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seb7701

Jab Plenum Chambers

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Hi all.

 

Just in the process of playing with the cooling on my newly re-vamped Jab and was wondering how many owners are using the original LSA style top cowl with the smaller front intakes, as well as the earlier plenum chambers / ram air ducts with the plug leads running externally?

 

While I have every intention to open up the front end a little more to match the ducts, I am wondering whether they can do the trick or not, given that I've read a few articles which suggest that they just won't cut the mustard.

 

Apologies in that I know Jab cooling has been done to death, but doesn't seem to be much on the old vs new ducts/cowls and I'm very curious to know who has the early stuff and if so, what was their keys to success!!

 

Cheers, Seb

 

 

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Seb ,opening up the air entry may improve temps, but extending the lip on the lower cowl of those earlier Jabs has been successful in lowering CHT's also .... Bob .

 

 

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Thanks Bob. I had a lip glassed on the lower cowl whilst up at the Jab factory, but I am thinking I'd like another inch or so on it!

 

 

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I presume your Jab is 19- reg, allowing modification?

 

Plenty of people have played with Jab cooling, some with success. I'm on my third version of the ram-air ducts, but nothing improves cooling like sealing the tiniest gaps in the cowl; you've got to have good suction to extract the heated air.

 

Some have had success with a fence around the top of the engine (like Lycoming and Continental).

 

My ultimate aim is updraught cooling, but I'll persevere to get the best out of what I have in the meantime.

 

 

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Seb

 

I have a 230, serial number 174 I have over 1000 hrs (first engine) I have found most important thing is oil temp make sure you have the latest oil cooler and cowl also as Bob has said above a lip on the bottom of the cowl. I also changed the exhaust 3 pipes on each side into a collector and a 2" pipe from the collector, removed the log muffler altogether made a huge difference to the cooling. Oil temps don't get above 85c even on a 35+ day out of Roma and CHT's run around 275f consistently. If you want some photos of my set up let me know.

 

Aldo

 

 

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Here's the lip - I think it needs increasing...maybe angled further backwards now?

 

There aren't any fences at the front of the heads, as these earlier ducts only extend down as far as halfway anyway, but thinking I might install some 2cm x 2cm angle pieces there anyway to send the air over the top of the fins/barrels??

 

IMG_0692.JPG.d7326d14c60441233559fd265337dd04.JPG

 

IMG_0690.jpg.108f353885974fc99783c81726b9d4fd.jpg

 

 

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...removed the log muffler altogether made a huge difference to the cooling...Aldo

I've heard that from a couple of sources, Aldo. I'd like to understand how.

 

 

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A very effective way to reduce temp on the jab is to cut out a piece of the cowling on top. Appr. 30 X 150 m/my and make a small scoop on top. That create a venturi action and will suck out the heat from the top.

 

 

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With due respect to all - cooling of an air-cooled flat engine is something of an arcane science. Ask any aero-engineer who has been involved in engine modifications/swaps/aircraft performance testing and you will find that it is a really, really tricky business to get right - and there is NO 'silver bullet' answer.

 

Some months ago, I spent some time with a J160 owner at Camden, who has managed to tune his standard Jab. baffles and cowl exit lip just about to perfection. He's a seriously competent guy, and his project for that has taken him many, many months of research-level trial - even down to wool-tufting the internal baffles and having a borescope camera inserted so see what the airflow was actually doing. Then - running a proper test sequence for each change: climbs at various speeds in various ambient temps and at various revs; cruise performance etc.

 

The degree of finesse required to get the standard Jab. ram ducts to work properly, is remarkable - and it varies between the sides. Then there is the cowl extraction set-up: too much lip at higher angles of attack causes a stall around the lip and reduced efficiency of extraction just when you need it most. And - the 'finessing' is in the area of millimetres, not just a bit of a thump with a mallet..

 

However - some basic principles still apply even to rather 'gross' changes. Basically, these are:

 

A p-delta drop between the intake side for head cooling and the extraction side of at least 2.5" is essential.

 

The cooling air needs to be at close to optimum pressure and velocity for the head fins to be really effective. Inadequate velocity will not provide adequate heat transfer; too much pressure is likely to cause stagnation at critical points - and the standard Jab. head bolt at 90-dgrees above the exhaust port is a major constriction for good cooling around the exhaust port. The standard Jab. ram ducts waste a considerable amount of the potential head fin area for cooling due to a too great velocity drop.

 

Excessive oil cooling does NOT improve overall cooling performance and has serious adverse effects: oil should be running at close to 100C to boil off the sulphides. Only if there is piston under-crown oil cooling,is there any reason to provide excessive cooling capacity - and that should be moderated by a TOCA.

 

Any opening on the top cowl is potentially dangerous: in the event of an oil leak, you will get oil mist on the windscreen and in the case of an under-cowl fire, you are dead in a very short time. Reducing the airflow around the muffler and exhaust stacks is extremely unsafe.

 

 

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Thanks for the insight Oscar- yep, your friend did what I dream of...oh for a wind tunnel and the knowledge to use it!!! Given your points above, what are your thoughts on adding an inch or two to the lip (pictured above), perhaps at a less acute angle and opening the front intakes by lowering the bottom edge by around another inch, to match the duct openings? Seems to be a few success stories from such a tactic.

