Jab Plenum Chambers

[seb7701]

Oh heck yes Ken, if you do happen to dig up the vid...I have often wished I had a wind tunnel so I could actually see what is going on.

 

Here's a stock intake, so not quite sure but looks a little smaller...

 

Fascinating overall and someone has put in a LOT of work - great to know what can be achieved at least!

 

One last thing - do you have the standard deflector inside the duct coming down over the rear cylinder? What's inside??

 

Thanks again,

 

Seb

 

 

 

[jabiruken]

Oh heck yes Ken, if you do happen to dig up the vid...I have often wished I had a wind tunnel so I could actually see what is going on.Here's a stock intake, so not quite sure but looks a little smaller...

Fascinating overall and someone has put in a LOT of work - great to know what can be achieved at least!

 

One last thing - do you have the standard deflector inside the duct coming down over the rear cylinder? What's inside??

 

Thanks again,

 

Seb

No,

No deflector, there is actually eight components involved in his modifications.

 

I should have taken lower cowl off so you can get better view. The cylinders are nearly fully Encased with Kevlar like duct and air is forced out bottom of cylinders nearer to engine block. Have moved recently so may take a little time locating the stuff you require

 

 

[seb7701]

No dramas at all Ken, will just take what I can get as you really seem to have the best figures I have ever seen!!!

 

 

[Bruce Tuncks]

Here's my figures after quite a lot of tweaking.

 

The first pic is if CHT's, on climbout on a warm day. I really don't like seeing the max at 170 degrees and usually do step climbs to keep it under 160.

 

The next pics are EGT's and CHT's on cruise. The EGT's as shown are a lot more even than before. I reckon they are ok.

 

 

[Bruce Tuncks]

oops...here they are

 

 

 

 

 

[seb7701]

Definitely alright Bruce - very nice. What was the OAT on that occasion? I too generally level out from a climb when my no.2 problem child reaches 165deg. On a 30deg day no.1 only sees 116 degrees, so really is some variation among our engines!!

 

Am interesting in the joining of the two ducts by the SCAT hose as per Ken's, but the rest is just odd. My right duct is stock and untouched and doing well. The left side as been recently enclosed as I touched on and made things worse!!!

 

Bruce - what's been your greatest mod? Old or new ducts, opened up intakes?? Spill the beans!!!

 

I am starting to wish there was a Jab get together some time where I could check out some of your aircraft before mine does my head in!!!

 

 

[Old Koreelah]

...I am starting to wish there was a Jab get together some time where I could check out some of your aircraft before mine does my head in!!!

Narromine's not far off, Seb.

You've probably done all the obvious things, but just in case: I thought I'd done a great job of sealing my ducts and engine cowls, but my temps were still too hot. Plugging some minor leaks made a big difference.

 

 

[Bruce Tuncks]

I wish I knew the answer to that question Seb. Bad science I know to do more than one thing at a time but that is what I did.

 

Opening up the air inlet holes was done at the same time as blocking up the ducting gaps and these things together worked to do about 15 degrees.

 

But in those days, my only temperature indication was the CHT on no 4, and this was a simple thermocouple with the "cold" junction on the engine side of the firewall.

 

As I was only seeing the difference between the hot and cold junctions, the CHT seemed to be constant regardless of the ambient temperature. For a time, I actually thought that 160 degrees was the max I was getting, and that I could keep it to 150 with care... wrong!

 

The other thing that helped was putting the vanes downstream of the carby ( thanks to jetjr for the suggestion). This cooled down the no 4 EGT from just over 800 degrees. .

 

And the ground temp on the day of those pics was about 30 degrees, with the max CHT pic taken at about 1000 ft on climbout.

 

 

[seb7701]

They're great temps for 30 degrees Bruce, no doubt there. I have done my best to change only one thing at a time, yet sealing the gaps and adding the oil cooler duct were the worst things yet???

