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Does trimming lower cowl to increase exit area really lower CHT?

R

rv7adriver12

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I have a RV-7A with an O-360 and fixed pitch prop. I routinely see CHT reach 410 or more on all 4 cylinders if I don't immediately throttle back after take-off and climb at a very shallow angle. It doesn't matter what the ambient temp is. It can be 40 degrees and I still exceed 400 degrees if I use full throttle in a shallow climb. CHT is always fine once I level off or throttle back. Baffle seals are all new and in good shape. I'm debating whether to trim the lower aft part of the cowling to increase exit area. But am hesitant because of a new paint job and feedback from one poster that this made his situation worse. However, if trimming off some cowling material is really worth the effort, I'm ready to cut. Is it effective, and if so, how much should be trimmed - 2" or so?
 
On my -8, I cut mine forward an inch at a time. I'm now at 5" and the temperatures came down incrementally.

I did a bunch of other "stuff" as well, lots of info in other threads.

The biggest remaining factor is getting the baffles "just right" in the very front of the cowl. I'm planning to install some "backing" for the rubber seals to help hold them in position.

Good luck...
 
A general rule of thumb is that you want the exit area to be twice that of the inlet area. I trimmed about 1 1/2" from the rear of the exit area on my 6A to approach that ratio and have had no issues with overheating.
 
Take a look at the servo actuated vents on the Anti Splat website. More exit area may lower CHT, but will also increase cooling drag. The servo actuated vents may help cool in climb and then can be closed in cruise so you dont add to drag
 
Well, it works IF, and it's a big if, you are getting enough high pressure inflow on top of the engine. I've solved cooling issues on about a half dozen RVs so far and while I'm not an expert, I can say that what I've found to be the two most consistent problems are:

1. Restricted inflow above the cylinders where the upper ramps are bonded to the top of the cowl. As a general rule, if you cannot get a clenched fist up inside your cowl, then the flow is restricted. Cut out the ramps and move them forward so they make a more abrupt transition to the inside of the upper cowl.

2. Leaky baffling. Make them as tight as you can. Consider a plenum if you just can't get them tight. Seal all the little slots, cracks, and openings around the engine to drive as much air as possible through the cylinder fins.

My previous RVs all cooled extremely well without the need for other "remedies". Work at it and I'm sure you can get the temps down where you want them.
 
Those kind of temps in climb are not a big deal. What are your temps like in level cruise at 75% power?
 
To answer your question directly, yes, increasing exit area will generally increase mass flow. I have actually taken that experiment to the extreme, as my modified cowl can be flown with no exit chute in place, i.e. with about half the bottom as an exit.

The cure is usually what Randy said, times two. Barring a poor engine setting, it's pretty much all there is.

That said, with 330F in cruise, 410 is about what I would expect for a fixed exit. Variable exit area would be the next step, so you could lower climb temps, and raise cruise temps to cut drag.

painless said:
A general rule of thumb is that you want the exit area to be twice that of the inlet area.
Click to expand...

Proven untrue, over and over again. Dan Raymer has a few words on the subject in his latest book.
 
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Use the search function. The solution to high CHTs on climb out in a carburetor engine is to have a bigger diameter jet in the carb. Don't cut your cowl!
 
How much time on your engine and new baffles? I went thru the same routine 3 months ago. I trimmed my exit 1" and it helped a little. Installed an anti splat cowl flap, it helped a little. Sealed every crack I could find and it helped a little. I also believe my baffles finally "broke in" suddenly and covered some spot that had been leaking because the baffles were not limber enough. Now I can climb hard and seldom hit 400.
 
High CHT

Vern, thanks for your suggestion about a larger carb jet. Mike Busch had also suggested trying that when I mentioned high CHT problems to him. Do you have any recommendations on jet size? The carb is a Precision Model MA-4-5. P/N 10-3878.
 
Before you change the jet, check to see if you have 200F, maybe 250F between your full throttle EGT and peak EGT. If not that much, install a larger jet. If you have 250 or more, a larger jet is just a dirty and expensive way to reduce horsepower. You can do it cheaper and cleaner by retarding the timing, or just pulling the throttle.
 
I think cutting the cowl is the last resort..because it's kind of permanent. Are all your gear fairings in place? what speed are you climbing out?

