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RV-12 alternate fuel system

Bill_H

Well Known Member
Well, in my mail today I found something interesting. And if you search on the net you can find information, including video and pics, of a full replacement tank system for the 12. Two tubular tanks are placed in each wing (9 feet long) and plumbed together as one single tank. They seem to be supported by the lightening holes with a polymer shield and tab mechanism to prevent lateral movement. Two more gallons capacity. Unusable fuel on the ground (not banked) is about a pint. Existing tank totally removed. Fill points in each wing, near the outboard ends. Quick disconnect fittings in the wing roots - pull the wing out about a foot to do the disconnect. Single low point drain. The claim is that retrofit is easy.

The existing single fuel shutoff valve is retained.

Some unanswered questions:
Difference in weight? Are they baffled? Unusable fuel seems quite low in an unbanked condition. Description of the modification process, i.e. wing skin removal? Hole cutting? Baggage floor removal? ("One day" might be optimistic!) I don't immediately understand the fuel level indication mechanism. Appears to be supported by the rib lightening holes? Structural analysis? If being installed in a plane with the ROTAX, is the existing redundant electrical pump retained, relocated, or replaced? I assume wing removal involves draining the system from the single low point? Is that also a sump point? Not sure I care for the Tygon tubing simply pressed onto barb fittings in fuel vent service. Are the filling caps vented? A schematic diagram would be really helpful.
 
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Some answers from the vendor: "Very little difference in weight. Each of the 4 fuel tanks weigh in at 2.5lb.
Each tank does not need any baffling since they are only 5.5 gal each. There is always an opposing tank, no matter what. No wing skin removal. Just pull the wing off, install plastic liner and fuel tanks. No baggage floor removal needed. Just drill as indicated in installation instructions and install stand pipe. It is a one day factory installation procedure. Be sure to use whatever time you need for your installation. Solidworks structural analysis showed worst case scenario having the combined fuel in the existing fuselage location and a superior distribution using the Viking fuel tanks. The tanks work equally in a Rotax or Viking powered aircraft. Fuel pumps and return fuel stay the same. Fueling is much better with the wing tanks. Yes, the drain is at the lowest point and all fuel can be drained there. Fuel does not have to be drained for wing removal. Just use quick disconnect fuel fittings. The tygon tubing is just your vent. No pressure and no problem. Caps should not be vented and are not. Venting is done through float bowls for a safe and secure vent system, without spillage. The system is identical to the existing system. 4 cells making up one tank."
 
They seem to be supported by the lightening holes with a polymer shield and tab mechanism to prevent lateral movement.

That would concern me a bit. Those ribs are designed for flight loads, not to support the up/down, front/back and sideways concentrated loads of full fuel tanks bearing on the edges of the holes in the web when flying in turbulence, maneouvering or in a hard landing for example. Not saying it wouldn't work, but just wondering what design checks they've done.
 
The main reason for putting the tank inside the fuselage was the issue of connecting/disconnecting fuel lines because VANs wanted a no-brainer wing removal feature (which is distinguishing the RV-12 from the rest of the LSA low wings crowd IMHO). This Viking initiative shows, as well as the comments in this thread, that with the proper fittings, this fuel line connection is a non issue. However, the solution offered by Viking looks to me as a kludge (as the successive fuel tank mods by VANs do btw:eek:), no offense to Viking as it's the best they can do not owning the design. Actually I hope Viking is successful because that would give VANs a pause and incline them to have a look at a factory option for fuel tanks in the wings, "a l? RV", i.e. the way they did it in all other models than the RV-12.
Enough of mods on the current fuel tank! :eek:
 
A few comments regarding some of the details I have read....

Mass distributed outboard in the wings -
This negates all factory spin testing results. Has a new spin test program been completed? If not, consider spin recovery ability to be a total unknown.

"Solidworks structural analysis showed worst case scenario having the combined fuel in the existing fuselage location and a superior distribution using the Viking fuel tanks." -
Solid works does not do load analysis. It is a design tool. You don't push a button and have it give you a green "good to go" or Red "needs more design work" indication.

Low wing airplane using fuel pumps to draw fuel from the tanks, but both tanks "T"ed together to the single (standard) shut-off valve is a major design problem.

4 cells making up one tank. No it doesn't when they are interconnected with small lines, particularly between the two wings.

Increased baggage capacity -
To 175 lbs? Really?
Moving the fuel to a different location on the airplane doesn't change the maximum gross weight of the airplane.
If the airplane occupants were light enough to allow for 175 lbs of baggage, the baggage area floor wasn't designed for it (the current fuel tank loads are not carried by the baggage area floor), and the CG would probably be extremely aft of the limit with full fuel.