 

Adding the bit in the photo below on the rear of no 4 was similarly pointless and possibly negative in fact!

 

Interestingly my plane only had a CHT ring on no.4 which, knowing what I do now, explains why they appear to have had a heat related incident on no.2 which would be why I just ended up replacing that cylinder the other day.

 

The ultimate quandary is why enclosing my left duct made it hotter and not cooler. Anyhow, I have a replacement 'old style' duct on the table here and will soon dig out the die grinder and open the intake RIGHT up, as 1.5cm off the bottom edge of it dropped around 10 deg off the CHT from memory, so perhaps there will be some more gains left in that realm.

 

Would love to know if balancing the ducts like Ken's really is the go...

 

Also been eyeing off the sonex baffle kit - complete sealing...hmmm....

 

Sonex Web Store - CAMit/Jabiru Universal Cooling Baffle Kit CAMit/Jabiru Universal Cooling Baffle Kit

 

 

 

[Oscar]

But in those days, my only temperature indication was the CHT on no 4, and this was a simple thermocouple with the "cold" junction on the engine side of the firewall.

As I was only seeing the difference between the hot and cold junctions, the CHT seemed to be constant regardless of the ambient temperature. For a time, I actually thought that 160 degrees was the max I was getting, and that I could keep it to 150 with care... wrong!

 

The other thing that helped was putting the vanes downstream of the carby ( thanks to jetjr for the suggestion). This cooled down the no 4 EGT from just over 800 degrees. .

 

And the ground temp on the day of those pics was about 30 degrees, with the max CHT pic taken at about 1000 ft on climbout.

The single ring-type thermocouple on NO #4 plus the lack of an ambient- temperature compensated cold junction has probably cost more Jabiru engines than any other single factor.. I think just about everybody who has moved from that set-up to a more modern set-up, such as using the MGL ( as an example) EMIS with temperature-compensation via their RDAC unit, and more accurate senders mounted between the plugs, has found that what they thought they were getting was very, very inaccurate and often very misleading. Jabiru has recommended full cylinder monitoring for many years now..

 

As Seb has found out - and we all should thank him profusely for sharing his 'Pilgrim's Progress' here - getting the cooling flow right is almost in the area of a 'dark art'.. And, lest he feel disappointed, I can say from the long, long experience of a family member who is an aeronautical engineer responsible for many engine swaps etc. - it is one of the hardest areas to get right.

 

There is an old saying - attributed to, I believe, either Harvard or MIT - that: 'under known and controlled conditions of pressure, temperature and environment, the subject organism will do what it damn well pleases'... and cooling airflow is right up at the top there...

 

It's not simply a matter of getting a big hole at the front and an even bigger one at the back... every change of direction, every change of area through which the air has to pass, creates its own influence. AND - just as an extra embuggerance - those changes, themselves change with pressure / airflow speed. What works well at say, cruise speed in flat flight, may NOT work at all well for climb.. ( and I KNOW I'm not telling anyone anything they haven't discovered)

 

The Jab. ram ducts are, I believe , a particularly sensitive methodology of delivering cooling air, because they are a compact design. Lycs. and Continentals generally use large plenums which are less sensitive. That, at least, reduces 'choke' points for the airflow. I have gone, in my own radical development ( as yet, unproven in flight and may be a TOTAL stuff-up, so don't use it as anything but an 'idea' at this time!) for the more 'traditional' plenum idea - and then complicated it by splitting off the head cooling from the barrel cooling, but I did that as a research project, to back up a potential development that will probably never come - sadly - to fruition as the engine design for which that project was intended is unlikely to ever reach production now ( thank you, CASA..you bastards)

 

 

If - in the fullness of time - it works well, I'll be happy to report on all the 'bits and tits' involved - it's taken hundreds of hours, from making new cowls onwards. If it doesn't, I will very probably slink away and take up macrame as a hobby..

