Engine Cooling Revelation

[Battson]

Great post.

I was surprised to see the louvers were making it worse!! Very surprising.

Our cowl flaps have proved very effective, as Whee said - we leave ours closed most of the time, but wide open on hot summer days. Our new cowl flaps are over 140 sq.in when fully open.
It's the opening area when the tunnel meets the firewall which matters most.
Having a smooth tunnel lip makes a big difference too.

We have the same engine as yours.

[John Bickham]

Haven't got to this point yet but have a question re use of cowl flaps. So instead of louvers or the weight of adjustable cowl flaps, would fixed and adjustable cowl vents like on Carbon Cubs be a viable option? The Carbon Cub has cowl cheek vents on both sides that have four fixed positions to choose (four season adjustments). I wont know till I put her on the scales but I've attempted to build light. If I encounter cooling issues I would want the lightest fix that delivers the needed results.

[swpilot3]

I think that would be fine as long as you're careful and plan power reductions well. Yesterday my wife and I flew to Sedona. I kept the CHTs at 380 in cruise but from the downwind through final with the power reduced the CHTs cooled to 340. I don't like them cooling that fast. In that particular instance, I would have wanted to close the cowl flaps from the cockpit. Mine is still fixed at the moment.

[John Bickham]

Thanks Bobby. That is a good point to consider. I'm a student of Mike Busch too. I need to go back and review if there was mention of rapid cooling of CHT's.

[swpilot3]

Excellent commentary everyone! Thank you for all the great additional information!

I’m not sure why my louvers didn’t work. Perhaps they were just poorly designed like svyolo pointed out. I’ve never seen louvers as large as mine, but I figured that louvers were usually smaller to minimize cooling drag. I researched the optimum angle for a louver and made them very large to penetrate the boundary layer and see if they made a difference or not. Perhaps I exceeded some diminishing return point.

Carl gave me the idea to put a scoop on top of the cowling that fit into the oil access door... just to see if it made any difference. It added 10.6” of additional inlet. It resulted in 0.23” less differential pressure and my CHTs seemed to be a few degrees hotter on average.

I’m beginning to feel a little like Thomas Edison. I’ve found 1,000 ways not to cool an engine! :-)

Without scoops or louvers, the only place that air can come from is the inlet to the tunnel outlet. My hypothesis is that air wasn’t going the direction I intended with the scoop or louvers. I did put tell-tale yarn around each. But it was so chaotic that it was really hard to tell what was happening.

Here’s the video: https://youtu.be/fcFGrSyavjY
Kinda makes you wonder how chaotic the air is as it enters the cowl through the normal inlets!

IMG_3614.jpeg

[zkelley2]

Excellent commentary everyone! Thank you for all the great additional information!

I’m not sure why my louvers didn’t work. Perhaps they were just poorly designed like svyolo pointed out. I’ve never seen louvers as large as mine, but I figured that louvers were usually smaller to minimize cooling drag. I researched the optimum angle for a louver and made them very large to penetrate the boundary layer and see if they made a difference or not. Perhaps I exceeded some diminishing return point.

Carl gave me the idea to put a scoop on top of the cowling that fit into the oil access door... just to see if it made any difference. It added 10.6” of additional inlet. It resulted in 0.23” less differential pressure and my CHTs seemed to be a few degrees hotter on average.

I’m beginning to feel a little like Thomas Edison. I’ve found 1,000 ways not to cool an engine! :-)

Without scoops or louvers, the only place that air can come from is the inlet to the tunnel outlet. My hypothesis is that air wasn’t going the direction I intended with the scoop or louvers. I did put tell-tale yarn around each. But it was so chaotic that it was really hard to tell what was happening.

Here’s the video: https://youtu.be/fcFGrSyavjY
Kinda makes you wonder how chaotic the air is as it enters the cowl through the normal inlets!

IMG_3614.jpeg

I think your louvers being so large were doing a lot of what you saw that scoop do, create a lot of turbulent flow. You want laminar flow over the louvers in one direction at high speed to create lower pressure outside them. If the flow over the louvers is turbulent it's being slowed and won't have the lower pressure. It's possible it could have been spilling over those and going in! Given your data, I'd say that is likely. Look at the louvers on the 206s and 207s. Sould give you a good idea of what it should look like. With the cowl flaps closed, the louvers are almost the only outlet, so they are very efficient.

At the inlet, ideally the flow would also be laminar. That's why you've seen modern cowls go to more round and gradually receding inlets. No sharp corners or edges. They all did wind tunnel testing with tufts I'm sure to get a laminar flow into the cowl.

If you really want to see what's going on, put tufts on the inlet and then inside the cowl and on the fins, place a gopro inside.

[swpilot3]

I purchased a cheap knock-off GoPro just for that purpose. Just in case it melts! :-)

[swpilot3]

That makes sense. Originally before I built those louvers I used some that I purchased from Aircraft Spruce. That was before I was keeping good records of numbers, but they didn't seem to help my high CHT problem.

I looked at some pictures of 206/207. They have, for lack of a better term, inverted louvers. They go in rather than out. Does anyone have good insight into if perhaps that's better?

image_7394.jpg
Here's the one I purchased from Spruce.

[svyolo]

I have noticed a few pictures with inverted louvers as well. I have no idea which is better. Even inverted they might provide some negative pressure due to "motive flow", if that is what they call it.
Try 'em both ways and see which works better.

[zkelley2]

I've seen them both ways. But all the Cessnas and Beeches have them as you describe. I just went and looked real quick and even the King air has them "inverted" vs what you did.

Here's a photo of a Bonanza cowl.

