter washers (which I have tried but with limited success)


bonding and rivetting a small bit of aluminium plate over the fibreglass of the offending bit of duct and then drilling a hole in the right spot for the retaining nut.


mounting a bracket and spring to the rear outside of the ducts (similar to the Jab spring/bracket on the inside front of ducts) but where to attach the other end of the spring?


Has anyone else been down this road?



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This is a common problem and I've had it myself. For me, I start with a very large penny washer (with a corner bent to make it fit) , when it wears past that I add a second penny washer and sandwitch the fibreglass between them, then after that a repair job. If the repair is thicker than the original it should last for ages as the original is pretty flimsy.


Our flying school Jabs have all had fibreglass repair jobs with success to date.



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I found that aluminium didn't last too long either, but 3 small pieces of thin stainless steel per side have done the trick for me for 500 hrs



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Guest Nelson Smith

The thin stainless did the trick for me.I drilled a strip of stainless 3 holes one for rocker cover and 2 for pop rivets.With the strip screwed to rocker cover,and in place,pop rivet holes are drilled ensuring all line up.Simple.





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We all encounter these things and find our way to separate solutions... my solution was to fabricate four stainless strips that go under the top rocker cover capscrews on the front two and rear two cylinders. The ducts are clamped under the strips. It seems to work fine. Can do a photo if anyone is interested.


Even after I did that, the ducts were still a bit wobbly at the front, so I put a light ally bar across the top to fix them to each other.


For me, this all came about because the floppy duct (with the flogged out hole) wobbled enough one day to loosen two spark plug leads on the back cylinder and the plane went all asthmatic. If this was GA there would have been an AD and a factory-approved fix .....


There was a story about one Jab jockey who was doing a very high speed pass over a field when BOTH ducts let go and suddenly he had no connected plugs (or not enough to matter). As he was over a field I gather it all ended happily enough.





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There was a story about one Jab jockey who was doing a very high speed pass over a field when BOTH ducts let go and suddenly he had no connected plugs (or not enough to matter). As he was over a field I gather it all ended happily enough.

Sounds familiar!


I partly lost a duct and lost 4 plug leads from cylinders 6 and 4. Mine ran more like an epeleptic than an asthmatic. Luckily I was at around 6,000ft when it happened and managed to coast around 25 miles with a descent rate of 100-200fpm and cruised into Latrobe Valley only just making the runway. Whilst I hadn't declared a mayday, the Arrow doing circuits was most upset when he had to go around as I joined straight-in. I had a Jab on each wing guiding me in for what it was worth 049_sad.gif.af5e5c0993af131d9c5bfe880fbbc2a0.gif



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Guest Flyer40

I heard the Jabiru rep at Cowra talking about the air ducts pulling plug leads off during his presentation. It's surprising they haven't been more proactive in providing a solution given they obviously know about it.


What's the likelihood of fitting a Sonex aluminium baffle kit under the hood of a Jab?







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  • 2 weeks later...

My J160 is yet to fly so I have modified the ducts after viewing a number of low time jabs with failing ducts due to excessive heat from the head contact damaging the fibreglass. This is apparent from the colour of the fibreglass at the failure point.


I have Araldited strips of stainless steel between the ducts and the heads on the inside of the ducts and drilled them to take the cap screws..


I have used stainless steel in preference to aluminium or copper as it has a substantially lower coefficient of heat conductivity than either of those metals.


This should result in a lower temperature on the fibreglass that it is bonded to provided that it is attached inside the duct and not the outside.


If the main heat supply is coming from the cap screws it may need an insulating washer under the cap screws.



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For what it is worth!


Somewhere on the net and darned if I can think of where I have seen it, there are photos of a Jabiru engined aircraft with a plenum chamber type cooling system.


DynAero use a similar plenum chamber system on their MCR's for the Rotax.


On the Jab engine, the engine bay is sealed by a wall in front of and above the flywheel.


The sides have a wall running down past the cylinder heads as per the ram air ducts or alternatively a sheet running from the middle of the cylinder heads out to the cowl .


The front is sealed between the engine and bottom cowling just under the air inlet openings.


The top of the chamber has rubber flaps all the way around so that when the top cowl is fitted the entire area above the engine becomes one big sealed chamber which is fed by the ram air openings.


The only way out for the cooling air is down through the cylinder fins.


It seems a very logical way of getting good and very even cooling across the entire engine.


I believe that there is a kit built Jab somewhere in Queensland that uses this system with very good results.



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For interest's sake here are some Coefficients of heat Transmission in BTUs per second through metals 1 inch thick per square inch of surface for a temperature difference of 1 degree Fahrenheit.


The bigger the number the greater will be the temperature on the cold side as more heat is being transmitted.


Aluminium 0.00203


Brass, yellow 0.00142


Copper 0.00404


Iron 0.00089


Steel, soft 0.00062


Steel, hard 0.00034 Best of the list to give the least heat/temperature on the cold side.


So to protect the fibreglass with only the choice above Copper would be the worst and hard steel would be the best for the same thickness of metal to give the lowest temperature on the cold side by a factor of almost 12 times and almost 6 times for aluminium.


Of course a non metal would have far better insulating properties but it needs to be heat and temperature tolerant.


Sourced from my old engineer's reference bible the "Machinery's Handbook" 16th Edition 1963 p.1923. It cost me Seven pounds and five shillings in about 1964.



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Ross's data above has triggered an idea which might minimise the heat transfer to the ducts which might be worth trying.


On our club's original Jab 55 LSA the exhaust muffler temps were starting to affect the fibre glass lower engine cowl to the extent that the inside of the engine cowl near the muffler was turning a solid shade of brown.


I ambled around to the local Holden agency and purchased two sheets [ each roughly about 300 mms square ] of their extremely sticky backed heat shielding material that Holden install between the exhaust pipe and [ plastic ? ] fuel tank to protect the tank.


This low heat transmission shield material is only about 3 mms thick and is very flexible as Holden have to stick it around the compound curves of the tank above the exhaust pipe and it has to stick there for the life of the vechile.


To be safe we put the blow torch on it first to see if it would burn before we installed it.


We then just peeled the protective backing material off the sticky side and just stuck it on the fibreglass cowl under the muffler to cover the heat affected areas.


The aircraft has been sold to club member and the shielding material is still there after a couple of hundred hours and no signs of the heat further affecting the engine cowl.


Just suggesting that a strip of this flexible highly heat resistant and very low heat transmission material installed between the duct and the head would protect the fibreglass duct.


And it is cheap as only a few dollars for the two sheets needed.



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