If for any reason one strake
is reluctant to transfer,
I can de-select the
tank that's
transferring OK and suck
fuel from
the reluctant tank (this would be impossible if the sump were vented).
Both "suck" and "reluctant" are relative terms since the
head pressure between the strake and the sump is quite small - 0.3 psi
with
full strakes, 0.1 when empty.
That's why I believe sumps should be a closed system .... Jim S.
I’d say
what works for you is
what works. But before adding potential problems
of electric valves, the requirement to use them, and the potential for
air or
vapor locked in the sump tank, I would determine why I had “reluctant”
gravity. There has got to be a reason, you just haven’t found it
yet. Agreed. I thrashed my brains out for over
a year. Spent a LOT of time on the Cozy and Velocity lists (as you
know). Nobody had ANYTHING that I hadn't already tried - except going
from 3/8" to 1/2" or 7/16" lines from the strake to the sump. The
valves were my last gasp, and much more of a workaround than a
solution. As you know, assymetric transfer is quite common in
Velocitys - mine is just the worst and most persistent. You
have a good ‘work around’ that you are willing to
manage, and that’s fine; I just wouldn’t generalize that to be the
best approach in all cases. Agreed. I didn't
mean to suggest that everyone would encounter what I did gravity
feeding a sump. But lots of people have lesser problems.
Assymetric/reluctant flow is something you have to watch out for
gravity feeding a sump from multiple sources. Reluctant flow could
give you problems if sump is vented. LARGE lines from strake to sump
should be a solution (that I might yet incorporate - I'm just reluctant
to open the bulkhead between the wing tank and the cabin).
You
also said: Your
sump basically
recycles completely every 4 minutes or so. Your fuel could be really
REALLY
hot just when you begin your takeoff roll.
Do
you have any reason to believe this;Sort of;
like any measurement? I estimated (key word) a
30-35 gph pump, 2-gal sump (your Vel?), 2 gph idle usage => >30
gph return => turns over 2-gal tank 15 times / hr. Pump capacity
and sump volume are BIG variables. 2 gph from main tank doesn't put
much of a dent in 30 gph return. There is always
some fresh fuel coming from the mains. How much heat is the fuel line
really picking up? With a 3 gallon sump and a 30 gph pump the recycle
is clearly
less than once every 6 minutes. Yeah, if I was on the taxiway more
than
20-30 minutes I might be concerned. By then I’d probably have turned
off
my engine a few times while I waited. I’ll plan on getting some temp
measurements on my sump tank temp and we’ll see. Good
idea. Measured numbers always better than SWAGs.
BTW,
my comment about a power runup before takeoff was to bring fresh cool
fuel into
the sump before takeoff. And as soon as I
apply full power for takeoff
roll the fuel temp in the sump tank is going down. (Very little
recycle, fuel
flowing in from the mains) If there were to be an issue, wouldn’t I
know
it at the beginning of the takeoff roll? I don't think so. With a 2-gal sump and 20 gph usage,
it will take upwards of 5 min to replace the sump
fuel. Any problem is going to happen long before that IMO. Paul’s
issue in the
earlier version was not just pumps higher than the sump, but also
restrictive
flow into the sump. I think badly located pumps
and restrictive flow (not quantified) were not his main issues. I
believe his biggest problem was returning fuel to a [small] sump. Was
his sump < 2-gal?
I
have no objection to returning fuel to the mains; that’s fine. I’m
not at all convinced that is a requirement that needs to be backfit. I'll be better able to speak to that when I retrofit
5/8" lines from my strakes to my sump :o)
I’m
headed of on vacation in the morning for about 10 days, so you can say
anything
about me that you wantJ. At the risk of repeating myself, by now we have a
pretty good handle on the down side of returning fuel to sump. I don't
recall hearing the up side (aside from it's already in place - sort of
like my 3/8" feed lines :o)