Cooling the Carbon Cub

[turbopilot]

Sorry for the intrusion by an outsider- These cubs are some really neat airplanes! I built and fly an RV-9A here in Minne with an ECI O-320 with a carb and dual EI (sounds very similar to your 340, same engine - no stoking). We have learned to effectively run the engine smoothly LOP, and it really does help with the cooling. The trick in our planes is to use partial or full carb heat to help balance the mixture going to each cylinder. This may be a product of the airbox and induction system of the RV, but it might work on the carbon cub as well. Our theory is that the carb heat adds turbulence and/or the heat atomizes the fuel spray in the intake.

The RV will run LOP 150 mph true on 5gph and 170+ mph true on 6gph. In each case, the engine runs in the low 300s for CHT.

I just wish I could land on a sand bar! Once again sorry for the intrusion........

Thanks for the input. I have tried this in the Carbon Cub but was not able to smooth it out very well. How many degrees LOP do you go to get smooth operation?

Update on my tinkering to bring down CHT's in the CC. Overall my efforts have brought down my hottest CHT by over 35 dF. I can now fly in 60 dF OAT air at 1,000' MSL at 80hp with all cylinders below 340 dF CHT's. This allows me to fly in the hot California deserts at 100 dF in the summer time with max CHT's below 400 dF. Climbing at full power in the desert is still a challenge.

The biggest single change in CHT's so far has been achieved by the use of a fixed cowl flap. However, I have recently found that the size of the cowl flap is much less important than filling in the sides of the existing outlet with small fillets.

Various sizes of fixed cowl flaps would yield 10 to 12 dF CHT reductions while just the small fillets alone will yield around 6 dF CHT reduction. I think what is happening is that the high pressure air is wrapping around the back side of the existing outlet and killing the low pressure at the cowl outlet of the existing cowl.

Here is my current fillet arrangement. Still working on the proper size for the fillets. Requires lots of testing. Good excuse to go flying. All sizes of cowl flaps I have tested give about 5 mph of cowl differential. The fillets provide about half of that differential. I am finding that about 1 mph of cowl differential improvement is worth about 2 dF improvement in lowering the CHT of each cylinder.



Here is a screen shot of the Dynon EMS with the fillets in place. The backup airspeed indicator in this shot is reading the cowl differential pressure.

[RVPete]

I can run 50-70 deg LOP with the RV. It does work better up high 7-9K ft

[randylervold]

Hey Pete, welcome to the Cub world, good to have you here!!
(Pete is a well known member of the RV community, which is where I come from too, and has a penchant for tackling difficult problems with a methodical and thorough approach)

[randylervold]

Bob, the new fillets look good and are better looking as well.

FYI, we had reason to suspect the Dynon might be reading CHT temps incorrectly but did some definitive tests last night that confirm its accuracy so that was a dead end.

[turbopilot]

Bob, the new fillets look good and are better looking as well.

FYI, we had reason to suspect the Dynon might be reading CHT temps incorrectly but did some definitive tests last night that confirm its accuracy so that was a dead end.

Darn, I was hoping for a little help from calibration. Next version of the fillets will include a small reduced angle 2" cowl flap blended into the fillets. You were right about having too acute of an angle on the cowl flap which stalled the air.

Time for a recap of all the fixes I have applied to my airplane. Once I have finished I will cover each of them in detail. I made improvements in 7 different areas. So far the best improvement I have been able to achieve is a blended average of 27 dF reduction per cylinder.

Here is a screen shot of my Dynon as I received the airplane in April. Adjusted average CHT (see definition below) was 355 dF @ 60 dF OAT.



Here is a screen shot of my Dynon with the best reduction of CHT's to date with a adjusted average CHT of 328 dF @ 60 dF OAT.



So from the beginning of my project to this point I have experienced a 27 dF reduction in the adjusted average CHT temperatures on the Carbon Cub with all the associated fixes. In addition to the reduction in adjusted average CHT's I have reduced the spread between the hottest and the coolest CHT from 75 dF to 26 dF. I have separately confirmed this reduction by flying formation with an unmodified Carbon Cub.

For those who are trying to make progress on cooling, I have found computing the average adjusted CHT from the Dynon to be just as reliable as measuring cowl differential pressure. Here is how you do it.

Just add up each of the 4 CHT values for a total raw CHT value, then adjust that value to a standard temperature (I use 60 dF). So you just compute the OAT above or below 60 dF, multiply times 4 and apply that number to the total raw CHT. Then divide by 4 and you have computed the average adjusted CHT temperature for your motor. This adjusted average CHT tracts directly with changes in the cowl differential pressure in my experience.

More detail on all the fixes when I am done. Still working it.

[RVPete]

Does anyone have photos of the intake, airbox, filter and carb heat setup on the carbon cub? I'd like to compare that to my set up.

Thanks

[turbopilot]

Does anyone have photos of the intake, airbox, filter and carb heat setup on the carbon cub? I'd like to compare that to my set up.

