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Thread: Cooling the Carbon Cub

  1. #11
    Senior Member turbopilot's Avatar
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    Default Re: Cooling the Carbon Cub

    Quote Originally Posted by seastar View Post
    Bob and all--
    It occurs to me that a simple extension of the inlet "ducts" to place them closer to the rear of the propeller might raise the inlet prssure.
    They may not be far enough out in the prop arc to be in an area of high pressure but it would be easy to try. A sheet aluminum replacement for the existing ducts could be hand bent to try the idea.
    Several manufacturers have used this trick.
    I had a Cheyene that had an STC that raised the inlet pressure in this way and provided additional engine power at altitude.
    Bill
    Bill, I will put that on my list. Current inlet's are pretty close to the prop. Medial edge is within an inch, lateral edge is farther away.

    I did fly a test flight with inlets removed. This decreased cylinder differential air pressure by about 1/2" of water and raised CHT's around 10 dF, so the current inlets are helping. I suspect the inlets would be more efficient if they had an aerodynamically round leading edge versus the sharp edge in the current inlet.
    Bob Anderson, CC11-00435, N94RA

  2. #12
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    Default Re: Cooling the Carbon Cub

    Hi folks,
    Sorry to not weigh in earlier but I have been out of the office visiting customers.
    The cylinder used on the CC340 is manufactured from the same head casting and barrel forging we use on our certified Titan cylinders. The FAA certification test for those cylinders set a Service Limit temp of 500 dF. In my experience few people have ever hit that mark and most pilots get concerned when temps cross 440dF in climb. When I give my presentations on Cylinder Anatomy I suggest a limit of 425 in climb and 400 in cruise. ECI owns and flies a Cessna 414 with turbocharged 520's and our VP of Engineering (and pilot) will fly comfortably at 425 in cruise in the Texas summers.
    I see the discussion regarding 380dF being an optimal temperature for cruise and can't disagree that is a great number but ECI does not have a problem with operation at 400dF in cruise. Baffling must be in great shape to direct the cooling air to the hot spots (you wouldn't drive to the lake with a hole in your radiator) and change that oil and filter every 25 hours.

    Tim Morland

  3. #13
    Senior Member RanRan's Avatar
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    Default Re: Cooling the Carbon Cub

    I'm not an engineer, mind you, but the problem looks to me to be back pressure within the cowling. The 'x' number of square inches at the IN openings shouldn't greater than the OUT opening. Remember, the exhaust pipe, itself, cuts into the number of square inches in the OUT opening.

    This looks like an easy fix...cut a larger opening around the exhaust.

    And for good measure...and for happy aft cylinders...add a couple quarter-rounds and mount them to the firewall to direct the hot air down to the OUT opening. - rather than banging up against the firewall and building up pressure.
    Last edited by RanRan; 07-04-2010 at 06:13 PM.

  4. #14
    Senior Member RanRan's Avatar
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    Default Re: Cooling the Carbon Cub

    One last thought - I think the Dynon allows you to monitor temps on up to 6 or 8 cylinders (or whatever). Why not mount sensors on the left and right forward firewall behind the pistons? And label them as such? Changes there should tell you how well back-pressure problems are being solved.
    Last edited by RanRan; 07-04-2010 at 08:03 PM.

  5. #15
    Senior Member turbopilot's Avatar
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    Default Re: Cooling the Carbon Cub

    Time for an interim update. Sorry not to have been updating more but I wanted to learn as much as I can before getting back to all the folks who have contacted me via email. I have learned a great deal about cooling the Carbon Cub in the last month.

    First, I want to preface my comments by talking about expectations. I bought the Carbon Cub with the understanding about the POH limitations on engine power levels. We all know those power levels are artificial constructs of the LSA regulation, but that is the basis of the aircraft certification. Engine heating will track with power produced and OAT. So I began this project looking for a solution that would allow my aircraft operate in the summer in Southern California on hot day within the operating limitations of the POH while keeping engine temperatures below a specific target. I like to see max CHT's below 380 dF in cruise and climb, while ECI suggests limits of 400 dF in cruise and 420 dF in climb. You pick the number. No matter which number, using either limit the Carbon Cub as delivered easily passes the limits on a day like today in Southern California where the atmosphere is 40 dF hotter than an ISA standard day.

