Adding fuel capacity has long been of interest to the RV market, and it can be done in many ways. Without getting into details of external or internal tank design, there are several important areas that will be impacted by adding mass to the wing.
1) Structural: IMHO, this is the biggest factor. Adding mass to the wing means that mass must be supported through out the airplane's structural envelope, plus safety factor. For our airplanes, that would be a multiple of 6.0 x 1.5, or nine times the symmetric load factor. <<edit: or placard the airplane to lower maneuvering limits with aux fuel on board. >> By that, we're talking about maneuvers that do not involve roll or yaw accelerations. With mass placed in the wings, these loads will relieve the bending moment on the main wing spar, which is why we like to carry fuel in the wings -vs- the fuselage. Fuel carried further outboard has more moment to oppose aerodynamic wing bending, so tip tanks are a popular idea.
During accelerated rolling and/or yawing maneuvers, (roll rate or yaw rate changes), inertial loads on the fuel mass must be analyzed. Some kind of inertial model of the fuel mass must be developed, and then maximum roll and yaw rates need to be known for worst case scenarios. For our RV's, yawing acceleration (theta-dot) won't be critical but rolling acceleration (phi-dot) will be since we can roll pretty aggressively. Reaction forces to the roll acceleration need to be analyzed, and they can be pretty large especially for tip tanks out there on the long arm of the wing.
2) Aerodynamic / Stability: Unless you're changing the shape of the wing, any internally carried fuel will not appreciably change cruise performance, but climb rate will drop noticeably with added weight. Remember, it takes fuel to carry fuel. A rough rule of thumb is 10% of fuel boarded will be used to carry the rest. Dynamic stability will be affected, especially in rolling and/or yawing maneuvers. Spin recovery will be adversely affected due to the larger rolling and yawing inertia of the airplane with tip tanks. Adding vertical tail area would be one way of countering this. As seen in Farn Reed's tip tank design, adding fixed wing area outboard of the aileron will also reduce roll rate due to increased roll damping.
3) Structural Dynamics: Mass added aft of the wing's quarter chord will reduce its structural flutter margins. I'm not talking about flutter in the ailerons here... I'm talking about fluttering the whole wing. The flutter divergence speed for our short and stiff wings is probably VERY high, and might not be a factor at at any subsonic speed. But, in principle, if you add mass aft in the wing (as my tank suggestion does) the wing must now react more mass moment about its aero center and so torsional flutter issues are magnified. I don't think this would be a factor for us, unless very large mass was added significantly aft of the main spar.
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Putting all the technical jargon aside, I like the idea of more fuel. The benefits generally outweigh the problems of adding tanks to the wing. Taking fuel stops out of your trip is just like adding speed, and lots of it. In some cases, you stock RV flyers could handily out run any of those six-bangers out there from A to B. Think of that...
I offered my own method for doing this a few years back, but nobody seemed interested. The fuel would be carried in the outboard wing bays, making tanks out of the main spar - drag spar - and the outer 2 or 3 ribs. Stiffen the lower skins, and make the wing tips removable for access. After burning off the mains, transfer the fuel into the mains with Facet pumps. (I prefer positive transfer mechanisms, not gravity for this) The tank vents would tee together and vent inside the wing tip. I've already worked up most of the loading and structural analysis to make this happen, but got off doing other projects instead.
