High CHTs on climbout - resolved

I know it's been discussed at length, and I followed most of the posts, so I thought it was worth the time to post my results and resolution.

I have the following configuration:
- RV-7A
- Lycoming O-360A1A
- 74" blended airfoil Hartzell CS Propeller
- Standard Van's cowl
- RV Bits Plenum

Problem: during climb out, or long climbs, CHTs climbed too quickly. I wanted more effective cylinder head cooling. Oil temperatures were good, and it was mostly cylinders 2 and 3 causing concern.

Solution(s):
1) my first "solution" was to completely seal off any spaces/holes under the plenum to ensure maximum pressure on the top side of the cylinders - no noticeable effect on CHTs vs my original installation
2) my next attempt was to increase the space behind #3 cylinder with a washer as recommended by many on this forum - this was quite effective in keeping CHTs below 420F during climbs, but there was still much room for improvement
3) my 3rd attempt was to smooth the plenum airflow by smoothing the transition from the Vans cowl opening...I used duct tape to create a smooth flow into the plenum - no noticeable difference in CHT cooling or airspeed...not effective in my configuration (although duct tape on the front of your airplane is obvious enough to cause good conversation at the fuel pump!!!)
4) installed 1 x AntiSplat cowl flap (bottom left side of cowl) - better (CHTs now below 410F on climb)....but still much room for improvement
5) installed 2nd AntiSplat cowl flap (bottom right side of cowl) - CHT cooling issue is resolved for now. On climb out or long climbs, with OAT @ 85F, CHTs do not rise above 390F.

I installed both cowl flaps on one switch which works very well. I replaced the very small switch provided by AntiSplat with a full size double pole-center off momentary switch so I can partially open the cowl flap as needed.
The AntiSplat switch worked just fine, I just wanted a momentary switch AND something a little bigger (you'll understand if you tried to solder the switch provided with the cowl flap kit). I used one indicator light (provided in the kit) for both flaps, so the light blinks "on" when both cowl flaps are completely open.

With both cowl flaps installed, I am penalized about 3 mph cruise speed IF I leave the flaps open....but I don't need them open during cruise as CHTs are not an issue with full airflow. With the flaps closed...cruise speeds are back to normal.

So...thanks to all for your advice and discussion. This forum continues to help me enjoy what is already a phenomenal little airplane.

Cheers,
Ackselle

What airspeed were you doing these climbs at? Did you try a higher airspeed climb?

Acksell

I did the same thing.
Drove me crazy that a plane with such performance couldnt climb out on a hot day at more than 300 fpm without excessive CHTs.

After trying all the cowling/baffling mods with minimal success, I gave up and did the dual cowl flaps deal. Expensive solution but does the trick.
Now nice temps below 400 on a hot day.

Fuel Flow

Bruce and Acksell, Thanks for the info and review of sorts. Good info.
Did you guys check fuel flow / richness too while trying to cool temps?
Thanks

More exit area usually works.

Neither of you guys mentioned ignition system choice.

What Dan said.

Also, the problem with #3 and #2 is well documented on the forum. And so is the fix (Hint: the "washer trick" is not it).

The stock RV cowl is adequate for stock engines - the stock Vans baffles are not. Fix the baffles and you will be golden.

Answer to some questions...

1) bret - Airspeed on climb out or long climb....I found anything 150 mph or lower caused my CHTs to rise too quickly. With the 2 x cowl flaps, airspeed is not nearly as much of a concern for cooling
2) DanH - Ignition System - 2 x Magnetos
3) Bob - mixture settings, throttle settings etc...yes...I tried various settings. I simply was not getting enough airflow across the cylinder heads
4) Toolbuilder - Vans baffles....I'm using the baffles, but I've replaced the baffle seals with a plenum

Increase the airflow....you're golden!

Cheers,
Ackselle

ackselle said:

1) bret - Airspeed on climb out or long climb....I found anything 150 mph or lower caused my CHTs to rise too quickly. With the 2 x cowl flaps, airspeed is not nearly as much of a concern for cooling
2) DanH - Ignition System - 2 x Magnetos
3) Bob - mixture settings, throttle settings etc...yes...I tried various settings. I simply was not getting enough airflow across the cylinder heads
4) Toolbuilder - Vans baffles....I'm using the baffles, but I've replaced the baffle seals with a plenum

Increase the airflow....you're golden!

