Looking for a fix- high CHTs on SQ2

stewartb said:

What’s inside that bottom fairing? Is anything faired inside or is it open? I don’t think a Cub’s cowl cheeks are magic but they are in airflow that effectively draws warm air out. Given how you’ve baffled behind the prop I’d say your inlet air looks like a typical Cub but the outlet air is very different. That’s where I’d focus my efforts. If your side scoops are still there I bet you could close off that bottom outlet and see improvement. It’s easy to visualize that rear area in the lee of that large ramp profile being in an eddy. It may not be but with those scoops there it would be easy to test.

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Good points, the area within the lower air exit or tunnel, houses the carb airbox and skat tubing from muffler to carb heat/airbox, also, the exhaust mufflers, one each side, are just behind and below the rear two cylinders it is a Vetterman exhaust system. It is rather congested in that area, but still plenty of room for air to exit in my opinion. There really isn't anything I could do without major changes to alleviate the way this is. The lower firewall behind the tunnel has no flange, I removed the flange in the hope of allowing a smother transition for air to exit overboard. Again, no change. The lip or fairing on the tunnel, which I added, seemed to help some, I've tried it both ways and even tried different sizes and the one on there now seems to work best, temperatures are worse without the fairing. I agree with your assessment that the problem lies with the exit air side of things.

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That is a very different design from the first generation of SQ cowling, I'm guessing it was a change made because of problems like I'm having.

stewartb said:

The Mackey version of the lower cowl isn’t coved. It’s square on the back. More interesting is how streamlined it is at the front.

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I've flown my SQ some more and the timing at 20 degrees makes things run much cooler. It is a lot more enjoyable flying the plane when I'm not staring at the two rear cylinders CHT bars bright red almost all the time. All of my manometer measurements I've done show that the upper area has sufficient pressure and the issue was in the lower area. It was always zero or even slightly positive on every test. I think that the root problem is related to the engine being lowered on the firewall. This is a great feature (and was a major selling point for me). It allows the pilot to see directly over the front of the plane (it provides much better over the cowl view than my Cessna 206). But having the engine lower seems to cause real airflow issues around the bottom end.
I'll spend this spring patching up my cowling, filling in holes I've made, replacing temporary stuff, and painting the front end. I might try 22 or 23 degree timing, but I'm happy with the performance at 20.

It's an odd mod but maybe an exhaust augmenter would help? Just a generic screen grab. And more: https://velocityxl.bdfserver.com/exhaust-augmentors/

Gary

Wish you guys lived closer. I'd like to have a few days to play with these airplanes.

Got to be honest- first time I saw this cowling I would have bet a steak dinner it would run hot. I understand the thought process behind an inlet area that is so large but unfortunately, that's not how aerodynamics work. The inlets are exponentially larger than they need to be. You can add all the ramps, plenums, widgets, gadgets and tricks you can dream up and it will yield marginal results at best. The outlet can't flow what the inlet brings in for cooling. The outlet makes matters worse by creating very little negative pressure due to the location and delta between it and the lower boot cowl.

Most lower cowlings extend back to the firewall or at least close. I think that helps reduce up flow or rolling reverse flow into the exit opening.

Edit: An earlier discussion> https://www.supercub.org/forum/showthread.php?56821-Lower-cowl-scoop-shape

I'd tape some yarn (non-combustable) to both the lower cowling and firewall lips. Mount a camera on the gear cabane. Go fly. Then note the airflow near that cowling exit area. Adjust the outlet size and shape to smooth out the airflow if possible. If the yarn hits the hot exhaust expect some grilling action.

Gary

BC12D-4-85 said:

It's an odd mod but maybe an exhaust augmenter would help? Just a generic screen grab. And more: https://velocityxl.bdfserver.com/exhaust-augmentors/

Gary

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I can't supply an engineering source, however I do know there are several considerations in the design of augmenter exhaust systems. Such as the diameter of the exhaust pipes, the diameter of the augmenter, the length of the augmenter, the relationship of the exit of the exhaust to the inlet of the augmenter, etc. Lots to consider.

