Jab Plenum Chambers

[Markdun]

And here is a pic of the previous cowl with 50mm lip. Exit area is about the same on both versions.

 

 

[seb7701]

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

Jetjr - The NACA style oil cooler inlet shown next to mine is actually from a 120 or 160 I photo'd up at the factory and is used with the small automotive oil cooler such as mine. The inlet is designed nicely to scoop the air running down the face of the cowl. The larger aero classics cooler is used with the box shaped inlet which does actually have more of a protruding lip into the airflow. Here's a 230 cowl- bit different to the NACA duct. I get the suction aspect, but scooping the air definitely has it's place, otherwise I wouldn't think the 230 one would look like this as opposed to a flush inlet.

 

Mark - Thanks for the shots. I have seen similar things on some Jabs - very impressive CHT reduction, hence my interest!!!!

 

 

[jetjr]

Scoop doesn't help much. A well designed NACA design is good but very small problems mean they don't bring much to benefit. In some instances air can push forward out of scoops even at high speed

 

Unless there's negative area inside cowl the air inside blocks any new entry, Can literally block an aggressive scoop by stopping air leaving elsewhere

 

by sucking it out, like your cowl mods, you allow more in.

 

As far as the Jabiru 230 cowl goes the cooler doesn't have much clearance and the intake design Id say is simply to get around it. Not very streamlined.

 

 

[Old Koreelah]

And here is a pic of the previous cowl with 50mm lip. Exit area is about the same on both versions.[ATTACH=full]48021[/ATTACH]

Nice plane, Mark. Looks like you have near perfect visibility past the wing.

 

 

[Markdun]

Yep....fantastic visibility. It's the third plane I've built and when I was deciding it was going to be the Jodel sovereign or the Cygnet. The low wing on the Jodel and my experience with a low wing Minimax swung me to the Cygnet. A few people are put off by the Cygent's geodesic wooden wing structure but it was really easy to build....definitely easier than the Minimax, Karatoo or the Carbon Dragon wings I've built. The plane still puts a smile on my face after 17 years of operation. The Jab engine had put 15-20kts increased cruise speed and 400fpm extra climb rate compared to the old 1835VW engine.

 

 

[Paul davenport]

I hope I got this right and the photos re attached of the oil stains on cowling of air flow ,interesting to see where the oil -air flows around this area

 

 

 

 

 

 

 

[Paul davenport]

This cowl looks like he has addressed the cowl issue

 

 

 

[Jaba-who]

I hope I got this right and the photos re attached of the oil stains on cowling of air flow ,interesting to see where the oil -air flows around this area

Interesting. But you can't be sure thats from in flight. Bit like spark plug colour. Its possible it does but it might also be related to what happens during the low speeds of landing and roll, taxi and getting back to the hangar etc.

 

 

[Paul davenport]

Interesting. But you can't be sure thats from in flight. Bit like spark plug colour. Its possible it does but it might also be related to what happens during the low speeds of landing and roll, taxi and getting back to the hangar etc.

You are quite right BUT I got out and checked it at the end of the runway befor take off and immediately went for a fly . I only put very small dibs on the cowl for that reason, what would be good I if say 5 of us did the same and we compared pictures. What would be really interesting would be if some one with really good temps did it to see if this is different between ac and why . How about it fellow 230 drivers feeling up to it. Maybe even a tuft test with some one taking pictures in flight from another ac??

 

 

[seb7701]

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

[ATTACH=full]48016[/ATTACH] [ATTACH=full]48017[/ATTACH] [ATTACH=full]48018[/ATTACH]

Mark,

 

Just checking, but out of interest, is that a modified Jabiru lower cowl that you've used? Just interested in whether the intake opening is therefore the same as mine. For the record, I am in the midst of figuring out how I am going to emulate your lower cowl, given your fantastic temp reductions!!!

 

 

 

[Markdun]

Yes it is a modified Jab cowl. I've also tweaked the oil cooler a bit by moving the bottom of the cooler forward so it sits more directly across the inlet, and I have flexible baffling between the inlet and the oil cooler to ensure no air leaks under the cooler.

 

 

[Markdun]

I forgot to mention that while the cowl is a modified Jab cowl the distance from the top of my firewall to the bottom is probably around 150-200mm shorter than a Jab, and I have added to that around a 50mm 'tunnel', and of course no nose wheel undercarriage. This gave me some challenge in getting the cowl to fit as the vertical height at the firewall had to be shortened, but it also means I had lots of room for a large exit area with only the exhaust pipe obstructing the airflow out.

