Jab Plenum Chambers

[seb7701]

Thanks Oscar - yep, I guess in hindsight it would be unusual for anything to be run in much by 8hrs!!

 

O.K. - todays little session seemed to be in the vicinity of 15LPH, give or take, but have been living on 2900rpm (aside from descending and climbing out to vary things). Looking at old records from this plane, even with the old heads etc, she was always very frugal. I am going to take a look at the jetting next - the carb is engraved with 255main/276 needle, which is interesting as the needle should be 278 according to Jab. Anyhow, will have a look - I know the main IS 255, but thought I might richen things a touch and see if it helps the hot side. (Just wonder what might happen to the cold side...)

 

Anyhow, the show will go on...not out of strategies yet thankfully!

 

 

[Old Koreelah]

Seb if your EGTs were fairly uniform I doubt that mucking about with the mixture will solve your "one side runs cold" problem. What happens if you tape off a small part of the cool side's inlet?

 

 

[seb7701]

O.K. - you've lost me Lol! You might have to explain a little as I'm not sure how taping off the cooler side intake will help? Main issue that has me stumped is that the front cylinder on the left is warmer than the rear. Small fence but might try fence above to direct air through fins more, as seems air is shooting straight down the back and out the rear , possibly something to do with it's whack of prop air coming from the upwards blast from the prop, whereas the right side is being forced 'down and in' over the heads a little more directly?

 

Oh, and uniform egt's usually only seen straight after takeoff. From there, 2 and 4 are elevated above the other two, just like the CHT's...

 

 

[Bruce Tuncks]

Seb, its got me puzzled too. With EGT's similar, I would expect the CHT's to be more even, unless as you say there is a bedding down issue causing more friction on those cylinders.

 

One thing which is different on each side is the swirl from the prop... 1 and 3 are getting downswirl and 2 and 4 upswirl. But I wouldn't have thought this would have a big effect.

 

 

[facthunter]

That's why lips etc make such a change. The gypsy motors have the opening designed to take account of the prop direction of rotation. Pressure rather than velocity works more predictably when cooling airflow is concerned. Nev

 

 

[seb7701]

Yeah, might try an direct the air from running along the roof of the duct from the up swirl just for curiosity sake.

 

Can't remember if I mentioned already, but just for kicks a few weeks back, I taped off both intakes completely and made a smal slit in each and poked the compressor nozzle in there to see where the air was exiting. On the cool side, air was exiting noticeably from down between there cylinders (perfect right?) while on the other side, it was shooting straight to the rear and out the back between duct/fins. Even after I taped off the gap (there is about 3mm more gap on hot side between duct/fins at the rear..) it was finding it's way straight out between the fins on no.4. I know this doesn't take into account the drawing effect from the lower cowl, but we'd see what happens with the manometer.

 

It seems the laws of physics operate differently between each of my bloody ducts!!!

 

 

[Old Koreelah]

Damned frustrating, Seb. Your idea to install a tab to direct air down onto No. 2 has merit. Early Jab ducts had something similar. One option worth considering is to extend the ram air duct right down, coming closest to the fins at the bottom. This should force air to contact the maximum surface area of the hot bits and speed up as it gathers heat.

 

 

[seb7701]

Yep, frustrating alright!! I should say a thanks in the mean time to you guys for your thoughts on this one a - much appreciated and thanks for hanging in there with this discussion!

 

Yep, it's a funny deal. For eg. after the success had by adding the tin 'fence' to no. 2, I tried adding a bigger one which covers the whole triangular outline of no.2 head. It ended up raising it's temp by 11 degrees, but dropped no. 4 by another 10 degrees! The aeronautical engineers out there will laugh perhaps, but I wonder that there isn't too much air pressure/velocity in the left duct, which is causing the flow to head straight for the back end. Doesn't explain why it isn't heading down between the barrels, which should be lower pressure.

 

 

[Old Koreelah]

Yep, frustrating alright!! I should say a thanks in the mean time to you guys for your thoughts on this one a - much appreciated and thanks for hanging in there with this discussion!...

On the contrary Seb, we should be thanking you for sharing your experiences. I've learned heaps from reading about other peoples' experiences. (And I could have saved myself an awful lot of bother by doing more reading before I make a modification...)

 

...Doesn't explain why it isn't heading down between the barrels, which should be lower pressure.