Cheers,
Ackselle

Click to expand...

Throwing more mass airflow through the engine does increase cooling - but also drag. The goal is to have "just enough" airflow through the cowl to ensure adequate cooling and also minimum drag.

As I said before, the stock Vans cowl is generally adequate. Put another way, if you are contemplating enlarging the exit with a stock engine there is a very good chance you are overlooking a problem. And that "problem" invariably turns out to be the baffle choke point on #2 and #3. (Many threads on this subject) Fabricate and install the bypass ducts as Dan, myself and others have done and there is a very good chance you can eliminate those draggy exit flaps. I have a 200 HP engine in the -8 and solved the problem you are having by using bypass ducts, not making the exit bigger. Also keep in mind that I'm flying in one of the hottest places around.

Remember that cooling drag is a significant portion of overall airframe drag, so the goal should not be to increase airflow, but to use the airflow you have more effectively. These ducts correct a design flaw, simple as that.

It's not just 2 and 3

I agree Michael....2 and 3 need ducting as discussed in the past. However, The problem is (or in my case was), it's not just cylinders 2 and 3...but rather all CHTs were rising too fast on climb. 2 and 3 were more problematic, but all CHTs needed help. Ergo...more airflow all around seems to resolve the issue....and I can pull the flaps up to eliminate the drag in cruise.

I believe I'll do the ducts on 2 and 3 in time, but the cowl flap solution was real simple and effective.

Cheers,
Ackselle

ackselle said:

I agree Michael....2 and 3 need ducting as discussed in the past. However, The problem is (or in my case was), it's not just cylinders 2 and 3...but rather all CHTs were rising too fast on climb. 2 and 3 were more problematic, but all CHTs needed help. Ergo...more airflow all around seems to resolve the issue....and I can pull the flaps up to eliminate the drag in cruise.

I believe I'll do the ducts on 2 and 3 in time, but the cowl flap solution was real simple and effective.

Cheers,
Ackselle

Click to expand...

I did the ducts on 2&3 with Michael's help. Both, especially #2's, worked, but one cowl flap added a couple months ago really helped get me the rest of the way to where I wanted to get.

ackselle said:

...I believe I'll do the ducts on 2 and 3 in time, but the cowl flap solution was real simple and effective.

Click to expand...

Its good that you have an airplane you can use now. Thats certainly more important than spending all of your free time chasing down the last knot or two.

I hope you will report back with your results after doing the bypass ducts. Maybe if enough people speak out Van will fix his baffles.

Just curious as to what your WOT fuel flows are. It seems another common problem with carbed RV's is too-low fuel flow, resulting in leaner mixtures and higher CHT's at WOT.

I spent a lot of time on baffling, including building a bypass for cyl #3, which helped, but was still too hot on climb-out, and found my fuel flows were too low at full throttle. I was fine in cruise with all temps around 360* on a warm day. I drilled out the main jet one size, and am now able to climb out normally on a hot day with temps under 400, and my fuel flows are more in line with what they should be.

Chris

Don't forget to inspect the cooling fins around the spark plugs for leftover flashing. It doesn't take much! I cleaned out relatively small amounts of excess flashing that was partially restricting airflow between the fins with a needle file. I saw 10-20F drop in peak CHT across all cylinders.

same issue

YellowJacket RV9 said:

Just curious as to what your WOT fuel flows are. It seems another common problem with carbed RV's is too-low fuel flow, resulting in leaner mixtures and higher CHT's at WOT.

I spent a lot of time on baffling, including building a bypass for cyl #3, which helped, but was still too hot on climb-out, and found my fuel flows were too low at full throttle. I was fine in cruise with all temps around 360* on a warm day. I drilled out the main jet one size, and am now able to climb out normally on a hot day with temps under 400, and my fuel flows are more in line with what they should be.

Chris

Click to expand...

Plus one for this fix!

Check your Mag timing.
It doesn't take much advance beyond spec to increase CHT's dramatically.

WOT Fuel Flow

Bob and Chris....My WOT Fuel Flow shows just over 16 GPH on takeoff @ 850' MSL (CJB3 field elevation)...which seems to be in line with Lycoming documentation. What are yours showing now that you've opened the main jet?