 

 

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Very early days into the testing, but cruise (not much more than downwind leg so far...) are mid 150's, but max values recorded are in the pic. Note - no 3 CHT not working... Buggered if I know why 2&4 are so much warmer, albeit nicely simiar!

 

IMG_0744.JPG.ac5d6a9d4fbbfe6b5e197bd0f70df3d5.JPG

 

 

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Seb , CHT's of 170 are a little too hot . Better if you can get them down to 140 or so . Do you have the thermocouple under the spark plug , or in the drilled hole between the plugs . Some T/c's under the spark plug can leak hot gas if not properly seated ,giving an incorrect reading . A portable thermal measuring device ( laser) can also be used to verify the MGL , simply a matter of removing the upper cowl after shutdown" and pointing beam at the individual heads .The CHT difference you have between 1 and 2 is not uncommon and mine is similar , although much lower . Interestingly the EGT's on 2 and 4 are also correspondingly high . Life was much simpler in the olden days, before we had access to digital monitoring systems giving us much greater accuracy ,compared to 'near enough' parallax error with the old analogue gauges, however I would be thinking maybe a few simple things for a start, such as replacing spark plugs and setting at .023" , ensuring correct fuel flow of around 16 l/hr at 2900 , running on Avgas , replacing air filter element if not recently done ,particularly if used in dusty environment ,and of course a leakdown check may also tell us something . EGT's can also be affected by simple things such as the positioning of the T/C 's from the exhaust stack , pipe bends etc. Oscar has done much work on these issues may care to comment also , good luck ! ..... Bob

 

 

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Thanks again Bob - CHT's are measure from hole drilled between plugs. Yep, 170 is too hot, but bear in mind engine is about 45mins old with completely new top end, so needs to settle a little before I get too carried away. That said, 2 & 4 need cooling! Ahhh, laser thermometer - bloody good idea!!! New air cleaner done, using AVGAS, but have to check jetting. Saw 16l/h on flow meter at around 2850ish on downwind, but really only glimpsed, so have to go give it a good run and see what's really happening!!

 

 

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Don't want to put a dampener on this, but a laser thermometer is affected by a number of variables like surface reflectivity. Mine showed 18 degrees below what a Mercury thermometer read. If you put enough thought into your probes and wiring your Guage it should be pretty reliable.

 

 

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Hi Seb - apologies for the tardy response, other stuff intervened..

 

I am absolutely NO guru - but I am fortunate to have access to some people who have decades of experience. I ask questions, listen to advice and then try to follow that as best I can.

 

Something I have had drummed into me, is that there is NO 'guaranteed' answer. In the case of an engine swap (for instance) to a certificated aircraft, the end result must meet the original cooling performance (with allowances for an accepted operating regime for the 'new' engine). Ask any ex-CAR35 / current Part 21M engineer and you will be told that ONLY by flight testing can compliance be demonstrated. Why? - because it is simply not possible - even with current airflow dynamics modelling programs - how the damn cooling will perform in real life. There are too many fine variables for accurate prediction from best design principles.

 

I believe you are well on the right track with your ideas, but only flight testing will tell. Re your anomalous cht reading - are you sure you've set up the MGL channel for the right probe? We're using two strato instruments for the engine test cell, and they do require attention to the set-up - but are pretty damn good when properly configured. We calibrated the cht readings from the probes ( CAMit probes) to the instrument using a special set-up, and it proved that they are within a couple of % across the entire range, using this set-up. It is a brass rod of 125mm diameter x 150 mm depth, heated by a stove element, with laboratory-grade mercury thermometers ( 2, for averaging), plus we used an IR meter just as back-up - and everything was damn close in readings.

 

1359028641_NakedDalek2.jpg.afbb4617bdd7379d890f025141393d42.jpg

 

It looks hokey - but it will pass CASA inspection for FAR specification for calibration accuracy.

 

WTR to your bottom-lip extension: the critical element there is that you achieve a decent negative pressure and extracting airflow in the lower cowl. A large lip at too steep an angle will stall and have a 'back-curl' effect that will choke off extracting the lower-cowl area. If you can modify it to have a rake of no more than about 15 degrees to the apparent air when the aircraft is in steep climb AND extend aft of the firewall, I think you will get better results. Combine that with a total air extraction of around 2.5 - 3 times the intake areas ( and you need to add to the head intakes the oil-cooler intake area, because it exhausts into the lower cowl.)

 

For our own aircraft, we have decided to go right away from the original cowl - but that was in part because ours was built for the 1600 engine but used as the test mule for the 2200 engine, and our cowl was a lash-up. It says somethiong for the tolerance of Jabirus that it did 2700 hours as mostly a training-fleet aircraft - but it also chewed through engines too quickly. I believe a lot of that was from bad operation and lousy-quality maintenance by an L2 who I would personally not let to maintain a half-brick. What we are doing is most definitely a 'work in progress' - and has a research element involved - so more description would be useless to you.

