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Thread: Trimming NACA duct aft edge

  1. #1
    Senior Member colospace's Avatar
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    Default Trimming NACA duct aft edge

    (S7 with aluminum radiator, if it matters)
    The manual states to trim the aft edges of the duct to have "a constant gap" between the duct and the face of the radiator.
    My question is, How close is close enough? Are most folks trying to minimize the gap (or even use a seal to fill it) to force air thru the radiator or going pretty generous on the gap.
    My thought was to go for a 1/4" gap to the end pieces (would be 3/8" or so more to the actual face), but would like to hear from others as to your setup and results.
    - Gary
    S7 SuperSport Tri-gear
    w/Rotax 912, Oratex, Dynon

  2. #2
    Senior Member jiott's Avatar
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    Default Re: Trimming NACA duct aft edge

    I think something on the order of 1/4" would be fine. Main thing is you don't want it rubbing and vibrating against the radiator. Trying to seal that gap or making it very close is probably not worth the effort; I doubt any measurable difference in cooling would be noticed.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #3
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    Default Re: Trimming NACA duct aft edge

    I agree with Jim on this - it took a lot of trial fitting to get this right - too close and it rubs the oil cooler when installing the lower cowl. I have more problem keeping temps up than down so I don't think the gap is that critical. My installation is a 912is sport - not sure if the same on others - but I noticed at the factory they fit a heat foil on that narca inlet to protect against heat from the muffler which is close to the fibreglass - I did the same - I found the sheet of heat shield on line and it wasn't expensive.

    cheers

    r
    Ross
    Mt Beauty, Vic
    OZ
    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

  4. #4
    Senior Member colospace's Avatar
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    Default Re: Trimming NACA duct aft edge

    Thanks for the feedback guys. I was trying to preserve as much to the final straight portion of the duct as possible by only trimming about half of it off and locating the duct itself a bit further forward since the aluminum radiator is thicker. Appears that won't work well as the duct has a "sweet spot" for matching the cowl curvature. I will probably need to trim all of that straight portion away to get even that 1/4" clearance. I do not have the spinner backing plate on currently, but can see where that will really restrict the cowl installation/removal. Setting the spinner/cowl gap to minimums, as I did, may bite me there.
    Ross, thanks for the heads up on the exhaust proximity. I'll have to get some heat shield material ordered too.
    - Gary
    S7 SuperSport Tri-gear
    w/Rotax 912, Oratex, Dynon

  5. #5
    Senior Member jiott's Avatar
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    Default Re: Trimming NACA duct aft edge

    You are right about the spinner back plate restricting lower cowl installation and removal. Mine is very tight and requires the prop to be in a certain position and the cowl to be slightly sprung to get past the backing plate. No big problem, just requires some care or you will be scuffing some paint.

    I am curious about the heat shield you talk about to protect the NACA duct from exhaust heat. I did not use any shielding material and have not noticed any heat problems for 3 years now. I would like to hear from anyone who has actually observed any heat related problems to the fiberglass duct or cowl. The build manual says nothing about this, unless its a recent addition.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  6. #6
    Senior Member Dave S's Avatar
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    Default Re: Trimming NACA duct aft edge

    I have the same scenario as Jim with the lower cowl clearing the backing plate for the prop. I normally remove the spinner, place the prop in the position that gives the least restriction then twist the cowl a bit to clear the prop blades - no problem.

    I am also interested in the heat shielding material used on the NACA duct.

    I currently do not have the heat shielding material used on the NACA duct; however, I have had no discoloration or other evidence of heat influence on the inside of the cowl. I did do two things in the interest of handling under cowl heat. 1) I painted the lower cowl on the inside with white epoxy primer with the theory that white would reflect radiant heat better than the somewhat natural yellow color of the of the fiberglass. 2) I fabricated a small aluminum heat shield between the back & top side of the muffler primarily to deflect radiant heat away from the lower rubber engine mounts. Before I made the engine mount heat shield, I noticed a little color bleed developing on the metal surface of the engine mount immediately adjacent to the lower rubber engine mount compared to the top engine mount.

    Sincerely,
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  7. #7
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    Default Re: Trimming NACA duct aft edge

    Jim,
    During prop balance static runups the right front header got the upper cowl hot enough to bubble the paint! I had never run it up over 5,000 RPM on the ground before. The exhaust header is less than an inch from the cowl only on the right side. After repairing, I used automotive header wrap on the exhaust pipe as a preventive measure.

    I have to kink the lower cowl slightly as well to get it past the spinner. To prevent scratching the paint, I loop a towel between the cowl and spinner.

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    Default Re: Trimming NACA duct aft edge

    Re the heat shield - remember mine is the 912is sport and the tolerances may be different - the one I saw it on at the factory was also a 912is sport installation. I remember seeing both Jim and Floog's spinner prop set up - mine has a bigger gap i think - a full inch - more a product of my bad fitting of cowl and having to cut a few more time than ideal! But that does make it easier to fit the bottom cowl!

    cheers

    r
    Ross
    Mt Beauty, Vic
    OZ
    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

  9. #9
    Senior Member colospace's Avatar
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    Default Re: Trimming NACA duct aft edge

    When Ross brought up heat shield material, this is what crossed my mind to use:
    http://www.aircraftspruce.com/catalo...p?recfer=13170
    - Gary
    S7 SuperSport Tri-gear
    w/Rotax 912, Oratex, Dynon

  10. #10
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    Default Re: Trimming NACA duct aft edge

    Yep, that's the stuff.

    r
    Ross
    Mt Beauty, Vic
    OZ
    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

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