Jab Plenum Chambers

[Paul davenport]

The more we learn the more we don't know? !

 

 

[Oscar]

Yes that's what I thought but Rod says differentStandard Jab hot air setup doesn't bypass air filter or the main scat hose

jj_ a HUGE apology!. I am starting to build a brand new airbox, and looked at the ( absolutely AWFUL) cobbled-up abortion on my aircraft, that was obviously done when changing from the 1600 engine to the 2200. Id I posted a picture ( that facility seems to have disappeared from the site), I probably wouldn't, to save people from throwing up their Weeties all over their keyboard.

 

I had removed it and thrown it away in disgust, to the back of the workshop - but I dragged it out to look at what I might - possibly - use in the new set-up.

 

You are absolutely correct: the majority of the hot-air passes through the airfilter, though there is a small by-pass in the hot air intake (about 1 inch square in area that exits after the filter.). That has the carby balance tube installed in it.

 

That arrangement is just not kosher: the hot-air intake is supposed to also provide an alternate inlet in the case of filter blockage. Further, it means that a % of hot and unfiltered air was ALWAYS being fed into the engine: power reduction issues and excessive wear issues...

 

If hot air is selected, it certainly DOES bypass the main inlet SCAT hose connecting the cold air intake on the side of the cowl to the airbox - it's a 100% bypass. You get full HOT air - but when cold air is selected, you don't get 100% cold air. In ambients of maybe 20C or lower, that's probably not an issue - but it could be an issue if trying to claw out of a short-ish hot-and high airfield at MTOW.

 

 

[Paul davenport]

jj_ a HUGE apology!. I am starting to build a brand new airbox, and looked at the ( absolutely AWFUL) cobbled-up abortion on my aircraft, that was obviously done when changing from the 1600 engine to the 2200. Id I posted a picture ( that facility seems to have disappeared from the site), I probably wouldn't, to save people from throwing up their Weeties all over their keyboard.I had removed it and thrown it away in disgust, to the back of the workshop - but I dragged it out to look at what I might - possibly - use in the new set-up.

 

You are absolutely correct: the majority of the hot-air passes through the airfilter, though there is a small by-pass in the hot air intake (about 1 inch square in area that exits after the filter.). That has the carby balance tube installed in it.

 

That arrangement is just not kosher: the hot-air intake is supposed to also provide an alternate inlet in the case of filter blockage. Further, it means that a % of hot and unfiltered air was ALWAYS being fed into the engine: power reduction issues and excessive wear issues...

 

If hot air is selected, it certainly DOES bypass the main inlet SCAT hose connecting the cold air intake on the side of the cowl to the airbox - it's a 100% bypass. You get full HOT air - but when cold air is selected, you don't get 100% cold air. In ambients of maybe 20C or lower, that's probably not an issue - but it could be an issue if trying to claw out of a short-ish hot-and high airfield at MTOW.

[Paul davenport]

Just back from a flyin to Mt Beauty and before I left I put some small dobs of oil on the cowling to check air flow and was surprised to see that the air flow was great around the front to sides of the cowling BUT it did not flow down and passed the cooling lip on the bottom cowl, there was a stagnant patch of air above the oil cooler inlet and interestingly air was actually spill out of the cowling at the inner cowling surfaces. Air flow down the cowling seems to split well before the nose wheel. My cht gauge indicates 250/275 on cruise 290to 300 on climb and the oil temps range from 70 to 100 depending on situation so any improvement will be great. Looking at the profile of the cowl it always looked like the oil cooler was shielding a large portion of the cowl lip and I was right. Now what ,any

 

 

[Paul davenport]

Just back from a flyin to Mt Beauty and before I left I put some small dobs of oil on the cowling to check air flow and was surprised to see that the air flow was great around the front to sides of the cowling BUT it did not flow down and passed the cooling lip on the bottom cowl, there was a stagnant patch of air above the oil cooler inlet and interestingly air was actually spill out of the cowling at the inner cowling surfaces. Air flow down the cowling seems to split well before the nose wheel. My cht gauge indicates 250/275 on cruise 290to 300 on climb and the oil temps range from 70 to 100 depending on situation so any improvement will be great. Looking at the profile of the cowl it always looked like the oil cooler was shielding a large portion of the cowl lip and I was right. Now what ,any

