52 Sport Pilot. (^) FOR RECREATIONAL PILOTS FOR RECREATIONAL PILOTS. Sport Pilot 53
idea. Finally, locations where relative move-
ment can be expected between pipe ends,
such as between the engine and the firewall
for example, are not suitable for rigid pipes and
flexible hoses should be used instead. Hoses
with appropriate temperature, pressure and
chemical resistance ratings are required, pref-
erably those protected with an external braided
metal sheath.
HAVING A GAS
Americans may insist on calling it gas but avia-
tion fuel is definitely a liquid and we’d really
prefer it to stay that way. Fuel pumps don’t like
pumping vapour and carburettors can’t meter
it, so allowing gas bubbles to form in your fuel
system is a really bad idea. Unfortunately for
us, combining Mogas (which is more suscepti-
ble to vapour problems than Avgas due to its
higher vapour pressure) with a hot Australian
summer day and high altitude creates the per-
fect recipe for vapour lock.
While an engine driven fuel pump is a great
idea from a mechanical reliability point of view,
it has some drawbacks when it comes to fuel
vaporisation. First up it requires fuel to be
routed close to the hot engine block, which
raises its temperature. Secondly the pump is
mounted on the engine and so sucks the fuel
up from the gascolator rather than pushing it,
which reduces the pressure in the fuel line.
Both of these conditions increase the risk of
vapour bubbles forming and need to be con-
trolled. Placing firesleeves over the fuel pipes
under the cowl gives fire protection but has the
side benefit of insulating against engine heat.
Any fuel systems requiring a pump should also
be fitted with a backup electric booster pump.
By locating this pump near the bottom of the
system will help avoid low fuel pressure in the
lines, at least while it is switched on.
BAD ATTITUDE
Aircraft fuel systems are exposed to much larg-
er changes in attitude than most ground based
vehicles. This presents some unique challeng-
es when it comes to keeping the fuel flowing.
For shallow and wide tanks the risk of attitude
changes causing tank outlets to un-port is al-
ways a concern. A header tank is a common
solution but installing baffles to reduce fuel
slop and hold fuel near the tank outlets is also
effective.
In a system with directly interconnected
wing tanks, fuel will naturally transfer from
one tank to the other any time the aircraft is
flown with the slip ball off-centre. This can be
irritating from a trim point of view, but taken to
extremes can also cause a dangerous weight
imbalance. When the tanks are full it will also
result in fuel draining between tanks and then
out of the aircraft through the vent system. Con-
nected tank outlets but separate vent lines can
also cause issues, especially in systems with
a fuel pump. If the vents are exposed to differ-
ent static pressures one tank can drain faster
than the other, increasing the risk of one tank
emptying and un-porting allowing the pump to
draw air into the system while the other tank
still has fuel in it. Of course the alternative is to
install a selector switch so only one tank can be
selected at a time, but that then relies on the
pilot managing the system correctly, and let’s
face it, pilots don’t have a great track record in
that department.
Roll is not the only problem - pitch changes
can cause serious problems too. I’ve not said
much up to now on gravity vs pump fed systems,
but pitch is likely to be the determining factor be-
tween which system to use. There are plenty of
tales of homebuilt aircraft experiencing an EFATO
on their first flight because the nose high attitude
had not been properly accounted for in the fuel
system design, resulting in fuel starvation on
climb out. If gravity can’t reliably deliver fuel at
adequate pressure in any attitude at which the
aircraft can reasonably be expected to fly, then
a fully redundant pump fed system is the only
choice, despite its increased complexity.
For fuel systems, as with just about every
other aspect of aircraft design, the devil is in
the details and adding complexity often leads
to lower reliability. There’s a lot to be said for
the simplicity of a well designed single tank
gravity fed arrangement... just remember to
keep the blue side up.
NEXT MONTH Cooling
DESIGN NOTES
joyce
(Joyce)
#1