Removing Fiberglass tank

Bandit · 30 Sep 2009

Could I remove the fiberglass tank and install the plastic tanks, or do we need the drag tube installed back in the rib bay ? Any body look into the seat/fuel tank or the external 5 gallon wing tanks that are attached on the jury strut?

jackak · 30 Sep 2009

Bandit,

Check out the pictures I have posted on the Yahoo Avid site under "Jack's Avid Plus 2". They show how I cut open my fiberglass tanks in order to install the plastic ones. The 14 gal.fiberglass tanks have enough internal room to hold a pair of the 6 gal. plastic tanks and the needed fittings and tubing. I think there may be some pictures on the site advertising the plastic tanks too.

Jack

dholly · 30 Sep 2009

Steve Winder once mentioned the tall, rectangular 5-gal poly fuel jugs fit in the Avid tubing where the original header tank is. There is a current Barnstormers ad for 'Avid Avid Mk IV HH w/C75 Cont' that kinda shows one in the ad pics. Go to www.thefind.com and search for '5 gallon fuel jugs' for lotsa options. I've also found a couple potentially adaptable poly and HDPE tanks of different shapes at www.usplastics.com too.

Re: the drag tubes and retrofit vs. 'clean' install of poly wing tanks, here's a copy 'n paste with pics from a post I put on the KF email list:

If you are concerned that adding two of the plastic tanks per wing will compromise wing strength, I believe the oem recommended install for both Skystar and Kitfox wing tanks requires removal of drag tubes and #2 rib with only silicone holding the tanks in place, right? I know there are several more Avids with Skystar wing tanks and no root end support tubes or #2 ribs (or, in the case of Avid wings with 12" o.c. rib spacing, the #2 and #3 ribs), all seemingly flying just fine. There certainly appears to be more than sufficient collective flying hours among them to confirm adequate wing strength, so I think a better question perhaps is whether wing structure after installing two plastic tanks per manufacturer's instructions is at least as strong in comparison. I don't know the particulars of your KF2 wing but the following info and pics should give you something to compare to. Because there is sufficient difference between a 'retrofit' (installed into an existing Avid fiberglass-wrapped wing tank as shown on the wingtanks.com website) and a new 'clean' install, it's probably best to look at each separately.

For reference, the Avid Heavy Hauler internal wing bracing includes, in part, (1) long 1/2" dia. steel drag tube from rear spar at root to front spar at #4 rib; and (3) short 5/16" dia. alum root rib brace tubes, (1) from front spar to root rib web and (2) from rear spar to root rib web. Adding the Avid 'fiberglass wrapped' wing tank replaced all of these supports plus the #2 and #3 ribs, but added a few layers of fiberglass mat wrapped all the way around the f/r spars to provide some rigidity and hold everything in place. The identical supports and ribs are replaced when installing (2) plastic tanks per wing. To install, (4) new alum support tubes for the plastic tanks to set upon are added between and perpendicular to the f/r spars. An extra rib is installed between the tanks and new upper and lower ply (or alum) skin is glued over the root to #4 rib bays and filled with expanding foam. Retrofitting the plastic tanks into an existing Avid 'fiberglass-wrapped' wing tank involves removing the tank top only, leaving the majority of the pre-installed tank intact. I have seen retrofits done with ply installed in the bottom of the old tanks for the new support tubes to set on (the recommended instructions I believe), but also without. As with the clean install, no additional drag/anti drag tubes are added or omitted, however, you lose the ability to add a new rib between the plastic tanks when doing a retrofit.

Whether either of the plastic tank installs result in a stronger wing structure than simply gluing the KF tank directly to the spars is up to you. More importantly, deciding whether any of these install methods are safe on your experimental aircraft is builder discretion. Personally, I'm ok with the plastic tank install procedures. As Leni noted, an experienced aircraft manufacturer took time to design and test both configurations before offering to the public, and the Avid owners who retrofitted them appear to be satisfied. The ethanol proof poly and ability to add a rib on my current project is very attractive, but the higher cost per wing, loss of 6gals and fittings between tanks which might be a b*tch to repair in a covered wing still have me a little gun shy. If the ethanol resistance of the new 13gal Kitfox wing tank resin is as good as John McBean said (ie. several months with 100% ethanol exposure and no degradation.) and they can be fit properly in the flat bottom Avid speed wing, I certainly won't dismiss them. I had Skystar tanks in an Avid HH and excepting the poly resin was comfortable with that install as well. -Doug

Hope that helps clear up some of the 'safety hysteria' inferred by some with regard to the omission of drag tube. FWIW, I am doing a 'clean install' but, if I did have the oem tanks installed, I would leave them in place like Jack did. Might give better rigidity and definitely a quicker, easier install than removing.