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Looking for a fix- high CHTs on SQ2

tedwaltman1 said:
I’m not quite sure what “mapping my leaning authority” means. If I get to say 50 degrees LOP or leaner my CHT’s go down. But 99% of the time I run about 50 ROP. Last flight, 70 degrees OAT, 2050 & 20.5”, 7900’ density altitude, running at about 390 CHT on all 4 cyl.
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tedwaltman1 said:
Thank you for the description of “leaning authority.” I have an injected engine, so no carb to fiddle with.

i will confuse to experiment w airflow changes, though the internal vane mentioned and pictured in my PDF is my last, current at least, idea.

As you say, high Hp and slow speeds are ultimately a problem. Couple that with starting density altitudes exceeding 7,500’ even at dawn and one has a hot engine!
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Ted, can you get a 150 degree EGT rise from full rich when you lean the mixture? If not perhaps you should consider having your nozzles reworked by these folks. https://airflowperformance.com
This is their procedure for tuning the nozzles. https://airflowperformance.com/wp-content/uploads/2015/12/Nozzle-Tuning-Instructions.pdf

" 2050 & 20.5”, 7900’ density altitude, running at about 390 CHT on all 4 cyl." This is a very low power setting for such a high CHT. Fuel cools.
 
Yes, I can get at least, likely more, than 150° EGT rise from moderately rich (never tried full at my typical operating altitudes). I'd love to get my nozzles optimally tuned. Thank you for the PDF link to the tuning test procedures. I will do that for sure. Yes, should have done it LONG ago! FWIW, my CHT are generally within 10 to 12° of each other throughout any power setting combinations.
 
Another thing Ted, Airpower's modifications to the nozzles involves removable orifices. Thus you can mix and match to your heart's content. You can even get a collection of different sizes for not much extra money. I did this to mine and am very pleased with the results. I operate strictly at sea level with a much higher percentage of power than you reported, so we can't directly compare your and my experiences. Only on hot summer days do I ever get a CHT exceeding 400*. Then I just richen the mixture a little to get it below 400.
 
d_kandle said:
Update on progress. I decided to go with pressure measurements as they are more direct than temperature (which is a result of air pressure). I'm using a 2 probe electronic manometer. A couple of things I found - first I can change the open areas (areas not sealed on top) by 4 sq inches and about zero difference. That lead me to look at pressures below the engine. What I found is that I had 1.5+" (i.e. higher than ambient) of pressure below the engine. Not much point in trying to get higher pressures above the engine until I stop the bottom from fighting movement. I added gills on the bottom (used the same geometry as the gills on my Cessna 206). That helped some, but still positive pressure. So I started shrinking the opening on the bottom. My thinking is that the large vertical gap on the bottom of the cowling is causing the air to become turbulent in front of the firewall and is pushing air into the cowling instead of pulling it out. The more I closed up the bottom the better things got. I have a few more changes to make but my goal now is to get at least some negative pressure (at least not positive) below the engine. It is clear that the pressure below the engine is the problem, not the lack of ram air pressure on top. The SQ may have a unique issue here because they lowered the engine to improve visibility over the engine. Thus, the vertical gap between the bottom of the cowl and the boot cowl is quite large.
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Been thinking on this. Adding lips to rear, then closing off from bottom firewall down to near lip with maybe 2” gap left... thinking I will make the top part adjustable, to make gap larger or smaller(cable? At least for testing).

I think that opening is to big for it to form a vacuum at rear of cowl and suck air from INSIDE of cowl, like normal planes. (Most have small openings) I think it’s sucking dead air down there instead of pulling it from inside cowl.... (is this what skywagon8a meant)

I’m not sure I understand your measurements, as higher vacuum is a larger number? Just so we are both talking the same?? Or ami misunderstanding your measurements????

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mike mcs repair said:
I think that opening is to big for it to form a vacuum at rear of cowl and suck air from INSIDE of cowl, like normal planes. (Most have small openings) I think it’s sucking dead air down there instead of pulling it from inside cowl.... (is this what skywagon8a meant)
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What I was mentioning was with a large outlet opening, the air passing outside the bottom cowl would hit the lower front section of the firewall. When it does this it can swirl back up into the engine compartment. When the air hit the firewall it creates a swirling motion which can provide a sort of air dam which restricts the normal out flow. Judging by your picture, I don't think this would be happening in your case.

That section below the basic cowl is a strange shape. If that runs hot I think I would extend the whole lower section further aft, at least as far as the firewall. I believe that it may draw better. A bit more like an augmenter.
 
d_kandle said:
I've got at least two more tests to do. I take some pictures soon and post. Nothing fancy. I'm making changes one step at a time to be sure that the numbers are always moving in the right direction.
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it's been 2 weeks.... any updates on your tests?
 
