Tony Renshaw wrote:
> Gidday,
> I have been following this thread for a while, because I started it.
> I am surprised there isn't anything definitive, but I appreciate
> thereis a difference between ID and OD. I do however "not" have my
> head around why you would need a restrictor in any fuel return line,
> irrespective of engine type?
At least on the 914, the maintenance-manual cutaway view of the pressure
regulator plus accompanying text looks clear at least to me that any
more than max specified back-pressure in the return line can deliver
excess pressure to flood the carbs. This is because the pressure from
the electric pump is substantially irrespective of engine RPM. But in a
mechanical pump, pressure is proportional to RPM but conveniently
coincident with engine needs, and at least in generic mech pumps (Rotax
same?) some degree of pressure regulation occurs, irrespective of
manifold pressure -- in contrast to the the 914. And without sufficient
restriction in a return line for the 912/912S, insufficient pressure
can result.
> Superficially, or based on "downunder" common sense (dare I say
> it......and I have this horrible feeling I am about to eat dirt),
> but, it doesn't gel. So, why is it so? and why isn't there a bit
> more definitive answers to the fuel dimension questions?
In following the thread also, I admit to some head-scratching, but at my
age that is rather harmless to the scalp. The diameter of the
fuel line is critical mostly to gravity-feed systems. If one studies
FAA's AC 23-16, Powerplant Guide for Certification of Part 23 Airplanes,
the required in-flight testing is broken down between gravity, pressure
(e.g. 912), and pressure-regulated (e.g. 914) systems. The test
requirements for the latter are indeed the least; gravity the most.
In fact, mere 1/8" I.D. of fuel line - if straight and not uphill,
should deliver to the pump about 4 times the max flow rate of the 914.
Even measured at the measly head pressure of the first gallon of usable
fuel as FAA requires for tests. The only issue is the pump's ability
to suck it out of the tank, any air in the line notwithstanding, and no
surprise that Rotax says mount the 914 electric pumps low and nearby the
tank as practical.
Downstream of any pump, it's all meaningless with sufficient pressure,
except for one thing. It's been established in University research that
the velocity of fuel anywhere in the line is relevant to tendency to
vapor lock. Narrower ID means greater flow velocity with one of them
squares in the computation, and resulting vapor-locking cavitation, but
a fat fuel line to slow her all down may not be a solution fwd of the
firewall due to greater time exposure to heat.
Further to the issue is that pre-pumped fuel flow is not so much
influenced by line internal diameter as it is by 1) altitude effects -
lesser head pressure especially affecting potential 914 operating
altitudes; 2) agitation in the tank - another vapor pressure problem,
and indeed there are potential tank baffling issues for production
approval; 3) hydrodynamics due to restrictions and bends in the system;
and 4) the head pressure as affected by "ram" air delivered by the tank
vent. Latter is proportional to another pesky square of the velocity,
and needs to exhibit positive pressure, relative to pressure someplace,
at low speed/high power/high density altitude, but not so excessive at
low-power/Vne speed/low altitude to damage or leak anything in the
system. It's also true that the constant stream of an electric pump can
be less insidious, agitation-wise, than the puslsations of an
engine-driven pump.
For the latter reasons, FAA requires of type-certificated manufacturers
to prove the entire system in flight testing under all extremes of
conditions, including as to vapor lock. One example is the effect of
any fuel-flow sensor which agitates especially higher VP auto fuel. But
as a practical matter, the roughly 5/16" ID of the soft line per Europa
is very similar to 3/8" OD aluminum - which will even be seen on much
higher-HP production aircraft, so like has there been any problem with
either of these on Europas? Still head-scratching.... :-)
Regards,
Fred F.
N3EU
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