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Thread: Lower cowl scoop shape

  1. #1

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    Lower cowl scoop shape

    Greetings All

    BCSC Rev 2 flying in VERY high ambient temps - our average day is around 80ºF-100ºF OAT while flying. On the ground we regularly see 105ºF and a couple of days a year we see 120ºF

    we bought the plane with a very poorly designed lower cowl scoop (builder had done his own thing, NOT followed BCSC), which we fixed temporarily by adding an aluminum lip which we riveted on to create more "suction", this seemed to work well. now we want to re-mould the lower scoop and I have some conflicting advice from tech's around me.

    side view cross section, prop would be on the left of the picture, back of plane on the right.
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    - Option 2 - some say "extend past the firewall, "cause thats how cessna does it" - I see this option constricting the outflow of air because it narrows the cross section. might work for a Cessna that usually see 70-90kts IAS but not for a BCSC that is slow.
    - Option 1 - this seems to be the most common cowl design out there from what I have observed.
    - Option 3 - my gut feel tells me this one will create the most "suction", it increases the cross sectional area the most.

    I know and understand that the more cooling drag I create the more it affects my speed, but since I operate most of the time at 40-60kts IAS, and when I am slow then down to 20-40Kts IAS, I dont care much for top end speed, I need cooling, cooling, cooling.

    advantage of Option 1 is that if we end up not needing the lip then we can cut it back and we have the cowl ending neatly at firewall edge.

    please let me know your thoughts/opinions.
    Last edited by bodumatau; 12-05-2019 at 03:14 AM.

  2. #2
    skywagon8a's Avatar
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    option 3
    This is my BC Cub https://www.supercub.org/forum/showt...ight=smith+cub
    Unfortunately the photobucket logo is bluring the pictures but perhaps you can get the idea.
    I have no temperature issues. Fuel injected 180 hp, electronic ignition and tuned nozzles.
    My normal flying temperatures in the summer generally are not over the mid 80 degrees OAT.
    N1PA
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  3. #3

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    yeah our WINTER day temps are around 80 and thats at 3100ft AMSL, in summer the combination of DA and hot air makes it a challenge to keep engines that were designed for temperate climates cool. I have found the "difficult point" on our engine to be around 90ºF OAT, below this I can hang it on the prop and temps stay sweet, above this need to run full rich and be careful that I keep airflow moving.... it will be interesting to see how the cowl changes this.

    remember I am not talking about OAT on the ground, but rather OAT when flying..

  4. #4
    BC12D-4-85's Avatar
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    Not #2. That sharp lower edge of the firewall creates turbulence (read that somewhere) and outlet flow should stay away as much as possible. The lip also adds stiffness to the lower cowl which if flexible can compress and close the gap under air loads. It does that on my Taylorcraft without the removable lip installed - but that's ok in winter and acts like an auto closing cowl flap.

    Gary
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    I would do #3 you could also try a small rounded edge cover on the upper sharp edge to reduce turbulence.
    DENNY
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    55-PA18A's Avatar
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    Why would anyone want to live in a place with those kind of temps? There are probably snakes there !!!

    Don't forget the other places in your cowling where air goes in or out. There are a lot of threads on this forum discussing those.

    Jim

  7. #7

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    thanks guys, @DENNY & Gary

    is this the sharp edge you are talking about?
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    how would you round it? put a half round on the front of it?

    cheers
    Heiko

  8. #8

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    Quote Originally Posted by 55-PA18A View Post
    Why would anyone want to live in a place with those kind of temps? There are probably snakes there !!!

    Don't forget the other places in your cowling where air goes in or out. There are a lot of threads on this forum discussing those.