I presume you've got some sort of butterfly patch between the barrels to reduce pressure loss there. The heads are the hottest and need most of the airflow.

 

 

[Oscar]

Seb, re the tube for the manometer: you might find that the clear vacuum tube used in milking machines could work. It looks like the standard Bunnings Aerospace clear tube but it has decent anti-collapse properties - not that you need that per se, but it MIGHT be useful if you're running from under the cowl down out the exit duct and back up into the cockpit via a footwell intake (which is how they did it on the early Jab. testing!) There is a place in Toowoomba that sells it, is a farmer's supply-type shop.

 

 

[seb7701]

O.K. - You mean gull wing baffles? Just had another look and seem to have missed a whole important aspect - the air needs to go through the head fins and not the giant gap between the barrels!!

 

Thanks Oscar - I haven't in honesty looked yet, but the small lengths that came with it are pretty tiny! I also happen to have a nice hole in the firewall which (is due to be covered...) will work a treat. Once I figure out how to drive it, the pressure investigation will be an interesting one!

 

 

[facthunter]

The air will always take the easiest path to lower pressure, so you have to give it no option but to go where you want it to. Making the head fins at the top form a fine point would aid flow over the exhaust area. Squared off forms an air dam. Nev

 

 

[Old Koreelah]

 

...for the manometer: you might find that the clear vacuum tube used in milking machines could work. It looks like the standard Bunnings Aerospace clear tube but it has decent anti-collapse properties - not that you need that per se...

I got away with using ordinary vinyl tube (about biro diameter) from Mitre 10. It also copes with petrol and all sorts of nasties. I went to quite some trouble to find the best coloured fluid (metho with food colouring) and set up my Guage out of the way behind my right boot. It turns out to be a stupid place; I have to be a contortionist to see it in flight and my little camera finds that all sorts of things get in the way of a good pic. It'll be relocated between my feet where I can more easily see it.

 

At Dafydd's suggestion, to avoid a pitot effect I used a small soldering iron to melt little holes near each end.

 

One thing I'm hazy about though is plumbing the tube. Mine taps the higher pressure air above the heads (inside the ram air duct) and the other end vents in the low pressure cowl flap exhaust area. That probably means I'm getting a bigger pressure differential than the ones in Dafydd's test cell, which vent inside the control room. At the end of my first test flight the fluid had been sucked out and stained the inside of the exit. Probably means I'm getting enough low pressure, but I'd like to actually get a reading at different speeds and cowl flap settings.

 

(This post was delayed by an earthquake at 1322...Windows rattled for about 5 seconds. A phone call confirmed a blast at Werris Ck mine, 16km away.)

 

 

 

[Jaba-who]

[ATTACH=full]45487[/ATTACH] [ATTACH=full]45488[/ATTACH] I got away with using ordinary vinyl tube (about biro diameter) from Mitre 10. It also copes with petrol and all sorts of nasties. I went to quite some trouble to find the best coloured fluid (metho with food colouring) and set up my Guage out of the way behind my right boot. It turns out to be a stupid place; I have to be a contortionist to see it in flight and my little camera finds that all sorts of things get in the way of a good pic. It'll be relocated between my feet where I can more easily see it.

 

At Dafydd's suggestion, to avoid a pitot effect I used a small soldering iron to melt little holes near each end.

 

One thing I'm hazy about though is plumbing the tube. Mine taps the higher pressure air above the heads (inside the ram air duct) and the other end vents in the low pressure cowl flap exhaust area. That probably means I'm getting a bigger pressure differential than the ones in Dafydd's test cell, which vent inside the control room. At the end of my first test flight the fluid had been sucked out and stained the inside of the exit. Probably means I'm getting enough low pressure, but I'd like to actually get a reading at different speeds and cowl flap settings.

 

(This post was delayed by an earthquake at 1322...Windows rattled for about 5 seconds. A phone call confirmed a blast at Werris Ck mine, 16km away.)

A couple of questions. I think I'm at the point of needing to make a manometer as well.

1. Why metho ( as opposed to eg. water) and if so what's the weight of metho? So far all the measurements I've read that it should reach about 2 - 2 1/2 inches difference ( were in inches of water ( so I'd need a conversion for the weight of metho.)

