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Thread: Looking for a fix- high CHTs on SQ2

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    Looking for a fix- high CHTs on SQ2

    I have a Backcountry cubs SQ2 with a 180 HP O-360. I have had high CHT issues since day 1. I've tried everything - spent hours fiddling with the baffling. Front cylinders are fine, the rear 2 are the problem. Has anyone tried scoops on top of the cowling to push colder air right down on top of the two rear cylinders? I hate to cut into the fiberglass cowling unless I'm fairly sure this will fix things.
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    supercrow's Avatar
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    Plenty of room near the back of those rear cyls for hear to get out?

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    mike mcs repair's Avatar
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    pictures!!!
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    What kind of mags? Pictures of baffling/cowling top, bottom. front, sides. What are the temps on all cylinders?
    DENNY
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    CamTom12's Avatar
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    Some pictures would help - do you have any ramps leading up to the front two cylinders to help push the air up over to the back?

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    tedwaltman1's Avatar
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    Hello Doug,

    I changed my cowl with “cheek” like a std Cub. Sealed every tiny hole/area in the baffling. Added fiberglass to smooth inside lip of entry top and bottom cowl. Opened up bottom rear exit area. Significantly larger front opening. Call me if you want to discuss.

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    Ted has a 16-18 page document with tons of pictures and explanations on cooling that I bet he could send you. That’s where I’d start given he has a similar cub.
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    tedwaltman1's Avatar
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    I’ll update and post a link to my cowl mods pdf in a day or two.
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    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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    mike mcs repair's Avatar
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    Quote Originally Posted by d_kandle View Post
    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.
    Excellent. Just starting install of one. And was looking at that huge bottom opening and thinking it’s to big.... now I need to reread this thread. Perfect timing.


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    mike mcs repair's Avatar
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    can you tape yarn on down there on things, and a camera on gear? and go fly?

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    mike mcs repair's Avatar
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    a picture of the vents & blocked of rear stuff would be nice also

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    mike mcs repair's Avatar
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    Quote Originally Posted by tedwaltman1 View Post
    I’ll update and post a link to my cowl mods pdf in a day or two.
    that would be useful

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    mike mcs repair's Avatar
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    pictures of your rear baffling? you make your own or use the rv stuff?

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    I did take a video. Not very useful to have the yarn at the back end of the cowl. Just flaps around. I think the yarn video works when you are looking at air flow over a surface and near the boundary layer. I thought about putting some up just inside the rear opening to see where the air was flowing back into the cowl but that requires the camera to be directly behind the cowl looking forward. Since I was borrowing the Gopro, I din't want to return it with an oil mist on it.
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    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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    skywagon8a's Avatar
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    In some cases ram air hitting the lower firewall up through the air outlet creates a swirling motion which in turn makes an air dam blocking the out flow. On some airplanes a curved piece of sheet metal attached to the lower corner of the firewall extending up and around to the firewall a few inches up on the fire wall stops the reverse swirling effect. When the swirling is stopped, the outflow is improved.
    N1PA
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    Eddie Foy's Avatar
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    I did this on my 180. I was looking for a little lower on #1. Click image for larger version. 

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    "Put out my hand and touched the face of God!"
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    tedwaltman1's Avatar
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    Thank you very much for the updates Doug. Great information! I wonder if anyone at BackCountry Cubs is paying attention to your hard work and taking notes regarding your productive results?
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    mike mcs repair's Avatar
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    Quote Originally Posted by tedwaltman1 View Post
    Thank you very much for the updates Doug. Great information! I wonder if anyone at BackCountry Cubs is paying attention to your hard work and taking notes regarding your productive results?
    Can you post your notes you mentioned above?


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    I see other cub clones using this cowling now. Is it lighter than a standard cub cowling with aluminum and cowl cheeks? They look nice and assume you have a little more visibility. Think they will fit a 4 place airframes fuselage?

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    mike mcs repair's Avatar
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    Quote Originally Posted by vj88 View Post
    I see other cub clones using this cowling now. Is it lighter than a standard cub cowling with aluminum and cowl cheeks? They look nice and assume you have a little more visibility. Think they will fit a 4 place airframes fuselage?
    Engine mount is different width at least on 4 place....


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    tedwaltman1's Avatar
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    I updated the PDF where I document all of the myriad changes to my SQ-2 cowl, airbox, oil cooler location, etc. Here is the latest version: http://eaerofab.com/docs/CHTprojectV3.pdf
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    mike mcs repair's Avatar
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    Looking for a fix- high CHTs on SQ2

    Quote Originally Posted by tedwaltman1 View Post
    I updated the PDF where I document all of the myriad changes to my SQ-2 cowl, airbox, oil cooler location, etc. Here is the latest version: http://eaerofab.com/docs/CHTprojectV3.pdf
    thank you, great information...

    one question, (I will add 2 pictures in a minute to this post.)

