Cooling air inlet duct opening.

[Flyboy66]

I am cutting the lower cowl for the radiator scoop. I would like to see the finished work (painted or not) of how people contoured the inlet, if anyone is willing to share. Did you use super fil on the edges? I was going to round the inlet. Do most make the transition a sharp corner, or a rounded edge? Does it affect airflow either way?

I did see tropicals work. If you did it same way as he did, no need to reply.

steve

[Dave S]

Here's some.....

Decided to reinforce the lower lip with a piece of aluminum

Also found later that the cowl behind the radiator/below the firewall was prone to flapping from engine vibration. Fixed that with a perpendicular aluminum bar riveted to the inside of the cowl behind the radiator/below the firewall.

[aviator79]

I don't have any good pictures on hand, but I used Super Fil and made smooth transitions. The newer versions of the lower cowl have a stiffener glassed in the location Dave mentions behind the radiator, and the instructions will have you bond in a scrap piece of fiberglass to reinforce the lower lip.

[jiott]

I used fairly sharp edges, using SuperFill. I like the look of crisp edges for NACA ducts better than rounded. Personal preference. Be sure you have the duct glued in solidly with Hysol before starting the SuperFill; a friend has his crack loose after several hundred hours.

[Flyboy66]

My cowling has a ridge glassed in (from factory) that is about 1/2” tall and is below, or maybe below and behind where the radiator will be located, and goes across the bottom where it is flat. I suspect it is to address that issue. Does your cowl have that?

I glassed in the duct, and will be laying strips over the edges of the duct extending onto the cowl.

[efwd]

I installed mine as Dave did. At the time, I had read about NACA duct design and testing. If I recall, they are designed for maximum efficiency by having the leading edge perpendicular to the relative wind as Dave has his shaped. Also, some people have cut the aft edge round as well. That gives a tear drop shape and you are able to look right into the hose. Google NACA Vent and see if you find the same article I read on the subject. Sorry, I can't recall the source.

[Flyboy66]

I installed mine as Dave did. At the time, I had read about NACA duct design and testing. If I recall, they are designed for maximum efficiency by having the leading edge perpendicular to the relative wind as Dave has his shaped. Also, some people have cut the aft edge round as well. That gives a tear drop shape and you are able to look right into the hose. Google NACA Vent and see if you find the same article I read on the subject. Sorry, I can't recall the source.

No need. After looking at my work, I think I like the crisp edge appearance rather than the extra work required for rounded edges. I don’t call it lazy anymore. I am becoming an expert at minimum effort.

[Flyboy66]

Alright. I have radiator attached to oil cooler. Waiting for some SS from Kitfox to make the brackets for hanging it on the airframe (aluminum temporary straps for now). It says to set it on some 3/16” shims on the floor if the cowl, but there is a ridge across the bottom (can be seen in comment before this one), and if I put shims under it, the radiator will extend even further above the duct. Modifying the duct is no problem, as I am a fiberglass guy, but I am wondering if this is the way everyone else is doing it. Picture is without shims under radiator.

I am inclined to cut the top of the duct and extend it up further to fully draft the oil cooler, and while I am at it, I think I would extend the sides if the duct back to force the air through the radiator rather than giving it an escape route around the sides.

[efwd]

That is interesting. My cowl didn't come with that stiffener. The manual directed me to cut the hole and then reinforcing the long edge of the hole by attaching additional fiberglass across that edge. I just used hysol to glue down remnant cowl piece. My oil cooler is mounted a good half inch lower than yours which allowed the NACA vent to slide right over the engine oil cooler after I cut notches in it to receive the hose fittings.
BTW, I recommend using something like cowl baffle rubber. I aim to add some of that and cut away some of the fiberglass as it makes it a bit of a challenge when you try to install the lower cowl. Two ridged structures meeting up make it challenging to install the cowl without screwing up some of the fins on the oil cooler.

[Flyboy66]

Ok, that makes more sense. I thought the oil cooler was supposed to be 1/8” behind the duct, but I did not try to wiggle the oil cooler in and locate the radiator up against the duct. Headed out now.

[Flyboy66]

So, my duct has a bow downward, so the assembly didn’t want to fit under the upper surface. I did some trimming to accommodate the fittings as well as move it forward. I am just going to do some glass work to build a contoured surface that will fit over the top and keep the air going through the coolers. A picture is worth a thousand words. Thanks again.

[efwd]

Yeah, that is what I was talking about adding Cowl Baffel. As you can see, in mine, I have restricted flow over the top of the oil cooler and I probably have significantly reduced that one inch worth of radiator between the oil lines. Wonder if it will matter when the summer arrives? I enjoy the fiberglass stuff so it will entertain me to make the correction.

[Flyboy66]

I am reconsidering moving the oil cooler to a lower position. Does anyone know the specifications of the stainless steel used to make the brackets? I might try to source locally if I can find some.

Steve

[Cherrybark]

As usual, a search of the forum turned up the answer to the next stage of my build - mounting the radiator, oil cooler, and getting the ducting set correctly. This thread answered my questions and I really appreciate Eddie's pictures.

[Flybyjim]

I am ready to set the duct flange in place in the lower cowl. How much space did you allow between the duct and the radiator? The instructions state "same spacing top and bottom" The photo in this thread looks like it is against but I'm wondering about vibration issues.

