R

RV8R999

Well Known Member
As I've written in a previous thread my #2 CHT has typically been 20-30 degrees higher than all the others whenever the cowling is on and even during flight. With the cowling off and the engine at idle all CHTs are within 5-10 degrees and typically stabilize around 290 after 15 or 20 minutes (std day).

During flight at 6000ft DA, 75% power, full rich, CHTs were nominally in the 330-350 range with #2 upwards of 360.

I checked every possible place for an induction leak, compression checks, and worked the baffling and filled every possible gap I could find and still temps on #2 were much higher. It was particularly bothersome while taxiing or at idle for extended periods of time in which #2 would climb to as high as 380 prior to take-off then into the 420's during climb out. It would always come back down to 360ish once I leveled off though. I just couldn't figure it out. Then I stepped back and noticed something which has been right in front of my face the whole time:

2mw6tep.jpg


Notice the 1/8" gap between the upper and lower cowl at the fwd outboard inlet. I never sealed this gap for some reason... so I tried this:

9up3jo.jpg


All CHTs dropped at least 20 degrees at 6000ft, 75% power, full rich, and more importantly #2 is within 10 degs of all the others (still the highest though) and never goes above 400 even during 100-110 mph climb full power.

Obviously enough air was passing through this gap, and the one on the other side, to limit cooling airflow through the entire engine.

To ensure this was the cure, I conducted the test with tape on and off three times in succession under the same test conditions. Results matched each time.

So for those still building - go ahead and seal this area. For anyone currently flying and have a little gap here, seal it with some good tape and see what happens. My cowl is secured with Skybolts exclusively and I've found they do have a little play - the result of which is this gap. Who knows it may even be a little larger during flight.

I'll probably layup some glass and resin and inside with the two cowl halves together using wax and release agent on the upper cowl surface.

Thought I'd share...

Ken
 
Glad you found your problem! It also appears that the stud is a little protruded. It may be just the camera, but I readjusted several of mine after about 50 hours of wear in. Are you using fixed receptacles on the "hinge line"?

Rocky
 
Thanks for posting this-------my cowl also has a small gap in this area, and I have been chasing high CHTs since the first flight.

Duck tape testing tomorrow for sure.
 
Yes, all fixed receptacles on the hinge line. I too have adjusted several but found some either protrude a little or are so tight they will not engage. The adjustment is too course, or at least the locking mechanism is in some cases.
 
Very interesting.

Ken, that gap leads into the lower cowl volume? You figure the open gap was increasing lower cowl pressure?
 
Excellent post, Ken.

I've been wondering why #1 runs warmer than average, even cut down the front flow blocker with no change. I will be checking that gap, I know I have one.

Thanks.
 
Ken, great catch. Never would have thought about that little hole being an issue.
 
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Yes indeed. Straight to the lower cowl section. Since the gap is to the outside edge of the cowl it is not as blocked by the thick prop section in close to the spinner resulting in much higher dynamic pressure at the gap face and into the lower cowl section. As we all know the cooling system is a fine balance between marginal differential pressures and doesn't take much to upset the flow. I'll be honest, I'm a bit surprised by the magnitude of change and is probably why I dismissed needing to close the gap in the first place.

I'd like to hear what results others find by closing the gap (if they have one).
 
I was pleased to see your post and also that fact that you repeated the experiment to confirm the results. All of my work has pointed to inlets as the place that should get the most attention in the cooling department.
I do not believe that we should jump to the conclusion that you are losing cooling air through that gap. It is quite possible that you are getting air from the lower cowling spilling out that gap and disturbing the airflow into the inlet. Either way sealing the gap has made an improvement to cooling. Thank you for reporting this to the group.
 
Mikey will try it, too!

I've been concerned about high CHTs this summer and I know Mikey has a gap in this area on at least one side. You may see him with a little tape on his chin at Oshkosh!
 
Amazing

I have had persistent high CHT on #1 on take-off and full power steep climbs (into the mid 400's) and assumed it was just the blanking effect of the pitch angle as the temps were in the low to mid 300's in the cruise configuration. I did cut down the air dam without much impact and just this week replaced the forward section of the baffle seal (vans stock material) with silicon to see if that would give a better cowling seal.
I read this thread this morning before I headed out to do some slow flight, spot landing and short final practice in prep for Oshkosh (first timer and in my RV-7) and I thought "what the heck, worth a try" and I taped over the same gaps in my cowling with silver aluminum tape before I went flying. Results were amazing, 91 degree OAT and lots of slow flight, full power touch and go's and go arounds etc and the CHT's never went over high 300's.
The tape is staying for now but later I will remove it to see the the results of the silicon by itself and asses the tape contribution. Looks like some cowling work this winter:D
 
RV8R999 said:
... As we all know the cooling system is a fine balance between marginal differential pressures and doesn't take much to upset the flow.
Click to expand...