Rick,
Do you know which oil cooler you're running (how many rows, etc.)? Legend Cub is suggesting two of the 8-row 10599R Aero-Classics He Series Aviation Oil Coolers | Aircraft Spruce
for our low compression Aero Sport IO375, but that seems excessive to me.
How have things turned out for you with CHT management?
Johnny

rotto789 said:

Hi Stewart, Engine is an Aero Sport 0320B2B, oil temp runs around 200 or slightly above on hot summer days, but for the most part oil temperatures are within normal range below 80 degrees fahrenheit outside temperatures.
Rick

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I'm running an AeroSport Power IO-375 B1B. Running LOP makes a HUGE difference. Have to do climbs to altitude very, if not full, ROP. Then lean out and excellent CHT's. But, as noted elsewhere, I've modified my SQ-2 cowl extensively too.

I have a single 13-row oil cooler. Oil never exceeds 220 even on hottest days.

JohnnyR said:

Rick,
Do you know which oil cooler you're running (how many rows, etc.)? Legend Cub is suggesting two of the 8-row 10599R Aero-Classics He Series Aviation Oil Coolers | Aircraft Spruce
for our low compression Aero Sport IO375, but that seems excessive to me.
How have things turned out for you with CHT management?
Johnny

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Hi Johnny,

Sorry about the late reply, I have done a lot of fooling around modifying my cowling and plenum and I still have higher than I would like CHTs but with what I have done it is much more manageable. my oil cooler is a single 8 row (not aero classic HE) mounted on the back of number 4 cyl and I can see some oil temps nearing 240-250F when the outside temps are over 80F I would probably benefit from a larger oil cooler, I can usually manage oil temps with power and mixture settings as well. Like Ted says in his post, I to, have to climb at full throttle to engage the carburetor fuel economizer valve for best fuel flow to cool the cylinders until I reach altitude and than I lean for best cyl temperatures trying to even them out across all four while watching EGTs and not exceeding 1430F on any one cylinder. at least this is how I do it and I am no expert. When outside temps are 70F or below I have no problems with CHTs and fly with normal power and sometimes LOP mixture settings without any worries.

Rick

rotto789 said:

my oil cooler is a single 8 row (not aero classic HE) mounted on the back of number 4 cyl and I can see some oil temps nearing 240-250F when the outside temps are over 80F I would probably benefit from a larger oil cooler, I can usually manage oil temps with power and mixture settings as well....while watching EGTs and not exceeding 1430F on any one cylinder.
Rick

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Here are some pictures of my rear mounted 8 row oil cooler (just an old one I had laying around) on my IO-360 engine. The oil temperatures never exceed 190* which is the vernatherm setting. Take notice of the baffle which is up tight against the back of the cylinder. This baffle prevents heat from that cylinder from going through the oil cooler and routes the cool air which is above the engine down into the cooler. The oil cooler is also tilted back for fluid flow reasons which I'll not delve into here. Also, Piper uses a baffle around the rear of the carb air filter to prevent ram air from entering under the engine. I left that baffle off, which allows cool ram air to circulate around the oil sump further cooling the oil.



The only time the CHTs exceed 400* is on a hot summer day when I get too aggressive in leaning. Richening the mixture 1 GPH solves that issue. Normally I burn 8.2 GPH at 22.5" and 2400 RPM. EGTs, you are aware there are no high limits on EGTs? EGTs rise as you lean the mixture until they reach their maximum. Then as you continue to lean they cool down. The only reason to notice the maximum is to use it for a reference at which to either richen or lean to 100* less than the maximum ... whatever it is. The early Alcor EGT instruments had no numbers at all on them. Only lines showing an amount of separation.

Keep in mind not to restrict the hot air openings below and the sides of the engine. If those outlets don't let out the hot air, the engine can not cool down.