Jabiru cooling

[Yenn]

I fail to see how copper coating of the fins can have any effect  on cooling. The aluminium fins get to X degrees due to the heat of combustion. Without plating they are in contact with the air and give up their heat.

 

Copper coat them and they are in contact with the copper, which gives up its heat to the air.

 

The only possibility that there could be any improvement is if the layer of oxidised aluminium is acting as an insulator.

 

What really matters is getting the maximum amount of air to contact the maximum area of fin. To do that you need ducting touching the top of the fins, to make the area between each fin into a duct for cooling air, Carry the ducting around the fins, leaving only enough room for the air to exit.

 

 

[facthunter]

File the fins to a relatively sharp edge on the heads that are running hot and check the difference (Where the air comes from not where it exits.) I would be surprised if a considerable improvement doesn't happen.  Squared edge fins are a real barrier to airflow. 

 

  Re the (very thin) layer of aluminium oxide which always forms straight away, I doubt it's effect is measurable. A Polished surface is not the best you can have either .Dull black anodised on a shot blasted surface would be much better. Anodising uses the Al oxide  to hold the colouring .Nev

 

 

[onetrack]

Well, the Yanks did extensive aircraft engine testing, and proved the copper coating on the aluminium fins provided improved cooling, simply because copper has a higher thermal conductivity rate than aluminium - around 66% better.

 

 

[facthunter]

 But it's a very small % of the cross sectional area,  and the heat has to get from the combustion chamber out and that's not affected in the non finned areas, so I can't see how. Someone may have claimed it and I've never known of it used in practice. Nev

 

 

[onetrack]

Nev, my impression is that the Americans did a vast amount of aircraft design testing during WW2, probably more than at any other time in aviation history.

 

They produced a vast amount of results that favoured certain designs - but as with all designing, it's the practical design that wins out.

 

In the case of the copper-plating of fins, it was proven to be beneficial - but I have little doubt, that the reason it was never used in practice, was simply the following practical reasons.

 

1. Copper was a strategically important metal during WW2, and it was deemed important to conserve what copper was available for more important and critical areas of use, such as wiring.

 

2. The process to copper-plate the fins is labour-intensive and time-consuming. None of the Allies had excess labour, nor lots of time to spare, to spend it electroplating aircraft engine fins for only modest gains.

 

3. Copper is actually one of the three important precious metals, and as such, is used in currency, and largely treated as a precious metal.

 

Thus no-one in Govt or the Military would have been willing to use up a precious metal on something as simple as cooling, when one of its major features is as a store of value.

 

 

[facthunter]

Copper's also pretty dense (Heavy). Generally the metals that conduct heat also are good conductors of electricity . Best  at heat transfer are silver copper and aluminium with aluminium at about 54 % of  the performance of copper so an .008" plate of copper would equal a .015" of extra aluminium which would probably be lighter.  Steel is  pretty poor as a conductor of heat. Far worse than aluminium. Accurately tapering the fins would probably achieve as much.

 

Plating down into between the fins is  extremely difficult and is normally only done by an electroless process. Really expensive. Normal plating with electron flow plates the closest areas more heavily so you would need  a (comb) type anode. and /or some pretty exquisite techniques. Today they have developed  reversing current pulsing techniques to even out coating thicknesses but it's still hard to get it to throw down to the bottom or into holes like electroless does.. Military rated engines are using higher output figures  than civilian use engines are and  have lower TBO's and less reliability. I would expect they would try whatever seemed like a good idea. The Porcupine AJS racing motorbike had pure silver heads and barrells and the silver was only on loan from the mint. It's about 10% better than copper..  also not very practical but it was a very notable bike, way ahead of it's time. Nev

 

 

[Bruce Tuncks]

Flying down to the farm, with the manometer between the 2 ducts still in place, along with the vacuum-hose joiner, there was a difference of 20mm pressure between the cooler RHS and the warmer LHS.

 

Here's what is next in line....