    Jim
    not ONLY snakes, we also have Elephants, Lions, Buffalo, Zebras, Giraffe, crocodiles, hippopotamus and a bunch of different antelope

    yep we are working on keeping our cowl/baffles/inlets/outlets all as good as possible.
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  9. #9
    BC12D-4-85's Avatar
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    Quote Originally Posted by bodumatau View Post
    thanks guys, @DENNY & Gary

    is this the sharp edge you are talking about?
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    how would you round it? put a half round on the front of it?

    cheers
    Heiko
    That might help. Experiment with a half round in front or 3/4 around the edge (metal or ? tubing cut to fit and fastened. Cessna on some models (like the 180/185) does what you did and formed a concave under belly for flow then added cowl flaps to vary the outlet area. Bellanca did similar but with a single cowl flap on their Scout that was able to be moved up and down but fixed when set. I made one for my Citabria that could be adjusted from the cockpit - piano hinge, flap, locking cable to panel. But we have winter and I needed the flap closed then. Is there room to remove material from the lower engine cowl to add opening area? That alone may help some. How about a pic of that area now?

    Edit: Notice the two metal "side curtains" Bellanca added to the boot cowl channel. I assume that helps smoothly direct exit air flow by reducing the chance of turbulent air entering that area from the sides

    Gary
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    Last edited by BC12D-4-85; 12-05-2019 at 03:33 PM.
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  10. #10
    hotrod180's Avatar
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    Quote Originally Posted by bodumatau View Post
    not ONLY snakes, we also have Elephants, Lions, Buffalo, Zebras, Giraffe, crocodiles, hippopotamus and a bunch of different antelope....
    And I always thought hitting a deer would be bad!

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    Cessna Skywagon-- accept no substitute!

  11. #11
    BC12D-4-85's Avatar
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    If this is the typical BCSC-Rev 2 cowl adding a lip as pictured or an adjustable cowl flap extended forward like the Bellance Scout pic above might help.

    Gary
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  12. #12
    skywagon8a's Avatar
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    Heiko,
    If this is your cowl, cut that reverse scoop along the sides. Replace that leading edge bend with a piano hinge. Extend the height of the sides and connect it to a cockpit control so that you can vary the opening in flight.

    N1PA
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  13. #13

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    Quote Originally Posted by hotrod180 View Post
    And I always thought hitting a deer would be bad!

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    I know this guy, he was researching wild dogs in Santawani and the giraffe ran across the runway as he had just rotated...... no options but to hold on and take the pain.

  14. #14

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    Quote Originally Posted by skywagon8a View Post
    Heiko,
    If this is your cowl, cut that reverse scoop along the sides. Replace that leading edge bend with a piano hinge. Extend the height of the sides and connect it to a cockpit control so that you can vary the opening in flight.

    yep, that is basically exactly what we are doing, except the hinge story as we never fly much below 70ºF, so never struggle with Low temps, and for the 5 flights a year we do have low temps then we can live with it. for some bizarre reason our plane builder made that scoop half the length and really deep.

    so... looking at the design above, would you make that lip flush with the firewall or would you extend it 3 to 4 inches aft of the firewall..... I think we should end it flush with the firewall, even maybe one inch forward of the firewall so the lip can be flush........my technician on the other hand wants to extend it 4 inches aft of the firewall "to stop air coming in when at high AoA", I disagree with him that the relative airflow will ever allow that unless you are stalled maybe with engine off.

  15. #15
    BC12D-4-85's Avatar
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    Keep the tech happy and let him design and own it. Put a camera on the gear to observe air create yarn flow in that outlet area at differing angles and airspeeds. If it works as expected everybody happy. If not then it's his to own and make right for both of you. Nothing wrong with learning and after all it is an experimental.

    Others will reply but that's the way I think at my age.

    Edit: It's -22F here in Fairbanks so cooling is assured.

    Gary
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    Quote Originally Posted by BC12D-4-85 View Post
    Keep the tech happy and let him design and own it. Put a camera on the gear to observe air create yarn flow in that outlet area at differing angles and airspeeds. If it works as expected everybody happy. If not then it's his to own and make right for both of you. Nothing wrong with learning and after all it is an experimental.

    Others will reply but that's the way I think at my age.

    Edit: It's -22F here in Fairbanks so cooling is assured.

