| Due to an undetermined fuel injection supplier, and the
possibility of
an 'alternative engine' (less likely... but who knows right now) we may
need to install a fuel return line to our fuel tanks. We've
researched these and have seen requirements for anything from a 1/4"
line up to a 3/8" line. In order to preserve the most flexibility
we're installing -6 size fittings which will handle the
larger 3/8" lines, and can easily be stepped down if a smaller
line is needed. We're installing these fittings on the outboard
side of the outboard most rib of the fuel tanks, and we decided
toinstall these now because we don't want to be
forced into cutting up our finished tanks at a later date. If
they are utilized, the return line will be run behind the fuel tanks
through the tank attachment zees. |
March 27, 2007:
Here are the fittings we're using... an AN833 elbow
fitting that will be fitted through the outboard rib, secured with an
AN924 nut and capped off with an AN929 cap. We want a bulletproof
installation that we don't have to worry about working itself loose due
to vibration or anything else, so the plan is to use a combination
of techniques Van's has employed in other parts of the fuel system on
it's RV's. On the exterior we're making these anti-rotation
brackets out of some 1/8" angle. Once riveted to the rib, the
elbow fittings won't be able to rotate. We'll also safety-wire
the caps to the bracket so they can't work their way off. And one
other thing... the brackets could be riveted with AD470 universal head
rivets, but just to ensure there would be no interference with getting
a wrench on the cap should we need to I countersunk the rivet holes and
will install flush rivets.
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March 29, 2007: I
spent a little time figuring out the exact location of the fuel return
fittings on the outboard rib. There are many thoughts on whether
these should be placed high or low, and we decided to go high... the
idea being that the pressure is lower resulting in less back pressure
in our return line (again, assuming we use it). The counter
argument is that as fuel is consumed and the level in the tank drops,
the fuel coming out of the fitting has farther to fall creating bubbles
and 'froth', which you don't want to introduce in your fuel supply
line. Since we're putting this in the outboard bay of the tank,
all the bubbles have time to dissipate before they get anywhere near
the fuel pickup located in the most inboard bay. Now, having said
that, here's the location we chose... Pretty much the highest point in
the tank... the upper rear of the outboard rib (the left tank is in the
cradle nose down in the pic)
Here's
a shot from the inside. The only thing we had to worry about with
the location we chose was the fuel cap. We don't really want the
fuel returning to the tank constantly pouring into the side of the cap
so we tried to squeeze it between the cap and the baffle. Once
located, the hole was drilled for the fitting. Next the
anti-rotation bracket was located on the exterior of the rib, and the
two rivet holes match drilled into the rib.
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March 30, 2007: The fitting and
it's anti-rotation bracket attach to the outside of the rib. The
fitting is held in place with the blue nut you see here. Once its
torqued down we plan to rivet an anti rotation bracket on the inside of
the rib. These are made out of some of the scrap sheet stock
included with the empennage kit, and will be cut to their final shape
and match drilled to the rib just before we rivet them on... this is
because we don't know the final orientation of the nut. Now, once
these things are riveted on we'll be forced to drill rivets out if we
ever need to remove these fittings... but that's really the point... we
don't want these things to ever come off, period.
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June 7, 2007: While prosealing the fuel tank skin stiffeners per page page 5, step 4 we had a bit or proseal left over. So, I glued
and clecoed our anti-rotation brackets for our fuel return fittings.
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June 9, 2007: While riveting the fuel tank skin stiffeners per page page 5, step 4 we also riveted the fuel return anti rotation
brackets. I used the squeezer on these. One thing
here… squeezing prosealed rivets can get pretty slippery.
I folded the first one over, so I made it a point after that to wipe
any excess proseal off the rivet prior to squeezing... helped some, but
it still wanted to slide around a bit.
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August 1, 2007: I knew I wasn't going to get very far this day so my goal was simply to
get to the point I was ready to mix up my next batch of proseal. The
only thing left was to cut my anti-rotation plates for the fuel return
fitting nut. I cut these out a while back but left the final shape
unfinished so I could get it oriented correctly once the nut was
actually torqued. This probably wasn't necessary because the torque
range for these nuts is pretty wide. Anyway... after torquing the nut,
I slipped the plate over in the best orientation and then trimmed away
at it until the shape fit...
After that I drilled two holes for attaching rivets, deburred
everything, etched the plate, alodined it, and then hung it up to dry.
Done for the night.
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August 4, 2007:
Today I'm sealing up the last few things in the left tank prior
to
closing it up. Among other things I sealed the fuel return
fitting in place, torqued it, and then Angela helped me rivet the
anti-rotation plate in place. After that I just sealed all the
edges. Note: you can see how the fitting exit inside the
tank is behind the fuel cap flange.
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