 

 

[Old Koreelah]

...If - in the fullness of time - it works well, I'll be happy to report on all the 'bits and tits' involved - it's taken hundreds of hours, from making new cowls onwards. If it doesn't, I will very probably slink away and take up macrame as a hobby..

Fear not, brave Oscar. I'm sure it will work.

But just in case, you won't have achieved nothing; the Macquarie Dictionary will benefit from your inventive language:

 

...- just as an extra embuggerance - CASA..you bastards)...

[Oscar]

OK, you are exceedingly generous..

 

THIS, is the sort of thing that happens when one thinks: 'heck, I can do better'..

 

I don't like at all, the Jab. airbox and associated airflow delivery to the carby. So....

 

Started with a new airbox.

 

 

There's a foam filter sitting under that spring-loaded flap that provides backfire relief to the filter.

 

(old SCAT used for trial fitting, new will be installed)

 

 

The Jab original (from their Installation manual) looks like this:

 

 

I am biased, but I reckon mine looks neater..

 

 

[scsirob]

..I am biased, but I reckon mine looks neater..

Your bias is well justified, a true piece of art!

 

 

[derekliston]

I didn't like the Jab air box setup either so I have electric carburettor heat and a pod filter. I don't currently have a balance tube from filter to carb, but it seems to function well like this.

[seb7701]

Hi Derek - hi went to the silicone hose myself, so good to see this one. I have wondered about the pod filter quite a bit, particularly when I've seen then so often on Rotax 912's.

 

Did you find any great changes when you strayed from the airbox?

 

Was leafing through a recent Kit Planes mag and was unsurprisingly intrigued by a plenum setup on a Titan air cooled engine fitted to a Kitfox. Bit like yours I reckon Ken, complete with some big outlets on the rear which I wondered could have been part of a balance hose setup too.

 

Here's the shots.

 

 

 

 

 

[derekliston]

Hi Derek - hi went to the silicone hose myself, so good to see this one. I have wondered about the pod filter quite a bit, particularly when I've seen then so often on Rotax 912's.Did you find any great changes when you strayed from the airbox?

Was leafing through a recent Kit Planes mag and was unsurprisingly intrigued by a plenum setup on a Titan air cooled engine fitted to a Kitfox. Bit like yours I reckon Ken, complete with some big outlets on the rear which I wondered could have been part of a balance hose setup too.

 

Here's the shots.

No, I had the Jab air box mounted on the left (from the cockpit) side of the lower cowling due to space considerations but could never achieve full static rpm, changed to the silicone hose and pod filter and static rpm is 3000. Filter is totally inside lower cowl but doesn't seem to be a problem.

 

 

[derekliston]

[ATTACH=full]52070[/ATTACH] I didn't like the Jab air box setup either so I have electric carburettor heat and a pod filter. I don't currently have a balance tube from filter to carb, but it seems to function well like this.

Hey biggles, why the caution!

 

 

[biggles]

Hey biggles, why the caution!

Sorry Derek . Was not aware I had done it . Now removed . Have never used that intentionally and don't ever intend to.... Bob

 

 

[seb7701]

Anyone have thoughts or knowledge of a big one-piece plenum like the green one above?

 

 

[jetjr]

I saw some attempts, lots of time and achieved Nil. My 2 cents (and only a current opinion) is that getting air out is the key.

 

Good to see all the pictures

 

I have spent huge amounts of time and effort on this and can summarize

 

1. Get temp probes sorted - under plug rings changed depending on each plug removal, also had cold junction in the engine area, finally had head temps 90-120

 

changes to Jabiru between head push in probes, temps jumped to 120-140, ended up with CAE probes and temps stable at 130-140 - same cowls and ducts (different engine)

 

2, As per Jabiru USA tech tip on top deflectors, this really works but takes time and doesnt help left to right bias

 

Learning from my trials and pics here that cowl exit is a BIG factor, notice pics here it is much larger and Jabiru have several versions of lower cowl and seems the big ones work.