 

And the second question is about the holes made in the tube end with a soldering iron. I'm having trouble envisioning what and where the holes are and how they achieve an anti-siphon effect. Can you explain.

 

On a second issue ( and maybe confirmed with that PDF about the Europa ) I was speaking to Jamie Cook last week and he said that they had some success ( in equalising egt's) with changing the cowl air inlet naca duct (for the carby intake) to a simple flat 2 1/4 inch circular hole. So that is my plan for this weekend. He suggested I try it at the current site and if it helps he had some measurements of where they got it to site best.

 

Now that article seems to perhaps explain why it might work. A flat hole will I assume have less efficient air entry and maybe even have a Venturi effect so turn it into a less positive air pressure down the tube. Maybe???

 

I'll try it this weekend and report back next week.

 

 

[Oscar]

OK - the manometer tubes in the test cell only measure the air delivery pressure to the heads from the 'squid', for the purpose of tuning the individual head air delivery - not the p-delta, because the test cell doesn't replicate 'installation'. (not meant to, for the specified engine tests: as a result of the tests, the engine manufacturer sets the various limits for operation and it's up to the aircraft manufacturer (or builder) to derive an installation and operational limits for that installation that keep the engine within those limits).

 

One of the interesting things we found when test running my engine, was that with the sump exposed to the full prop blast, there was overcooling of the engine oil. Since we had oil temp gauges both in the normal (bottom of sump) position AND in the line to the oil cooler (for control purposes, as some tests require running the oil very hot indeed for periods to see what the sensible temp. limit is), we could verify that happening - we had to wrap the sump in foam insulation to get the O/T to sensible! In a normal Jab installation, Rod designed the oil cooler to exit its air over the sump, but would have found that without the prop blast, the sump is not very effective at all; what happens is that there is enough cooling from the sump to have a boundary layer of cool oil but beyond that, the oil is hot and thin and that's what gets recirculated. The standard Jab. O/T probe at the bottom of the sump really is NOT measuring the temp. of the oil circulating through the engine, but the temp. of the oil sitting in the bottom of the sump... (another good reason to use a CAMit TOCA).

 

 

[Old Koreelah]

A couple of questions. I think I'm at the point of needing to make a manometer as well.1. Why metho ( as opposed to eg. water) and if so what's the weight of metho? So far all the measurements I've read that it should reach about 2 - 2 1/2 inches difference ( were in inches of water ( so I'd need a conversion for the weight of metho.)

And the second question is about the holes made in the tube end with a soldering iron. I'm having trouble envisioning what and where the holes are and how they achieve an anti-siphon effect. Can you explain.

 

On a second issue ( and maybe confirmed with that PDF about the Europa ) I was speaking to Jamie Cook last week and he said that they had some success ( in equalising egt's) with changing the cowl air inlet naca duct (for the carby intake) to a simple flat 2 1/4 inch circular hole. So that is my plan for this weekend. He suggested I try it at the current site and if it helps he had some measurements of where they got it to site best.

 

Now that article seems to perhaps explain why it might work. A flat hole will I assume have less efficient air entry and maybe even have a Venturi effect so turn it into a less positive air pressure down the tube. Maybe???

 

I'll try it this weekend and report back next week.

I used metho with food colouring so I could see it during flight, in the dimly-lit area around my feet. For some reason coloured water kept separating into segments in the tube. No idea of the impact on readings of metho having different density to water, but I'm assuming bugger-all.

 

How the ends of the plastic tube sit in the airstream is important. Facing into the airstream might create a pitot "ram" effect (watch your ASI while someone slips a sealed tube over your pitot.) Facing back could cause a suction effect (like happens with some over-wing fuel vents).

 

Little holes around the ends should nullify such effects.

 

Jaba I'm keen to see how your carby inlet changes work. Today I'm making a start on yet more major surgery to my cowls and the damned NACA duct for the oil cooler (that I wasted weeks on) is going. Trouble is the carby shares that inlet, and more recent reading on NACA ducts say they're not ideal for carby intakes. I am therefore open to design suggestions. Most of the work will happen after I get home from North Qld. I'll be in Cairns next week and plan to look up Frank Farri. Is your Jab at the airport?