    Gary
    bloody marvellous idea Gary, I think I will do EXACTLY that.

    yeah we get -22ºF in Maun...... but only at FL280

  17. #17

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    this was our lower cowl when we got it, the "scoop" was only half the length of the cowl.

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    ended about 5 inches forward of the firewall

    then we put this aluminium lip on as a quick fix.
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    that worked reasonably well

    we were struggling with exhaust fumes in the cockpit
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    so we took the light bar off, that made a big difference, no more exhaust fumes in cockpit

    so now my techie has made a temporary aluminium extension for the lower cowl that looks like this.
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    we will test this and if it works then rebuild the whole bottom section without the sharp front end of the scoop, similar to the green BCSC posted a few post above just with a deeper sided scoop.
    Last edited by bodumatau; 12-06-2019 at 02:02 AM.

  18. #18
    BC12D-4-85's Avatar
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    Smoother overall would be my choice to encourage surface flow. If desired add some metal side curtains like Bellanca did above for assumably a good reason to the under cowl adjacent to the exhaust pipes. It probably (?) encourages the air nearby to flow smoothly aft and creates a larger low pressure area relative to the outlet (?). It's always easier to delete material than add.

    For CO intrusion also seal leaks in the fuselage near the gear and any open areas near the tail.

    Gary
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  19. #19
    skywagon8a's Avatar
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    Quote Originally Posted by bodumatau View Post
    ...we were struggling with exhaust fumes in the cockpit
    so we took the light bar off, that made a big difference, no more exhaust fumes in cockpit
    Quote Originally Posted by BC12D-4-85 View Post
    For CO intrusion also seal leaks in the fuselage near the gear and any open areas near the tail.

    Gary
    Due to the aerodynamic shape of a fuselage it is natural for the ambient pressure in the cockpit to be lower than the pressure outside. This tends to draw air in from any openings at the tail. Thus exhaust fumes can travel along the bottom of the fuselage, enter an opening in the tail then move forward to the cockpit. Generally, sealing off the inside of the fuselage aft of the baggage area will minimize or eliminate this issue.

    This picture of your nose bowl indicates that someone has reduced the size of the cooling air openings. The prop is covering the openings in the picture. That was not a good idea for a slow airplane. It appears that the reason for doing this was to mount a oil cooler to the nose bowl? In addition to opening the outlet on the lower cowl you should consider opening these intakes.
    N1PA
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    Yeah, I thought the same when I first saw that photo, but it is the reflection of the hangar and the upper cowl,

    I have however cut the right side intake back by about an inch at the bottom

    in this picture my finger indicates about what I cut off.
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    here two more pics, I dont have another cowl to compare directly to but from other pics it looks about the same as the other BCSC cowls.
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    here you can see i have also closed that oil cooler hole off, it now sits behind #3 on the baffle

  21. #21
    skywagon8a's Avatar
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    Quote Originally Posted by bodumatau View Post
    ...here two more pics, I don't have another cowl to compare directly to but from other pics it looks about the same as the other BCSC cowls.
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    You could alter that baffle shelf in front of the cylinder to a ramp. Slope it up from the lower cowl opening to the center of the cylinder. This will smooth the airflow up and back over the top. As it is now, it is possible that the air swirls around creating an aerodynamic dam, thus reducing the amount of air which actually is able to flow back over the engine. Similar to the slope on the opposite side. On the opposite side remove that vertical section which is creating a dam. Let that air flow through the fins. If you want that piece to minimize chafing on the fins, bend it down instead of up.
    N1PA
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  22. #22

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    Lower cowl scoop shape

    Lots of Back Country owners have fought high engine temps. I never used my kit’s cowl and went with a home made Cub style cowl instead. No temperature issues. If anything my oil cooling potential is excessive, but not enough to change right now. Easy engine access. The first two pics are my cowl lip. That’s mostly to move air past the mufflers. Space is tight in that area. Lots of heat in very close quarters. I also have typical Cub cowl cheeks. Look at pics of Ted Waltman’s or Kevin Quinn’s Back Countrys. They added big outlet ducts on the side cowls. CC uses louvers.