1. Do the light test to check for any leakage path...all air needs to either go through the fins or the cooler. EVERY other path, no matter how tiny is a cooling killer
2. Clean up the exit area of all clutter. Add internal fairings to direct the flow if needed.
3. Timing backed off a bit.
4. Ramps and inlets installed properly

FYI...
my -7 has a James Cowl inlet size is 4.750" ID, outlet hole on mine trimmed flush with firewall is 14" along the top, 3.5" tall and has a radius edge to a 12.5" bottom lip. I have Vetterman 4 pipe (1.75" OD) So here is how I calculate the areas:

Inlets=35.44 sqin
effective Outlet=35 sqin
or roughly 1 to 1 ratio.

I had some cooling issues early on, but filling the holes and cleaning up the outlet helped drop CHTs about 30F in cruise and oil temps down below 200 except on the hottest days. Extended summer climbs can push the oil to 215-220, but I tip the nose over to help that. CHT's never above 400, usually 330-360. Some day I would like to reduce the outlet to limit drag while cruising.

Good luck
 
Could you add a bit of info regarding your 330F cruise number? What power and mixture settings... LOP? Variable timing? Compression ratio and which cylinders (Lycoming vs, ECI vs. ?)

I've battled CHT problems with ECI nickle carbide cylinders, IO-360, 9.2:1 compression, dual P Mags, WW 200RV prop and the solution that worked for me to fix my climb temps was to add the cowl flaps.

My cruise temps are mostly influenced by mixture/power settings. 23 squared, LOP (around 7gph) and mine are in the 340 to 350F range doing 150kts true. If I want more speed, 24 squared, ROP(around 9gph) and I get around 165kts but my temps are in the 390 range, but that is still not a concern for me. I start paying attention above 400F and start changing my behaviour when it's 410 and still climbing.

For climbs, before cowl flaps, I had to frequently flatten my climbs to 130 kts and they would still shoot rapidly past 415F. Now I can climb out at 100 to 110 and they almost never go over 400 even on a very hot day.
 
Lyc parallel valve 360, AFP FM200 injection, 9.2:1 comp, Pmags("A" curve 24*), James log cowl and plenum. I cruise at WOT 2380-2450 RPM(depending on the mood) and LOP slightly.
Last week at 11500, OAT as 6*C, 2430rpm, 20"mp, burning 8.1gph, CHT's 343, 346, 350, 352. Oil was 192. Showing 148ktias, 179KTAS.
 
jets

I had the same problem with mine. I have a cowl flap, however it didn't make enough of an impact. I upper the Carb jet size to a #38 if I remember correctly and during a really hot day I see 403F of the hottest cylinder during a 90kias climb. During cruise I adjust mixture and cowl flap accordingly, not to exceed 380F. This reduces engine wear and cooling drag. I can close the cowl flap beyond the nominal exit size to almost half the nominal exit size at all most power settings. Only at 75% do I open it more, but still less than nominal, to keep CHT's below 380F.
I just recently replaced the GA200 prop with a C/S Hartzell, and I use the same procedures and it works.
You also need to normalize your CHT to ambient temps. All CHT's should be normalized to a 100F day. So if you are flying on a day where it is 80F, you need to ADD 20F to your indicated CHT's in order to get accurate CHT numbers.
 
FWIW when I was testing my cowl flap installation, I tested the effects of leaving them open in cruise. I my installation I found that while it cost 5-7 its to leave it open, there was virtually no difference in CHTs in cruise, flaps open vs closed. The only difference, other than speed was a slightly audible rumbling. Dan Horton can probably explain why (please!) I'm guessing that at low climb speeds and higher AOA the exit area is the limiting factor but at higher speeds and lower AOA the intake and exit area are quite closely balanced. I could be completely mucking up the concepts though.:D
 
jjconstant said:
...
I've battled CHT problems with ECI nickle carbide cylinders, IO-360, 9.2:1 compression, dual P Mags, WW 200RV prop and the solution that worked for me to fix my climb temps was to add the cowl flaps.

...
Click to expand...
You didn't mention if you are running the jumper in the P-mags, forcing them to the "A Curve" or if you have adjusted the timing with the EICAD program or EICommander.

The "A Curve" sets the timing at 26* BTDC which is probably a bit high for 9.2:1 pistons. I would recommend retarding the timing a little bit on your P-mags, say to 22 to 23* BTC.

