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Different Fuel Tank - Questions for You

f1rocket

f1rocket

Well Known Member
Well, it's winter time, the condition inspection is done, and I'm looking for a project. I am giving some thought to building an alternative fuel tank for the -12. I am just getting ready to dive into the engineering and CAD/CAM aspects of this, but before I do, I have a few questions for those of you smarter than me. First, let me say that I'm not convinced of either the need or the feasibility of doing this. This is more of a mind exercise at this point to see if something better might be possible.

First, my design thoughts. I'd like a tank that fits over the spar, behind the seats, and frees up baggage space. I guess the driving force behind this change is to get more room in the baggage area. While I can make the current limited space work, it's not altogether convenient and it is downright impossible for anything longer that a couple of feet. Capacity is also an issue. I would love to get 5 more gallons on board for those times when I fly by myself cross country. I fully admit this change, if made, appears to have marginal benefits. Like I said, I'm just looking at this at this point.

My initial concern is with the center of gravity shift forward of the weight of the fuel. Today, I fly mostly in the forward regions of the CG envelope. In fact, I find it impossible to get anywhere near the aft CG limit due to my weight, and the typical weight of any passenger. Moving the fuel CG forward is going to make this problem worse. I suspect, and will need to prove this once the engineering drawings are done, is that I will be forward of the forward limit at takeoff at gross weight.

So my question to those who have worked on this on the -12 before (both alternate tank design and wet wing design) is where did you calculate your fuel CG to be with your design? Do you have issues with the CG forward limit? Did you have to make other changes, like move the battery or add weight, to get the CG back in range?

If, and it's a big if, I can create a plug in tank to replace the current tank and not have to reengineer a bunch of other changes, then it might be worth putting together both kits and completed tanks for sale. I have free access on the weekends to an advanced machine shop so my primary investment is raw materials and time.

Comments and suggestions welcome.
 
Having just built a new tank using the sending unit and float guage from the old tank..... Not a huge fan of the multitude of proseal and rivets. I realize that even wing tanks use rivets and sealant to hold them together, but the -12 tank has a lot of pop rivets holding it together. If it's something you don't do every day, it can be a process..... Messy. I like the welded designs that have been presented in another thread, but my guess is it's pricey. All custom work from a machine shop. I don't mind where the tank is located or how it is attached. Perhaps a poly tank of the same shape? Even viable wing tank option that keeps the fuel near the wing root..... Unlike the Viking option with the tubes that extend to the tip, that worries me with CG and loading effects on the wing (spin testing anyone?). We only remove wings for the condition, so a "quick disconnect" setup or even standard AN fittings at the wing root would work. I'm not an aeronautical engineer but I do believe there is always more than 1 way to solve a problem. I'm sure vans did a lot of work on the standard tank and it's the way it is for many reasons. Not demeaning their engineers or testing process, as the current tank does serve its function well, but I think it could be improved upon.
 
Thanks for the link. I knew someone had built a behind-the-seats tank, just couldn't remember who. Joe did a nice job.

I see from Joe's pictures and thread that the new tank did not increase the capacity AND my worst fears about moving the CG forward are real. If I use Joe's calculation of 100" aft datum for the tank, at full fuel and two passengers without baggage, the airplane ends up outside the forward CG limit after burning approximately 8 gallons. That won't work for me anyway.

I guess I could look at a flat tank that goes across the floor of the baggage area. I'll have to chew on that one for awhile. Time to think out of the box.
 
f1rocket said:
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I guess I could look at a flat tank that goes across the floor of the baggage area. I'll have to chew on that one for awhile. Time to think out of the box.
Click to expand...

With a flat tank design I would be interested how that design would affect minimum fuel quantity for takeoff or go-around and also consideration should be given to minimum fuel quantities for various degrees of bank. It would be a most interesting tank baffle and fuel pickup design to cover those variables.
 
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f1rocket said:
I guess I could look at a flat tank that goes across the floor of the baggage area. I'll have to chew on that one for awhile. Time to think out of the box.
Click to expand...

Randy,
That was my first thought...a fuel tank that would go all the way across (as well as fore & aft) of the baggage compartment. Vary the height during the design process to give you the capacity you want. Making it fit the full size of the baggage compartment should keep the fuel CG pretty close to stock (as opposed to moving it forward). The tank bottom would slope down (following the fuselage profile) to put the "sump" up front near the spar...minimal unusable fuel. The only big downside would be that the fuel tank top is now the floor of your baggage area...perhaps thicker material to prevent punctures would take care of that issue?

Good luck with your ruminating. Let us know what you settle on.
 
I agree that a fuel tank covering the entire width of the baggage floor would be nice... except you'd have to pull the tank during the annual inspection to get access to the tunnel. Careful design of the baffling and a suitable top skin thickness could take care of sloshing, slips with low fuel, baggage weight, etc. You'd probably want to build it with full height internal ribs, maybe with some little hinged one-way flappers similar to what's used in the RV-7 tank with a flop tube.
 
Randy, I am out of town right now without access to my drawings. Send me a private message with your email address and I will send you what I have when I get home. About CG, consider how far forward the CG moves in a stock RV-12 when Van's tank is near empty. As long as your planned tank does not move the CG more forward than that, it should be OK. I also have an Excel spreadsheet to calculate the CG with my tank.
 
I've completed some back-of-the-napkin calculations on building a new tank and I can easily see how the current tank got to the size and position it is in. When you start looking at alternatives, you start a domino effect that quickly gets out of hand. My conclusion if that wing tanks are probably the better choice for getting rid of the current tank. I'm not going to do the engineering for that, that's way above my comfort level.

To summarize, I found every option I looked at had some major drawback associated with it. On Joe's design, my most favorite option, I just couldn't get the CG to stay behind the forward limit unless I started moving the battery around or adding weight to the tail. Neither of these were worth the benefit. For a floor tank, removal becomes a problem. Maybe split a floor tank? That's even more complexity but doable. Worth it? Not sure.

I think a behind the seats tank like Joe's would work for lighter pilots, but not for me. If anyone has any other ideas, I'd love to hear them.
 
Randy, I sent you another email.
If anyone is interested, here is a link to an Excel Spreadsheet for calculating the RV-12 weight and balance. Click on the arrow that points down to download the file to your computer. Change numbers in the YELLOW cells only.
 
I use a weight and balance app on my iPhone called "Aviation W&B". It works pretty good too.
 
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