S_P_2015_04_

(Joyce) #1
are more convenient to build and test and with
some work can usually be removed for mainte-
nance. Bladder tanks, basically being a flexible
fuel bag designed to fit inside the structure,
fall somewhere between the other two types.
They turn up quite often in military aircraft and
racing cars due to their crash worthiness and
ability to provide self-sealing properties. They
aren’t cheap and I’m not aware of any produc-
tion ultralights which use them, but there may
be some out there.
From a construction perspective compos-
ite tanks have the advantage that they can be
manufactured in almost any shape. However
correct material selection is vital to ensure
long-term resistance to fuel - including the
ethanol which is progressively sneaking into
Mogas. This is not just a case of avoiding leaks
or fuel contamination, but critically for an inte-
gral type fuel tank, there must be no significant
loss in the mechanical properties of the struc-
ture over the whole life of the aircraft.
Discrete tanks made from welded alumini-
um should theoretically be leak-proof(duh), but
riveted tanks will always require sealing to be
usable. This is achieved by painting or sloshing
the inside of the tank with special elastomeric

sealants - a messy and unpleasant task for a
discrete tank, let alone one which forms part
of an airframe. On the subject of sealing, it’s
important for designers to contemplate where
fuel will end up when the inevitable leaks do
occur and ensure drain holes are provided to
avoid the potentially explosive build-up of fuel
vapour in the airframe.

PIPE DREAMS
With tanks out of the way it’s
time to look at the plumb-
ing. Aluminium 5052-O
tubing is the usual choice
for fuel lines in GA appli-
cations. However copper
pipe is more readily avail-
able and so does turn up
on some homebuilts despite
being heavier and more prone
to fatigue. Minimising the number
of connections in a system means fewer
opportunities for leaks, but there will always be
some connections required, for which a wide
array of aircraft grade AN fittings are available.
While not a design consideration, it is worth
noting that AN fittings are designed to accept

a 37° pipe flare, not the 45° usually found in
automotive applications. The two are not inter-
changeable, so if you are building or modifying
your fuel system, getting the correct flaring tool
for the job is essential.
When routing fuel lines there are several
important considerations. Firstly, there should
be a downhill slope with no intervening low
points between the fuel tanks and
the main drain (usually on the
gascolator) to ensure that
water and contaminants will
only collect where they can
be easily removed from
the system. Secondly, vi-
bration leads to fatigue
failures and should be con-
trolled by providing support
at regular intervals along the
pipe run - every 250mm is a
good rule-of-thumb for small diam-
eter pipes, but larger pipes will tolerate
larger spacing. Care must be taken to ensure
the pipes cannot chafe on support brackets or
any other part of the aircraft structure, routing
them away from areas where they may be in-
advertently kicked or trodden on is also a good

FOR RECREATIONAL PILOTS. Sport Pilot^51

design


notes


The energy required


to get you into the air has


to be stored somewhere


FIGURE 2


“fuel


is definitely


a liquid and


we’d really prefer


it to stay that


way”

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