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Old Koreelah

Jabiru cooling ducts

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A question I have never heard discussed: could a Jabiru engine be effectively cooled using a standard aero engine cowl setup? I mean no ram air ducts, just a simple horizontal "fence" around the pots and heads and a vertical "fence" over the top of the engine, so that incoming air can only get out by going down past the cooling fins. It would be simpler to install, and access to plugs, etc would be a doddle.

 

Why not?

 

 

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Peter Anson, a Sonex builder has done just that,

 

PA44.JPG

 

The installation has had some challenges, its documented here: http://sonexaus.wikispaces.com/Peter+Anson+-+Sonex+894

 

The Bristell did use a fence as well but I believe they have gone back to Jabiru style ducts.

 

Cheers

 

Tony

 

 

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I have often wondered if anyone had used a lycoming style plenum over the entire engine top with the gull wing deflectors between the cylinders at the bottom. It may help to even the temps between heads.

 

Current thoughts with the Jab ram air ducts is to install deflectors at the top of the duct between each pot & put the gull wings on the bottom a la lycoming. The big main issue in my opinion is getting good smooth airflow through the engine. I have custom intake nacelles that provide a smooth flow to the heads with no fence that Jabirus have to turbulate the intake air. My oil cooler is mounted on the firewall with its own NACA duct providing ambient temperature air to the cooler with a direct exhaust and a full fuselage width (1200mm) wide 80mm deep air exhaust with a 25 mm lip to help suction. I also have the old style ram air ducts which have a smaller internal volume (sit lower on the heads) with the plug leads on the outside so plugs can be accessed by just removing the rubber grommet.

 

 

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I would much rather a simple fence setup, KG. Getting rid of those damned ugly ram air ducts would make access easier, but I'll persevere with them a bit longer. I'm about to apply some lessons learned from this excellent source:

 

http://www.customflightcreations.com/sitebuildercontent/sitebuilderfiles/cooling102.pdf

 

Their research proved the importance of attention to detail and properly sealing air gaps. Now that I have fitted a more accurate temperature sensing instrument, I can see the results of slight modifications.

 

 

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The right term is plenum chamber, and to make it work you need to ensure that all the air coming in passes through the cooling fins, and not out of any other leak-holes. I reckon the Jabiru ducts are easier, but there is nothing special about them from a cooling point of view. Since you are measuring the CHT's and you are aware of the gap detail then you will be ok.

 

 

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What complete ignorance. It is essential for adequate cooling to get a p-delta difference between the intake and the exhaust sides of the heads and barrels of around 2.5"; adding high-pressure air to the exhaust side of the cowl area will reduce the exhaust negative pressure and therefore reduce the p-delta significantly - possibly even stagnate flow through the head and barrel fins.

 

 

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Of course it wouldn't be wise have cooling air coming from both directions, but I can see Bex's point. Who says air needs to go down through the fins? Blasting cool air onto the hot exhaust pipes first might reduce the heat conducted back to the heads.

 

Perhaps the conversion of forcing air down from above is based on where it normally exhausts: underneath.

 

 

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In theory it has always been better to have the flow from below UP through the head/ barrel assemblies but it isn't desirable to exit it all at the top, because residues may end up on the windscreen and you would be worse off in an engine fire You may wish to augment the cooling with the exhaust or run an adjustable cowl flap which is all better underneath (or each side low) Nev.

 

 

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Bex's post , is how ive cooled the Avocet ,

 

image.jpg.ed2319cc46027b46de1a24ffee89df2d.jpg

 

The black holes send cold air to exhaust pipes and is vented before going through the radiator .

 

 

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You have a lot of flexibility with that design as far as cooling goes. The later CAMit engine has the starter gear (on a long shaft) near the propeller, like a Lycoming. That would suit your set up. Nev

 

 

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In theory it has always been better to have the flow from below UP through the head/ barrel assemblies but it isn't desirable to exit it all at the top, .

It doesn't have to exit at the "top".

 

Air travels from high pressure to low pressure, your use'able friend in nature.

 

 

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Update: I did lots of research and shied off the fences idea, because the incoming air would be heated by contact with the crankcase and barrels before passing down thru the head fins, where cooling is most critical. A call to the factory confirmed that people have tried fences and gone back to the ducts.

 

Spent a week grovelling around inside my shipping container fitting a pair of sheet aluminium ducts.

 

(Could have done a neater job in half the time with fibreglass.) Based on what others have written, I eliminated all leaks and extended the ducts over 2/3 of the cylinders (and fitted large butterfly baffles) and right to the bottom of the heads, with less than 5mm clearance to force air thru the fins.

 

Test flew on Wednesday: 166 degrees C- no cooler than the old-style fibreglass ducts. What a disappointment. Spoke to the factory who stressed the importance of sealing leaks to get low pressure in the lower cowl. Had a closer look and found a few small gaps. Sealed them with rubber and silastic. Test flew yesterday: over twenty degrees cooler!

 

image.jpg.a5a761bd7b524cbe96fd0b9593dbf99d.jpg

 

image.jpg.5966715057d5219dfe709d10b63c8361.jpg

 

 

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Congrats , it does take a lot of careful work to get them right , getting the majic 20 c cooler over a standard installation has got to be good for the longevity of the engine !

 

 

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