I'm looking for some peer review on this fuel
system to either validate my choices or tell me I'm a fool! But
first I should explain some things.
Some may notice that the regulator return line is routed to
the filter which is unusual. I had intended to route this back
to one of the main fuel tanks, similar to what Dave has done
in his diagram. However when I found this Parker filter
housing, it has 2 inlets and 2 outlets plus an extra port
which was plugged. This port is plumbed to the inlet side of
the filter and could be used as a vent or inlet. So I decided
to try it as a regulator return.
So far all ground tests show it to work well. The only
exception was as I've previously mentioned, after an hour of
heat soak, which likely would be the same regardless of
where I returned it to. My reasoning on this is that with
vapor in the hi-press pumps I can't produce the 45 psi
required to allow the regulator to open and pass the fuel to
any destination. routing the excess fuel from the regulator
back to the tank would certainly mitigate this problem, but
would it eliminate it?
Why I like the momentary-on purge system is it can
rapidly clear air/vapor as there is no significant
back-pressure, so even the Facet boost pumps (7psi) (mounted
in a cool location) can easily clear air/vapor. So with that
in place, is there any reason to route the regulator return
back to a tank?
To give some idea as to my thought process on my fuel
system, I should give some background as to how I arrived at
this current point. Back when I started my plane, I was
inspired by crazy bloke from down under that was flying his
RV-4 around the world. Not once, but 3 times. First going
east, then going west, then going north and south. Some of
you will remember Jon Johanson. Well like most builders they
start with less grandiose ambitions and build an airplane
with far less fuel capacity than is required for an
adventure like this. Then as their horizons expand they are
forced to add fuel capacity, usually in less desirable
locations that result in a dangerously aft CG and lots of
fuel in the cockpit. Max, just went through this.
Now at that time, I said that I wasn't necessarily
planning on following Jon's path, but that I wanted to
ensure all options were open. Now all these years later, it
seems much less likely than ever as so many responsibilities
keep me closer to home, but I am not ready to give up on my
design philosophy.
Before I began my wing construction I made the
decision to make the entire leading edge wet as well as the
tips as this, while it increases my GW, does not result in a
dangerous CG. It also keeps all of the fuel out of the
cockpit and has far better span loading. But it also results
in 6 tanks that I have to manage, which any way you slice it
is going to have a complex fuel system.
Right from the start and even today I'm aware that a
large portion of GA accidents involve poor fuel management.
So simple is better. So why, in the face of other proven
alternatives, am I pursuing an apparently needlessly complex
fuel system????
At the risk of increasing complexity, I wanted to
ensure that no single failure or leak would prevent me from
accessing fuel on-board. Loosing fuel is bad, but not being
able to access fuel on board is even worse. I didn't like
the idea that a failed transfer pump could result in having
a tank full of fuel that I couldn't use. Not only would that
leave me with not enough fuel, but I'd still be packing the
weight of it, likely also be quite unbalanced, requiring
significant trim to correct, increasing drag, thereby
decreasing range. And any resulting forced landing (assuming
it's not over water) would still have fuel to feed any
possible fires.
I know that significant fuel tank leaks are rare,
but that was something else that concerned me, even though
now looking back it really wasn't a legitimate concern. But
at the time I wanted to avoid the possibility of returning
fuel to a tank that could be leaking, thereby reducing my
range greater than if I'd just lost that tank alone. The
more reasonable concern would be returning fuel to a full
tank and loosing it out of the vent.
So you can see that my design was driven by my fear of
being over water or inhospitable terrain without enough
accessible fuel to reach a safe destination. This is why I
have optical fuel sensors to allow me to run a tank bone dry
and know the moment it does so. So far my test experience
has shown that any air in the system from this is quickly
cleared through the injectors of a running engine, however,
the purge system ensures this can be rapidly vented if
required. The vent system has a manual isolation valve to
back up the solenoid valve, to ensure that a valve leak
doesn't result in insufficient fuel pressure along with a
loss of fuel. This could and probably should be routed all
the way back to a tank. My choice to tie into the vent
system, stems from an conveniently accessible capped tee in
the vent line that was originally used to vent my long ago
removed header tank.
And my valve arrangement will minimize any risk of
not being able to access any fuel. And it will keep all
tanks isolated from each other. So if it becomes clear that
a tank is loosing level on it's own, that tank can be
selected to use the remaining fuel before it is all lost,
then the tank is isolated and not a risk to loose fuel from
another tank. Not returning fuel to any tank will eliminate
the risk of loosing fuel through overflowing through a vent.
Now this one is easily mitigated by use of a timer and well,
just being a responsible pilot!
Now with my reasons for this design and resistance to
adopt a simpler, proven system design explained, I am open
to comments, criticism, etc. At this point I'm ready to haul
it to the airport and would really rather not make any
changes, but since the continued forest fire situation here
is still keeping our airport too busy to accommodate any
flight testing that I'd like to do, if I have to make any
changes I'd rather do it now.
Max's recent adventure has only strengthened my resolve
to take this bird on at least a few long distance
adventures.
Todd. Gonna have to learn to pee in a bag! :-P