Those marginal differential pressures are typically 5 to 10 inches of water.....only 0.18 to 0.36 psi. The little gap would not leak a lot of mass, which is probably where we get misled. It wouldn't take much of a leak to increase local pressure below that front cylinder. And that particular spot should have good pressure recovery. I think it would be quite a lot higher than internal lower cowl pressure, and probably higher than pressure immediately above #2.

No need to guess. Poke a manometer tube up inside that corner and go fly.
 
I did the duck tape thing today, saw a small improvement of apx 10*

I am still higher that I would like in cruise, but I wonder if the engine is just running at its own particular norm???

IO 540, 9 to 1 comp, LASAR ign, and I was seeing the following numbers.

9500' WOT 21"mp, 2000 rpm gave me a CHT spread from 378 to 394, oil was at 180*, running 50* lean of peak, seeing 194 MPH, and 10.6 GPH. OAT was 73*.

I know the higher compression and the LASAR will cause higher temps, do these numbers sound reasonable??
 
another interesting result was lower oil temp by about 10 degrees. All summer with OATs in the low to mid 90's my OT have been very stable in the high 180s during 75% cruise, ROP (150 deg) in the 5000-8000ft DA range. After taping those little gaps OT were high 160's to 170's.

My oil cooler coincidentally is mounted to the angled engine mount cross members on the #2/4 cyl side, low toward the cowl exit. The oil cooler exit basically lies in plane formed by the engine mount. Any higher than normal pressure in the lower cowl section would decrease flow through the oil cooler as well as the cyl so the results match the theory.

I can probably blank off even more of the my OC inlet to get temps a little higher - as it is now the 3" inlet is blanked by 50%. I have SW oil cooler.

Dan - I agree it isn't a mass flow problem but rather a flow path problem. The flow is favoring the lower pressure #1, 3 and 4 lower cowl areas due to the locally higher pressure under number 2 causing the temp imbalance.
 
Thanks for the tip. I always have high CHTs during climb out. This morning I taped my cowl and the CHTs were much lower.
 
I guess my "hinged" cowl.............doesn't need this mod... :)

Actually, I looked inside the inlet, and rubber baffle seals that portion. High temps have not been a problem.

L.Adamson --- RV6A
 
I have suspected for years that there were some flow issues with the cowl hinge line. On my RV-6 with the naca vents directly behind the hinge line, in the dead of winter I always got warmer than ambient air thru the vent. To fix this on my Rocket I bought some extruded hinge that was wider than the hinge typically used and offset it so there weren't any gaps exposed in-between the hinge eyes as they were on my -6.
 
Yo Dan

Very insightful piece of info regarding the pressure differentials!! What is the total differential in inches H20 and PSI between outside air, inside plenum, inside lower cowl and outlet area. Just looking for a swag.

I am thinking about buying a magnehelic gage to do some under cowl analysis and need to quesstimate an appropriate measuring range.

Thanks and hope to see you at beer call Monday.

Fly safe!!
 
I have an idea for sealing the entire perimeter of the junction between the two cowl halves.

I'll first put down some electrical tape along the edge of the upper cowl and then wax the entire perimeter of the upper cowl edge inside and out.

Then place electrical tape along the lower cowl edge and attach the cowls to the plane normally.

I'll then mix up some pro-seal and fill the gap along the entire perimeter using a putty knife the create a nice flush surface.

Then pull all the electrical tap off leaving nothing but the fillet in place. Once this sets, remove the upper cowl and clean off the wax leaving a tight flexible gasket around the entire perimeter.

Thoughts?
 
gereed75 said:
What is the total differential in inches H20 and PSI between outside air, inside plenum, inside lower cowl and outlet area. Just looking for a swag.
Click to expand...

Depends on cowl design, execution, and velocity among other variables.

I am thinking about buying a magnehelic gage to do some under cowl analysis and need to quesstimate an appropriate measuring range.
Click to expand...

Zero to 1 psi (27.6" H2O or 2.035" Hg) should be enough for differential pressures.
 
I'm not using a Lycoming, but I do use the cowl air inlets. I made fiberglass ducts that fit into the inlets, all the way to the front. This way I can control airflow, prevent leaks and hopefully prevent excess duct turbulence.
I guess the James Cowl and plenum cover does the same thing.
 
Thanks Dan

Maybe I'll get the mini gauge and convert it to an AOA indicator after the undercowl testing is complete!!

