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

[jjconstant]

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.

[N941WR]

Originally Posted by jjconstant

...
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.

...

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.

[Toobuilder]

Originally Posted by f1rocket

... 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...

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.




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.

[erikpmort]

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.

[Jaws]

Originally Posted by erikpmort

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.

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.

[RV6_flyer]

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.

[DanH]

Originally Posted by jjconstant

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.

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.

Originally Posted by Toobuilder

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.

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.

Originally Posted by Jaws

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.

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.

[turbo]

Also taping the forward seam of the upper and lower cowl dropped my cht by about 7 degrees

[jjconstant]

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!

[lr172]

Originally Posted by turbo

Also taping the forward seam of the upper and lower cowl dropped my cht by about 7 degrees

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