Thanks

Here you go.

[RanRan]

One of the things about some RV designs that I find perfectly reasonable is the use of an angled firewall to direct hot ram air downward and out. A flat (90 degree to ram air) firewall is so common but creates nothing but back pressure and thus real cooling. I intend to build an asymmetric conical parabolic air deflector over the flat firewall that directs ram air where I want it to go. Better put: where it needs to go.

If that means side vent cowling flaps, then so be it.

An over heated engine is trash the moment it is overheated. It will run, but, man, the harm done is irreversible.

[rowdyroyce]

Just wondering, has anyone had performance tuning (such as LYCON) done on a CC340 motor?

[turbopilot]

The Carbon Cub is Running Cooler

Thought I would update folks on my project to cool the Carbon Cub. As background I operate my Carbon Cub in southern California. It gets very hot at altitude in the summer over this part of the country. Typically we see temperatures +30 to +40 dF above ISA standard temperate usually associated with strong inversions around 2,000 to 3,000'. So it is very challenging to operate an airplane at these temperatures in a manner that preserves engine life.

Since June I have worked on lowering the temperatures of my airplane along with some significant assistance from CubCrafters. Here are the areas you should consider to cool your CC.

1. Plenum and Baffles - The plenum was pretty well sealed with high temperature RTV from the factory but I found several areas that required more RTV sealing particularly around the two aft cylinders. The baffle seals on my airplane extending from the plenum intakes to the front cowl did not meet the cowl in my airplane from around 7 o'clock to 4 o' clock on the left intake and from 8 o' clock to 5 o' clock on the right inlet. The gap was around 1/2" in those areas allowing pressurized air to escape from the plenum to the lower cowl.

2. Sealed the top cowl "decorative" inlets. These inlets connected directly to the low pressure side of the cowl system lowering cowl differential pressure.

3. I worked on several variations of a fixed cowl flap attached to the aft outlet of the lower cowl. I finally settled on a compromise between how it looks and how it works. Bigger is better but big does not look good and adds more drag. See picture below for my current version.

4. As I have posted before the oil cooler provides more cooling than necessary, in my opinion. The oil cooler has 20 square inches of exposed fins across the back of the plenum. In my airplane the oil temperature would often run low (below 180 dF) while CHT's were at or above a comfortable level. I have found that restricting the oil cooler to about 13 square inches of surface area provides the best balance between CHT's and oil temperatures. As you obstruct the oil cooler in the plenum the differential pressure across the cylinders increases and the CHT's drop. I simple drilled 1/4" holes in some .016 aluminum and inserted it in front of 1/2 of the oil cooler inlet. See below.

5. I have added front cylinder fences. These fences level front to back differences in CHT's. They simple allow the hotter running aft cylinders to enjoy a little more of the cool air from the front significantly lower aft CHT's while increasing the front CHT's. Since the thermal performance of the airplane is limited by the hottest cylinders these fences help a great deal to operate in higher temperatures. See pictures below.

6. Aerodynamic smoothing of cowl inlets. As you can see in the images below the cowl inlets have filler added to aerodynamically smooth the airflow entering the plenum.

7. The heater inlet in the left cowl inlet is covered by aluminum tape. We don't need the heater in southern California during the summer. I will take the tape off in the winter.

8. And finally CubCrafters has just released to production cowl "gills". These "gills" are installed on both sides of the lower cowl. The gills are made of carbon fiber and can be set in three ground adjustable positions. I am currently running my gills in the middle position. See images below.













So how does it all work.. Atmosphere today over southern California varied from 30 to 35 dF hotter than ISA above the inversion at around 1000'.

Today I took off on a climb test. With temperatures this hot I will only cruise climb at around 90 mph. With that I was still climbing at 1,000 to 1,200 fpm at full power. As I climbed out today I passed through the top of the inversion with air at 86 dF at 2,500'. From that point temperatures slowly declined but stayed around +30 ISA up to 7,500'

I climbed with around a 40 dF spread hottest to coolest CHT. Maximum CHT in the climb was 418 dF on #4 at 5,500'. From there up to 7,500' all CHT's declined. As I reached 7,500 at just under 7 minutes of full power #4 CHT had dropped to 411 dF.

Once I reached 7,500' I set up for cruise at around 45% power (6 gph). CHT's continued to fall with a 35 dF spread and #4 the hottest at a nice cool 381 dF. Note the oil temperature was less than 200 dF at 7,500' with a +30 dF ISA atmosphere.



So my Carbon Cub is now running much cooler. Bottom line is that the Carbon Cub is generating a lot of power and heat while moving relatively slowly through the air. So owners will have to pay special attention to the engine in the summertime to make sure that engine temperatures are compatible with long engine life. Operating in the deserts of the US southwest during the summer is a big challenge. Those operating the Carbon Cub in the northern United States will not have the same challenge.