 

 

[Jaba-who]

I used metho with food colouring so I could see it during flight, in the dimly-lit area around my feet. For some reason coloured water kept separating into segments in the tube. No idea of the impact on readings of metho having different density to water, but I'm assuming bugger-all.How the ends of the plastic tube sit in the airstream is important. Facing into the airstream might create a pitot "ram" effect (watch your ASI while someone slips a sealed tube over your pitot.) Facing back could cause a suction effect (like happens with some over-wing fuel vents).

 

Little holes around the ends should nullify such effects.

 

Jaba I'm keen to see how your carby inlet changes work. Today I'm making a start on yet more major surgery to my cowls and the damned NACA duct for the oil cooler (that I wasted weeks on) is going. Trouble is the carby shares that inlet, and more recent reading on NACA ducts say they're not [ATTACH=full]45489[/ATTACH] ideal for carby intakes. I am therefore open to design suggestions. Most of the work will happen after I get home from North Qld. I'll be in Cairns next week and plan to look up Frank Farri. Is your Jab at the airport?

Unfortunately no. I used to have it at the Cairns airport for years. Rented a spot in a hangar with a mate who had an EC120 helicopter. But he sold his hangar to Nautilus Helicopters when the airport owners started screwing us all with logarithmic cost increases.

 

There's very few private aircraft left on the field now.

 

I keep it up at Atherton (about an hours drive south west). Farri lives south of Cairns about 45 minutes drive south at Deeral. (Well he used to. I think he's still there. I used to fly my old R22 into his strip quite a lot. )

 

Sharing an intake there sounds like it might cause trouble - (If the theory on that pdf discussed before is true. Two conflicting requirements - the oil cooler needs positive pressure inflow and the what those guys were saying is the carby requires NOT positive pressure.

 

I'll let you know how it goes.

 

 

[Old Koreelah]

Jaba you're probably right about my NACA air inlet. For years the carby breathed quite successfully from a simple forward-facing inlet on the side. A simple rubber flap valve on the top of the air cleaner box seemed to relieve any ram air pressure.

 

I'll be driving thru Atherton next Sunday to visit friends in Ravenshoe. Last time I was there (via bike) I found hangar space advertised. When I finally fly up there I hope there's still space.

 

 

 

[Jaba-who]

Hangar space is variable depending on events around the place. At present I think there's some. I'll be up there this Sunday. Drop in.

 

First hangar on left as you turn into the road between hangars that parallels the runway.

 

 

[Oscar]

Jaba you're probably right about my NACA air inlet. For years the carby breathed quite successfully from a simple forward-facing inlet on the side. A simple rubber flap valve on the top of the air cleaner box seemed to relieve any ram air pressure.I'll be driving thru Atherton next Sunday to visit friends in Ravenshoe. Last time I was there (via bike) I found hangar space advertised. When I finally fly up there I hope there's still space.

That rubber flap (if it was a standard Jab airbox) is there for backfire pressure relief, of course.. but certainly sufficient to relieve any ram-air pressure.

 

Is that your K100? if so, what fuel pressure do the injectors require? I just happen to have a K100 engine complete in my workshop, which I bought for a WIG project that didn't happen, but I have in mind to use it to make a (road-going) leaning trike... once my Jab is completed and the workshop is looking bare of ridiculous projects..

 

 

[Jaba-who]

That rubber flap (if it was a standard Jab airbox) is there for backfire pressure relief, of course.. but certainly sufficient to relieve any ram-air pressure.snip....

Well you may be right, but the current info does throw some suspicion on it.

 

That pdf alluded to earlier says they did lots of pressure readings and found that any positive pressure made a negative difference. It was in a Europa and I dont recall reading whether they had a jab (or any other) mixer box or if they did whether it had a pressure relief flap.(They did say they initially cured the problem by completely removing tubing and having a 4 sq. inch hole straight into the carby throat.

 

But Jabiru are now saying that using in intake attached to the NACA scoop is made better by removing it and attaching it to a small circular flush hole. No statement as to why, just that it does.

 

I guess we can speculate on what might change but the obvious is that the ram pressure will change (easily explainable) and its reasonable to suggest that if that's the cause of the problems then the pressure relief flap was not enough to drop the pressure. The presence of the flap relief might negate some of the pressure rise of the NACA system but maybe only does to certain extent and the change of the intake tubing may still be enough to drop the pressure to a more favourable level.

 

I guess all will be revealed on the weekend!!