 

 

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No probs Oscar - always fascinating to hear this stuff. Good point re the MGL, as it does actually appear to be reading similar to the last one, which is simply ambient...

 

You mentioned something I wondered, and that was to overlap the firewall with the cowl extending past it, as I have seen often before a few times. In all, I think opening the intakes is on the cards, along with making sure that the low pressure area underneath is sufficient to do justice to the intake.

 

 

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seb: I think you are on the right track (but that's just my feeling - not to be taken as any sort of authoritative statement!)

 

I assume you have looked at the Jab. engine install manual? (JEM2202-7, available from the Jab web-site, is I think the latest version). It provides quite a lot of useful info.

 

FWIW - and I stress that this is my interpretation of general information provided to me by people whose knowledge is far better than mine and whose experience suggests they know what they are talking about, NOT any definitive statement of results for which I have figures to suggest I am correct - I consider that the early intake shape is pretty damn awful. The narrow outer ends of the intakes are only just getting into the effective area of blade shape for getting decent prop-blast flow from the prop. for starters.

 

When Alan Kerr (who did the development and certification testing for the original 2200C engine) was looking at an installation of the Jab. 2200 for Boeing's prospective entry to the Australian Drone contract for the RAAF (which didn't come to pass, different story) he found that in a cross-wind, the downwind intake for the heads was actually flowing backwards! Worse for the 'downward-prop' side ( the PAX. side, in a Jab.). In other words, the prop blast effect was useless when stationary and at around idle revs. - the heads on that side of the engine were sucking the hot air from underneath the ram-ducts!. Now, put that into a FTF context: holding on a busy strip, lengthy pre-take-off check sequences with a trainee, then moving onto the strip and putting max. power/low airspeed for the next several minutes of initial take-off onto heads already damn hot - no wonder that FTF operation has had the worst run of Jab engine problems..

 

If you are thinking abut modifying your intakes, I'd be trying to get a bit more intake area at the outside edges to pick up more prop blast. This is what I have done with my revised top cowl:

 

Topcowlmodspainted.jpg.537c853b8fb2d65649de8221ac95e61c.jpg

 

It's a completely different profile to the standard LSA55 etc. cowls (hence the necessity for a 'bulge' to clear the starter!) NO - I cannot say that it works - it will be many months before we get to flying tests - but it might give you some ideas you may wish to try.

 

 

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Thanks Oscar - love to see what goes for cowls these days and what mods people have down. Quite simply, if the increase in the intake size isn't effective, I will know to go back downstairs to the outlet and perhaps increase lip size etc or add some tab style lips as a trial. So far, anecdotal evidence indicates that intake enlargement is quite successful, so we'll see. The fact is, there are perfectly satisfactory LSA cowled Jab in existence, so I am not of the impression that drastic measures are required, just some tweaking. Next after that though, I am curious to remove that big barrel muffler from behind the trusty oil cooler, as I imagine it could be creating all sorts of complexities in terms of air output from the cooler striking it, as well as the radiated heat.

 

This shows exactly what I have in mind. There appears to be quite a few people I know of locally who have success to the amount of 10-22 odd degrees reduction.

 

INTAKE1.JPG.edf759e1bc1bd722feae41c78b9e49b5.JPG

 

INTAKE2.JPG.e68b6505536cfdc93082bf4e31feaccb.JPG

 

 

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Seb - that looks good to me, though JUST stuffing more air in won't be a magic fix if it doesn't get to the the right places; testing will prove the pudding and the old adage of 'one change at a time' is a good one.

 

Re changing the muffler: be cautious. If you are running a 2200C engine, it was certificated and the back-pressure figure for the engine was carefully tested; don't make up something that introduces an increase in back-pressure as it will affect engine performance. ( I haven't seen the test stuff for ASTM certfying but I imagine it has a similar requirement). That 'log' looks like a tin-can stuck on but a bit more work than shows went into it!.

 

 

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2200A -solid lifter. Got my base line figures today for cruise, so tomorrow will check what it's like after stuffing more air in...

 

As far as the exhaust is concerned, I was thinking of trialling a straight through just for a brief test to see the effect, as I note the likes of Sonex etc don't use a muffler with their Jab installations.

 

 

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Excessive oil cooling does NOT improve overall cooling performance and has serious adverse effects: oil should be running at close to 100C to boil off the sulphides. Only if there is piston under-crown oil cooling,is there any reason to provide excessive cooling capacity - and that should be moderated by a TOCA.

Oscar

 

If you are referring to my comments above it's probably not relevant to my situation as I do a 25 hr oil change approximately every 2 weeks. I personally am more concerned with oil temperatures than anything else from my experience.

 

Aldo

 

 

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Seb, I have a 22A solid lifter and went through the process you are doing. My cowl has the bigger holes in front to better match the ducts. You have got good advice here, I can only agree how important it is to monitor the temperatures, including the EGT, on each cylinder. The EGT is the heat source, the CHT is the result of heat in and heat out.

 

My engine has gone well for 510 hours ( that is if you ignore a ground-running event where I lost the no4 valve seat) and is now running cooler and with more even temperatures than ever before. Getting to this has been hard work but fun too. Keep at it.

 

 

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