Fly Baby N48ML Auxiliary Fuel Installation

Text and Photos by Matt Michael

June 2010

Everybody had a different "mission" for their Fly Baby.  Most of us use our airplanes for local fun flying, but Matt adds the phase "...and serious cross-country machine..." to his mission statement.

To support his mission, Matt has performed some real cool modifications to his Fly Baby.  He's added a canopy and a baggage compartment, and in this installment, explains how he added another ~hour to the range of N48ML by adding an auxiliary fuel tank.

Most Fly Babies don't need an additional tank... the standard fuel tanks give 2-3 hours of range, and for me at least, I'm *quite* ready for a break by that point.  Matt's system is nice because it can be added to planes after they're built.  So don't think you have to put one of these in during construction, although if you are positive you'll need the range, the job will be easier.

The impact of the tank on CG...and the obvious dangers involved in routing fuel around the something that every builder/owner should be aware of.   This is a description on how Matt added a fuel tank in a fashion that met his personal safety standards.   No claim is made that this solution is approved or approvable by any other standard.   Any changes to the basic design of the Fly Baby are at the owner's own risk.


I struggled long and hard to come up with an auxiliary fuel tank design.  The final installation is so simple, cheap, and straightforward you would laugh at all my initial concepts.  I considered individual tanks for the spaces under the floor panels and even something external like the baggage bomb.  I mocked up a cardboard custom fitted tank for the baggage compartment but when I showed it to my welder buddy I about fainted at the cost estimate.

Somehow after looking at dozens of pre-made gas tanks I settled on this simple, cheap, rugged marine tank that in retrospect is practically custom made for this application!  It’s got a nice handle, a screened pick-up tube that is threaded for a standard marine quick connect, and a vented cap.  Best of all, it cost less than $40!  All I had to do was put a little valve in place of the cap vent and I was ready to plumb it in. Its’ capacity is 6 gallons and it weighs 5 lbs. empty.

I ran a small plastic tube from the top of the baggage compartment rearward and down to an inspection panel on the bottom of the fuselage.  One small hole drilled in that inspection panel was all that was needed to run the vent line overboard.  The line plugs into the vent valve on the cap when installing the tank and once in place the valve is opened to vent the tank.  For removal I just close the valve and unplug the line.

The main fuel line runs first to a small electric fuel pump that transfers the fuel to the tank up in front of the instrument panel.  I did a lot of searching for a manual pump but couldn’t find anything either durable enough or cheap enough to do the job.  Wobble pumps aren’t very common these days and are either too big and heavy or remarkably costly.  The pump I selected was designed for RC model fuel transfer stations and was noted by various sources as very reliable and leak proof.  It’s made by Sig and is amusingly called the Gas Passer.  It cost about $35 and weighs half a pound.

I showed it to my local rocket scientist-aircraft and engineering guru and he approved though recommended mounting it above the tank to prevent a massive fuel spill in the event it did spring a leak.  We tested it to draw 2 amps and pump about half a gallon a minute, well within the perimeters of my miniscule electrical system. Transferring one tank (6 gals) would use about half an amp and my gel cell is 12 amp/hours.  This leaves plenty of juice for running the radio and even the position lights and strobes.  The pump was bolted to the roof of the baggage compartment and its’ exit line then runs down to the bottom of the fuselage and forward under the floorboards.  It emerges from below the floor just forward of where your knee is when seated and runs up along the side wall till behind the instrument panel.  Here I used clear tubing so that I could see if fuel is flowing.

I placed a small shut-off valve in that section within reach so that it can be closed to prevent the header tank from siphoning back to the auxiliary tank and overboard through the aux vent.  This could only happen when the main tank is full but would be a real drag.  The aux fuel line enters the main tank through a small fitting near the top of the tank.  These fittings are a screw-in type available from Spruce or Wag. 

The final detail in the auxiliary system amounts to securing the tank in the baggage compartment. A couple of bungee cords hold it in place but I wanted more support to prevent it overloading the baggage shelf when full.  36 lbs. is nearly the max weight for that station and at several Gs, well in excess of what I felt the structure could endure.  The simple solution was to run a couple of flat webbing straps under the tank and up to the top of the baggage compartment.  Here, they pass through the ply and over the top of the structure.  I even added a couple of reinforcing strips to the plywood that  provide extra strength by making it virtually impossible for the webbing to pull through the roof of the baggage compartment.

With these straps the auxiliary tank doesn’t even need the baggage shelf to support it.  A couple of quick links in the webbing facilitate removal and installation though it is fairly fussy.  It’s strong, light weight, and cheap which is what really counts.

A simple 12 volt circuit breaker switch and a couple of wires to the pump complete the installation. 

The pump generates considerable static howl in my headset during operation but this turns out to be a good thing as there is no question as to whether or not it’s running.  If I accidentally bump the switch on there is no chance of running my battery down as the howling in my headset alerts me. This noise varies depending on if the fuel transfer valve is open or not so I don’t have to even look at the valve or the line to know if fuel is flowing.

Having 6 extra gallons of fuel to add to the 16 in the main tank is really wonderful and totally worth the effort I put into the installation.  Even at full power I have about 4 hours of duration with reserve.  This is great for long cross country cruising but it’s also really nice for local fun flying.  I have a lot more time to play around without worrying about fuel and can fuel up at the home airport with cheap car gas instead of paying top dollar at airports.  And, if I ever get into a tricky situation were things aren’t working out as planned that extra 6 gallons in “reserve” will be awfully nice to have.

Matt Michael

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