I
T is an inescapable fact that no mat-
ter what sort of flying you do, the energy
required to get you into the air has to be
stored somewhere. And yes, that statement
includes gliders (Keeping your fuel in the tow
plane/winch motor is clever - but it doesn’t
mean it doesn’t exist).
For the majority of our aircraft, the energy
needed to fly will come in the form of Avgas or
Mogas, so a means of storing and reliably deliv-
ering this volatile and flammable fluid to where
it’s needed forms a vital part of any design.
At first glance this may appear to be nothing
more than a mundane plumbing exercise, but it
doesn’t take a genius to spot what will happen
if the designer gets it wrong. A sudden engine
stoppage is bad enough but an in-flight fire is a
truly horrifying prospect. So now I’ve got your
attention, it’s time to take a look at the chal-
lenges and pitfalls of designing a fuel system.
First up, fuel is heavy. If you squeeze thefull 135 litre capacity into a Jabiru J160 (that’s
the fluid equivalent of 15 slabs of beer) you are
looking at slightly over 100kg of fuel - that’s
almost 19% of the MTOW. Given the controlla-
bility impact of even small movements on an
aircraft C of G, it’s pretty clear you are going to
want to store all this weight as close to the C of
G as possible - to avoid ending up with an un-
controllable aircraft when the tanks are empty.
But that brings us to a second problem -
where to put it all? Slap-bang in the middle of
the plane may be the weight ideal - and there
are plenty of aircraft out there with a fuel tank
behind the instrument panel or under the seats- but sharing the cockpit with a sizeable quan-
tity of fuel is not appealing to most people. I
personally prefer wing tanks, if only because
they guarantee leaking fuel won’t soak into my
socks. While crash-worthiness is often cited as
a significant benefit of wing tanks, I’d suggest
that in the event of a crash severe enough to
cause structural damage, a tank’s resistance
to rupture is much more important than its
location. Especially if you consider that follow-
ing a crash, what’s inside and what’s outside
the cabin may no longer be quite so clear cut
and wing tanks - especially those in the lead-
ing edge - are in a position far more exposed
to damage.
BUILT LIKE A TANK
Fuel tank construction methods divide into
three types - integral, discrete and bladder.
For sheet metal and composite airframes, in-
tegral tanks are common and offer the lightest
option for fuel storage. By designing appropri-
ately strengthened and sealed compartments
into the airframe, the structure can perform
double service as a fuel tank. In the case of
wood, fabric and tubular framed aircraft inte-
gral tanks are not an option, so discrete tanks
are the norm. Constructed separately from the
airframe they do work out slightly heavier, butFOR RECREATIONAL PILOTS. Sport Pilot^51dave daniel
50 Sport Pilot. (^) FOR RECREATIONAL PILOTS
design
notes
More fuel you?