Installed some cowl inlet ramps looks good and I think a marginal improvement. Also had some vg,s left over so installed them under the prop and under the oil cooler lip, with dobs of oil on cowl off I went , air flow under cooler is better but the top vg,s did nothing (so off they come) . So what can I relate to you all NOTHING but I had a great end of the day flight so it was not total loss . Next step to make up some perminant lower cowl cooling ramps

 

 

[seb7701]

This is interesting stuff Paul, as I am currently investigating what seem to be high oil temps on my Jab courtesy of the oil cooler inlet being a cross between the old and new.

 

Those I have seen with great cooling seemed to have more of a 'scoop' thing going on. I can only hypothesise that this stops the airflow simply running down the face of the cowling below the prop, with only the occasional lucky bit heading into the cooler?? Mine is the first photo and the later version is the second. You can see how the indentation on the second type would at least cause the air hit the lip and go in. Also no rounded lower edge. Ahh, more glassing I suppose....

 

 

 

 

[Paul davenport]

This is interesting stuff Paul, as I am currently investigating what seem to be high oil temps on my Jab courtesy of the oil cooler inlet being a cross between the old and new.Those I have seen with great cooling seemed to have more of a 'scoop' thing going on. I can only hypothesise that this stops the airflow simply running down the face of the cowling below the prop, with only the occasional lucky bit heading into the cooler?? Mine is the first photo and the later version is the second. You can see how the indentation on the second type would at least cause the air hit the lip and go in. Also no rounded lower edge. Ahh, more glassing I suppose....

There is a picture of an oil cooler some where on this thread that has been encased in a chamber that allows it to discharge its heated air to a low pressure area not into the engine cooling lower cowl area,that has merit and I may next oil change try some thing similar. Give the dirty oil dot on the cowl trick and let us know what your results are

 

 

[Paul davenport]

There is a picture of an oil cooler some where on this thread that has been encased in a chamber that allows it to discharge its heated air to a low pressure area not into the engine cooling lower cowl area,that has merit and I may next oil change try some thing similar. Give the dirty oil dot on the cowl trick and let us know what your results are

Just a thought I wonder why jab went and put there cooler there and not where the normal Cessna piper mob put theirs ???

 

 

[seb7701]

Yep, that's probably the one from the Europa in the states. Looked awfully good, but I should just be able to either extend the lower section a touch or indent above. I am close, just need to knock some degree off!

 

As for the location, I guess there was a reason somewhere!!

 

 

[kasper]

Just a thought I wonder why jab went and put there cooler there and not where the normal Cessna piper mob put theirs ???

I'd go for evolution ... original Jab engine had no cooler separate from the engine just the sump exposed to airflow with a small opening in the lower cowl ... need more cooling make hole bigger ... need more cooling ... mount a cooler off the front of the sump - just in front of the sump- after all that's were the hole in the cowl is already ... make the cowl openings bigger/better/different as time goes by on various installations ...

 

 

[horsefeathers]

fotos please to show flat plate. Please

Hi Geoff.

 

Attached are a couple of photos showing the (admittedly rough) flat plate that floats between the muffler and the oil cooler.

 

Its attached to the front muffler shield, and loosley attached to the bottom of the oil cooler brackets.

 

 

 

 

[Markdun]

Guys, I've been following this thread as I've just installed a Jab2200 rollercam in my Cygnet which previously had an 1835VW, and I've had the usual issues sorting out cooling. I've now got about 23 hours with the Jab motor.

 

My CHT temps at cruise with 25C OAT and at 90kts are 140, 122, 135, 120 & EGTs at around 15.5lph are 695, 650, 690, 645 at between 4000asl and 5000asl. Rpm 2900. They max if I throttle back to 14lph and 80kt and 2750rpm at 720, 660, 720, 645. No matter what I do I can't get the stb cylinders cooler. No 1 & 2 were my hottest cht wise, but baffles in the pressure ducts were adjusted to get rear cylinders chts as close as possible to the front.

