I would advise against the mod. I've seen serious safety and build problems after doing tech inspections on rv wings that have had the outboard tanks added. Unless you plan to perminantly rivet them to the spar web. Every extended range tank mod that I've seen done has had nut plates added to the spar web where the out board skin normally rivets in place. The problem with this is edge distances can't be met on a pre punched structure. The fuel tank skin on an rv is thicker (.032) than the leading edge skin outboard of the tank. So to save time people use the inboard skin on the out board section of the wing as well. So now you have prepunched holes in the spar web and pre punched holes for screws in the tank skin. Both of them competing for space. The result is a series of holes left open in the spar web with improper edge distance. This will eventually lead to spar cracking. My advice is to land for fuel one extra time on your cross country and enjoy all of the small airports we have at our disposal.
BK
I have done the mod on my 9A, and BK's warning above is valid. I chose to build the tanks as well as I possibly could, slather on a metric ton (or close to it) of proseal on the seams to ward off leaks, and I riveted the outboard tanks in place using the original prepunched holes. I also put the Z-brackets between the tank baffle and the spar, just the same as the main tanks. Those tanks are now non-removable, and if I ever develop a serious leak in them I will simply abandon them in place and discontinue their use.
Here's a few of the reasons I wouldn't opt for additional outboard leading edge tanks.
1. There's no point in having additional tanks if you can't 100% reliably access the fuel in them. I've seen builders tackle auxiliary fuel supply numerous ways but all of them have some shortcomings, and all of them add to the complexity of the fuel system which makes it less reliable.
2. Vans wing spars are not engineered for the loads that might arise with outboard fuel tanks. In particular, landing with the outboard tanks full or partly full of fuel could easily overstress the wing spar. You might think that you won't land with outboard fuel but there are many reasons why you might HAVE to land.
3. Outboard leading edge tanks are a MAJOR structural and systems modification and when you come to sell this aircraft many potential buyers will be very wary of what you have done, particularly given that Vans would not approve it and you are unlikely to have any approved engineering calculations. It might thus be wise to consider resale value of the aircraft.
Vans used to sell the Jon Johansen auxiliary tip tanks but they discontinued selling them many years ago. It might be constructive to speculate as to why they made that decision.
This is also a valid comment - but there are some other considerations as well. With respect to #1, I put a small non-fuel bay between my main and outboard tanks, and put a Facet fuel transfer pump between them for positive transfer. I also have flow-through venting on my tanks (see my builders site for more, or PM me) so I actually have a passive and an active method for transferring fuel from the outboard to the inboards. Proper design is critical here, fuel system mods have the highest likelihood of inducing an emergency in the air of any mods I can think of. Caution is advised, but it can be done.
With respect to #2 - this is also absolutely true. Landing with any fuel at all in the outboard tanks should be considered an emergency condition, requiring a wing spar inspection before further flight. The wing spars are NOT engineered to take that kind of load on a landing. Taxi and takeoff with those tanks filled should be OK as long as you taxi gingerly and have good smooth pavement for the taxiway and runway. The bending loads on the spar at the root are greatly amplified during ground ops with fuel in the outboard tanks.
With respect to #3 - this is also true - Vans won't approve it. BUT there are quite a few people who have already done it, a lot more that ask about it, and many that want it. I discount the probability that you couldn't find a buyer fairly heavily. It's certainly true that most buyers would turn away - but some will actually prefer the aircraft BECAUSE it has longer range tanks. YMMV.
No engineering study was done by me; I just followed the example of two -9 builders who went before. Not yet flying, so I can't comment on that, but I can tell you that 1. it's a LOT of extra work, and 2. it's almost certainly the lightest way to do it. I weighed all the bits & pieces to add mine, and IIRC, it was under 2 pounds. Don't remember if that included the extra fuel line, but I doubt it.
Yep, I'll agree to all that as well. I followed Pat Tuckey's design, with a few tweaks specific to the 9A wing and change to the vent system. No "proper" engineering study has been done to support this, it's not recommended or supported by Vans, and you're going to be a member of a very small club of people that have done it, which means if you break it you own both pieces. No guarantees anywhere.
I spent about 50 hours and maybe a couple hundred dollars extra on my outboard tanks, adding 15.5 gallons each side, and I love having them. I will caution you again - it's something that can be done, but tinkering with the fuel system as I have done is NOT a thing to approach lightly. It's the quickest way I know to snap your fingers in the mousetrap if you're not very careful. Caution is advised.
You might also consider this as an alternative -
http://www.vansairforce.com/community/showthread.php?t=142539&highlight=smoke+oil+tank you'll still need to figure out some plumbing and fuel transfer issues, but this is removable and leaves the wing structure as-designed. He has tanks available in a variety of sizes, including some that are hidden behind the seat backs and don't interfere with the baggage space. He also has some that give 7 gallons on each side behind the seats and only chew up 3" of baggage space.