    The good news is that several owners and CubCrafters are actively working on this problem. The purpose of this update is to share with other owners my progress in hitting the target. My airplane is running much cooler now with a couple of simple modifications. It will now operate within the ECI temperature limit recommendations even on a day like today at +40 dF over ISA.

    Please read the opening post on this thread for the baseline information on my aircraft. Since making that post I have made many, many test flights evaluating different ways to bring down the engine temperatures.

    The first task was to use aluminum tape on the front cylinders to balance the very wide front to back temperature differences. This worked well which is a big help in getting to the goal since the goal is based on the hottest running cylinder.

    Next I began to look for ways to increase the differential pressure across the cylinders. This has been a very time consuming job. I have tested a wide variety of fixed cowl flaps on my Carbon Cub. There is a lot going on aerodynamically around the lower cowl so this job has been tougher than I thought. I have now settled on a fixed cowl flap with a 4" lip and a 20 degree aft pitch. This prototype cowl flap is made out of 5 pieces of aluminum and attached to the aft cow lip with 5 screws. Aluminum tape covers the flap to block out voids between the aluminum pieces. It is not pretty but this is a prototype. I expect the final part to be done in carbon fiber to match the cowl.



    This version of the fixed cowl flap increases differential pressure about .7" of water from the base line of around 2" of water during an 80hp cruise. So the fixed cowl flap increases differential pressure across the cylinders by about 30%. This resulted in a significant decrease in CHT's.

    In my opinion, the oil cooler on the Carbon Cub is actually too large for the motor given in inlet air available to the plenum. The cross sectional area of the oil cooler is 25 square inches. Most of the face of the oil cooler is exposed to the inside of the plenum robbing important cooling air from the cylinders across an oil cooler that has been keeping the oil too cool. So the oil cooler is grabbing too much air from the plenum. Once I increased the differential pressure in the plenum with the external cowl flap, the oil temperature dropped even more because the same increased differential pressure was increasing the efficiency of the oil cooler.

    So my next task was to begin taping of parts of the oil cooler. Limiting airflow across the oil cooler results in two improvements. First it raised the oil cooler temperatures up to a range of oil temperatures more appropriate to the proper running of the engine (ie > 180 dF). But most importantly, taping off the oil cooler blocks cooling air normally flowing through the cooler instead diverting the air to the cylinders by raising the differential pressure across the cylinders even higher. I currently have 4.8" (out of 12") of the oil cooler blocked. This has raised the differential pressure another .5" of water. The differential pressure across the cylinders with the fixed cowl flap and the oil cooler taped is now 3.2" at 80 hp in cruise up from the baseline of 2" of water when I started. More importantly in a full power climb at 90 mph the differential is 4.4" of water which has significantly enhance cooling across the cylinders in full power climbs.

    So how does it work flying? Today in southern California is a typical July day. OAT's aloft are very high, about 37 dF higher than a standard ISA temperature atmosphere at 5,000'. Below is a screen shot of my Dynon after 5 minutes of a continuous full power climb at about 90 kts IAS today. This is a cruise climb not a climb at maximum rate. The standby airspeed gauge is measuring differential pressure in MPH across the cylinders (see conversion to inches of water in first post). The OAT at 5,000' is 77 dF. In this test the inversion actually peaked at 83 dF at 3,000'. The maximum CHT had stabilized at minute 4 in the climb at 411 dF as well as the oil temperature at 205 dF. So I could have climbed several more minutes at maximum power and maintained around the same engine temperatures, using the ECI CHT temperature recommendations.



    In cruise the temperature profile is much better even though the OAT was 82 dF at 2,600'. The screen shot below was taken with the engine developing 80 hp (max continuous LSA power). Maximum CHT is cool at 369 dF while the oil is running at 205 dF with the differential pressure around 3.2" of water (81 mph).



    This project is far from complete. There are several other modifications to the cowl system that will allow for even lower temperatures while running at higher power settings. Other owners and CubCrafters are working on those fixes.

    My testing to date confirms the Carbon Cub can operate in high summertime temperatures restricted to the POH LSA power limitations with acceptable CHT's and oil temperatures with the addition of a fixed cowl flap, oil cooler flow restriction and blocking the forward faces of cylinders #1 and #2 for better temperature distribution across the cylinders.