 

 

[Old Koreelah]

...it also means I had lots of room for a large exit area with only the exhaust pipe obstructing the airflow out.

That pesky obstruction (the exhaust pipe) can be a big help in sucking air out of the cowl. I regret having removed my exhaust augmenters (they were mounted on the sides of the cockpit and the noise was intolerable) but they sure helped with cooling. I recently measured the outlets under a ruddy great Cessna and was amazed to find they're no larger than the ones on my little Jodel. They each have an exhaust pipe in the middle.

 

 

[Markdun]

An exhaust augmentor was going to be my next project but as my temps are now OK, I'd rather save the weight. I am going to try wheel pants (and a general drag tidy up) to see if I can get my cruise speed from 90 to 95 kts. And here are my EGT & CHT for my last flight....35C OAT at 2400, most of flight at 4500 at OAT 30C. Note EGTs spread wide and increase on decent and throttle closed to 13lph.

 

 

 

[Bruce Tuncks]

Good work Mark. I personally agree with your priorities, that is to get the temps right first and worry about drag next. There is a trade-off here in that more cooling has a drag cost. My son's Lancair has a much smaller cowl outlet than you have and it runs too hot for my liking, but they optimized the Lancair design for low drag.

 

Do you have any theory about why a stbd side ( cylinders 1 and 3) should run hotter than the port side? It is the opposite for mine, and I always put this down to the fact that 2 and 4 are further back. There must be something I'm missing here.

 

 

[Bruce Tuncks]

It's not much different with gliders. The pilot ventilation air is a source of drag and getting rid of this is a big problem. All I know is that some exits, like where it might cause a nasty separation, are worse than others. But you can't get away from the fact that taking in ambient air and releasing it at near the plane's velocity has an energy cost.

 

 

[Old Koreelah]

... I am going to try wheel pants (and a general drag tidy up) to see if I can get my cruise speed from 90 to 95 kts...

Carefully designed spats should easily achieve that improvement, Mark.

 

However I'd advise you to look long and hard at the problem before you commit to it. I built mine a couple of years ago (two months work, mostly outdoors in a heatwave) and was amazed at the difference, particularly in reduced fuel burn. In retrospect it wasn't the best idea. My cruise speed crept even closer to VNE and the load on the engine was reduced- the last thing a Jab motor needs.

 

 

[Markdun]

Bruce, and I thought my starboard cylinders were hotter because they were further forward. The first question is whether the difference in CHTs is due to EGT differences or not. The way my CHTs go, I think that mostly the difference is EGT related, but there is still a residual that makes the stbd side hotter if EGTs are the same. I have thought it might be to do with the fuel pump, fuel hoses and gascolator (on firewall) which are on the stbd side that somehow interferes with the flow of air out of the heads in a different way to the scat hoses on the port side. I also have a stbd offset to counteract the engine/prop P factor and a 4.5 degree downthrust, but I wouldn't think that would make that much difference. Others have postulated that its the downsweep or upsweep of the prop, and have put little fences on the cowl inlets. But I reckon the big issue is the exit. The RV guys have found that modifying the bottom of the firewall edge makes a big difference, such as a curved sheet of aluminium attached to the bottom of the firewall coming fwd towards the engine and up and around back to the firewall to give a 200mm diameter curve to the fwd facing rear side of the exit. It could also be due to different compression ratios. Perhaps my crankshaft is a bit more to the starboard side, and yours is a bit to the port side? I don't know what the Jab tolerances are on that sort of thing. On the EGT side of things, I think it could be differences in the flow diverter (again we dont know what the tolerances are), or simply how the needle rests in the needle jet in the carbie. We all seem to think it sits centrally in the needle jet, but I think it tends to rest on the side of the needle jet and this effects how the fuel plume comes out of the jet, ie. if the needle tends to rest on the port side, more fuel will come out the stbd side. Of-course with the breeze in the carbie throat you would think the needle would be hard pressed to the front of the needle jet. And then there is the atomizer that the needle jet sits in. I have seen that the South Africans at one time were suggesting making the four holes in the side of the atomizer larger (more air?) and that this reduces EGT spread. But equally it could be more to do with the orientation of the holes. Anyway, lots of guesses. Maybe its just animal spirits....I know my wife seems to think its easier to start one of her Ducati motorcycle singles if another one is already running to avoid the shame of not running!