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Cessna 205
440px-Cessna205C-GHOR03.jpg
Cessna 180
ff7f6cd18c7122a9423dfd2a.jpg

[Nev]

Carl gave me the idea to put a scoop on top of the cowling that fit into the oil access door... just to see if it made any difference. It added 10.6” of additional inlet. It resulted in 0.23” less differential pressure and my CHTs seemed to be a few degrees hotter on average.

The scoop might be reducing the dynamic pressure from the normal air intake by feeding air to the rear of the cowl area.

Neat what you’re doing with the experimentation. Keep posting results !

[swpilot3]

I tested moving the rear baffle wall back. I moved it back 1/2". It certainly has a very quantifiable effect! Cylinders 5 and 6 used to be the hottest, now are my coolest! They dropped by about 30 degrees! However, the other cylinders suffered a bit. Number 2 is now the hottest and 20 degrees hotter than it was before. Numbers 2 & 3 are also 10 degrees hotter than before. Number 1 is about the same.

The differential pressure went way down with this setup.
92kias = 3.82" (0.63" less than before)
105kias = 4.97" (0.93" less than before)

So, it appears that moving the baffle wall really does help cool the back cylinders better but at the expense of the other cylinders. Makes sense. I just overshot a bit. I'm going to try a 1/8" gap now. I read on a few RV forums that they just put a washer between the cylinder and the baffle and that did the trick for them.

[swpilot3]

Cylinder #2 is still the hottest... however, I noticed it flucuating about 20 degrees after landing. I think that thermocouple has gone bad (I've had this before). I ordered a new one. It might not be as hot as its indicating.

[John Bickham]

Thanks for update. Good info. Besides replacing CHT probe, future plans?

[swpilot3]

My plans now are to re-build the fixed cowl flap into a controllable one. Then, in no particular order, I'm going to try inverted louvers, build a carbon fiber plenum for the baffling, and putting a go-pro inside the top cowl to watch the inlet tell tales.

[svyolo]

Something from my non-aviation distant past. If I remember right for a manifold to function as a equal pressure manifold the manifold needs( at an absolute minimum) to have double the volume of the sum of the outlets. If not, you end up with each outlet having different pressures/flow rates. The bigger the manifold compared to the outlets, the better. Without that you end up having to control the flow to each outlet to equalize the flow rates, if that is a requirement. It was for what I was trying to fix.

I am not sure that helps you fix it, but it might help to understand the problem at hand.

[swpilot3]

This is pretty interesting. So I would assume that the cowl under the engine would be the "pressure manifold" in this case? Did I understand that right?

[svyolo]

Yeah too much exit (flow between cylinders) volume are compared to manifold volume. We are stuck with that ratio as we are not going to build a bigger cowling. I know RV-10s and Rockets have a couple of little tricks to equalize cylinder cooling but I can't remember what they are at the moment.

[svyolo]

I just saw a couple of other aircraft that had "inverted" louvres. My guess to try what works better (right side up or upside down) might not be just a bit of jest.

[zkelley2]

Ran across this today with the Van's louvers. They are also completely opposite of what you have.

You should flip those louvers around so they are oriented like all the other stuff out there and take a measurement.

cat-med_cowl-louver.jpg

FP19082019A0001F.jpg


Here's some on a bearhawk mounted what appears to be the correct way.

100_8944.jpg

[swpilot3]

Thank-you! I do plan to flip the louvers to see what they do. I've already patched and riveted where they used to be, but I think it's worth investigating further.

[jaredyates]

This thread is just so fantastic!

[svyolo]

I was looking at various pics to see how different GA companies dealt with engine intakes and cooling. I looked hard at the Mooneys as they are fast not because they are overpowered, but they are low drag. Most of the newer Mooney's look like they intake all or most of their air from the two cowl cooling intakes. There are a couple that have a large NACA duct on one side. I am guessing that is either for an oil cooler, or possibly a turbo intercooler. I am talking specifically the 201-231-252 models. I haven't looked at the really new ones. The 252 in particular is supposed to cool really well, including the turbo intercooler.

I am kind of wondering if the problem sometimes is leaking air from the air intake system into the lower cowl. It seems like we always concentrate on what is going on above the engine for leaks, but It seems to me leaking air into the lower cowling from outside is also a problem.

Bobby's louvre dilemma might add credence to this. For some reason his louvre design was drawing air in and reducing the pressure differential from the top. Wouldn't a leaky carb air intake scoop do the same?

[swpilot3]

I've had that same concern in the back of my mind. On mine, I have silicone baffling material sealing against the filter frame. I will take a good hard look at it and see how well it's sealing.

As a very interesting side note, during my cowl flap design modifications, I ended up with a 2"x6" gap at the leading edge of the cowl flap on the bottom of the cowl. I was going to seal it but decided to put some yarn tell tales around it and see what the air was doing.

What would you think? Would air be drawn out of the cowling? Or would it be sucking air in? Take a look at the video for the answer, not what I expected. https://youtu.be/Yw0g1niN9lo

[svyolo]

I would say too many variables for me to take a guess. I would go the same path as you and experiment.

[svyolo]

Which way does your the baffle material on your air filter bend? I would think it needs to bend forward so the pressure seals it. Maybe some Gorilla tape (or a pop rivet or two) to make sure it can't move?

[swpilot3]

It's sandwiched between the filter bracket and the cowl. So the pressure does seal it. Taping is a good idea. I'll take some differential pressure measurements before and after.

[swpilot3]

Sealing that gap at the leading edge of the cowl flap increased the differential pressure by 0.2" of water.

[zkelley2]

Could you not just install the cowl flap so that the material that is used to rivet it to the lower cowl is inside the cowl instead of outside?