REFLECTOR: FUEL FEED

[Al Gietzen]

Response to Jim’s response:

 

1)    Main tank vents from the highest point in the tank, continuous upward
slope to the manifold point that is well above the upper level of the tanks.
You do not want fuel in the vent lines.

Highest point in the tank varies considerably with airplane attitude.  Most
vents are to aft inboard corner of strake.  In a climb, a point about a
little aft of the forward most point of the strakes will be several inches
higher than the aft corner.  That means several (3"?) of fuel in the vent
line.  A LOT of chatter recently on the Cozy list.  Consensus was:  if you
want acceptable venting at full fuel you need two separate vents for each
tank - one at aft inboard corner, another a couple of feet further forward.
One or the other will be

 

I brought my vent lines forward along the top inside surface of the strake
about 12-13” from the rear bulkhead to the highest point. Checking the
current manual I see it suggests the fitting at the back corner.  But I
think in any case we a talking about ½” or so of difference in height, and
there is more than enough head difference to draw the fuel out of the line
for air to come in.

   

About the sump tank vent - 1.) the tank must be vented to allow it to fill,
2.) because of the low vapor pressure of fuel (particularly auto fuel) you
do not want to depend on "sucking" fuel anywhere because of the possibility
of forming vapor at the pump (vapor lock); 3.) if you have fuel injection
with recycle flow back to the sump, you want a sump vent for air or vapor
bubbles from the return line.

I respectfully disagree.  There is about 6" head pressure from a half full
strake to sump at cruise AoA..  That is about 0.16 psi.  Damned small
pressure.  A damned small aberration (like a damned small leak in a fuel
cap) will make it go away.  Gravity feed can be stopped entirely by a
problem (cap leak again?) that you can't even measure!  That's mainly what
drove me nuts for so long.

I agree that it is a small difference, but it takes very little pressure
head to drive 20 gph through a 3/8” tube.  But I respectfully disagree with
disagreement (are we carrying diplomacy too far :-)) that it only takes a
damned small leak because a ¼” (or 3/8”) vent line provides enough air flow
to offset what I would consider a fairly large leak; one that could be
easily measured; maybe difficult to overlook.  I can’t think of many
aberrations that stop gravity in the tank beyond a serious cap leak.  But
then, small is relative.

 

Further, it seems to me; if you vent the sump for it to fill, and then close
the vent, you eliminate gravity flow from the mains because you have fixed
the level in the sump (somewhere up the vent line) at whatever the level was
in the mains at that time, and you now are depending on the slight negative
pressure created by the fuel pump to draw the fuel from the tank. Not the
best situation.

Beg to differ.  Assuming you mount your boost pump as low in the bilges as
you can, you are much better off.  You've made the strake(s) and the sump a
single system.  Head pressure to the pump inlet is the distance from the
pump inlet to the surface of the fuel in the strake.  That's (depending on
aircraft attitude and how full the strakes are) maybe a foot (and possible
18") higher than if the sump were vented.  Sump being maximum a foot from
surface to pump inlet, you've at least doubled the head pressure at the pump
face by capping off the sump vent.

 

You are correct, but perhaps I am also correct because we are looking at two
different aspects.  The head at the bottom of the sump (barring any other
difficulties) is the same in either case.  The level in the vented sump is
the same as in the strake; the fuel will go up the sump vent line until it
reaches the level in the strake. You have to include that as “head” as well.

 I was considering only the driving force from the tank to the entrance of
the sump.  If the fuel level in the sump is the same or higher than in the
tank (and it will be including up the sump vent line, and the vent line is
closed), what is the driving force from the tank to the sump?  But, OK;
taken by itself it is probably irrelevant.  



FURTHERMORE, try a little experiment for me.  Can we assume that the
airplane deck angle (angle between fuselage reference line and the deck -
climb angle + AoA - is 10*?  If not, figure out a deck angle of your own for
say 2000 fpm climb at say 90-100 kts and use it.  Let's further assume that
the tanks are full.

NOW:  A)  Cap off your sump vent;  B)  Fill your strakes with fuel;  C)
Jack the nose to a 10* deck angle;  D)  Open your fuel line at the engine
driven pump and let it drain into a bucket.  E)  Observe (keeping time) what
happens.  I think your strakes will drain, but slowly.

NEXT: A)  Open your sump vent;  Repeat steps B thru E above and measure what
happens.
 I will hazard a guess that since your vent line is flooded, and there's
maybe 6" - 8" of fuel in the vent line, that only your sump will vent.
Absent suction from your fuel pump, your strakes cannot drain.  

 

Like I said above; the 6 or 8” of fuel in the main tank vent line only
represents a ½” or so of head (the line is basically horizontal, no?) and is
easily drawn out for the tank to vent and the fuel to flow.  The only way it
may not is if the main vent is plugged. In the case you present here, unless
I misunderstand, it makes no difference whether the sump is vented or not.





I can think of no explanation for the problems the Jim S. experienced
because he tried every fix, but there is something wrong somewhere.
Possibly different flow characteristics from the two lines; and/or less than
ambient pressure at the vent exit.

Neither can I.  I fancy myself a pretty good engineer.  I have looked at
this problem from every angle I can think of and tried every fix (except
switching from 3/8" to 1/2" strake-to-sump lines - which I suspect may have
solved the problem).  

 
Like I said; I can think of no explanation for the problems the Jim S.
experienced.  But if I can get 20 gph feed from one tank through a 3/8” line
how does it help to go the ½” line for 4-6 gph; unless there is the real
problem that you haven’t found yet.  I have to think that the problem is
specific to your case, not to the system design.  But I appreciate that you
have been very thorough about this issue, so I’m not able to be of any help.
You have a solution that works; go with it.
 
Al
 
 
 
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