Sunday, February 7, 2016

Fuel Cells

Although I have had a little prior experience with fuel sealant (Proseal, Flamemaster are brand names), I really still consider myself a rookie with it.  Good news is that it really is not as hard to work with as I was anticipating - though it is still quite messy and it does seem to find its way onto skin or clothing no matter how clean you try to be.

First step in the process is getting jigs built - the plans include a template cut-out that worked well.  I made it a little larger than the template and lined it with foam weatherstripping to help avoid scratches.  Since the tanks will obviously hold fuel, all ribs and skins were completely cleaned of any ink or markings that might subsequently dissolve and contaminate the fuel.  Skins and rib edges were roughened to form a better surface for the sealant to adhere to.

Both fuel cells were initially clecoed to ensure the parts fit well and to help strategize the order of final riveting.

Test fitting the fit of the fuel cells - I placed the skins in the jigs first,
and then placed the ribs in, followed by the backplate.
After verifying the pieces were going to fit, the rivnuts were installed on the end ribs with dabs of fuel sealant to ensure no leaks around the rivnuts.  The fuel sender will be attached to the fuel cell with bolts later on.

Rivnuts installed on end rib.  Note holes for fuel pick-up and return lines (both of \
which need to be enlarged before inserting the fittings).

I used a fay-sealing approach.  A thin layer of fuel sealant was placed on the rib edges and on the skin surface.  Popsicle sticks were my primary tool for applying and spreading the sealant.  I tried to keep the layers thin to maintain the overall tertiary structure.  The ribs were carefully set and clecoed on top of one side of the skin in their final intended location.  After one side of each rib was clecoed, the skin was folded over and the opposing side of the ribs was clecoed in place.  The skins were then placed in the jigs and allowed to sit overnight to allow the sealant to partially set, after which the ribs and skin were riveted using the provided sealed rivets.

Fay-sealing a rib - the rear of the rib (in the clamp) is not sealed until the backplate
is attached.  Note that I just used an excess square of aluminum as a disposable mixing
pad for the sealant.
Clecoed skin, ribs and stringer were placed in the jigs overnight to partially
set prior to riveting.

A bent rivet stem worked great to spread a layer of selant around each rivet tail
for additional protection against leaks

Skin, stringers, and ribs of both tanks riveted.  Note that the backplates are not
attached at this point.  

After riveting (and cleaning up the excess sealant) I test fit the tanks on the wings to ensure everything still lined up correctly. The fuel cap, fuel drain, and the fuel fittings were then installed (note that it might have been easier to install the fittings on the ribs before attaching to the skin). Holes in the fuel drain and skin needed to be enlarged to allow use of the longer (and larger diameter) rivets.  Care was used when attaching the drain to ensure that any excess sealant would not prevent a clear path for contaminants to be drained. The fuel pick-up bracket was also installed at this time.
Fuel cap outer ring is installed with rivets.  The fuel cell insert is then sealed in place using
only sealant.  
Fuel cap insert is attached using only fuel sealant.  A weight (titanium bucking bar)
was used to keep the insert in place while the sealant set.
The final step was attaching the backplate and attachment brackets. As per the instructions, the backplate is initially attached with sealant and clecos. To ensure the backplate sets in the correct position, the cell is clecoed to the wing spar.  I also threaded bolts through the fuel cell brackets to ensure that they set in the correct positioning as well.  After the sealant has set for a couple days, the backplate is riveted in position while the cell is still clecoed to the main wing spar. After riveting, the fuel cell is removed from the spar the final steps.  On my first wing, excess sealant had spread from the backplate to the spar, effectively sealing the pieces together - it took quite a bit of work to get the pieces apart.  For the second wing I used a thin layer of plastic to keep the cell from sticking to the wing spar - worked much better as the pieces were separated with ease.
After the sealant is allowed to set for a couple days, set, the backplate is riveted on while the
cell is clecoed in position on the wing spar.

Excess sealant was squeezed out of the backplate and resulted in the fuel cell sticking to the
wing spar. It took a lot of work to pry the fuel cell off the spar.
A thin layer of plastic (strips of a garbage bag) were used on the other wing
to keep the cells from sticking to the spar while the backplate set in position.

The fuel sender was then attached to its support plate and the plate was bolted to the cells.  Final step was to check for leaks (and fix any that did appear).  I initially just tried to attach a balloon to one of the fuel fittings, but this did not work well.  I therefore used some left-over components for the pitot/static tubing to attach a balloon to the overflow tubing. AN caps were used on the other fuel fittings to seal them. This method ended up working very well.   The balloon is used to provide slight positive pressure in the tank so that leaks can be identified using soapy water - I found that even though I had a couple small leaks, the balloon still stayed inflated - so a balloon alone is not enough to verify a lack of leaks.  After completion, the tanks were set aside until final installation on the wing.

Verifying the fuel sender works prior to final installation.
One of two leaks I found - this one is from the fuel drain valve itself.
The other leak was around a fuel fitting that was rectified using another
layer of sealant.  I did not find any rivets that leaked.

This worked well.  AN caps were sued to seal the fitting on the other side of the cell.
Tubing and a t-valve were used to attach a balloon to the overflow fitting.
The short arm of the T has a plug that is removed to enable the
 balloon to be blown up.  This stayed inflated for at least
3-4 days (soapy water was still used to verify a lack of leaks)

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