 

I originally had over 120C egt spread and that was mostly fixed by putting a fence of about 2cm high in front of the air inlet (which is simply a hole in the side of the cowl). This makes sure there is no ram air. Drawing cowl air made no difference. I also have flow straighteners in the airbox outlet and carb inlet. I do not have a simple cross because, if placed in the nsew orientation, the bottom interferes with the emulsion tube air inlet on the Bing and minute changes produce crazy results. When I installed the fence on my air intake my egt spread dropped and the average egts dropped so I've gone one size smaller on main and needle jets (230 & 2.8 if my memory is ok).

 

I was getting over 140C CHT on no.1 in cruise and oil temps over 90C so I ditched the 50mm lip on the cowl outlet and replaced it with a ramp for more suction. Result was a 5-10C drop on all CHTs and oil temps now mid 70s...so my next step is to put in a plate to exhaust the oil cooler air separately from the cylinders and adjust for mid 80C OT.....I'm worried about winter. Downside is higher Chts on 3&4 when taxiing (+135C).

 

My problem is on climb. When I go WOT, no.4 goes lean and the egt will, if allowed, runaway over 720C. So I throttle back to 3050 rpm where my egts are 650, 670, 645, 690 (Fuel flow somewhere around 25lph). CHTs on climb at 70-75kts don't go above 150C so far. If I rotate the carb to even mixture on cruise, it makes it much worse on WOT. And yes, the butterfly throttle is limited to 80 degrees open. By throttling back I lose about 50 to 100 revs on climb.

 

So now I'm reasonably happy and thinking there is just some tweaking on the cowl outlet with perhaps a bit of exhaust augmentation. I'm thinking my right bank is running hotter because that side is leaner and that may be because the inlet plenum diffuser is not exactly centre, or because of the orientation of the 4 horizontal holes in the emulsion tube/ atomizer which the needle jet sits in, or how the needle lays in the needle jet, eg it might lie against the stb side causing the fuel spray/plume to favour the port side?

 

I get a small drop of oil at the front crankshaft oil seal. Is this normal? Motor runs very smooth.

 

 

[Paul davenport]

Guys, I've been following this thread as I've just installed a Jab2200 rollercam in my Cygnet which previously had an 1835VW, and I've had the usual issues sorting out cooling. I've now got about 23 hours with the Jab motor.My CHT temps at cruise with 25C OAT and at 90kts are 140, 122, 135, 120 & EGTs at around 15.5lph are 695, 650, 690, 645 at between 4000asl and 5000asl. Rpm 2900. They max if I throttle back to 14lph and 80kt and 2750rpm at 720, 660, 720, 645. No matter what I do I can't get the stb cylinders cooler. No 1 & 2 were my hottest cht wise, but baffles in the pressure ducts were adjusted to get rear cylinders chts as close as possible to the front.

 

I originally had over 120C egt spread and that was mostly fixed by putting a fence of about 2cm high in front of the air inlet (which is simply a hole in the side of the cowl). This makes sure there is no ram air. Drawing cowl air made no difference. I also have flow straighteners in the airbox outlet and carb inlet. I do not have a simple cross because, if placed in the nsew orientation, the bottom interferes with the emulsion tube air inlet on the Bing and minute changes produce crazy results. When I installed the fence on my air intake my egt spread dropped and the average egts dropped so I've gone one size smaller on main and needle jets (230 & 2.8 if my memory is ok).

 

I was getting over 140C CHT on no.1 in cruise and oil temps over 90C so I ditched the 50mm lip on the cowl outlet and replaced it with a ramp for more suction. Result was a 5-10C drop on all CHTs and oil temps now mid 70s...so my next step is to put in a plate to exhaust the oil cooler air separately from the cylinders and adjust for mid 80C OT.....I'm worried about winter. Downside is higher Chts on 3&4 when taxiing (+135C).