    I see you also have the cylinders with the fins cut down/tapered.... did you fix inter cylinder baffles on bottom to account for that? I can't see them installed in your pictures in the PDF

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  25. #25
    tedwaltman1's Avatar
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    Yes, I have those same inter-cyl baffles, and sealed every other possible micro-opening, with a LOT of RTV. I put a very bright light above, or below--depending on the area--and made sure by the time I was done with liberal RTV that there was no light shining anywhere. i'll get more pictures next time I'm at the hangar (but that may be a while as I have to go out of town).
    Last edited by tedwaltman1; 06-16-2020 at 01:45 PM. Reason: wording
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    Quote Originally Posted by tedwaltman1 View Post
    I updated the PDF where I document all of the myriad changes to my SQ-2 cowl, airbox, oil cooler location, etc. Here is the latest version: http://eaerofab.com/docs/CHTprojectV3.pdf
    great info Ted,
    thanks for sharing.
    curious if you mapped your leaning authority at different power settings while addressing high cyl temps?
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    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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    I went thru a similar situation after install a new Lycon engine in a pa 18, difference being, I was flying behind a proven cowl design that was more than adequate at cooling previous engine.
    Maiden flight #3 cyl was flashing 450° (JPI) and climbing as I passed thru pattern altitude. I immediately returned to the field thinking I had left a rag or something on top of the engine. Thorough inspection revealed nothing obviously amiss so I went back up, same thing, all cylinders temps were Climbing thru 450°, it was impossible to push the engine hard enough to insure good break in.
    landed, thoroughly went over the engine, checked mag timing, fuel flow at carb, etc.. nothing amiss.
    I had sent the engine to Lycon for their hot rod treatment and was now embarking on a long arduous journey to figure out how to make it flyable.
    Fortunately, this occurred during “winter” (so cal),
    waking up at predawn i was able to take advantage of 30° mornings and fly enough to successfully break cylinders in.
    Recreational flying was pretty much out of the question.
    Thru a tip I picked up on the vans chat board, started to focus on carb mixture and “leaning authority” - how many deg rop the engine was running at various power settings with mixture knob full rich.
    pen and notebook in hand, I embarked on a test flight recording temp spread between full rich and peak while leaning, beginning at 2100 rpm thru 2700 rpm on hottest cyl.(#3)
    At 2500rpm, full rich, I was 20 rop.
    that will destroy a motor in pretty short order. (Ideally looking for 150° rop at 2500)
    long story short, solution was to drill main “jet” in carb incrementally until I was able to run 100° rop full rich at 2500
    this made the plane flyable, and combined with several cooling mods similar to what Ted has described, I’m now happily climbing out on 80° days at 380° hottest cyl.

    Horsepower creates heat, cub type aircraft fly too slow to get rid of heat, adding a long prop makes it worse.
    For what it’s worth, this was the solution to my high cht issues, SQ2 is a different animal..

    disclaimer: “drilling” the fuel nozzle requires proper tooling - lathe, drill followed by reamer followed by de burring, I’m not encouraging anyone to take this operation lightly.
    Last edited by Oliver; 06-16-2020 at 11:45 PM.

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    BC12D-4-85's Avatar
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    Ask what procedure determines the main nozzle rate of fuel flow via air pressure differential...diameter of main jet hole, the position of its outlet in an air pressure reducing venturi, or airflow via volumetric efficiency at various power settings that creates a low pressure zone through the carb. There's no universal fix so .....

    Gary

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    tedwaltman1's Avatar
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    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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    skywagon8a's Avatar
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    Quote Originally Posted by tedwaltman1 View Post
    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.
    Quote Originally Posted by tedwaltman1 View Post
    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!
    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-co...structions.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.
    N1PA

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    tedwaltman1's Avatar
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    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.

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    skywagon8a's Avatar
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    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.
    N1PA
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    mike mcs repair's Avatar
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    Looking for a fix- high CHTs on SQ2

    Quote Originally Posted by d_kandle View Post
    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.
    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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  35. #35
    skywagon8a's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    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)
    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.
    N1PA
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  36. #36
    mike mcs repair's Avatar
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    Quote Originally Posted by d_kandle View Post
    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.
    updates?

  37. #37
    mike mcs repair's Avatar
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    Quote Originally Posted by d_kandle View Post
    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.
    it's been 2 weeks.... any updates on your tests?

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    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.
    Click image for larger version. 

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    The other change I made was to add gills on each side. This made the biggest change.
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    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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  39. #39

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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.
    Thanks tedwaltman1, mike mcs repair, d_kandle, Farmboy thanked for this post

  40. #40
    mike mcs repair's Avatar
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    Looking for a fix- high CHTs on SQ2

    Quote Originally Posted by Oliver View Post
    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.

    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...
    Last edited by mike mcs repair; 07-28-2020 at 04:13 PM.

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