Replies to This Discussion

Permalink Reply by Danal Estes on February 13, 2013 at 5:13am

Last summer, I purchased a CH750 built and flying, Jabiru 3300 and Sensenich carbon prop.  Despite the original builder's efforts, and my efforts, CHT #5 (right rear on a 3300) is running quite a bit hotter than all the others.  Last summer, this was a limiting factor in flying the plane; this winter it has been somewhat less of an issue. 

I am located in North Texas.  For months at a time, the temps do not drop below 90F until after midnight.  There were times this summer when I couldn't taxi to the active without going over yellow line. Climbouts were an issue. And so forth. 

At this moment, I have a Rotec Liquid Cooled Head kit for the 3300.  I am still debating somewhat whether to install it, as there are a few tweaks I can make to cool the engine further.  For example, I haven't made the mod to the cowling that puts air on the fins on the front of the engine sump.  Caleb tells me this is worth -10F in Oil Temp.  I also haven't fully explored changes to the fiberglass ducts... and so on. 

On the other hand, I see the Video that Zenith published called "Flying on a Hot Day" and I look at the shots where you can see the engine monitor. Their #5 runs warmer than all the others.  Their #5 is running 15 to 16 degrees below yellow (continuous limit) at cruise.  Given my OATs are warmer than theirs, I believe that even the Factory's "more tweaked for cooling" airplane would have problems operating in my temps. 

Then there are engine life considerations.  I really like the idea of 200F to 210F heads ALL the time!

In short, I am seriously considering the install. My biggest barrier at this point is choosing a radiator, and modifying the cowl. I'm curious about your path to those two things: What radiator did you choose?  How/where did you mount it?  What cowl mods or scratch build or ??? What will the duct look like? Did you move the oil cooler?

And a few basic questions: Who's doing the install?  Did you re-use valves?

Thanks,

Danal

BTW, I am happy posting all on the forum, and/or, if you want direct contact, I am danal dot estes at gmail dot com

 

• ▶ Reply

Permalink Reply by John Austin on February 13, 2013 at 8:56am

Danal - l have the "new style" (#2331 and up) heads (deleted external oil lines and have more head fin area), so this may not be relevant, but I conquered the cooling! We had some unusually hot and humid weather last summer and my engine did fine with #5 at 280F at the very worst and usually more like 250's. I could climb as long as I wanted without oil or CHT concerns. Here's a summary of what I did - of course, one would want to make incremental modifications and see how effective each is:

1)  Extend cowl lip 2" (suggested for high drag airframes in Jabiru installation manual)

2) Raise ram air duct dams approx 1/2 the distance to top of exposed cylinder fins with a curved ramp over front cylinder barrels to apex of top of barrel.

3) Oil sump cooler diverting air off left/pilot side ram air duct in front of dam. (Cooler oil can't hurt! A substantial percentage of cooling is done by the oil - my design is good for 10-15 degF drop.) Some say the Zenith oil sump cooler design upsets the cowl pressure differential and can make CHT's worse!

4) Close off gaps at end of ram air ducts, extending downward the rear of right/passenger ram air duct to divert air down backside of #5

5) Install end plate baffles between tappet covers to seal off leakage between cylinders and through access holes to cylinder head bolts (I found this more helpful on right/passenger side, less so on left/pilot side which ran cooler, anyway.

6) Made "fences" between tops of tappet covers to hold tabs on ram air duct more tightly against cylinder heads.

7) Sensenich wood prop - I don't know if this is a myth or not, but some claim this prop cools the engine better than other props. Theory is that props with broad blades near hub push more air in ram air ducts? There is definitely something going on there - the left bank (the side prop wash is most prominent) always runs a bit cooler - that's why I borrowed my oil sump cooler bleed air from that side). Also, this prop, coupled with the 6 cylinder Jabiru, is delightfully smooth!

 

Sounds like a lot of work, but it was fun experimenting and really not too difficult or time-consuming. I think this is much preferable to adding liquid cooled heads wtih the cost, weight, complexity and potential multiple points of failure involved. However, as a caution, I really don't know how my approach would work on engines with the "old" heads, but i suspect it probably would work. Best part, most of these mods are almost "free" as far as material costs - fabricated from scrap leftovers - I understand the Rotec heads aren't free? LOL!