 

My problem is on climb. When I go WOT, no.4 goes lean and the egt will, if allowed, runaway over 720C. So I throttle back to 3050 rpm where my egts are 650, 670, 645, 690 (Fuel flow somewhere around 25lph). CHTs on climb at 70-75kts don't go above 150C so far. If I rotate the carb to even mixture on cruise, it makes it much worse on WOT. And yes, the butterfly throttle is limited to 80 degrees open. By throttling back I lose about 50 to 100 revs on climb.

 

So now I'm reasonably happy and thinking there is just some tweaking on the cowl outlet with perhaps a bit of exhaust augmentation. I'm thinking my right bank is running hotter because that side is leaner and that may be because the inlet plenum diffuser is not exactly centre, or because of the orientation of the 4 horizontal holes in the emulsion tube/ atomizer which the needle jet sits in, or how the needle lays in the needle jet, eg it might lie against the stb side causing the fuel spray/plume to favour the port side?

 

I get a small drop of oil at the front crankshaft oil seal. Is this normal? Motor runs very smooth.

[Paul davenport]

Back from an early fly around the traps on Saturday cht at 300 on climb and 270 in cruise at 2900 rpm, back off to 2600 rpm and temp drops to 250 ,I am using around 21 lph at cruise,a quick call to Jabiru and a new main jet is on its way ,stay tuned

 

 

[Bruce Tuncks]

Good work Markdun. You have done better than me on the same job.

 

Could you give some more detail where you say how you "ditched the 50mm lip on the cowl outlet and replaced it with a ramp..." ?

 

Thanks, Bruce

 

 

[Oscar]

Bruce: if the 'lip' on the bottom of the cowl is too steep, in climb it will stall and the backwards flow of the outside extraction air will actually block the cowl air exit. The Jab. engine install manual shows this, and I have also been so advised by a J160 owner who has done the best work on the cooling set-up of which I am aware.

 

Just for the giggle, this is the radical (and as yet, totally unproven!) approach I have taken with my installation, to match my entirely new cowl(s) :

 

 

The barrels are fed entirely separately from the heads. This is for research purposes.

 

 

[seb7701]

Hi Markdun.

 

As Bruce said, you've done a excellent job alright and I too would really love to know a little more about the lip you mentioned.

 

'I was getting over 140C CHT on no.1 in cruise and oil temps over 90C so I ditched the 50mm lip on the cowl outlet and replaced it with a ramp for more suction. Result was a 5-10C drop on all CHTs and oil temps now mid 70s...'

 

That's a really dramatic drop in oil temp!

 

As per my previous posts, my problem child is no.2 cylinder and you have things covered with regard to preventing ram air for the intake also. I am just about to investigate my oil cooler inlet, but now I am fascinated with your result!

 

 

[Jaba-who]

Hi Markdun.As Bruce said, you've done a excellent job alright and I too would really love to know a little more about the lip you mentioned.

 

'I was getting over 140C CHT on no.1 in cruise and oil temps over 90C so I ditched the 50mm lip on the cowl outlet and replaced it with a ramp for more suction. Result was a 5-10C drop on all CHTs and oil temps now mid 70s...'

 

That's a really dramatic drop in oil temp!

 

As per my previous posts, my problem child is no.2 cylinder and you have things covered with regard to preventing ram air for the intake also. I am just about to investigate my oil cooler inlet, but now I am fascinated with your result!

Your expirement has gone too far. Your oil temps are now too cold.

 

Shell used to have a really good video discussion about aviation engine oil temps and jabiru have several times advised people to look at the video and ensure their temps reach Into the mid 90 degrees range.

 

Engine oils collect a lot of contaminants and water and unless they get to these temps they don't evaporate out the water and volatile contaminants.

 

This is the same reason you should not simply run your engine for a few minutes on the ground in times when you can't fly if. You need to run it at full temps for long enough to get rid of the water etc.

 

 

[Bruce Tuncks]

That divided plenum looks good Oscar. I bet several of us will be interested to hear what results you get.

 

Now that the hot weather is here, it turns out that my CHT's are not as good they were in winter ( no surprise huh, now that I am measuring real temperatures and not just the difference between hot and "cold" junctions )

 

Yesterday I saw 165 C max on cylinder no 4 on climb, with 145 on cruise. The port side cylinders ( 2 and 4) are noticeably hotter than the stbd ones and I would like to fix this but don't know how.

