Basking in the Sun

You know that summer is here when you see shy fuel tanks, basking in the sun!

In this case, we were able to capture the image of our Dream Tundra fuel tanks, stripped out of the airplane and catching some rays to set up Pro-Seal patches on some leaking welds. It's all part of experimental aircraft ownership especially ownership of a kit or plans-built that doesn't have a lot of examples flying. The Tundra is a good workhorse, but we are discovering a few bugs - and the fuel tanks appear to have a couple. We've seen leaks at the welded edge seams, as well as leaks from some spot welds that hold the internal stiffeners/ribs in place. Unfortunately, consulting with other owners of flying Tundras has shown that we are not alone - similar tank leaks have cropped up on a number of other operational planes, and have been dealt with in a number of ways.

Dream recommends having the seams re-welded, and if we had an aircraft-quality aluminum welder handy, we might have done that. Some owners have had replacement tanks fabricated by Dream, and sent out. We are pursuing that approach, but at the same time, have adopted the "Pro-seal the heck out of it" approach to see if we can keep flying while waiting on Dream. While the best way to apply Pro-Seal (Polysulfide Sealant) is to the inside of a tank, its just not feasible in this case - so we made U-shaped aluminum clips that go on over the welds on all of the edge seams, pre-sealed the seams, clipped the aluminum over the Pro-seal, then slathered on more Pro-seal. The spot welds were roughed up with a blue Scotchbrite wheel, cleaned, then daubed with the gray goo.

A little bright sunshine helps the cure of the nasty stuff - so out into the sun they go to catch some rays. Reinstallation will come when I am happy with the cure. We'll let you know if the 100LL starts seeping again.

A day in the life - catching some sunshine, then back to work!

Paul Dye

Paul Dye, Kitplanes® Editor in Chief, retired as a Lead Flight Director for NASA’s Human Space Flight program, with 40 years of aerospace experience on everything from Cubs to the space shuttle. An avid homebuilder, he began flying and working on airplanes as a teen, and has experience with a wide range of construction techniques and materials. He flies an RV-8 and a Subsonex jet that he built, an RV-3 that he built with his pilot wife, as well as a Dream Tundra they completed. Currently, they are building a Xenos motorglider. A commercially licensed pilot, he has logged over 5000 hours in many different types of aircraft and is an A&P, EAA Tech Counselor, and Flight Advisor, as well as a member of the Homebuilder’s Council. He consults and collaborates in aerospace operations and flight-testing projects across the country.

6 Replies to “Basking in the Sun”

  1. In my opinion as Mech. Engineer for what it's worth, sealants of any type have no place in a fuel tank of any type. All design work should consider cracking due vibration and temperature swings.
    A good friend died going the cheapest route.

  2. In reply to Oliver, all large aircraft use sealants to keep fuel in the tank. Many a time my KC-135 was in the fuel cell getting the integral tank leaks fixed by scraping out the old sealant and resealing. As for Mr. Dye's repair, the only thing I would have done different is maybe drawing a low vacuum to help draw in the sealant.

  3. Welds in aluminum sheet metal are notoriously weak. The lap joint is the preferable weld joint in thin aluminum, and should be kept in shear alone. Note that welding any aluminum essentially returns the properties of the base metal to that of pure aluminum. Spot welds along the bottom probably should be supported by aircraft structure. I understand from others that the mounting of these fuel tanks in the aircraft structure should allow them to be only partly constrained, as any airframe strain will be transferred to the tank and tank welds if the mounting is not thought out well. As a side note, I have welded test coupons on 5052 using 1/8" and 3/16" diameter spot welds, with and without weldable rivets, and neither is very satisfactory. I can pull them apart by hand. They may be suitable if they're kept near the top of the tank where the stress is lower, or if they're supported from the bottom.

  4. In boats and other marine applications we went to roto-molded polyethelene for fuel tanks and even with the pounding the hulls and tanks experience we had no problems.

    For the cost of having a welded aluminum tank made a roto-mold can be made and a HDPE tank molded from that mold. Most, if not all, motor vehicles are using HDPE fuel tanks.

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