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Thread: wet wing fuel tank

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    wet wing fuel tank

    I am in the process of building a Sonerai Stretch . The plans have a fuel tank behind the instrument panel but it only holds 10 gallons. I have been thinking about using the wings as long range fuel tanks. I don't want to add any unnecessary weight. The plans call for wing skins of .025 and .025 ribs with rivet spacing of every 2 inches. If I close up the rivet spacing to 1 inch will .025 be thick enough to carry fuel? There is about 8 inches between ribs and I want to carry the fuel aft of the spar, so the tank length would be about 30 inches. If necessary I could go to .032 but this adds about 7 - 10 lbs

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    Registered User Othman's Avatar
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    Re: wet wing fuel tank

    This is a question that should be addressed by the manufacturer.

    Before structure is considered for wing tanks there some other issues that should be looked at first.

    1. Center of gravity. Will the CG still be within limits with full tanks? What about with partially full tanks and some climb angle. With the fuel behind the main spar an AFT CG position is probably the critical condition.

    2. Flutter (wing bending and torsion modes). Although most light aircraft wings like the sonerai's are pretty stiff, adding weight to the wings should not be taken lightly. Especially when the wing CG is shifted aft, critical flutter speeds will be reduced.

    Those are a couple of issues that immediately come to mind.

    With that said... I would stay away from wet-wings and would recommend installing an actual tank. The few homebuilts with wet-wing that I have had experience with always seemed to have some leak problems. Others here may have different experience.

    You may also consider modifying the wing tips to become fuel tanks. That would be the easiest way to integrate tanks on the wings. However, adding weight to wing tips reduces critical flutter speeds... bringing us back to point 2 above.

    Talk to the manufacturer or consult with an engineer before pursuing fuel carriage in the wings.

    Ashraf

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    Re: wet wing fuel tank

    thanks for the info, I don't want to fool around with flutter at all. I considered installing a removable belly tank but didn't like the thought of a crash landing with the fuel underneath me. Thats why I wanted it in the wings, to get it out of the cockpit.

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    Re: wet wing fuel tank

    Quote Originally Posted by pilotsailing View Post
    thanks for the info, I don't want to fool around with flutter at all. I considered installing a removable belly tank but didn't like the thought of a crash landing with the fuel underneath me. Thats why I wanted it in the wings, to get it out of the cockpit.
    Also, I believe that is a pulled rivet kit is it not? I'd guess that those would be less than ideal with a wet wing, so you'd probably have to use solid rivets.
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    Re: wet wing fuel tank

    Yes, solid rivets shouldn't really be any problem. I would really like to put the fuel in the wings if possible. Does anyone know of someone qualified who could engineer this for me and how much it would roughly cost ?

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    Registered User Othman's Avatar
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    Re: wet wing fuel tank

    I would ask the kit manufacturer first. They're intimately familiar with the aircraft and have all the data and experience necessary... this should be more cost effective than hiring someone that has to spend time studying the design and deriving data.

    Also it may be a mod that they already looked into. If they haven't then maybe they might be interested in marketing the mod... in that case they should cut you a break on the development costs.

    Ashraf

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    Super Moderator orion's Avatar
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    Re: wet wing fuel tank

    Of course you're assuming that the plans/kit seller is actually the original developer - as far as I know, that's not the case. Great Plains is fabricating and marketing various components for the airplane but they are not the developers. Given that this airplane has been around for decades, it is possible that the original designer is no longer around.

    Although a popular classic, in today's terms the airplane may be somewhat archaic and as such, have only limited potential in the market. For this reason I don't think Great Plains would be all that interested in doing or in any way supporting the necessary analysis and development work necessary to cover the full spectrum of associated issues. But it may be worth a call since as Othman says, someone may have already done this.

    However, having said that, I think it should also be pointed out that the small amount of fuel we're talking about would probably have pretty negligible affect on the Sonerai's wing flutter characteristics, especially if good incorporation practices were used in the modification. One of the best ways to go about this is to look at how other small aluminum kit airplanes have done it.

    Realistically you have only two choices to consider - the fuel could go in the leading edge or it could go aft of the main spar, between two of the root ribs (or if the landing gear is there, you may have to go one rib bay out). The lowest risk approach, and the one with the fewest seams to seal, is to place the fuel in the leading edge ahead of the main spar. Granted, I don't remember the structural details but there should be sufficient room in both wings so that you could eliminate all fuel storage in the fuselage.
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    Registered User Othman's Avatar
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    Re: wet wing fuel tank

    I often mistake the sonerai for the sonex... but know that you've mentioned Great Planes, I'm back on the right track.

    Flutter will most likely not be a concern. To further yourself away from any flutter issues it would be more favorable to add the fuel into the leading edge.

    I don't know how the ribs are layed out, but you may have to extend the tank across multiple bays then some ribs will have to modified to act like baffels.

    As I mentioned before, I would recommend building an actual tank to be placed in the leading edge. rather than trying to seal the wing. The tank can be built on your bench, and the seal can be perfected and tested easily while out of the wing. You won't be trying to slosh sealant in the wing and trying to access through small openings.

    Also the tank will allow you to design in a crumple zone in the leading edge... just in case you get some leading edge damage the risk of rupturing the tank will be low.

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    Super Moderator Midniteoyl's Avatar
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    Re: wet wing fuel tank

    Make a template and have two tanks (left/right) blow molded to fit? Plastics light and seamless. For that matter, find ready made tanks that will fit the wing roots.
    Jim

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  10. #10
    Registered User Othman's Avatar
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    Re: wet wing fuel tank

    Forgot to mention that you'll also need a filler neck and cap. If the tank is in the leading edge you may have to get a set custom machined.

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