 

1. Improve the baffles on the hot side as Yenn suggested. It already has a smallish  "A" baffle between the cylinders, pulled up against the underside of the fins with a spring setup.   I can use this on the top and put a larger thing on the bottom.

 

2. Sharpen the fins as Nev suggested. This idea was new to me but I like it.

 

3. Enlarge the diameter of the connecting hose and make it smoother . There must be a big reason why the RHS has more pressure than the left, but I'm at a loss to know why after the failure of the deflector which was going to improve things but instead made things worse.

 

Ducting air to the oil-cooler as keenaviator suggests is also a good idea but it has already been done, in that the lower-cowl inlet has been blocked and the smaller amount of air admitted there goes only through the oil-cooler.

 

Cowl-flaps are another idea, they could consist of skirt sections which could be retracted. But quite complex to construct and then live with.

 

The temperature difference with the LHS hotter than the right does seem to be caused by the pressure difference, but I'm starting to doubt that common-sense prevails here. The term "dark art" comes to mind.

 

Thanks guys for the comments and ideas. I'll do those 3 things and tell you what happens.

 

 

[jetjr]

The term "dark art" comes to mind.

 

Sure is, and in terms of air flow through cowl, (and fuel flow from inlet) it seems no to Jabs are the same

 

The big difference is made by sucking more air out - lower cowl lips,

 

then controlling the path that air takes - duct deflectors

 

I think sealing gaps etc is essentially playing around the edges.

 

There is a point where lower lip modifications can result in less flow and can cause exhaust to enter cockpit

 

When modifying older Jabiru heads, be careful as you cant easily get them anymore unless something has changed

 

 

[Old Koreelah]

The term "dark art" comes to mind.

 

Too right, Jet. Plenty of clever people have been beaten by it.

 

I think sealing gaps etc is essentially playing around the edges...

 

I disagree. I've put lots of work into redesigning my cooling setup and the most dramatic improvement came after I discovered some tiny gaps I'd overlooked. After plugging them, the CHT's came down 20-odd degrees immediately.

 

 

[facthunter]

 If you are using the top of the engine as a plenum, (Pressure chamber) good sealing is essential. In a reasonable cross section plenum, pressure is essentially EQUAL every where as the air is at relatively low velocity The reduction of speed gives the pressure. at the cowl opening, You then direct the required air through an appropriately sized channel to where you want the cooling to happen. Blast tubes to the alternator etc as well as around the hot parts of the engine .Even cylinder cooling is more likely this way. Nev

 

 

[Paul davenport]

Pictures of my drop in cylinder baffles. Last picture is of my cowl lip

 

 

 

[Yenn]

The Jab design is basically two plenums, one for each side. The pipe between them is only attempting to even up the pressure in each plenum. The difference between the Jab method and a Lycoming for example is that the plenums are smaller and there is a lot of cylinder which does not get ducted air, plus there is air escaping between the fins and the plenum and also as I noticed this morning when I looked at my setup, it is possible for air to escape beneath the plenum and out between the rocker boxes. If I was trying to get the max cooling I would blank off between the rocker boxes and duct air down between the sides of them.

 

I have only just seen Pauls contribution above, looks good and what I am suggesting is the same again on the outside.

 

 

[Paul davenport]

I have done the same to the heads but cannot find pictures. 

 

 

[Kenlsa]

 

Also sealed around oil cooler, CAMit gull wings on the bottom of jugs, extraction skirt.

 

I tried enclosing the rear of 3 and 4 but this increased the temp on those by 15 deg. Ian Bent suggested that the heated air pooled at the rear of the system coz it doesn't like  tight 90deg bends down the back of the fins as it slows the airflow. Returned them to the standard outflow at the rear and temps were lowered.