Very little difference in weight -
The current tank installation weighs 9 lbs ( I measured it).
Based on the quoted tank weights, my guess is this system would weight (at a minimum) about 15 lbs installed... net increase in empty weight of at least 6 lbs

Fuel does not have to be drained for wing removal. I guess it depends on how much fuel is in the wing... full tanks would add another 66 lbs to wing weight... more than 1/2 again what a wing weighs.

Influence on wing structure at high G loads is a big question...
The wings bend/flex at 4 G's. How is that influenced by the tanks? Does it cause rib crushing, etc.?

Just some of the many questions anyone considering the mod should think about...
 
Scott - have you ever seen ANYONE support something like this in rib wing holes?
 
I think some of the add on aux tanks for other models are installed that way. Not sure what level of engineering anal. was done.
Ribs in the RV-12 wing are .020 material. Doesn't. Look good to me, but I haven't done any anal. so I don't know for sure.
 
Thoughts from an aero engineer

After spending 30+ years as a design specialist at WPAFB I can offer a few thoughts-- THOUGHTS only, no analysis done.

Fuel in the wings is a good thing-- IN FLIGHT. Lift bends the wings up, weight of fuel reduces that.
Fuel in the wings, and especially outboard in the wings is a bad thing-- DURING HARD LANDINGS. Puts a negative G loading on the wing. Having admired the spars in our wings while building, I don't think that would affect the spars. Effect on those .020" ribs is another thing indeed, not to mention all those reinforcing doublers and bolts we had so much fun installing. Don't know how much negative G load the tanks can take either.

Not exactly what the effect would be on the fuel system during a prolonged slip. Less sure what the effect would be during a climbing turn (aborted landing). We already have a 4-gallon minimum for takeoff due to the fuselage angle. Add a steep bank to that at minimum fuel and...."

I DEFINITELY want someone else to be the test pilot during spin tests!!

My first though on getting the postcard was-- yeah, I'll go for that! Following thoughts are listed above.

If you always make nice gentle landings, never stall/spin, never do a steep climb at minimum fuel, and avoid slips, you're good to go. I'd like to see those concerns addressed before spending I pony up for the conversion.

Wayne 120241/143WM
 
Well dang Bill, your thread is apparently more interesting than mine, but I don't mind as I am enjoying the dialogue about this Viking proposed fuel alternative. What I still don't understand tho is if it's so hot why did it take four years after you first saw it to be be offered to us RV12 flyers? That is really curious. Anyway the topic is interesting and the comments thought provoking.
Dick Seiders
 
I have no idea about the 4 year remark? Dick - are you think about a different Bill? I did not know of this until I got the card in the mail. And the info I posted came from the products blog that same day.
 
Regarding baggage compartment weight capacity without the tank there. Gross wt and CG range limits still apply. The pilot side of the baggage compartment obviously is good for 50 pounds. Since they are symmetrical in the plans, one might reasonably assume that the other side can carry the same. Which would likely be enough at 100 pounds total.

But maybe that symmetry assumption is wrong. OK, the full fuel tank weighs about 129 pounds. It does not sit on the baggage compartment floor, but is supported by the two frangible forward bolts and one aft bolt. One could presumably make a shelf of some sort supported by those same connections which could carry up to 129 pounds minus its own weight. So the total baggage compartment capability is about 179 pounds. N412BR empty is 750 pounds so a 201 pound pilot brings that to 750+179+201= 1130 pounds. That leaves 190 pounds to gross weight for the sum of a new fuel system, fuel, and passenger. I haven't looked at what the moment arm looks like for the replacement system, the moment arm of the new baggage area is the same as the fuel system it replaces.
 
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Four years? Well that comes from me reading Bills join date vs the posting date of his post. Sorry, but I haven't been on VAF for a few weeks and I read the wrong date after he responded to my "Wing Tanks" thread telling me he beat me to the news. Dang, that's another mistake on my part making it the second one this year I think. Best thing I can say about that is that I answered my own question. It's still an interesting topic and please don't let my slip bamboozle anyone from contributing good out of the box comments,
Dick Seiders
 
So who has implemented the Viking wing tanks and what's your feedback? Also, would you suggest any changes to it?
 
I installed the Viking wing tanks, I think there are about a dozen of us in all. I have not flown with them yet, but others have with none of the suggested destruction of their plane so far..
My view is this is experimental building, and we all get to experiment, with engines, tanks, and other stuff. If it don't work out to my satisfaction, I will do away with them. I pulled my wings last week after filling for taxi testing, those winge get mighty heavy when full of fuel! I could have drained them, but just did not want to bother with it.
 
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