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    Sent from my iPad using SuperCub.Org mobile app
    Last edited by stewartb; 12-06-2019 at 08:44 AM.
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  23. #23
    fobjob's Avatar
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    Measure the suction side pressure with respect to static. If it’s zero, getting any further gains in cooling are problematic.
    Ill bet it won’t be zero....
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  24. #24

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    Your exhaust pipes need more down turn and added length, and shrouding them from airflow will make your fumes problem worse.
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  25. #25

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    Quote Originally Posted by BC12D-4-85 View Post
    If this is the typical BCSC-Rev 2 cowl adding a lip as pictured or an adjustable cowl flap extended forward like the Bellance Scout pic above might help.

    Gary
    The picture shows the factory demonstrator Rev 3. There's no "typical" Rev 2 cowl. Some of us got our cowls well after the kits and the fit was.... let's just call it challenging. I don't mean to criticize BCSC in the least but their cowls are constantly changing. While SQ2s usually used hot rodded 360s most Revs used IO400s and now IO390s. Part of that evolution was because the planes got bigger and heavier and needed more power and prop and the hot rod motors were too hot to manage. The bigger displacement and angle valve heads addressed power needs and heat management.

    What guys overlook in that photo? That's an IO-390 powered airplane with a ground adjustable 3 blade. Performance reports are surprisingly good. My interest in that plane centers on the flaps but the prop evokes a bit of curiosity as well.
    Last edited by stewartb; 12-06-2019 at 11:51 AM.
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  26. #26
    BC12D-4-85's Avatar
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    There's something like this locally (overall black with bright green cockpit). I know the owner and will have a closer look.

    Gary

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    fobjob's Avatar
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    Enter into Google.. site: supercub.org CHT
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  28. #28

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    Quote Originally Posted by fobjob View Post
    Enter into Google.. site: supercub.org CHT
    thanks Fobjob, we have, subsequent to the photos above, put a vertical fence up the front of #1
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    roughly the size of the red rectangle, this brought #1 temp in line with #4

    then we cut off the stiffener in front of #2 and replaced it with another underneath
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    roughly where the red line is and this brought #2 in line with #1 & #4

    we still have about 20ºF higher temps on #3 throughout the operating range, but I must add that we have our 13 row oil cooler behind #3 stealing air from that back corner.

    we will try the ramp in front of #1 and let you know how it goes.

  29. #29
    skywagon8a's Avatar
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    Quote Originally Posted by bodumatau View Post
    Click image for larger version. 

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    That is an interesting prop hub spacer. What are you using for a propeller? If all else fails to get your temperatures in line with your wishes you could try another propeller with more chord near the root of the blades. This will grab and push larger volumes of air into the engine. In the past there have been propellers with airfoil shaped rubber cuffs placed over the roots of the blades for cooling like on this Stinson L-1.
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    Quote Originally Posted by bodumatau View Post
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    With that propeller spacer moving the prop forward away from the cowl you have room to extend the top of the openings forward creating an air scoop which would be able to grab more air when flying at low speeds/higher angles of attack. Though difficult to see, this Bellanca has scoops extending forward from the top of the cowl openings.

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    Last edited by skywagon8a; 12-07-2019 at 06:12 AM.
    N1PA
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  30. #30

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    Quote Originally Posted by skywagon8a View Post
    That is an interesting prop hub spacer. What are you using for a propeller? If all else fails to get your temperatures in line with your wishes you could try another propeller with more chord near the root of the blades. This will grab and push larger volumes of air into the engine.


    With that propeller spacer moving the prop forward away from the cowl you have room to extend the top of the openings forward creating an air scoop which would be able to grab more air when flying at low speeds/higher angles of attack. Though difficult to see, this Bellanca has scoops extending forward from the top of the cowl openings.

    Click image for larger version. 