 

• ▶ Reply

Permalink Reply by Danal Estes on February 13, 2013 at 3:21pm

Note that all "results" mentioned below are documented in Dynon Skyview data captures, that are carefully compared on the ground after back-back flights. 

1) Cowl lip: Do you have a photo?

2) 1/2 way & curved ramp: Did that.  Negative result, or possibly neutral. 

3) Airflow over front oil sump fins: Haven't done that yet... may do it your way or the Zenith Factory way...  I do have a 10 row Oil Cooler at this time,  and added baffle that the original builder omitted.  Oil temps are OK almost all the time.  Doing your and/or factory mod that Caleb tells me will drop -10F will make them acceptable year round.  Having Oil a bit cooler may help CHT a little bit.  

4) Gap behind #5 in factory ducts: Added aluminum to direct flow. VERY VERY negative result.  Had to turn back to field before reaching pattern altitude. 

5) Alu end plate baffle: Haven't done that.  May try it. 

6) Alu to make fiberglass fit better between cyls: Haven't done that.  May try it. 

7) The carbon prop is pretty round near the root.  This could be part of the problem that gives different results from 1 through 4. 

And... I have purchased a brand new set of fiberglass ducts so that I can re-work the right bank and really, really seal it up.  I may try this before going LCH. 

So, I still haven't decided yet... but one of the things that really bothers me is the Zenith factory video "Flying on a hot day".  They have the same Air Cooled heads I do,  I've seen that plane, and there were a lot of little pieces of alu in the ducts to direct air.  It also had the oil sump fin mod.  Yet the video shots of them at cruise show a #5 temp that, assuming the OAT they are flying and the OAT I will be flying, is not acceptable. 

Not saying their airplane is the absolute final word in Jab cooling... at the same time, I'm not arrogant enough to think that I can get my airplane to cool BETTER than theirs by 15 to 20 degrees... I just don't have the luxury to fly every day, and work on the airplane in between every flight. 

So, still undecided how much air cooling development I will do, vs just putting on the LCH.  Still open to input.  Debating how far to "fly into summer" while working on airflow, before either being grounded by temps, or just cutting my losses and going LCH. I don't expect them to be perfect, but I am hopeful the debugging/testing will be very short.  Extensive ground runs, followed by a few flights, I hope. 

• ▶ Reply

Permalink Reply by Danal Estes on February 13, 2013 at 3:23pm

By the way, on one test flight, working through the steps above, I had a ramp baffle come loose (my bad, repeat: me, not the factory or Jab) and had #5 up over redline for 8.5 minutes, as high as 425.  Scared the cr-p out of me. Call Jab and asked if I need to pull the head, or inspect, or what.  They said to re-torque head bolts and that's all.  To this day I wonder if I damaged my engine.  

Maybe that experience is why I have a LCH kit sitting in my front hall. 

• ▶ Reply

Permalink Reply by John Austin on February 13, 2013 at 5:44pm

Danal - It's my theory that with the 750's slow and high-drag airframe, it's a lot easier to get cooling air into the cowl (at least with the wood prop!) than to get air out !  You didn't say if you had done the cowl lip extension or not, but I suspect a lot of the other mods would produce negligible or even negative effects if you're not creating more suction/negative pressure with a cowl lip extension. The extension also blocks the opening in climb, avoiding ram-air effect into the lower cowl opening and raising pressure. I feel it's imperative to do the extension first, (as recommended by the Jab engine installation manual) before proceeding with the other mods - in a word, it helps everything ! Pic:

Made the extension from .040 and flush riveted it to the existing lip. Covered the extension with 3M polyurethane tape (same as used on helicopter blades) for chip protection.

I also have the 10-row oil cooler. Unfortunately, I didn't add my oil sump cooler 'til after getting the 10 row. I now have to block-off about a third of the 10 rows to keep the oil temp up this winter and disconnect the oil sump cooler, too. (I use the duct to feed the cabin heat in winter.) My gut feeling is if I had kept my 7-row cooler, the oil sump cooler would have been enough for summer. Again, though, all the oil cooling - sump cooler and oil cooler - is dependent on negative lower cowl pressure to get the cooling flow - i.e., extend the cowl lip!