I've made a bunch of changes to the cowling, but none of them seems to make a big change in the pressures or cooling. I thought that adding some scoops on the top of the cowl would make a big difference, but they didn't. I've been using a long pressure averaging tube to obtain my pressure readings and I think I'm going to change to a much smaller tube (something more like 1 sq inch). Just big enough to average out any pulses which might make the measurement device inaccurate.
I put two scoops on the top. Due to the location of the stiffener in the upper cowl, I had to make the openings farther forward than I wanted. My next change will be to build something to direct the air entering the scoop aft more over the rear cylinders.
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The other change I made was to add gills on each side. This made the biggest change.
IMG_5583.JPG

I also enclosed more of the opening at the bottom of the cowl. Changes in this area varied in impact. Moving the opening aft helped to a point but then moving all the way aft of the firewall seemed to make things worse.
I have a 70+ degree difference between the front and rear cylinders. Adding 14 sq. in. of frontal opening (with the two scoops) made no difference. That doesn't seem logical to me. I'll probably run some tests where I measure pressure difference between the front and rear of the upper and lower decks.
 

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Interesting article to add further mystery to cowl airflow:
(Forgive me if I’ve already posted this)


“I spent over a decade doing intensive research into baffle design and cooling. Everything we learned was directly opposed to what I had learned in A&P school and was frequently contrary to logic.


We instrumented for testing by having six CHT probes on every cylinder evenly spaced circumferentially to be able to see how even the temps were around the cylinder. We placed safety wire through the cowl with dozens of yarn tufts attached with a movable from the cockpit lipstick camera so we could watch what the airflow actually does. Everything we observed was contrary to what we knew to be true about air movement under a cowl!


In the interest of brevity, here are the Cliff Notes on what we learned:


1) the airflow moves into the front of the cowl along the far outboard edges of the cowl opening ( about the outer 2”). It then passes along the outer edges of the cowl or plenum until it hits the back plate. Upon hitting the back plate it turns inboard and hits the airflow from the other side. These two air masses become one and pass forward along the engine spine and out of the cowl behind the low pressure area behind the spinner and over the top of the cowl and over the wind screen. This is why you get oil on the windscreen when you have an upper deck oil leak. The tufts placed on all but the outer 2” of the cowl opening pointed directly TOWARDS the prop!!


2) Along the way, about 25-30% of the air that came into the cowl becomes turbulent enough to work its way through the cylinder fins to the lower pressure area of the lower deck.


3) There is way too much air entering the cowl than is used for cooling and it’s passing out of the front adds a lot of drag.


4) unless you can test the results of baffle design changes, you have no clue what effects you are having. Logic serves no function. You need to have six CHT probes on each cylinder.


5) the worst problem is the creation of hot spots on cylinders which make them run out of round. A single probe system cannot tell you these things. In applications where a particular cylinder is the most common one to fail, it almost always is running out of round. It’s better to have a hotter, ROUND cylinder than a cooler egg-shaped one.


6) creating a highly efficient cowl design is a tremendously difficult task. The bad news is that many of the truly genius ideas we had to address problems made them worse rather than better no matter how logical the notion. There’s that pesky testing thing again.


Did I mention that unless you can test your work in detail, you have no clue what the result of the changes are. (Aargh)


A truly efficient cowl design was so complex as to make the effort not worth the time and the cash expenditure not sensible.


I’m better off putting that time and cash towards fuel and having a good time.
 
Oliver said:
Interesting article to add further mystery to cowl airflow:
(Forgive me if I’ve already posted this)


“I spent over a decade doing intensive research into baffle design and cooling. Everything we learned was directly opposed to what I had learned in A&P school and was frequently contrary to logic.


We instrumented for testing by having six CHT probes on every cylinder evenly spaced circumferentially to be able to see how even the temps were around the cylinder. We placed safety wire through the cowl with dozens of yarn tufts attached with a movable from the cockpit lipstick camera so we could watch what the airflow actually does. Everything we observed was contrary to what we knew to be true about air movement under a cowl!


In the interest of brevity, here are the Cliff Notes on what we learned:


1) the airflow moves into the front of the cowl along the far outboard edges of the cowl opening ( about the outer 2”). It then passes along the outer edges of the cowl or plenum until it hits the back plate. Upon hitting the back plate it turns inboard and hits the airflow from the other side. These two air masses become one and pass forward along the engine spine and out of the cowl behind the low pressure area behind the spinner and over the top of the cowl and over the wind screen. This is why you get oil on the windscreen when you have an upper deck oil leak. The tufts placed on all but the outer 2” of the cowl opening pointed directly TOWARDS the prop!!


2) Along the way, about 25-30% of the air that came into the cowl becomes turbulent enough to work its way through the cylinder fins to the lower pressure area of the lower deck.


3) There is way too much air entering the cowl than is used for cooling and it’s passing out of the front adds a lot of drag.


