Extra Fuel
After flying the FSII for a while,
we discovered (as the Goracle would say) an Inconvenient Truth:
since the FSII does not fly with the fuselage tube level, the
rear tank runs dry well ahead of the front one, and unless you
put yourself at risk of sucking air, you end up having only about
8.5 or so gallons of usable fuel. Here is how to fit 3.5 gallons
more usable fuel into the same space, with minimal expense and
aggravation.
Directly behind the passenger seat
and on either side of the front tank, we added hopper tanks. The
tanks are made from 4" OD PVC pipe, sealed top and bottom
with flat caps, purchased from Home Depot. Lowe's sells caps,
but they have a slight dome shape, Home Depot's were quite flat.
They had some raised letters on them, which were sanded off. There
are three sorts of PVC which we looked at. One was very thick
wall and heavy, another was thick wall and had a foam core, and
the third was somewhat thinner wall. That was the one we used.
We made them as long as practical.
The plumbing is a bit complicated,
but not too bad. Both stock five gallon tanks drain out the bottom,
T together, and then T into the bottom of the hopper tanks at
the lowest point, to the sight gauge and to the gascolator. The
hopper tanks have two fittings per tank at the bottom. The rear/lowest
one is where the main tanks feed in, which is the one also tied
in to the gascolator and to the sight gauge. Each tank also has
a forward/higher fitting which ties each tank together and also
T's to the fuel pump. It is high enough above the rear one that
water pickup should not be a problem, leaves less than a quart
total unusable fuel for all four tanks.
I found some steel C channel in the
scrap box that was a perfect size to fit over the stock brace
tube. Welded a vertical flat to it. Cut another piece of flat
stock to secure the top end and bent it to tie into the stock
rear seat retaining bolt. The band clamps fit around the caps,
which are pretty thick at those points, and we reinforced the
lower part of the tanks by cutting off some scrap from the PVC
tube and doubling it around the tank just to give the clamp a
little extra support. The hopper tanks/fittings just barely clear
the sides and the bottom fabric, but they are quite rigidly mounted
and are at least as secure as the stock tanks.
We used different fittings for the
top and the bottom of the tanks. At the top, we just used rubber
valve stems with the guts removed, as all they need to do is act
as air vents and a tie-in for the sight gauge. At the bottom,
we used NAPA metal valve stems with flat flanges. These have a
flat flange as part of the inside base, then a thick stepped rubber
gasket against it, and then a flat metal washer fits on the outside,
held in place with a jam nut. The stems were progressively drilled
out to around 3/16" (??if memory serves me right??) and all
valve stem installations were completed and in place before the
caps were glued on.
The sight gauge is clear taped to a 1"
white PVC tube with black numbers running up the right side, we
did it that way to make it easily readible just by glancing over
your shoulder. Maybe it seems like a lot of work to only gain
two gallons, but we actually gained a bit over 3.5 gallons, because
now the unusable fuel is only about a pint, whereas before we
only had around 8.5 or so usable (and risky, because you couldn't
see when the back tank was going to go dry) and now we have a
full twelve gallons. Since the 582 needs to be kept down around
5,000 rpm to keep the airspeed in check, fuel burn is never over
3.5 gallons per hour, so now we have a good three hour range,
plus the ability to closely monitor the exact fuel state when
it does get down low. Cost was less than $150, not too bad.
Index