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Mark Henderson

Well Known Member
After the latest SB came out for the fuel tank, I started doing serious research on alternative fuel tanks. Several builders suggested automotive fuel cells. From a safety standpoint, a true fuel cell is probably the safest tank you could use. Why do you think every automotive racing body requires them. From an aircraft point of view, especially for an already completed RV-12, there are problems. ATL and Fuel safe make cells that could be made to fit in the baggage area. The ATL Bantam 22 gal cell is 21 lbs, and it is about the lightest. Fuel cells are also expensive, $1200-1700. You can find cheaper ones with steel shells, but they can weigh 30 lbs. The shape on any fuel cell will take up more real estate than the existing tank since their shape is not optimized for our baggage area. Lastly, the design of the cells dictates a central cutout in the top of the tank which uses a fill plate for the filler and fuel fittings. This would require rerouting our fuel and return lines. It would also entail a longer fuel filler neck.

Another alternative are pre-made tanks. The can be made of aluminum or molded from heavy plastic (3/16) Again the shape is not optimized. Summitt racing has an aluminum 20 gal tank that is 24 x20x10, and a plastic 22 gal that is 33 x 17.5 x 9.5. There are other shapes, but not would fit as well as the current tank. The price is reasonable, $200-300. They do not list the weight. I did not bother to research further since again the location of fuel fittings and shape were not what I wanted.

This leaves custom made tanks. I initially looked at welded .050 or .063 5052 aluminum. I made inquiries to several tank manufactures. The minute I mentioned "airplane" I was told they no longer will make anything for aircraft. The only one I found that would make an aircraft tank wanted to make it of .125 5052. This weighs in at 1.8 lbs per sq.ft. .050 and .063 weigh approx .72 and .88 lbs per sq. ft. respectively.

I eventually went to a local fabricator that I have used in the past, although not for anything aircraft related. I explained what I wanted, and he had no problem with it being for an airplane. He did have reservations about using aluminum. The welded seam on aluminum is weaker than the surrounding sheet, and can crack. He does a lot of work on aluminum sprayed tanks for the agricultural industry. It is not uncommon to have a crack develop after a couple of years. Their use may be more prone to vibration than we would see, but in any application welded aluminum may crack.

I have a number of friends with older planes using welded aluminum. Most have never any any issue, even after 50 years of use. I wonder if this is because 50 years ago we had real craftsmen building aluminum tanks for airplanes. What ever the case, my only purpose is an alternative tank would be to eliminate leaking or breaching of the fuel tank.

My fabricator did suggest a possible solution, 304 stainless steel. Here in Napa all wineries use stainless tanks and equipment. They are quite experienced in welding SS tanks. Welded SS is far stronger, far less prone to cracking, and more corrosion resistant than aluminum. The price isn't much greater than aluminum. The only real drawback is weight. 20 gauge 304 weighs 1.5 lbs per sq ft, and 18 gauge is 2 lbs. A replacement for our existing tank is approx 10 sq ft. The weight penalty using 18 gauge would be in the range of 17 lbs over the standard tank. The 20 gauge is lighter, but harder to weld, and as a result more expensive.

The benefits are a much stronger tank, and would be equipped with a flapper valve on the fuel filler so the fuel is contained in the event the airplane ever flipped upside down. The tank could be mounted the same as the present tank, or by means of straps. If mounted to the center u channel, the tank should be much stronger than the frangible bolts.

I am leaning towards using the 18 gauge SS. 98% of my flying is solo. I am not worried about the extra weight subtracting from my baggage capacity. I always pack light. I can pack for a 2 week trip and fit everything in one carry on, and stay under 25 lbs. The tank will be stronger and safer in the event of a mishap. It will incorporate many of the features of a fuel cell.Plus, it will look pretty cool! Cost will be about $500. I realize that it may not fit the bill for everyone, but then I am looking solely at my mission needs.
 
Nice write up Mark. So your fabricator will make you a custom, 18 gauge SS tank for $500. Does that include necessary baffles, fittings, etc? Also, as long as you are getting a custom tank, any ideas about adding a few more gallons to the size?
 
My guess is that if another 4 or 5 gallons could be added, and it was pretty much a drop in, at $500 there would be a lot of takers.