 

3. Oil cooler inlets stuff up flow.and mean process has to be looked at again.

 

3, EGT, spread is a massive problem with every airframe being different. Actual temps are sucessfuly adjusted with jets or leaning valve.

 

Air box and inlet NACA do induce swirl and RAM pressure, which Bing seems especially sensitive to.

 

Newest hole inlet is next to be tried or at least running with cowl hose off. A small amount of carb heat really makes a difference to EGT spread, not because of heat but disruption to flow. A pod filter would do similar.

 

Almost a black art but critical to get engine working right

 

A catch up at YNRM is possible, I have a place in hanger there

 

 

[facthunter]

In essence you need a pressure differential to create airflow and direct it to the right places. Since the ambient conditions vary and the aircrafts "situation" varies a lot also why not go to an adjustable (from the cockpit )cowl door/flap/ intake louvre etc.. When you block /open areas you are altering a "fixed " aperture trying to cover a multitude of changing circumstances. Ambient air temps should not have a dramatic effect. Its the temperature DIFFERENCE that determines the rate of heat flow/ dissipation (all other things remaining equal)..

 

I have never understood why the jab installation doesn't use in between cylinder baffles like just about every other aero aircooled motor. You have this massive LEAK of air doing almost nothing but reducing the pressure differential,. Without some form of air dam blast tubes etc don't work. Nev

 

 

[Old Koreelah]

Good points, Nev. Sealing air gaps makes a difference. Adjustable cowls are the ideal, but I've discovered they are also another distraction for the pilot. An automatic actuator shouldn't be too hard to rig up.

 

 

[facthunter]

You can make them so the normal position does cover most things. Having had to land to make changes en route to aircraft (I didn't own) more than once, the adjustable set up is a no brainer.. You can close them on descent to keep the engine warm and to warm up in cold conditions on the ground quicker. Also only using the minimum cooling air you save drag. You have only one engine to worry about and many had throttle, Manifold pressure (boost) mixture, fuel flow, pitch and cowl gills to manage in the past. Surely we are capable of giving the engine a bit of attention to optimise it's running . You monitor them probably a bit too much with all the gauges especially if something is running a bit too hot . You increase airspeed on climb if the heads or oil are getting a bit hot. That's probably more effort than just moving a cowl lever a small bit. Automatic equals less reliability. Something more to fail.

 

Just a comment. 120 degrees C for a head temp is pretty cold. The P&W round motors used 235C as upper limit. I'm not suggesting you go there as long as you don't need to clear a mountain or such emergency. Oil should run about 95 max for normal ops but tolerate say 110 if you run synthetic and there's some reason why it's happening. Nev

 

 

[jetjr]

You're correct of course Nev but the Jab style engine is a sensitive thing and in this area comparisons to other engines has to be done carefully.

 

I recall material limits on their heads is just 180 C or so and CAE are a little higher. With regular operating temps just 20 odd deg below this , not much has to change to have long lasting problems

 

Old solid lifter engines had baffles on top, many refitted below. CAE versions has larger ones underneath and whilst they didnt make any dramatic difference i reckon they stabilised temps significantly.

 

I am yet to see a good adjustable cowl flap setup on a Jabiru, Id say it would work well but is another thing for basic pilots to manage and simple things like cowl removal etc gets harder.

 

Cutting two pivoting sections into lower cowl wouldnt be too hard but how to actuate or hold them in position?

 

 

[Old Koreelah]

It always amazes me that most aircraft get away with dumping waste air out the bottom, into what is inherently a high pressure zone. That little lip is expected to create enough localised low pressure to suck cooling air out at all flight attitudes.

 

After lots of trials (and even more errors) my system is working pretty well: the air exits into the low pressure zone above the wing leading edge. I can idle for indefinite periods without overheating the heads.

 

After congratulating myself for this achievement I happened to look under a bloody great big Cessna with an engine many times larger than my Jab; it gets by with cowl flaps that are smaller than mine.