[Shadowrider]

I had the same concern and I was assured the rotax don’t vibrate like the lycoming or continentals. I was advise to not set it too close making it hard to remove cowling. I ended up setting mine about 1/8” to 1/4” of space. I used a paint stick and set it onto of a paint stick for spacing.

https://lh3.googleusercontent.com/rliOAQsHcPBsm1X8fSyXMl4Luz1wgth3elxEmYgwHOhh9hVDnQ 3flRzwHwDMrNFQq04cuEwkeQL7LQSUFyfomGzvzeJIojxkUCs7 QRL6UADW6e92ShamLIYuE_2DFScC-HlGc3QYrlDGPWfkObWrK7b0mEmtLafsjvMsNEKkzAgjvcDaNaQ 5Mrb3JAOAz_fG3Yjyn2snphSAon7nfLRZ4FG7xP-z8uBcvaDFYhhp7w1ASM77MAlHm9lMZsNwtxTATMsNQYgLeyVUf o0JX0NSbJNGugmuOVNlR4MEhWc66HOEoBAPtcPIG9ChgRhGfB3 CkR4arL1AbXLdrkK3R4Mvbppb5TJ3VMjClhf5G83kUoZ_kNNwB JChCLSxrRY-nJ2ZLifezK8R8_LtKr-D3NpRmvEyvzBeAy7esfI61LXmNRXE0S2LR6zbRcQPEIdF0OVnH n8o7izFivVNV6mfll-m-cnfrDp4HVGWF7v_Y6ilfle9gH7TheI4kI746sF3qlNKC-_j-rmeAbfb5rb6r0h3mmbpE08YAz4zKNZXoKWvAhnuOAHzDRZyZgH melMxlRsLjCdlPfV5OOmgYxG-CSJ1vVnzR3swUUmUPPK5CsyXmHJHDrm-_tg68yTPaLl1Vsn7IZhRK91iR0ABOLEV79g7L0HR6x3XX5N6kr wgMk_nes1NyZFd8AEiA-qOdkJRjPRUZL9fGlRbkGpkmwJe1x9WWpSpAQizOW9DN0p9FlB2 z8ff8kMHzw=w777-h1036-no

[HighWing]

Didnt catch whether this was mentioned or not. But in some research I did, I read that for maximum efficiency, the edges should be sharp - not rounded.

[efwd]

I set the lower cowl at 1/8 inch from the bottom of the radiator. My NACA ducts you see in the previous post, are very close, if not touching and I don't find any kind of chaffing. It may have been me that recommended some additional clearance of the NACA to radiator simply because this is where I have some issue with lining up the lower cowl when installing. The clearances I have add challenges to getting to duct to slip up over the oil cooler. I have damaged the delicate cooling fins on the oil cooler. I don't have any cooling issues as I thought I might have. I use an 18 inch long (or about) segment of fire sleeve to block the air flow that was slipping around the radiators bottom edge. It helped cooling a bit. Enough so that it keeps my temps down under redline.

[Cherrybark]

Those fins on the oil cooler are very delicate and a strip of firesleeve is a clever way to close the gap while protecting the fins. I assume the firesleeve is attached to the fiberglass cowling. How did you go about that?

I've been a little concerned about long wait times for engine warmup without a thermostat. What have you been experiencing Eddie?

[JMH]

Carl, my 912iS doesn’t have a thermostat, and it warms up quickly. I’ve flown on frosty mornings and it takes less than 10 minutes to get to operating temperature. In Texas I don’t think you have anything to worry about. Those living where it really gets cold may benefit from having the thermostat. My challenge was plugging all the drafty openings in the turtle deck.

John

[GuppyWN]

http://www.formula1-dictionary.net/naca_duct.html

[jrevens]

I left a gap between the bottom cowl & radiator... don't recall exactly how much. I sealed that gap with some nice soft silicone rubber "D" section that I glued to the cowl w/ silicone sealant. The cowl comes off & re-installs very easily.

Carl - I flew this morning when it was in the low 30's, and I was up to adequate temperature by the time I taxied to the run-up area, but I do have oil & water thermostats.

2390223903

[efwd]

My firesleeve is just wedged into the space and secured from coming back the way it went in by a piece of velcro. The gap is too close fitting to allow it to go through toward the exit.
The Rotax manual states that the engine should be run around 1800 for two minutes. I do that then bump up to 2200 and that is enough to taxi (too much) but like John, It is warmed up by the time I get to the run up area. It takes about 4-5 min run time.

[Cherrybark]

Thanks for the feedback on time to operating temperature. And I really like these ideas for closing the gap around the radiator.

[jiott]

Eddie, FWIW even though Rotax may recommend idling at 1800, I was taught by Stick & Rudder to never idle below 2000 rpm. Their huge experience with the Rotax 912 indicates 2000 is a definitely smoother idle and easier on the gearbox than 1800. This doesn't mean you can't set your minumum idle below 2000 (mine is at 1600), just don't run it on the ground for warmup, taxi, etc. below 2000 rpm.

[efwd]

Hmmm 🤔 Paul had me idle at 1800. All in an attempt to protect the gearbox.

[jiott]

That's very interesting! When I trained with S&R they were running the 914 turbo and 2000 was the number. Maybe with the newer 912is 1800 is preferred. I know with my 912uls there is noticeably better smoothness at 2000 vs. 1800.

[jrevens]

Paul had me keep the idle above 2000 also. I believe he made the comment that brake pads were cheaper than gear box repairs, or words to that effect. I reason that it is probably to keep solidly driving the prop and help reduce or eliminate any gear clatter due to backlash... I might be wrong. I also read somewhere that warm-up should be accomplished at 2500 rpm. Except for perhaps in very cold ambient conditions, that doesn’t seem an unreasonable rpm to me with these particular engines. I generally taxi at 2100 to 2200 rpm, but my idle stops are set for as low as possible without the engine quitting on short final (like it did on my first test flight), maybe 1550 rpm?

[jiott]

My 912uls operator's manual says to idle at 2000 rpm until oil temp reaches 90F, then increase to 2500 rpm until oil is 120F. Then you are good to do the 4000 rpm mag check and T/O.