BillyBob said:
How about sharing all of the above....
I can expand on some. Firstly CofG. The max weight I can have (taking into account my weight and takeoff fuel (43 Gal in main tanks) is 200 lbs. But that value puts me at the far back CofG and during takeoff I burn from the left tank. So in the climb and for the first hour or so, the CofG moves aft and exceeds the rear value. So you need to play with CofG issues. My baggage load is set for 150 lbs. There are other issues as well for myself including the 2nd fuel tank in the passenger seat. Obviously you need to play with CofG and track the change in CofG as the fuel is burnt. The basic method I use is burn 1 hr left tank, transfer that amount of fuel from aux tanks. I then switch to right tank (to ensure continued flow) for 15 min and back to left tank until all aux fuel tanks are empty.
Safety - fuel in cockpit is not great. A risk you need to take depending on your goals and reasons for all the fuel. Keep in mind as well, that gross weight may be exceeded (ferry permits allow this) and landing within the first few hours of takeoff is not a option. At least if your want the gear to stay in place.
Size - Depends on your canopy style. You can't build a tank (36" by 21") to fill the baggage floor. You won't be able to get it in and out. Test with a cardboard tank first. You can get bladder tanks made as well.
Weight - see CofG.
Securing - bolts to bulkhead near flap control tubes as well as into floor ribs. My baggage floors are removable with flat head screws. If you riveted the floor or used dome screws, you need to rethink how to secure the tank.
Plumbing - I have 2 fuel selectors. One normal one (left and right) to feed the engine. The second selector is labelled baggage tank and passenger tank (wife doesn't like that name). The fuel transfers into the left tank only via 5/16 alum line, fuel pump, and check valve.
How do you know how much has been transfered and how much is left in the tank? A few options. You can measure the fuel flow and turn the transfer pump on for a predetermined amount of time. Compare to insitu fuel gauge reading after the transfer. You can plumb a clear fuel line on the outside of the aux tank to see the level of fuel left (same as low cost high wing fuel gauge). I use a combination of timing (fuel flow) and a fuel press gauge. When the tank is empty, the fuel gauge oscillates from 0 to 5 psi. During transfer is about 2 psi. The guage measures "transfer press." not fuel press at engine.
Venting - needs to be obviously. My vent is the same as the main tanks vents. Goes out he bottom (under baggage area) and is cut to face forward. Don't put it right behind one of the exhaust pipes.
Other issues - how to fill the tank? Who wants to hold a big heavy black fuel hose while sitting on the wing? The hose hits the canopy rails, canopy fiberglass, leaves black marks and dirt. Not great. Extend the fill up hole to reach the top of the back of the seats. This is where the clear fuel hose outside is useful so you don't over fill the tank and spill into the cockpit.
I'm in the early stages of finishing and testing mine. You mileage may vary.
Steve
RV7A