Part V - Fuel Tanks

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Part I-Skeleton
Part II-Skins
Part III-More of it
Part IV-Ailerons
Part V - Fuel Tanks
Part VI - Finishing

 

Now, ladies and gentlemen, The fuel tanks
I am also building my fuel tanks from scratch.  Following my welder's advise, I used 0.040" 5052-H32 aluminum alloy.  This alloy is very easy to work with. I made the ribs about 3/16" smaller than the wing nose ribs, except for the aft side which is cut about 9/8".  I also cut an angle on the aft top side to allow for the installation of DZus fasteners for a fuel door.  The pieces at the bottom are bulkheads to reinforce the tanks and to control the displacement of fuel... OK... with all those big holes, they won't control much displacement!  I wanted the big rear hole to allow for the inspection of the finger screen through the filling hole.  However, I decided to close the leading edge hole with a 0.016" piece of aluminum.
Assembling a fuel tank
The assembly is quite straightforward.  Besides ensuring that there is no twist, this does not require the same level of attention as for the rest of the wing.  Clecoes are installed where needed.  Once assembled, the tanks were sent to a professional welder.
powing32.jpg (35784 bytes) Fuel tanks being welded
This picture is out of sequence, but I thought it made more sense like this.  The reason for the large welding spot in the middle of the tank is that I overinflated it while I wanted to test it for leaks.  When I inflated it, the skin detached from the rib and the tank became more or less like a baloon!  This guy, while not being totally happy about this, had a good reason to laugh at me...
powing33.jpg (26414 bytes) Both tanks after being welded...
but still before being tested.  Talking about tests... I used three techniques: 1. fill with water and look for leaks.  Works quite well, but I missed several holes.  2. fill with pressured air and put soapy water on seams (used dishwasher detergent with bubble bath stuff).  This is very effective, but don't blow too much!  In the end, I blew only with my lungs---equiv to about 1 or 1.5 psi).   3. fill with coloured water (with Blue Methylen for example), pressure and leave overnight.  This is what I am doing as I write these lines.  Note on August 7, 2000: I decided to stop worrying about possible leaks and I finally sloshed them with Randolph 912 tank sealer (the one compatible with Alcohol).
PFuel01.jpg (32806 bytes) Ouf!  They still fit!
I verified the fit of the left fuel tank on wing.  It does fit, but not as originally planned.  It will be very difficult to have an hinged door (I will need flush rivets under the skin).  Note after Oshkosh visit:  I decided for another style of fuel door... see below.  To ensure a tight fit of the fuel tank in the wing, I covered it with 4mm thick cork (that I bought at Reno-Depot) glued with Contact cement.
PFuel1.jpg (104294 bytes) How to ground a fuel tank
My mentor told me that his Datcon fuel sender seals come with some sort of a wire that is installed through the seal.  This wire is in contact with both the fuel tank and the sender.  After thinking about various ways to ground mine, I called him back for more information.  I took the center wire of a TV cable.  For the right sender, I will get a bigger wire. (note that I installed two wires on this one) The liquid is Noalox, a product which prevent oxidation when two different conductors are attached.
PFuel2.jpg (91108 bytes) How full is full?
After I adjusted the sender for full and empty and installed it on the tank, I filled the fuel tank to check the fuel gauge readings for various volumes.  I found that the gauge is not linear at all.  At 2 gal, needle goes on top of Empty.  7 gal=1/4, 10 gal=1/2, 11.5 gal=3/4, 12,5 gal=Full.  I added another 0.25 gal.  All values are approximate.
PFuel3.jpg (50366 bytes) Locating holes for fuel cap, drain hole and vent
To locate the holes I used a simple technique.  I took a sheet of paper and punched a hole identifying the center of the hole (1/4" for vent and drain and smaller for fuel cap).  Then I positioned the sheet (skin removed) and I punched at holes in spar and ribs.  Finally, I transfered the hole location by attaching the sheet to the skin.
PFuel5.jpg (70098 bytes) My fuel door... simple design
Here is an Idea I stole at the Zenair site at Oshkosh (in July 2000).  No hinge and very simple... One concern.  Can water intrusion be a problem?  Please e-mail me if you have any opinion or experience with this.
PFuel6.jpg (71988 bytes) Fuel door details
The door diameter is 110mm while the fuel cap hole in the skin is 2-3/4" (how do you like my consistency in measurement units?). To curve the door, I removed a tile from the ceiling in the basement to access the toilet drain pipe and I pulled the door around the pipe. Simple method hey?  I wonder how many aircraft shops use toilet drain pipes to form aircraft parts??? :-)  Aluminium tubing was used for the spacers (5mm long).  On this wing, the fuel cap sits about 4.5mm under the wing skin. 
 
 

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