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    we have 2 props, a CATTO 84x43 and a Whirl Wind 82" ground adjustable, both props have given us very similar cooling results. WW needs balancing for which we currently dont have a tool so we have switched back to the CATTO for now. spacer is a 2,25" from SABER, different spacer for the CATTO and the WW (both made by SABER) as the WW needs 2 flush lugs for the Ground Adjustable body.

    interesting idea on the scoop for the upper side of the opening, trying to think about how to make a temporary install without drilling any holes..... will let you know if I come up with something....

  31. #31

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    Do you have any pics of the 13 row cooler installed on the rear baffle? I've thought about changing my own coolers but can't see how such a big opening in the rear baffle could work. How are your oil temps on a hot day? Why was the cooler changed and relocated from the nose bowl?
    Last edited by stewartb; 12-07-2019 at 12:36 PM.

  32. #32

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    Quote Originally Posted by stewartb View Post
    Do you have any pics of the 13 row cooler installed on the rear baffle? I've thought about changing my own coolers but can't see how such a big opening in the rear baffle could work. How are your oil temps on a hot day?
    lets not talk about the oil cooler on the baffle ...... ah heck, why not, maybe some will learn from my mistakes..... so.....the story goes like this.

    when we got the plane the baffles were terrible, badly made, gaps of over an inch square and we immediately struggled with temps from the moment we took her out of the container and put the wings on.... time pressure was on and we had a small window to ferry her up to Botswana, luckily it was the start of winter and temps were cool a cool 70ºF... so my plane partner flew her up and then grounded the plane till we had fixed the baffles.... I ordered a set of IO360 baffles from VANS and they arrived.....then the cowls came off and the baffle fitting began.....till now the oil cooler had been up front under cyl #2 and we were struggling with oil temps and with cyl #2 temps......so the old 10 row oil cooler came off with the old baffles, while the fitting of the back baffle plate was started the oil dipstick kept getting in the way so it got taken out and plugged with a temporary plastic plug..... this is about the time I arrived on the scene... and I got stuck in with gusto, we prepared the baffles, got them all built up and shaped and fiddled and faffed and then fiddled and then faffed....you all know how it goes. then one evening I was trying to figure out how we were going to do the oil cooler, I was walking around the engine (no cowls on) with the new 13 row cooler in my hand and found a wonderful spot behind cyl #3 where the oil cooler fitted in snugly between the baffle, cowl and engine mount......it was like it was MADE for a 13 row cooler..... so I suggested we put it there and all got busy busy busy and we fitted it..... then came the end of the project where you start figuring out what do to with the handful of left over nuts and bolts that were supposed to go into the project and what you had missed and it struck me that one of these items was the oil dipstick tube.....which now no longer fit in because there was a great big damn 13 row oil cooler in the way

    but by now I had a serious case of "gottoflythisdamnplanerightnoworiamgonnadie" so we made a plan with a short oil dipstick that is a SON OF A beautiful old lady to use and check oil but hey.... it works for now..... end of story....

    I have been quite happy with the oil temps since we have put this oil cooler in though, highest I have seen is 208 (at 105ºF OAT for a couple of hours flying) and normally when temps are around 95 oil temp will sit at around 190-195

    remember though that we aren't able to lean much at these high temps because our CHT's then go over 400 which we dont like.

    so we are considering 2 different options:
    1. fit the oil cooler back below cyl#2, but about 4 inches lower than it was before and with a better opening scoop (before it was directly on the front skin of the cowl and we dont think it was getting clean airflow through).....but....even though we have a kit from BCSC for this it is going to be a mission as our starter motor is different to the BCSC IO390 starter so we will need to modify the brackets significantly. - also this puts the weight of the oil cooler up front which the REV2 doesn't like, it likes a light nose..
    2. fit a carbon scoop where the oil cooler is currently that feeds into a 4" SCAT tube, this should allow the oil dipstick enough space to be fitted again, and mount the oil cooler on the firewall lower down where there is space.

    both options have their challenges, I am worried that we won't get enough air through the 4" SCAT tube to maintain the oil temps we want.

    here are some pics of our oil cooler behind cyl#3, note the re-inforcements on the back baffle to carry the cooler.
    Click image for larger version. 