Fly safe, see you at Osh
 
Hmmm

This also gets me to thinking about how much lower cowl pressure might be generated from leaks around the inlet to FAB connection at the bottom of most 6 cowls???

If there can be significant pressure from Ken's little gap slot, there must be some from any FAB inlet leak.

Details Details Details!!
 
agreed! I'm working on this part too. I have about a 3/16" gap between the seal and the lower portion of the FAB air inlet. This is tougher to seal because the baffle material is on the outside of the inlet which reduces its sealing ability as the pressure increases. I'm going to attache some baffle material around the cowl inlet and let this seal to the inside of the FAB baffle material. It may be more difficult to remove/re-install the lower cowl however.
 
ken

Sounds like the approach I would like to take. After my cowl exit work is done.

I was thinking of a fiberglass sleeve, moulded to the shape of the inlet that extends back into the mouth of the FAB. Sort of like a trumpet mute.

That way the cowling could go on and off without interference. Once it was on, the sleeve could be inserted, sealing the transition. The sleeve could be held in place by 1 or 2 screws and nutplates in the stock opening.

The seal part (aft end of the sleeve) would maybe have to be somewhat flexible, maybe via Dan's rubberized glass material, to allow for movement.

Just thinkin'

Maybe you got an AM who could do some of this fab for us!!
 
Duh!

I've stared at this gap on every RV I've worked on. Nothing in the plans and nobody ever said boo about it until now (except for looks).
352i2w7.jpg

Ken, is this kinda what you're talking about for the FAB?
8yut51.jpg
 
a bit hard to tell but if the baffle material is situated such it will seal more thoroughly as the pressure increases then yes. as it is in the plans the seal is attached to the fab and seals around the outside of the cowl inlet which will be less efficient as pressure increases.
 
The seal is riveted to the FAB snout all around and fits into the stock lip with no tunnel or foam, etc. It seems to seal completely. I've done a 6, 8 & a 10 this way and its easy actually. Trim the FAB snout just enough to allow easy cowl removal, rivet in an over length wrap of baffle seal and trim the forward edge to just fit in the lip with no wrinkles. Usually indicates 1-1.5 in above ambient MAP.
Anyone got pix of a way to seal that other gap we were talking about?
 
Just so i'm understanding this... Are you saying that the rubber seal on the FAB should sit *inside* the snout? ie. so on the ground, if you ran your finger along the rim of the snout and then slid it in, you would hit the edge of the rubber seal? On mine, the rubber seal fits perfectly around the lip of the fibreglass... If you run your finger into the snout you slide smoothly off the fibreglass lip and onto the rubber seal. Is that backwards?

My cowl has those gaps at the front too, i'm going to take some electrical tape out tomorrow to close them off before I go for a flight... I've got to do an oil change so a flight before and after might be interesting to compare.

Having recently installed CHT probes (only had EGT before) and changed from an analog to an MGL gauge, I now have CHT numbers for all cylinders but I don't know what they "should" be nor what they were before the gauge was installed. Sometimes I wonder whether I now just have too much information... :)
 
CHT results after minor cowling mod

As I reported at the beginning of this thread a couple pieces of 200MPH tape placed over the outboard inlet joint on both sides significantly reduced my #2 CHT and all others during climb and cruise. From it's first flight the #2 CHT was always about 30 hotter than the other cyl during climb and 15-20 in cruise. I almost never could keep in below 400 during the climb unless I was climbing at 145mph or greater. After the tape worked so well, I removed the cowling and laid-up several layers of resin and cloth to form a permenant internal seal. The process was simple - I waxed the heck out of the top cowling near the inlet (inside and out). Put the two halves together, mixed up the resin and laid precut strips of cloth along the inside. When it dried, seperated the two cowl halves and trimmed to lay-up to about 3/8" high. That is it. Total time about an hour of work.

Results: (3 consecutive climb snapshots and one cruise @full rich as a baseline and one leaned to 10GPH)

2ir23wh.jpg


23k4vmc.jpg


jfcj8z.jpg


1z63ymx.jpg


This last shot is leaned to 10GPH (about approaching 100 ROP)

15eg0ol.jpg
 
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pictures

ken, can you post some pictures of your permanent fix. I am having trouble visualizing it.

Thanks
 
To cover that gap, I have made a fairing (for the lack of better word) that will cover the gap and the piano pin hole. I will take a pic and post it next time I go to the hanger.
 
Please post more photos

Aerhed, sure would appreciated a picture of just the FAB and seal material and a close up showing how the seal sits in the the cowl nozzle.
 
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