 

 

[jetjr]

Seb, the cooler inlet you have pictured is Its a Naca design used in older J200 and 4cyl models.

 

In 6 cyl the movement to larger Aero Classics boxier cooler needed more room so a deeper cowl and opening required.

 

You can buy this larger cowl floor from Jabiru for retrofitting

 

Air is sucked through a cowl by outlet not forced in by scoop

 

 

[Oscar]

Bruce: nobody will be more interested than me! But it'll be many months yet before we can think about test flying, I still have a lot of work to do, and whether it works will only really be told by BOTH temps. achieved AND pressure-drop recording, as I've also made up a radically different airbox to try to eliminate, or at least significantly reduce, the swirl effect at the carby throat from the Jab. set-up.:

 

 

 

[jetjr]

Oscar, I notice you have a scoop on the inlet to your airbox? Is that correct?

 

Latest ideas says this works best as a flat hole pretty much where you have it - pictures here somewhere

 

Found it - Ausfly 2016 - apparently fair bit of testing went into this - front NACA is blocked off, this is std intake location, new one is plain hole. This improves mixture distribution and is a pressure neutral location

 

 

 

[Oscar]

Oscar, I notice you have a scoop on the inlet to your airbox? Is that correct?Latest ideas says this works best as a flat hole pretty much where you have it - pictures here somewhere

Yep, at the moment it is the old, original scoop on an LSA55. Given the latest info from Jab., that may well change, but I'd like to put some wool tufting ahead of it and video it from a go-pro on the strut before I remove it. My new cowls are a rather different shape from the standard Jab. cowl (for an LSA55), so airflow back there may not be similar.

 

It's important to remember that every element of a system affects everything else; the initial intake pressure you may be getting MAY not be a problem - it could be that the carby balance tube is not feeding the right info. to the Bing. This is a real 'suck it and see' ( little intake physics joke there) situation. For example: in the original certification testing of the Skyfox Gazelle, pulling hot air resulted in an instantaneous loss of nearly 2000 rpm - the carby balance tube was vented to the ambient cowl pressure. When it was changed to vent to airbox exit pressure, that revs drop went to almost nothing!.

 

THEORETICALLY, a reasonably neutral airflow pressure at the carby throat SHOULD reduce the effect of both swirl and butterfly opening position to a minimum. Given the airflow restrictions of the filter and the airbox convolutions ( including the friction losses from lengths of SCAT tube), a slight positive pressure at the air intake SHOULD be beneficial.

 

However - and this is something that every Jab. owner should take into consideration before just changing things - it's all a balancing act. The proof of any change lies in the recorded performance.

 

I have gone out very much on a limb with my mods. They could turn out to be an absolute packet of poo tickets!. However, we are resigned to a serious regime of professionally-conducted testing, by someone who has undertaken the certification of a number of similar aircraft over more than 20 years, with all his test gear. If it doesn't work, we will know...

 

 

[Markdun]

Here are some shots of my latest cowl, sans lip. Had a big headache last week. Bushfire started by a windfarm came to 150m from my hanger......my 750m airstrip was used by the RFS as a major control line and now has a rough bulldozer track up one side. Close call! The guys on the ground said the DC10 waterbomber was what stopped the fire. Anyway after the fire and in over 30C OAT my max CHT was a tad over 150C on climb at 65-70kts, and on cruise 138C; starboard side still hottest. Oil temp now (Shell W100plus) maxed at 82C. EGTs are still a work in progress.....spread is OK 650 - 715 on cruise (No.1 hottest, No.4 coldest) and 620-700 on climb (No.4 hottest, No.2 coldest). Oil drip on front seal has stopped after balancing the spinner, but it really wasn't that much out of balance. Now I have to upgrade my oil vapour recovery system as I'm dripping a bit of oil from some of my leaky aluminium welding. Mark

 

 

 

 

 

[Oscar]

Mark: that cowl exit is very likely to be more effective that the 'lip' solution; it creates a low-pressure area in all flight attitudes.