EXTERNAL FUEL TANKS
ON A
RANS S7S
2011/11/17
There are
times when a little more fuel than the stock 18 gallons would be nice to have.
I wanted the additional fuel but did not want it as far aft as a baggage tank would
put it because we were planning a trip which would include some survival
equipment and a rear seat passenger. I also wanted tanks that could be easily
removed for the bulk of the time when I was flying locally.
This is the
terrain that got me thinking about having lots of fuel:
But
additional endurance isn’t the only reason for the tanks. Most of my flying is
on floats but now that I have the 100hp engine, marina gas is not high enough
octane so carrying extra fuel could be helpful. There is another advantage to
having two separate fuel sources (I can select left or right external). High
octane gas is only needed for full power operation. It is perfectly acceptable
to cruise using lower octane gas. So now
I could run with high octane in the wing tanks which would be used for
takeoff but I could fill up the external tanks with lower octane marina gas
which could then be used during cruise power settings (or visa versa).
A third
reason for these tanks is that they allow you to get full use of the fuel in
the mains knowing that you have more time in the fuel in the external tanks.
These tanks
are available from dealers who stock parts for the Chinook and Beaver. I got my
pair from LEAF but they are available other places including the Chinook
manufacturer, ASAP, in
The tanks
hold about 5 gallons each. The installed weight of the tanks total is 15 lbs. I
estimate the cg to be at 42” which is well ahead of the wing tank arm of 52”
I made use
of the existing spade/eye bolts at the jury struts to bolt on the SS brackets
(.090 304 SS). Strictly speaking these bolts should be another dash number
longer to get a few threads showing but
they cost over $20 a piece. I plan to weld up some new ones from stainless. The
Chinook uses a similar system but only clamps the brackets to their struts with
2 stainless clamps. I used the bolt and one clamp at the front and just the
bolt at the rear. The aluminum channel is made from 040 6061 and takes a bit of
fitting to clear the caps on the tank. The tanks come with two 1/4"
aluminum lined holes for the mounting bolts.
The horizontal bolt at the rear strut has a tube on it so that the
aluminum, channel won’t be deformed when you tighten.
You can’t
make a simple U bracket at the front because the tank needs to be moved to the
rear about 3/4" so the filler cap can clear the strut. That is why it is
an odd shape.
This
mounting feels pretty solid.
The filler
cap and fuel outlet cap are stock plumbing parts. The fuel line cap from the
dealer has a long barb going straight thru which I didn't like because it would
result in a big loop of fuel line above the tank. I bought new ones (3/4"
MIP plastic plugs) and made up barbs into a 90 degree path. You can buy the
screen/check valve pick up from the dealers. Most people get rid of the check
valve because they are unreliable. I just expanded mine to let the balls drop
out. That is a quick disconnect in the line. The lines are held on to the rear
strut leading edge with aluminum tape.
Below the
flap handle I mounted a left/right/off valve. After the existing Rans fuel shut
off, I put in a T to connect to this new valve output. The new valve moves
pretty easily and once when I was poking around there I inadvertently moved it
to on. Without check valves and with the main valve open fuel will drop down
from the main tanks into the selected outboard. To prevent this I’ll mount a
plate with a slot in it to hold the valve handle in the off position. The plate
will flex up to release the handle. With
this plumbing you can select either the mains of one of the external tanks.
Here is the
routing near the fuselage:
There could be a better method than punching
a hole in the fabric like maybe a quick disconnect at the fuselage. Let me know
if you have one.
The engine
mechanical pump easily pulls in the fuel from these tanks. For safety on
subsequent installations where there was no backup pump, I’ve added and
electric pump after that T with a cigarette lighter plug for power.
This is a
link to Chinook ASAP who sell the
tanks: http://www.ultralight.ca/Framesets/chinook/chfuelfr.htm
Probably
the guys with show quality planes might not like the cosmetics of this
especially when these lines would stay on even when the tanks are removed.
Functionality suits me just fine however.
Peter
Peterc at
pipcom dot com