 

Ken

 

 

[Paul davenport]

Bares out what I found on the 230.  One side on the 230 is more closed (pilots side) and number 6 is the originally monitored temp, when I opened up the plenum it immediately cooled. I to fitted ramps to the front openings and this helps . My engine cooling system is now a bit like grandpas axe had two new handles and a new head but it’s still the same axe.  GREAT STUFF fellas keep it going

 

 

[Bruce Tuncks]

We had some terrible weather here lately, with 46 degrees a few days ago, just short of the record. So not much flying and I tried to do 3 things: Firstly, add a skirt to the lower cowl exit lip ; secondly, make and fit some bigger " gull wing " deflectors to the LHS duct ; thirdly, to sharpen the fins on the no 4 cylinder.

 

Sharpening the fins was harder than I thought it would be, and it will need to wait till I get the right bit for the Dremel to finish the job properly.

 

Not very scientific to do more than one thing at a time I know.

 

This morning ( ambient 19 C ) it was test-flown, and there was a definite improvement.

 

Pic 1 is before starting up, the engine was still cool from the night. I reckon this tests the senders, well at least at ambient.

 

Pic 2 is the manometer between the ducts, taken on climb. The RHS is still higher pressure and the joining duct is obviously at work. That's about 1 cm pressure difference, which is less than before and I think the gull wing baffles must be the reason. 

 

Pic 3 is the climb temps. They are more even than before, and I reckon it was easier to cool down 2 and 4 with step climbing. Mind you, this needs checking out on a 30 degree day.

 

Pic 4 is the skirt. Unfinished at the moment, but I think it helped a lot.  It must slow the plane down a bit, but I couldn't notice the difference.

 

Thanks guys for the good advice. As you can see, I have implemented quite a few of your ideas.

 

 

 

 

[Jabiru7252]

wow - that's a fine looking digital temperature meter. ? Don't slice yourself open on the sharp fins....?

 

 

[Paul davenport]

Thanks for the update. I had ordered my dremel grinding burs some days ago and will use them with the routing attachment to achieve the taper I want . Do you think we could have some pictures of under the cowl and of the fin mod.  Do you think the fin mod was responsible for very much of your cooling success ?

 

 

[Bruce Tuncks]

Yep, all this stuff was only possible with that digital temperature meter... Thanks Jab 7252.

 

For years, my plane operated with just one CHT operating from a ring terminal under no4 spark plug, with a moving-coil VDO display, and the cold thermocouple junction in front of the firewall.  Wish I knew what temperatures it really got to then.

 

 

[jetjr]

Jabiru sell lower cowl pieces to modify cowl

 

theres an old sb about it

 

 

[Old Koreelah]

...Wish I knew what temperatures it really got to then.

 

Remove your tappet covers and the colour of the head metal will give you a fair idea; someone on here posted the temperature required to "cook" the oil, so I guess the colour of oil baked onto the metal might be a rough guide.

 

[facthunter]

 You can't grind aluminium with a dremel and mounted stone. You need a cutter (rotary Burr) or even a small  warding file will give you a more controlled removal  A flat edged fin is like a dam to the air, the flow is so bad.. Nev

 

 

[turboplanner]

 You can't grind aluminium with a dremel and mounted stone. You need a cutter (rotary Burr) 

 

I'd second that and add; the burr works quite well but you need to hold the tool in both hands and stick your tongue out the side to keep the cut straight.

 

Clamping a smooth piece of wood somehow to rest the tool wold make it easier of possible.

 

 

[Bruce Tuncks]

Thanks jetjr, yes I did that sb years ago. They supplied a moulding which replaced part of the front lower cowl leaving more room for the oil cooler and actually lowering the drag.

 

And for the fin-shaping job, I agree that mounted stone type dremel tools would be no good, and it will need a side-cutting carbide burr of some sort. A small file will do the job, but gosh its slow and hard work. It is intuitively obvious that airflow down the bottom of the fins will be improved, and I hope to be able to report at least 2 degrees more cooling. 

 

 

[facthunter]

 You might have success with a steel bladed scraper more or less the reverse of a knife sharpener. I repair by welding broken fins on motorcycle barrels /heads where you try to get them looking as new. The top of the Jab fins is quite straight so you could mount it on a piece of wood like a moulding plane in principle. Nev