    I am now looking for a source to fabricate the fixed cowl flap as a carbon fiber composite. The cowl flap will be easy to install just requiring 7 screws and no more than one man hour. If anyone knows of a source for carbon fiber fabrication let me know.
    Last edited by turbopilot; 07-19-2010 at 07:51 AM.
    Bob Anderson, CC11-00435, N94RA

  6. #16
    Senior Member RanRan's Avatar
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    Default Re: Cooling the Carbon Cub

    Quote Originally Posted by turbopilot View Post
    While my presumption is that the cowl outflow issue is the biggest problem...
    I agree. It needs to be larger. Or add periphery cowl vents at just the right places. Pressure in - Pressure out. MOVE the air. Quickly. That is the key, isn't it?

    Question: Have you tried running with the engine cowling off? It would serve as a benchmark, if nothing else.
    Last edited by RanRan; 07-17-2010 at 08:03 PM.

  7. #17
    Junior Member jmpstl's Avatar
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    Default Re: Cooling the Carbon Cub

    Just a data point: In #133 I've been seeing the same sort of temperatures as others posting, but have been too busy with other aspects of getting the new plane settled in at home (such as learning how to land it more predictably!) to be systematic about it. From memory: my hottest cylinder is reliably about 40-50 degrees warmer than the coolest, and in the hot St. Louis summer I'm struggling to stay below 400dF at rated RPM on the #4 cylinder (#3 is always 20dF cooler and #2 about 10dF less; #1 will be at 10dF cooler still). Oil temperatures run between 180-190, so are not a problem.

    Under identical atmospheric conditions and altitudes (same day and time, but not same speeds, obviously), my Cirrus SR22T runs with all cylinders even at 340dF at 65% power, so I am hopeful that something can be done to cool the Cub down a bit. Thanks to CubCrafters and the forum members for working on this. It is a beautiful thing and a joy to fly.

    Michael, N551BP, N552BP

  8. #18
    Senior Member turbopilot's Avatar
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    Default Re: Cooling the Carbon Cub

    Quote Originally Posted by jmpstl View Post
    Just a data point: In #133 I've been seeing the same sort of temperatures as others posting, but have been too busy with other aspects of getting the new plane settled in at home (such as learning how to land it more predictably!) to be systematic about it. From memory: my hottest cylinder is reliably about 40-50 degrees warmer than the coolest, and in the hot St. Louis summer I'm struggling to stay below 400dF at rated RPM on the #4 cylinder (#3 is always 20dF cooler and #2 about 10dF less; #1 will be at 10dF cooler still). Oil temperatures run between 180-190, so are not a problem.

    Under identical atmospheric conditions and altitudes (same day and time, but not same speeds, obviously), my Cirrus SR22T runs with all cylinders even at 340dF at 65% power, so I am hopeful that something can be done to cool the Cub down a bit. Thanks to CubCrafters and the forum members for working on this. It is a beautiful thing and a joy to fly.

    Michael, N551BP, N552BP
    Unfortunately, the Carbon Cub will not run LOP like our SR22TN's. That would really help with temperature management. It is very interesting to fly these airplanes back to back. SR22TN stick in the left hand, throttle in the right. Carbon Cub stick in the right hand, throttle in the left. Talk about switch hitting.
    Bob Anderson, CC11-00435, N94RA

  9. #19
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    Default Re: Cooling the Carbon Cub

    As you can see in the image I uploaded, I'm climbing out at full power, about 4-5 minutes into a climb, in cool (53dF) air, around 82 mph. The back cylinders are heating up nicely, while the front ones are reasonable. In other climbs, I tried to pitch down, which brought the flow in more evenly, but the temps still rose (albeit more evenly) to the 415+ range. I've been told numerous times that the redline is 450, you can climb out at 425+, etc, but I just don't feel that great about it. Also factor into this the fact that at an OAT in the 40s and 50s, when you do back off power, the temps come down quickly - so quickly I worry about this too!

    Bottom line - Can we somehow create an operable cowl flap? somewhere on the lower cowl?
    Attached Images Attached Images

  10. #20
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    Default Re: Cooling the Carbon Cub

    Quote Originally Posted by turbopilot View Post
    Unfortunately, the Carbon Cub will not run LOP like our SR22TN's. That would really help with temperature management. It is very interesting to fly these airplanes back to back. SR22TN stick in the left hand, throttle in the right. Carbon Cub stick in the right hand, throttle in the left. Talk about switch hitting.
    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........

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