Originally Posted by blaplante
This is mostly a brainstorm that probably won't ever get to the front of the list
Feel free to (politely please) poke holes in this.
Goal: ways to add significant temporary extra fuel to the RV6 (could work in the 7 etc. as well). Significant being 12-15 gallons. Not required when flying with a passenger. It seems to me that a solution could be as follows:
Take out the passenger seat pads. Build a tank to sit on the floor and lean back on the seat back. Dimensions could be 15.5" wide, 27" high, and around 8.5" fore/aft. Weight all up is about 100 pounds. Use the harnesses to retain it. Probably additional hold down mounted to the seat cross-bar.
Vent overboard via hose and a new vent (location TBD).
Ground strap from tank to airframe.
Use a small electric fuel pump to pump from the tank into a Tee in the feed line from the right tank to the selector valve.
Use: Once the right tank is down (let's say to 6 gallons), switch to the left tank.
Engage the aux tank pump, verify the number of gallons pumped by looking at the right tank gauge. Most likely you'd do this in two rounds - pump 7 gallons to the right tank, and repeat later with the remainder.
Some advantages I see:
By pumping into the right tank we can drain the aux tank. Any un-porting of the fuel pump only interrupts flow to the right tank, not to the engine.
Pretty simple to install.
Weight is located at a fairly good position for w&b.
No need for fuel sender or gauge as the gauge in this scenario (dipstick to record starting fuel is all that's needed).
Downside is that I can't find a suitable size pre-made tank, so it would need to be scratchbuild, similar to an RV12 tank.
Some inspiration at https://www.kitplanes.com/adding-an-...nk-to-an-rv-8/
A low mounted tank like in the link above is another possibility, but strikes me as harder to tie down in the RV6.
I have an aluminum tank that holds 22-22.5 gallons, sits in the passenger seat of my RV-6 and works great. It does not use a pump, plumbed into Vans standard three-way fuel valve using 1/2" Rubber Marine fuel line, a short section of clear Tygon tubing (designed for fuel-not the cheap vinyl stuff you can buy at most hardware stores), Use head pressure to supply fuel to the regular system through the standard 3-way valve from Vans.
Tilt the seat all the back, strap in using the seat belt-have abrasion protection that was added, the vent line needs to go up front, not out the back. My AP/IA had me install it into the NACA inlet.
Bought a roll over protected vent valve from ebay.
Plumbing starts as 1/2" NPT-the stainless steel valve has NPT on the inlet, 8AN outlet to the filter, once past the filter it uses the rubber marine fuel line. Before using 8AN fittings again, have a section of the clear Tygon tubing to verify fuel flow. If bubbles show: know that it is getting low and I switch to the mains. You will need to verify accurate fuel flow at your cruise RPM so that you know when to start watching closely and more often if you want to burn the fuel to the empty level.
I've tested the system up to 17,500 to verify no problems. 1-gallon unusable.
Was able to complete a 1009-mile leg from Petersburg, Alaska to Hermiston, Oregon in 9-hours. Still had over an hour of fuel in the mains.
A friend who made marine fuel tanks for forty years TIG welded it up for me, he knew enough to add a filler neck/tube to about 2" from the bottom so that the filling is not just spraying fuel around inside.
Used .090 weldable type of aluminum (5200 series if I remember right).
Use two alligator clipped cables for the ground strap, portable and easy to remove or clamp into position. One on my side the other near the fuselage wall.
Fuel cap and neck came from Aircraft Spruce & Specialty.
15-1/2" Wide to fit between back seat angle-makes a nice fit and doesn't move around. Supported on both sides by the seat cushion angles.
Front to back is 12-3/4", this gives plenty of room for full deflection of the passenger seat control stick-no mechanical stop which could be real bad...
Height is 28", plenty of clearance just in case of turbulence, wouldn't want it making a hole in the canopy...
I stop filling at 22-gallons, there is more room, but 22-gal works just fine. No, splash that way.
The seat belt and shoulder harness lock it into place. Has handles for easy removal.
Weighs in at 25.2lbs including fittings valves, filters, fuel line.
Per my AP/IA he said to use the Permatex AVIATION Form-A-Gasket to seal the NPT fittings.
Also use the fuel line hose clamps instead of normal hose clamps you buy in a hardware store. They are designed to bite into the rubber hose, locking it into position better than the other cheaper style.
The AN "bulkhead" fitting for the vent line is located facing forward into the airstream to supply positive air flow to the tank. To mount this bulkhead, drilled a hole in the NACA inlet used for the cabin ventilation.
A final note-Make sure you label the Vans fuel control valve with Left/Right/AUX/Off, and keep an AN plug to plug off the fuel valve, if you leave it open, big possibility that you will dump fuel in the cockpit, when changing main tanks.
Weight of fuel 6lbs x 22 gallons = 132lbs + 25.2lbs = 157.2lbs or less than a 170lb passenger. Has not has any effect on the normal CG for weight and balance.
Planning to add some retention cable straps to be bolted down, holding it tighter to the seat frame, the seatbelt shoulder harness does allow a little vertical movement.
I have the fuel tank here at the house, could take some photos for you if that would help.