4) unless you can test the results of baffle design changes, you have no clue what effects you are having. Logic serves no function. You need to have six CHT probes on each cylinder.


5) the worst problem is the creation of hot spots on cylinders which make them run out of round. A single probe system cannot tell you these things. In applications where a particular cylinder is the most common one to fail, it almost always is running out of round. It’s better to have a hotter, ROUND cylinder than a cooler egg-shaped one.


6) creating a highly efficient cowl design is a tremendously difficult task. The bad news is that many of the truly genius ideas we had to address problems made them worse rather than better no matter how logical the notion. There’s that pesky testing thing again.


Did I mention that unless you can test your work in detail, you have no clue what the result of the changes are. (Aargh)


A truly efficient cowl design was so complex as to make the effort not worth the time and the cash expenditure not sensible.


I’m better off putting that time and cash towards fuel and having a good time.
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that's very interesting, about the air going out behind the prop.... but it makes sense.... think of like a 140 nose bowl, small openings outboard only.... and not huge exits......

would love to see the videos he is referring to...
 
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Thank you for input on recent tests Doug. And your thoughts/experience much appreciated too Oliver.

Std certified Cubs have cowl "cheeks." I'm thoughtfully optimistic that the cheeks I added to my SQ-2 did indeed make a difference. Another friend with an SQ-2, and same exact engine as mine, built a plenum inside his cowl. That interestingly enough did not make any difference at all for his cooling.
 
tedwaltman1 said:
Thank you for input on recent tests Doug. And your thoughts/experience much appreciated too Oliver.

Std certified Cubs have cowl "cheeks." I'm thoughtfully optimistic that the cheeks I added to my SQ-2 did indeed make a difference. Another friend with an SQ-2, and same exact engine as mine, built a plenum inside his cowl. That interestingly enough did not make any difference at all for his cooling.
Click to expand...

should be relatively easy to test closing off front center of cowling and just leaving maybe 8" at ends open.... maybe try a couple sizes of openings, or make it controllable with a cable from inside....
 
d_kandle said:
. I thought that adding some scoops on the top of the cowl would make a big difference, but they didn't.
View attachment 50223

Adding 14 sq. in. of frontal opening (with the two scoops) made no difference. That doesn't seem logical to me.
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Im thinking like the upper cowl like a bucket that is ALREADY FULL of water(air). Adding more(scoops) just spills more overboard..... and directs flow out front like the other post mentions....


so blocking the center opening off to limit the inlet size/volume might be helpful or at least a good test.
 
Today I added a deflector just in front of the scoops which goes from the cowling (upper) down to the case and extends across about the middle 1/2 of the engine. That should force any air coming into the scoops aft and not allow much air to move forward near the center of the engine. As soon as our temps gets below 100 here I'll so some test flying.
 
At this point I can say Oliver is correct. I added a dam on the upper cowling to block air going forward from the two scoops I had already put on top. It made a huge difference. I did the two sides slightly different (by mistake but I'll say it was on purpose). On the more complete covered side I got a 60 degree improvement. On the less covered side 30-40 degrees. Adding the scoops alone did nothing. But blocking the air from coming forward of the scoops seems to do the trick. The photograph is looking in the front over #2. I'll probably never know if the same thing could be achieved without the scoops as they are already there (better is the enemy of good enough).
At this point I'll probably just clean things up a bit.
CowlChange.jpg
 

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Be interesting if someone will try blocking front middle off at cowl itself. just leaving openings outboard in high pressure area. Bet the effects will be same as your air dam. Thanks for reporting your results back.


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d_kandle said:
At this point I can say Oliver is correct. I added a dam on the upper cowling to block air going forward from the two scoops I had already put on top. It made a huge difference. I did the two sides slightly different (by mistake but I'll say it was on purpose). On the more complete covered side I got a 60 degree improvement. On the less covered side 30-40 degrees. Adding the scoops alone did nothing. But blocking the air from coming forward of the scoops seems to do the trick. The photograph is looking in the front over #2. I'll probably never know if the same thing could be achieved without the scoops as they are already there (better is the enemy of good enough).
At this point I'll probably just clean things up a bit.
View attachment 50426
Click to expand...

You could also do a quick test by taping over new scoops to block them off. I bet there will be no difference, as your dam is blocking the excess air up top from setting up a current flow and exiting front as easily.


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I’m assuming this picture is looking forward over the fins of #1? I’m trying to envision all the above text & pics and am confused.
 
It would interesting to see if air is actually entering the top scoops. I would think that’s a low pressure area - at least lower pressure than what’s in the upper cowl from ram air . I had thought of placing scoops further outboard near baffle seals. It would be fun to tuft the upper cowl from edge to edge where you could see them from the pilot seat.
Maybe no need for scoops at all, 60° is really significant! interesting progress you are making.
thanks for keeping us in the loop.
 