 

 

[Old Koreelah]

...Maybe its just animal spirits....I know my wife seems to think its easier to start one of her Ducati motorcycle singles if another one is already running to avoid the shame of not running!

The opposite of my experience, Mark. My Ducati was such a Prima Donna that if another bike was getting all the attention she'd refuse to start!

 

 

[Jaba-who]

Not sure if you guys ( Bruce and markdun) are aware of the role of the rotating vortex of air that goes through the scat hose and the carby throat and the effect it has on one side of engine running hotter or cooler than the other. My apologies if you have already considered it.

 

Essentially the air flow goes through the carby and picks up fuel. If it is vortexing (usually in the direction such that it tends to load the cylinders usually on starboard side but I would be happy to accept you could get it swirling in the opposite direction ) it loads one side of the engine with rich air. The port side then gets lean air:fuel. The vortex is caused by normal flow dynamics in tubes especially if they are very bendy or have sharp lips or ridges in the pathway through the scat hose and hot air mixer box.

 

So jabiru recommend a baffle in the intake of the throat to stop the vortex and straighten the flow. I did a heap of trials and found a crossed baffle worked best in mine.

 

But just to complicate matters is the aerofoil shaped post in the induction chamber. That further effects the split and flow by the Coanda effect. Such that if you have a hot side ( after adding a baffle) tilting the carby ( and I forget if it's top to hot side or cool side - I did it ages ago and it made a big difference) but tilt the carby to one side will change it further.

 

Then the next complication is that if you have a jet with big space around the needle you can get bigger fuel droplets in the air and the inertia tends to throw them further forward and give leaner air at the back. So that can affect front to back lean/rich difference.

 

Lots to consider and to effect the egts.

 

 

[Markdun]

Thanks Jabba. I'm sure we do. I think mine is sorted...at least good enough...all within specs now. I have two 'criss-crosses' upstream from the carbie....one at the carbie entrance and one at the airbox exit, no bends, just dead straight. No.4 goes hot (lean) on WOT and No.1 on cruise, so tilting the carbie make one or the other worse. Bigger difference is inlet air pressure - less pressure gives more even mixture but less power at WOT, and of-course at cruise (less air being sucked) and faster airspeed means higher pressure (depending on where one's inlet is).

 

 

[seb7701]

Out of curiosity, does everyone run the flattened fibreglass 'cobra head' between the scat hose from the airbox and the carby or have some gone to straight hose?

 

I am about to trial a short section of silicone intake hose instead of the scat hose, 'just to see....'.

 

 

[Jaba-who]

Out of curiosity, does everyone run the flattened fibreglass 'cobra head' between the scat hose from the airbox and the carby or have some gone to straight hose?I am about to trial a short section of silicone intake hose instead of the scat hose, 'just to see....'.

What is a "flattened cobra head?"

 

The standard cobra head for over 11 years (at least) is a circular cross section cylindrical short piece of fibreglass pipe (Just a bit over 40 mm diameter) bent in a shallow curve. It connects to a piece of 2 1/4 inch scat hose that goes to the airbox. Nothing "flat" in the pathway.

 

You need something flexible in there because the carby is attached to the engine (notwithstanding its by a rubber flange) so will vibrate when the engines running while the mixer box stays static on the firewall.

 

I have heard or read of people using a smoothed out inner surface scat hose to try to get rid of the roughened surface. Some say they have had success. Other had troubles. One that I only vaguely recall was a type of scat hose that was either manufactured with an inner smoothing lining or was custom modified - but the lining degraded with time or use and broke up or folded in on itself somehow. I have no idea how or why.

 

The other issue was kinking but dont recall if that was in a jabiru setup or another model with a sharper bend from mixer box to intake of the carby.

 

The idea sounds OK as long as: Will smoothness fix the spiral flow or will it make it worse?

 

 

[jetjr]

At some stage Jabiru worked on a flat cobra head fibreglass connector to carb, was designed to steady airflow into carb

 

Only fitted on 2200 installations i think, sont recal being a great fix for every case.

 

Good quality silicon should be ok for air intake connector, widely used in automotive installs and racing.

 

 

[Jaba-who]

At some stage Jabiru worked on a flat cobra head fibreglass connector to carb, was designed to steady airflow into carbOnly fitted on 2200 installations i think, sont recal being a great fix for every case.

Good quality silicon should be ok for air intake connector, widely used in automotive installs and racing.

Ah. Ok. I've managed to miss that mod. Done lots of work on 3300s for over 10 years and only a few 2200s of fairly recent (last 5 or so years) vintage.