If you don't have the jumper in the P-mags, your timing is around 30* BTDC, which is way too much advance and will easily explain your high CHT's.

Check your PM's.
 
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f1rocket said:
... I've solved cooling issues on about a half dozen RVs so far and while I'm not an expert, I can say that what I've found to be the two most consistent problems are:

1. Restricted inflow above the cylinders where the upper ramps are bonded to the top of the cowl. As a general rule, if you cannot get a clenched fist up inside your cowl, then the flow is restricted. Cut out the ramps and move them forward so they make a more abrupt transition to the inside of the upper cowl...
Click to expand...

I'd like to respectfully offer up a recent experience which appears to contradict your "general rule".

Just last weekend I was messing around with the upper cowl and changed one side from the standard "abrupt" ramps to a long, gradual ramp. I tried to get the classic 7 degree divergence, but the cylinder was in the way. I ended up with about a 10 degree divergence, but the transition is very gentle. Anyway, the space between the ramp and the top of the cylinder fins is pretty tight. I think a grilled cheese sandwich would fit, but certainly not a clenched fist.

1ij252.jpg



I want to hold off on a definitive report until I get some more hours on the airplane, but the initial flights show a noticable improvement in temps, and unexpectedly, my misbehaving #4 and #5 have evened out.

I'm not declaring victory yet, but this mod is certainly no worse than it was before. The tight fit did not hurt my airplane at all.
 
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I'm so glad it's getting cold.

Another year of high chts over. I'm to the point I don't even fly when it's over 80

I did louvers, cut exit, worked on baffles, it helped. But I still can't climb worth beans until oat goes around 70 or lower. 430 can be hit very easily at 125 climb speed with 19" and 2400 rpm. Yeah sucks so I pretty much nurse it up to altitude until it cools off

Anyway end rant. I almost drilled jet and read things that scared me out of it. Otherwise I love my plane. I would pay good money to take it to someone to fix it.
 
erikpmort said:
I'm so glad it's getting cold.

Another year of high chts over. I'm to the point I don't even fly when it's over 80

I did louvers, cut exit, worked on baffles, it helped. But I still can't climb worth beans until oat goes around 70 or lower. 430 can be hit very easily at 125 climb speed with 19" and 2400 rpm. Yeah sucks so I pretty much nurse it up to altitude until it cools off

Anyway end rant. I almost drilled jet and read things that scared me out of it. Otherwise I love my plane. I would pay good money to take it to someone to fix it.
Click to expand...

Just my two cents worth...I find that the CHTs, on my RV6A (o360, carb, with 10:1 pistons and a CS prop) are lower when I climb out with full power and RPM rolled back to 2400.

Most of the time, even with the PHX summer heat, I can climb to 4 or 5K feet before leveling off to cool down. When I used pull the power back on climb out in attempt to manage CHT, I could barely get to pattern altitude before the temps got uncomfortably high.

I read someplace several years ago...can't recall where or find it right now...that the additional fuel supplied at full power provides significant cooling effect. I went through the evaluation process years ago and don't recall exactly how much cooler but it was enough for me to adopt the process as mine.

I've tried many of the techniques and changes talked about here and I guess the cumulative effect makes CHTs manageable now. Now I get overheated before the engine does! That said, no matter what I do, our summer ambient temps (over 100 degrees F for most of the summer) drive constant CHT awareness and management.

Love fall, winter, and spring flying in AZ!

YMMV.
 
18-years ago

I cut 2" off the back of my cowl exit scoop and it helped. As a side benefit, I think the sloping cowl exit looks better.
 
jjconstant said:
FWIW when I was testing my cowl flap installation, I tested the effects of leaving them open in cruise. I my installation I found that while it cost 5-7 its to leave it open, there was virtually no difference in CHTs in cruise, flaps open vs closed. The only difference, other than speed was a slightly audible rumbling.
Click to expand...

Logic says the CHTs do not change because mass flow does not change. Mass flow through the fins is driven by pressure delta, upper to lower. So, if you're sincerely curious, start with a differential pressure measurement to confirm or deny that logic. Email for a how-to sheet if you like.