Oliver said:
It would interesting to see if air is actually entering the top scoops. I would think that’s a low pressure area - at least lower pressure than what’s in the upper cowl from ram air . I had thought of placing scoops further outboard near baffle seals. .
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The scoops are 1.5" high which is puts them well above the boundary layer. I'll likely take Mike's suggestion and tape them off to see what happens. I may also do the heavy black oil test where I put a drop of really thick black oil on something and see where the track ends up going.
 
As Oliver said - it's complicated. I made small changes and they make big differences. Not always in the direction I would expect. I also flew today at different angles of attack (i.e. speeds). The very surprising thing was that the pressure differential was higher at lower speeds than higher. At 100 mph (about my max) the pressure was about 1/2 of what it was at 70 mph. I would have thought that the increase in ram air would drive the number.
I've decided that I've had enough of random testing and I'm going to do the yarn tufts tests. Today I ordered an endoscope which I hope will survive the heat.
Some of the changes almost look like a modal flow (I really hope that is not the case).
Once I get the endoscope installed with some tufts and I can understand how air is moving around I'll start a new thread.
 
d_kandle said:
). The very surprising thing was that the pressure differential was higher at lower speeds than higher. At 100 mph (about my max) the pressure was about 1/2 of what it was at 70 mph. I would have thought that the increase in ram air would drive the number.
.
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Thanks for the update!!

How did taping off the top scoops do??

The top cowl is overflowing with air, setting up a out flow sucking from rear from middle out, from what I get from reading here... (think how you set up a flow siphoning gas..) that’s why faster isn’t as good as slower.....

Getting rid of that center low pressure area(air exiting and pulling) at front of cowl and only leaving the outer high pressure areas/inlet I think will be the key.




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The endoscope didn't work very well. It is designed to use in conduit or other applications with a very short distance between lens and object. So, didn't get anything useful from that experiment. I did raise the scoops up more into the wind and that did help raise the upper deck pressure. My lower deck pressure is still positive (>1" H2O) and I believe it should be negative. So that is where I'm going to move my effort. I've blocked off most of the path from scoops forward so I don't think there is any significant air moving back towards the front of the engine. It does appear that there is a forward flow or a circular flow under the cowl. I may try putting on some yarn with grease on them to see if I can tell which way they were flapping.
Another side effect of the changes I've made is to increase the CHT on#2 which is where I have the oil cooler mounted. That cylinder was as cold as #1 but now it is 20+ hotter.
I'm going to start attacking the positive pressure on the lower area. Except for a few simple experiments I'll can do, this will be a fall/winter task. In the near term I'll try different exit geometries at the rear of the lower cowling to see if I can reduce the pressure. Past that I'm going to move the oil cooler to the firewall and change the profile of the lower cowling. That will require fiberglass work which I'm not proficient at. Once I get some results from that effort I'll start a new thread and post pictures.
 
tedwaltman1 said:
Thank you for the description of “leaning authority.” I have an injected engine, so no carb to fiddle with.

i will confuse to experiment w airflow changes, though the internal vane mentioned and pictured in my PDF is my last, current at least, idea.

As you say, high Hp and slow speeds are ultimately a problem. Couple that with starting density altitudes exceeding 7,500’ even at dawn and one has a hot engine!
Click to expand...

The 7500 ft density altitude should help cool your engine, because unless it is turbocharged, it won't be able to generate full hp. As you ascend in altitude, your engine produces less hp, right? Mike Busch (SavvyAviation) has some lectures on engine leaning-ROP-LOP etc. He says the worst cht to operate at is, I think, 40 to 50 degrees ROP. This is where the most pressure and heat are generated in a normally aspirated engine. I'm no expert, but it is quite interesting to hear what Mr. Busch has to say. https://www.youtube.com/watch?v=X-tKyiUZ3ts&t=3344s
 
supercub9675 said:
The 7500 ft density altitude should help cool your engine, because unless it is turbocharged, it won't be able to generate full hp. As you ascend in altitude, your engine produces less hp, right? Mike Busch (SavvyAviation) has some lectures on engine leaning-ROP-LOP etc. He says the worst cht to operate at is, I think, 40 to 50 degrees ROP. This is where the most pressure and heat are generated in a normally aspirated engine. I'm no expert, but it is quite interesting to hear what Mr. Busch has to say. https://www.youtube.com/watch?v=X-tKyiUZ3ts&t=3344s
Click to expand...
I believe you meant to say EGT instead of cht. EGT is related to fuel/air mixture ratio. CHT is just the temperature of the cylinder metal.
 
FWIW, my Lycoming manual states to use peak EGT when below 75% power. The engine cooling should be sufficient to support that.

Interesting thread. Lots of SQ guys have fought temp issues. I’m glad I have a Cub cowl.
 
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