KF3 tanks in an Avid STOL/HH wing

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It will work, but some tweaking necessary.

Issues: Avid wing has 2" factory twist, slightly longer KF wing has 1.75" factory twist. No, the ribs are not the same. The KF rib has a thicker section at the rear spar and the rear spar hole in the KF rib web is lower, resulting in KF tank sitting above the adjoining Avid rib cap strips on top and below the adjoining Avid rib cap strips on the bottom. You will need to double up on the Avid HH root and #4 flaperon hangar rib top and bottom cap strips to clear the tank. The #2 and #3 short ribs are omitted because of the tank, of course, and you can modify and reuse the Avid rib tails behind each tank. You will also need to modify or raise your butt ribs to match, which will require redoing your skylight to butt rib attachments etc. Be careful and test for clearance before doing anything permanent, there is a higher probability of interference when folding wings after these modifications. You may need to move the root rib out slightly to prevent interference.

Alternatively, you can replace the Avid butt, #1 root and #4 long ribs with their Kitfox counterparts without negatively effecting the flight or stall characteristics. This will allow the tanks to sit below the cap strips as designed, and smoothly transition height-wise to the Avid #5 and out ribs without being noticed. It also eliminates the need to modify butt rib height or root rib spacing (increased root rib height causes wing to hit fuse near rear carry-thru tube when folding).

Pics show difference between Avid STOL/HH (blue tape) vs. KF1-3 ribs, also tanks sitting on spars with measure above top and bottom cap strips. Tanks were the standard 13 gal vinyl ester resin fiberglass wing tanks for a KF3 made by Skystar. Note fuel outlet is changed from under (Avid) to the end (KF) of the tank. I built my Mk-IV HH wings with 1.5" twist and the tanks rocked ever so slightly when set in the spar coves before securing with Dow 739 Silicone Adhesive.

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