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    so its real easy for me to point a finger and say "but I wasn't there when they took the oil dipstick off" but I think its more important to learn from my mistake and learn to make sure that I FIRST take a look at all the parts when doing something and make sure I have a plan for ALL of the parts..... we live and learn
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  33. #33
    skywagon8a's Avatar
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    Quote Originally Posted by bodumatau View Post
    Click image for larger version. 

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    That cooler is very close to the engine mount tube. Normal shaking of the engine could damage the engine mount and/or the oil cooler.
    N1PA

  34. #34
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    Take a look at my photos section...I have some diagrams on airflow optimization through an oil cooler. In support of those, I was told once that putting a one inch spacer between the baffle and the cooler helped performance. Try running a jet engine without a long inlet duct....

    *that’s NOT to say that scat tubing is an efficient duct.....

    http://www.supercub.org/photopost/se...searchid=10055
    Last edited by fobjob; 12-07-2019 at 02:51 PM.
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  35. #35

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    Quote Originally Posted by skywagon8a View Post
    That cooler is very close to the engine mount tube. Normal shaking of the engine could damage the engine mount and/or the oil cooler.
    It's just the angle of the photo, we have done about 60hrs since and it doesn't touch

    Sent from my SM-G930F using SuperCub.Org mobile app
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  36. #36

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    If CHTs are the focus? I'd lose that cooler, close the baffle, and put the cooler back on the nose with an outlet duct so it doesn't pressurize the lower cowl. I've got the opportunity to reduce cooler size or eliminate one of my nose mounted coolers right now but my new engine will make more heat so I'll probably leave things alone this go-round. I've dreamed up a few new cooler scenarios for my plane. Like a firewall mounted cooler fed by cowl ducts from the nose. Composite cowls would be easy to incorporate ducts into.
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  37. #37
    fobjob's Avatar
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    After a LOT of experimentation and measurement, the following things were the most effective at lowering chts.(67 cub, 150hp, @100smph 5500 ft msl. 2450 rpm.)
    1. Ramps in front of cylinders, 20deg F ( to avoid raising front chts, drill 12-1/4 inch holes in each one.)
    2. Boxing in the front of the engine to eliminate air leakage down the front of the engine, 27 deg F
    3. Rear cowling lip, 10 deg F.
    4. Deflection vane over #4, along with reducing the space behind #4, 20 deg F on #4.
    5. Finding a binding rocker arm on #4, another 20 deg F.
    Not effective: extending side cowl cheeks....(45 deg at 1.5 inches protrusion)
    things I am trying: a half round (at least 3 inches diameter) on the bottom of the firewall. Incidentally, the two bottom cowl “vents” are good for 5 deg F each........ all this got me from 390F above ambient to 310F above ambient....fob

    *note that these changes were measured sequentially, not separately.

  38. #38

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    slight deviation from the topic, but has anyone out there mounted their oil cooler to the lower cowling? (not to the engine but to the cowling directly), I have seen this done on a few ROTAX installations, they graft a scoop into the front of the cowl that can receive the oil cooler, your flexible oil pipes must be long enough to take up vibration and movement, the oil cooler is held into the receptacle by a set of bungee cords, so when you drop your lower cowl you unhook your bungees and your oil cooler hangs on its flexible oil pipes, or you put a bungee in place that hangs it on an induction tube or something while it is out of it cowl mount, just so it doesn't hang down too much.

  39. #39
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by bodumatau View Post
    slight deviation from the topic, but has anyone out there mounted their oil cooler to the lower cowling? (not to the engine but to the cowling directly), I have seen this done on a few ROTAX installations, they graft a scoop into the front of the cowl that can receive the oil cooler, your flexible oil pipes must be long enough to take up vibration and movement, the oil cooler is held into the receptacle by a set of bungee cords, so when you drop your lower cowl you unhook your bungees and your oil cooler hangs on its flexible oil pipes, or you put a bungee in place that hangs it on an induction tube or something while it is out of it cowl mount, just so it doesn't hang down too much.
    yes, see the wildcat thread...

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