I'm NOT an engineer, but I immodestly think my oil sump cooler design is better than Zenith's - it produces equal or better results plus no further cowl mods/holes plus no additional high pressure air being introduced into the lower cowl than what was coming through the ram air ducts prior to the mod plus no connection with the cowl to fiddle with when removing/replacing the cowl. All you have to buy is the 2" flange from Spruce and a piece of 2" aluminum tubing and the rest can be made from scrap! Try it ... you'll like it!

Here's a pic of my extended dams and ramp over the cylinder - I got positive results, but I had already extended the cowl lip:

By the way, do you have "old" heads or "new" (#2331 or newer) heads? I'm sure the newer heads are easier to cool, so that may explain differing results.

• ▶ Reply

Permalink Reply by Danal Estes on February 13, 2013 at 6:20pm

I have not yet tried the lip.  I agree that it may be the magic that makes all other mods work.  North Texas is already having some 60 and 70 degree days... I may add the lip and see how much it lowers temps.  I believe I can predict if I'll be OK for the summer, based on a 70 degree day, after the lip. 

I am serial # 2108, so I do not have your heads.  There were some very early "small fin" heads, and I am well after those.  So I have the "big fin" heads, but not the most current ones like yours.  In particular, I do have external oil lines. 

• ▶ Reply

Permalink Reply by John Austin on February 14, 2013 at 9:43am

Danal - Is that top pic after removing the higher dam and curved ramp? As my pic shows, I curved a ramp all the way back to the very apex of the cylinder barrel and my dam goes up much higher. Of course, the idea is keep more cooling air from dumping down around #1 and #2 and never making it further back. Despite my dams/ramps, #1 and #2 still hover around only 200F. My tweaks got to the point that now #4 is usually the hottest, not the infamous #5 ! I wound up with nearly no ram-air duct baffling above the cylinders, but I'm tempted to add a little back to the one above #4 to see if I can knock it down a few degrees - not that it's too high, just an exercise to see how even I can get them. Since the front cylinders are so cool, it might work just as well to extend the dam in front of #2 some more to see what that does to #4. Guess a piece of tape would give me a quick answer! I probably should wait 'til warmer weather to expect to see much results from experimenting.

Of course, all of this is with the extended cowl lip - so I wouldn't recommend doing much of this unless that is done first.

• ▶ Reply

Permalink Reply by Danal Estes on February 14, 2013 at 10:22am

That pic is actually the Pilot's side, and yes, the ramps that run over 1 # 2 had been removed.  Actually, I only tried the ramp on the Passenger side... but anyway, yes it had been removed. 

The more we talk and detail what we've tried on each airframe, the more I agree that the exit lip is the key.  And agree that all other mods should be re-tried, one at a time, AFTER installing a much larger lip.  

Therefore, I have a path in my head, as it warms up here in North Texas:

Data capture (BTW, Dynon 5.1 and above now logs to files that can be copied to USB... no more serial logging!!) and analysis before and after each of:

1) Big Lip on exit. 

2) Oil sump fin cooling "Austin Style".

3) Replace right hand duct with a more tightly sealed new duct.   Including bracing to make it fit against rockers and etc. better, as required. 

4) Alu pieces at cylinder ends

5) Alu piece to re-direct flow behind #5

6) Alu ramp at front of #1 and #2 halfway up/over. 

I may vary the order of 4/5/6, depending on exact CHTs. 

Only after trying these things will I make a go/nogo on Liquid.  If I decide not to install, I'll sell the kit I have on hand.  I bought it second hand (but new, never installed) and can easily sell it for what I paid. 

• ▶ Reply

Permalink Reply by Scott Hunter on February 24, 2013 at 1:17pm

Daniel- also have older styled Jab 3300, S/N  485. As TTSN 30 hrs. before having to tear the 601XL for B conversions + augmenting many other custom changes, I had excessive CHT's  also on run-up taxi tests. Keep me in mind in the event you part ways on the liquid head conversion, as I will attempt similar changes initially as well. Didn't obviously state what you had to have, could go to another communique if preferred.