Auxiliary Fuel Tank

A common criticism of the Sonex is the limited fuel capacity. The tank will hold between 16 and 17 gallons, depending on how you fill it on the ground. Fill to 17 gal (tank completely full) and when the tail comes up to level some may vent out the tank breather tube. This means 16 is about the max you can reliably count on (at least when using the plans-style tank vent).

With a cruise fuel burn of 5-6 gph, 3 hours is about it until tanks dry. Throw in a VFR reserve and now you're looking at stopping every 2 hours or so. It would sure be nice to have an extra hour's worth of fuel.

I reviewed several ideals and methods that other builders have used for auxiliary fuel:

This research helped me define the criteria I was looking for in an aux fuel system: light weight (under 5 lbs added to the aircraft weight); inexpensive (under $200); simple to execute (no major airframe mods or complicated fuel system mods required); convenient (no filling the tank inside the cockpit, no significant loss of utility when using the system).

None of the existing aux fuel systems met all the criteria. The Jerry can occupied the pax seat but was simple. Tubular wind tanks were heavy and expensive, and not loner available which meant custom fabrication. The aluminum seat-back tank was the leading design in utility (it occupies unused space and stays out of the way), but was difficult to fabricate and would need an external filler port and associated hardware.

The solution seemed to be a light-weight prefabricated tank that could occupy a portion of the baggage compartment. Add a fuel pump and power cord to the tank itself and the whole thing is easy to remove from the airplane for filling, and adds no weight to the airplane when its not being used (take it out completely - 90% of the time I won't need or want aux fuel anyway). The only modifications required to the airframe or fuel system is to add a fill port into the main fuel tank. For this I used the upper sight glass fitting already molded in by Sonex. A check valve installed in the port would allow fuel to enter the tank but keep liquid and vapor sealed inside, and a short length of fuel tubing with a quick disconnect fitting (i.e. 12" pigtail) would allow me to easily connect the aux tank to the main tank without fumbling around behind the instrument panel.

The items I selected are listed below:

Assembly is pretty straightforward. The hose that comes with the tank was trimmed down, the pump and fuel filter were plumbed in, and the wiring harness was connected to the pump. That's pretty much all there is to it!

You may notice a black device attached to the hose near the tank body. Mercury calls that a "Demand Valve" and describes its function as preventing vapor from flooding into the engine (going back to its outboard engine origins). Essentially it's a check valve that allows liquid to pass, but holds back vapor only. It requires a slight "suck" from the engine side to open the valve and flow fuel. In the Sonex system this is not really required as any vapor can just go to the main tank. However, it does help keep the tank from burping vapor out the hose end when the tank is disconnected. The quick disconnect on the end of the hose has a shutoff valve in it for this same purpose, but the two working together make this work out great.

The demand valve comes pre-installed on the mercury tank, as well as a mercury quick disconnect. I re-use the demand valve and some of the original hose, but get rid of the outboard motor style connect in favor of the ones I list above. All this work out really well. The tank holds between 6.5 and 6.8 gal, and will transfer that in about 30 minutes leaving about 1/4 gal left inside unusable. The only thing you have to do is remember to open the vent valve or the pump will quit transferring at about 3 gal because of the vacuum that forms inside with the vent closed.

The system works great, and best part is that it comes out easy and adds very little weight when you don't want it, but is easy to use when you do. The tank fits snugly behind the pax seat and is not likely to shift much in flight. For extra security, though, a couple of straps can hold in in place. The power cord simply plugs into the 12 volt accessory plug (cigarette lighter) in the corner of the panel, and the switch is easily accessible in flight.

The final consideration is the effect on the CG with a full aux tank in the baggage compartment. The empty weight of the tank and hardware is 3 lbs, and with a full 6.6 gal it weighs 43 lbs. The arm for the tank is 102", and that moves my CG aft approx 1.1" to 1.5", depending on the exact loading scenario. From a practical standpoint, in my plane I can have 40 lbs of aux fuel, 40 lbs of additional baggage, 450 lbs of pilot and pax, and only 6 gal of fuel in the main tank and still be within CG range (way over gross, but still in CG!). Under a typical cross country loading (350 lbs pilot and pax, 40 lbs aux fuel, 40 lbs baggage, full fuel in main tank), I am right at gross and well within CG at takeoff. As fuel is burned from the main tank the CG moves aft until it reaches the aft limit with approx 3 gal remaining in the main tank. When fuel is transferred forward to the main tank, the CG again moves forward to within CG range. Easy!

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Updated: 15 Feb 15