Toobuilder said:
Just last weekend I was messing around with the upper cowl and changed one side from the standard "abrupt" ramps to a long, gradual ramp. I tried to get the classic 7 degree divergence, but the cylinder was in the way. I ended up with about a 10 degree divergence, but the transition is very gentle. Anyway, the space between the ramp and the top of the cylinder fins is pretty tight. I think a grilled cheese sandwich would fit, but certainly not a clenched fist.
I want to hold off on a definitive report until I get some more hours on the airplane, but the initial flights show a noticable improvement in temps, and unexpectedly, my misbehaving #4 and #5 have evened out.
Click to expand...

I note that's your Rocket cowl. Pretty sure the inlets operate at a higher Vi/Vo ratio than a standard Vans cowl, so there are some differences in how things work.

If inlet velocity is high (high Vi/Vo, velocity through the inlet is a high percentage of freestream velocity), an abrupt inner lip would probably cause flow separation along that upper, inner surface, which would spoil some of the dynamic-to-static conversion. You obviously recognize this detail, which is why you added the nicely diverging ramp. Note that now velocity (and dynamic pressure, Q) may still be pretty high at the constriction over 1 and 2, so some of the Q-to-static may be taking place further aft.

A lower Vi/Vo inlet makes the upper, inner lip diverging angle less critical. The slow flow of a truly low Vi/Vo entirely eliminates the need for a diverging ramp; look at a Cessna Corvalis or Mooney Acclaim at the next fly-in. At the same time, the slower flow requires more space above 1 and 2.

Jaws said:
I find that the CHTs, on my RV6A (o360, carb, with 10:1 pistons and a CS prop) are lower when I climb out with full power and RPM rolled back to 2400.
Click to expand...

Sure. That's a note right from the 1930's NACA studies. Same cooling mass flow, fewer combustion events in a given time. The NACA wizards phrased it as "for a given mixture and ignition timing, cooling demand is proportional to engine internal mass flow"...a fancy way of saying proportional to RPM.

Just remember that with high CR and a very hot cylinder, running highly oversquare at high MAP puts you close to the detonation limit.
 
Also taping the forward seam of the upper and lower cowl dropped my cht by about 7 degrees
 
Bill, I should have mentioned that I have the jumper in and the timing is retarded to 22 degrees. I tried that after verifying that I had enough fuel flow and rich enough mixture. I even left the M.P. tubes off to leave it at 22 with no advance. It helped a bit but not enough. I put a bullnose on the exit area to smooth airflow out the exhaust area. There was an incremental improvement of 5 - 10 degrees. I then faired in all the nose gear tubing to smooth the exit flow. Again an incremental improvement but still not enough. There is no flashing between the cylinder fins and I've had everyone around the airport, including those folks who run too cool look for leaks in the baffles and they can't find any. The only thing that worked was the installation of the cowl flaps and it has worked well.

Dan, thanks for the offer of the how to sheet. I'll email you for it.

Fun discussions!
 
turbo said:
Also taping the forward seam of the upper and lower cowl dropped my cht by about 7 degrees
Click to expand...

That gets me thinking. Can I achieve the same effect by riveting baffle seal to the upper cowl, covering the hinge? It wouldn't be a tight seal and I wonder if the air would just finds it's way under the opening.

I could try this, as I had to re-rivet parts of my upper hinge and will need to re-paint this winter.

Larry
 
The tape at the hinge line is a simple trick to try before you look for a more permanent fix. I did it on the RV some time ago and was not able to duplicate any positive result, but clearly some have.
 
erikpmort said:
I'm so glad it's getting cold.

Another year of high chts over. I'm to the point I don't even fly when it's over 80

I did louvers, cut exit, worked on baffles, it helped. But I still can't climb worth beans until oat goes around 70 or lower. 430 can be hit very easily at 125 climb speed with 19" and 2400 rpm. Yeah sucks so I pretty much nurse it up to altitude until it cools off

Anyway end rant. I almost drilled jet and read things that scared me out of it. Otherwise I love my plane. I would pay good money to take it to someone to fix it.
Click to expand...

What is you're EGT spread between full rich and peak at say 60-65% power?
 
I did this above 8k full rich wot to lean and I'm trying to remember what the spread was. Say it was 200 what would you tell me to do ? I think I recall from my last experiment I was told to drill to more rich.
 
You will need good data first. But if it is a too lean condition I would call Marvel Schebler and tell them what -10 number carb you have and see what carb setting they recommend. I recently "fixed" a 300 hour -9A that was built with a factory new lycoming and ran way too hot in all phases of flight. A new carb and some baffle work did the trick.
 
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