Tom
 
just thinking out loud here. so me it seems that the material thickness [1/32''] is the weak link in the present tank. even with a thicker wall though a crush to a rivited edge would more likely cause cause a leak than an edge formed by a bend.
that new subsonex has a spun poly tank made just for it so hard and expensive is it to make one? how many will monnett ever make?
also, seems like securing with straps , whatever the material, would put less stress on the tank.
 
Not sure, but you might give Marv at smoke system helper a call. I know he is building some special tanks for a RV6 that look great. Possibly he can figure something out.
Tom
 
I have looked at increasing the capacity. The present tank is a very odd shape if you look at it closely. Vans did a good job shaping it to fit the baggage area. At the front, it is 17.5" wide, tapering to 16" at the rear,23.5" long, and 12,5" tall. In cross section it is rhomboid, not a rectangle. The front and rear are at approx an 83 deg angle. In order to add capacity you are limited to increasing the height or the width. The length is pretty much fixed. Each increase in width is approx 1.25 gal. If you make it wider, it must be removed for the annual condition inspection. No big deal. in 18 gauge stainless 4" adds 4 lbs and 5 gallons. If you fill the tank, you have used all 50 lbs of baggage capacity.

I didn't like the option of making the tank taller. To get the same 25 gallons, it would need to be about 3.25" taller. The fuel probe I was considering using is 12" tall, so it would be useless. If I decide to add capacity, I would make it 4" wider.
 
R&D vendor?

I recall Allan at Anti-Splat saying they spec'd spun poly tanks pretty regulary, so evidently he has done this before, and has a manufacturer lined up.
Perhaps he'd be interested in a decent order?
 
Interesting stuff. I have been playing with the idea of two alum (.063 5032 or 5052 not sure but half hard) bent corners where feasible and welded seams and AN fittings. I can't find any room for more capacity, and frankly am happy with 20 gal as 2 to 2.5 hrs of flying is my limit. Tank size would be roughly 8.5 wide x 22' long with 10 1/4" hgt at rear and 14.5 at front. The rear would be attached as is done now. The front would be strapped to floor (which would need some reinforcement) and the straps would be held in tension using a pr of springs. It would require two filler pipes, but so do wing tanks so no big deal. The size allows removal of rear baggage wall for inspection w/o removing tanks. The two would be joined by a 1/2" id. flexible SS line with a few inches of slack in event one of the two tanks would be moved differently in an unpleasent event. The two tanks would share a vent setup that is connected to each and joined at top so the vent facilitates a common fuel level between the two. It would then travel to the bottom of fuse as Van's design does, and employ a siphon breaker as well. Moller fuel gauges would be used . There would be no sending unit so Dynon would't know how much is in tanks, but the consumption would be indicated by the burn rate anyway. Weight about 9.5# each. OK, so how much am I overlooking here? Just a thought, but I am interested in people more knowledgable than me (that includes everyone) giving me some feedback.
Dick Seiders 120093
 
Hey all. I sure find it interesting that with all the b------g about tank mods that this thread just died. Did readers not like the comments? They were mean't to be thought provoking which usually generates helpful ideas, and/or useful commentary. Or is all the fuss about the tank just that (a bit of fussing over nothing).
Dick Seiders 120093
 
1. While a racing tank might be a safer container, that doesn't necessarily mean that a re-engineered fuel system for it is an overall safer system. It might have different failure modes or have other issues.

2. Welded 300 series stainless had slightly higher ultimate tensile strength than the aluminum specified, but approximately half the yield strength.

Dave
 
Sometimes life's a compromise!

Dick, because at this time Vans has no desire to change the tank beyond what you see now it is what it is.;) It's kind of like the rear window or a trailer. Until a single individual decides to spend some money and his personal time on his own we have what we have.:p Because of this form and most RV12s being all built the same way we are unique in being able to compare notes on these compromises. So for now guys will speculate but no action will be taken.:rolleyes: Not to brag but my tank has made 530 hrs with no leaks and it has the window in it. We mostly run premium unleaded with no ethanol.:p Who would have thought!
 
Thanks guys. I just knew there were folks out there interested in the topic. John, where do you get unleaded 93 octane no ethanol mogas? All of mine contains ethanol, but like you I have had no leak problems, and still have the sightglass as well.
I am tempted to try the concept I have outlined earlier, but am wavering a bit as I am waiting for Van's to come out with it's new tank design. I would much prefer to use a Van's design/tested alternative. I may be all wrong about that happening, but I can't believe that the latest mod is the final resolution to the tank breakaway problem. Time will tell.
Dick Seiders 120093
 
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