Kitfox Aircraft Stick and Rudder Stein Air Grove Aircraft TCW Technologies Dynon Avionics AeroLED MGL Avionics Leading Edge Airfoils Desser EarthX Batteries Garmin G3X Touch
Results 1 to 6 of 6

Thread: Air entrapment loops in gas line

  1. #1
    Birdseyeview's Avatar
    Join Date
    Sep 2017
    Location
    Oregon, Ill
    Posts
    198

    Default Air entrapment loops in gas line

    In preparation for my upcoming airworthiness inspection I was rereading my build manual and in the final assembly section there is a page that talks about the gas lines from the two wing tanks coming down to the header tank. Specifically it warns that the lines need to run continuously downhill from each wing tank towards the header tank without any high spots that could entrap air and cause an interruption in the fuel supply to the engine. This isn't usually an issue for the line down from the right wing tank because the header tank is more directly below that wing tank. However, the line down from the left wing tank has to come across the fuselage to get to the header tank and the cross over can easily cause a high spot in the line, especially with the straight barbed fitting it connects to in the top of the header tank (per the build manual). I noticed mine had such a high spot but I easily solved it by replacing the straight barbed fitting with a brass 90 degree street elbow ($5 from ACE hardware). Then the original barbed fitting comes out of that street elbow at a better angle and virtually eliminates the possibility of a high spot in the line. See my before and after pictures below.

    IMG_8318.jpgIMG_8319.jpg
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #2
    Administrator DesertFox4's Avatar
    Join Date
    Aug 2008
    Location
    Arizona
    Posts
    3,562

    Default Re: Air entrapment loops in gas line

    That looks good Larry. Small change makes an important improvement. 👍


    DesertFox4
    Admin.
    7 Super Sport
    912 ULS Tri-gear


  3. #3
    Birdseyeview's Avatar
    Join Date
    Sep 2017
    Location
    Oregon, Ill
    Posts
    198

    Default Re: Air entrapment loops in gas line

    My suspicion is that small details like this line loop are easily overlooked and although the build manual has a warning about such loops it doesn't go the extra step to mitigate the issue. I've gained so much from other builder's ideas on this forum and my hope is that sharing this small improvement might help someone else.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  4. #4
    Senior Member Kitfox Pilot's Avatar
    Join Date
    Aug 2019
    Location
    Spring Garden Illinois
    Posts
    859

    Default Re: Air entrapment loops in gas line

    Thanks for that update. I'm just getting ready to run those lines.
    Harlan and Susan Payne
    Sold Piper Archer
    Flying FarmFox STI Kitfox N61HP
    Rotax 915is, Airmaster prop
    https://www.youtube.com/channel/UC5A...oCVUP15G0uB-Yw

  5. #5
    Senior Member jiott's Avatar
    Join Date
    Jun 2010
    Location
    Portland, OR
    Posts
    2,959

    Default Re: Air entrapment loops in gas line

    Larry, no big deal, but personally I would have stayed with your original setup, just shorten that hose enough to take out the sag. I think staying with the straight fittings on top of the header tank is a better deal than replacing one with a 90 degree (significantly more flow restriction). On a gravity fed fuel system (I don't have a backup electric pump, maybe you do) lowest restriction possible is a good thing if the mechanical fuel pump ever fails. As I said, its probably no big deal either way, but I just wanted to add my 2 cents.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  6. #6
    Birdseyeview's Avatar
    Join Date
    Sep 2017
    Location
    Oregon, Ill
    Posts
    198

    Default Re: Air entrapment loops in gas line

    Jim,

    Good points and I get it - restrictions are bad but the street 90 I put in isn't the smallest diameter along the line and the 90 turn inside it has so little pressure drop that there's no problem there. The ID of a street elbow is actually large in comparison to the ID of the straight barbed fitting. My recent static fuel flow test proved to me that the lines have way more flow from just gravity than I will ever need. I actually thought about your idea when I was originally running my lines and I had a shorter run mocked up to try to not have a loop and it seemed to come close to kinking the rubber line (it may have been mocked up too short). It probably wouldn't have pinched it off but it made me nervous at the time. Then when I installed the actual line it got a little long and formed a loop which I let go for a while until I recently rediscovered the issue. Like you said, no big deal either way as long as the loop is eliminated in the process. The point of this whole discussion is really to remind others to eliminate the loop and there are many ways to accomplish it safely.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •