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Other popular methods exist, such as the Great Planes Fuel Filler
Valve, that block the fuel flow into the engine while filling the tank,
to prevent accidental engine flooding. However, sometimes such systems
require longer fuel-line runs to the engine. It is often a good idea in
such cases to use a third line anyway.
The filler valve connects to the tank, as would a fuel dot, allowing the
engine's fuel line to be made as short and direct as possible. Block off
the unused port with a short piece of fuel line capped with a small 4-40
bolt. There are so many onboard refueling systems available that you
should check your local hobby shop to find the ones that seem best to
you.
Hooking the lines up inside the tank is also fairly simple. The cutaway
photo shows how to position a three-line system inside the tank. If the
tank has a bubble section, position the muffler pressure/
overflow brass tubing inside the bubble for maximum tank capacity. Most
fuel tanks include enough brass tubing to fabricate any three-line
system.
Try to reach into the bubble with as straight a brass tubing "run" as
possible. This helps prevent the fuel "pickup" line from wrapping itself
around the vent tubing and getting stuck in a full forward position,
which could cause the engine to quit during the next vertical maneuver.
Some modelers prefer to use rigid plastic tubing on the pickup line to
prevent this, but sometimes that also prevents the engine from receiving
fuel during long vertical dives or spins. That is a modeler's choice,
however.
To further reduce the chance of pickup-line fouling, the fuel-inlet
tubing (the fill line) should be a straight line into the tank, as
shown. Most fuel pumps have no trouble filling the tank against any
extra pressure this may cause. Squeeze the filling line while installing
the fuel-dot cap or quickly close whatever fueling system you are using
to prevent spilling fuel once the pump line is removed.
Bending the brass tubing is fairly easy, but you must be careful to
avoid kinking it if you bend it by hand. Several great tubing benders,
shown, prevent kinks while providing just the angles needed. There are
many others, so get the one you prefer.
If you accidentally bend the tubing, carefully apply pressure on the
sides of the spot using pliers. This makes the tubing round enough to
allow operation if you do not have a spare brass tube (available in most
hobby shops).
Some flexible tubing—called fuel line—is also required to connect the tank's brass tubing to the engine, muffler, and fill port. At one time
there were many types of fuel line available, but only two are commonly
used today.
Pure silicone fuel line is used inside the tank. This semiclear tubing
is extremely flexible and allows the pickup line to conform to the
aircraft's maneuvers without kinking or leaving the fuel itself. It is
also fuelproof and is unaffected by model fuel. It lasts almost forever
without stiffening or degrading.
Click on photo to view large image with caption
On the other hand, pure silicone fuel line is prone to cracking
or rubbing wear. It also tends to slip off the brass tubing if it's
improperly secured. It is great inside the tank but does not last long
outside of it. Therefore, a form of "reinforced" silicone fuel line has
become popular.
As is pure silicone, reinforced silicone is totally fuelproof. Unlike
pure silicone, it is resistant to cracking and vibration wear and tends
to stay connected. Fuel lines, which were once problematic, are now
nearly trouble-free for years. Just make sure there is no firm contact
between the fuel line and the fuselage structure, to prevent wear caused
by vibration.
What about the fuel line's size, or diameter? This is not as critical as
it once was since engine-fuel draw has greatly improved. Consider it,
but don't lose too much sleep over it. Small-diameter fuel line is good
for up to .25 cu. in. engines. Larger engines, up to .65 cu. in.,
require medium-diameter fuel line. Engines larger than .65 cu. in. work
well with large-diameter line.
The best way to know for sure what size to use is to compare the
engine's inlet diameter (inside measurement) at the high-speed needle
valve to the inside fuel-line diameter. Try to match these diameters as
closely as possible.
A slightly larger-diameter fuel line is preferable to a smaller size if
a perfect match is impossible. Just make sure that the fuel line has a
firm grip on the engine's fuel inlet and will not slip off.
In emergencies, I have used medium-diameter fuel line on 1.40 cu. in.
engines without noticeable differences, so fuel-line diameter may not be
extremely important.
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