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David Staten wrote:
Jim Sower wrote:
I think that makes the sump tank way too
"busy". Submerged pumps are nice but not pivotal. Returning fuel from
the rail to the sump is what got Paul Connor in trouble.
Gravity feed main(s) to sump, return to main(s) ... Jim S.
Dave,
How do we KNOW that the return to the sump is what caused the prob? Are
we talking his redesign or the original install?
That was the consensus after looooong
discussion. Paul had a small unvented sump to which he returned fuel
from the rail. When things got hot and he returned vapor, it had no
place to go, so it accumulated in the sump, displacing fuel until the
inevitible happened.
If we are talking about his first engine out, if I remember right, he
had the pumps mounted high, and his fuel lines were un-sleeved/
un-insulated.
Yes we are. Yes they were. But he might well
have gotten away with that had he not returned hot fuel (/vapor) to the
sump.
The reason I am SO interested in this particular issue is that the
Velocity has a sump tank as well, but it holds nearly an hour of fuel
at lower power settings.
I am aware of that. I have a Velocity too.
I've had hideous problems with assymmetric [gravity] feed to the sump
from the strakes. It doesn't take much to disturb that flow (at least
with 3/8" lines). I've had a flame out from the left strake not
gravity feeding properly to the point that the 5-gal sump ran dry with
a nearly full wing tank.
It HAS the room for an in-sump fuel pump, if desired (even though we've
closed it already), and with the volume it lists, I am suspecting that
it has enough thermal "inertia" to prevent the sump from over-heating
the fuel to the point it would vapor lock.
The fact that it has room for a pump does not
in and of itself make it a good idea to put a pump in there. I'm
against submerged fuel pumps on principle. Just a strong opinion
(driven mostly by instalation and maintenance issues). There are
plenty of other places to put fuel pump(s) and lots of injected cars do
just that.
My original plan was to return fuel to the sump, but after Paul's first
engine out to also have the sump vent be capable of overflowing into
one of the wing tanks (both of which gravity feed into the sump).
I think returning fuel to the sump is a terrible
idea. It's too small to properly cool the fuel, and it's very
sensitive to transfer issues. Paul "solved" his problem by mounting
his pumps lower (not the cause of his first failure IMO but a very good
idea) and venting his [heretofor unvented] sump to the main tank (a bad
idea IMO). One possible scenario that I can see on his accident is the
long (10' - 12') EZ/Cozy-type run of fuel lines from strakes through
selector valve to sump could have very well presented more resistance
to fuel flow than the vent line back to strake presented to air flow
(into sump), causing gradual depletion of sump fuel until he ran out of
gas.
Trying to figure this out and follow the data.
Dave
My own (personal) recommendation is:
Large (>1/2") gravity feed from Right
("supply") strake to Sump;
Sump feeds engine; return from rail to Right strake;
Have a facet pump that transfers fuel from Left ("transfer") strake to
Right;
Sump vent is capped off (and only uncapped to initially fill the sump);
ONLY fluid path into sump is from "supply" tank;
"Transfer" tank feeds ONLY the "supply" tank.
Dirt simple, as reliable as it can be, does require some fuel
management (which if I can't handle, I shouldn't be flying unsupervised
:o). Simple warnings and other simple gizmos can assure more timely
fuel management.
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