 

 

[Old Koreelah]

That rubber flap (if it was a standard Jab airbox) is there for backfire pressure relief, of course.. but certainly sufficient to relieve any ram-air pressure...

Mine is home-made on the top of a square plywood box with a Corolla filter slid in diagonally. I made enough room for a "still air" section before it gets sucked into the carby. (I first saw a still air box on my 1978 Honda CX-500. It took up mobs of room under the seat, but must have worked, cos the bloody thing breathed very well: over 10,000 rpm from a pushrod engine!)

 

...Is that your K100? if so, what fuel pressure do the injectors require? I just happen to have a K100 engine complete in my workshop, which I bought for a WIG project that didn't happen, but I have in mind to use it to make a (road-going) leaning trike... once my Jab is completed and the workshop is looking bare of ridiculous projects..

If you look closely Oscar, you'll see it's only got three crank throws and the cut-down seat: it's a K75c (should have gone to Specsavers...) a terrific tourer I bought in Darwin for a song and rode 5,000 km home. I had planned to sell it when I got home but I keep putting that off...

 

After I replaced the in-tank fuel filter it started running like it had lumpy petrol and a lady with pram overtook me-twice! Turns out I'd ruptured the rubber shroud around the pump. The resulting leak had fuel spraying around inside the tank, which looked like fairly low pressure.

 

A brilliant engine that's said to run almost forever with minimal maintenance. Ideal for putting in a plane? I was told that a couple of blokes in Atherton (where the picture was taken) were working on just that idea. Unfortunately very heavy.

 

 

[Oscar]

O.K. - You mean gull wing baffles? Just had another look and seem to have missed a whole important aspect - the air needs to go through the head fins and not the giant gap between the barrels!!Thanks Oscar - I haven't in honesty looked yet, but the small lengths that came with it are pretty tiny! I also happen to have a nice hole in the firewall which (is due to be covered...) will work a treat. Once I figure out how to drive it, the pressure investigation will be an interesting one!

Seb - I'll be very interested to hear your opinion of the digital manometer when you have some experience of it - I'm thinking of buying one of that model myself. I notice it has a 'record' function', that should be capable of correlating to recording from any EFIS (we will be using an MGL 'Extreme EFIS) to revs, ch's, egts and airspeed. I'm thinking of making up a double-gang shuttle valve so one could have inputs of both air intake and exhaust pressures on the heads on both sides ( the p-delta) that one could switch between in flight.

 

 

[jetjr]

There is something happening in Jab intake box/naca duct for sure.

 

Others have over time removed air intake hose and used under cowl air and seen dramatic changes in EGT and distribution, I have seen another with taped off NACA and then a round hole who swears it made a difference (similar to what Jaba who says)

 

I have seen myself similar EGT spread problems on two totally different engines in the same airframe. Somehow nearly gone when CAE engine fitted........

 

Theres some intake effect there but seems it can be fought somehow

 

 

[Oscar]

Mine is home-made on the top of a square plywood box with a Corolla filter slid in diagonally. I made enough room for a "still air" section before it gets sucked into the carby. (I first saw a still air box on my 1978 Honda CX-500. It took up mobs of room under the seat, but must have worked, cos the bloody thing breathed very well: over 10,000 rpm from a pushrod engine!)

 

If you look closely Oscar, you'll see it's only got three crank throws and the cut-down seat: it's a K75c (should have gone to Specsavers...) a terrific tourer I bought in Darwin for a song and rode 5,000 km home. I had planned to sell it when I got home but I keep putting that off...

 

After I replaced the in-tank fuel filter it started running like it had lumpy petrol and a lady with pram overtook me-twice! Turns out I'd ruptured the rubber shroud around the pump. The resulting leak had fuel spraying around inside the tank, which looked like fairly low pressure.

 

A brilliant engine that's said to run almost forever with minimal maintenance. Ideal for putting in a plane? I was told that a couple of blokes in Atherton (where the picture was taken) were working on just that idea. Unfortunately very heavy.

I blame that on my screen!. But I DID look at your piccy enlarged and thought: I think that's a K75.. which as I recall, got away from the harmonics of the K100 at around 100 kph that made them speed-ticket-bait. Had a CX500 Sports, bought as a rebuild project, but never got to it - I liked them very much, but other things intervened. And yes, the airbox was LARGE. I'm about to develop an airbox for my Jab. project and it won't be anything like what was originally on it.