REFLECTOR: FUEL FEED - One New Idea

[Al Gietzen]

Jim wrote:

 

I wold bet that the highest point in the strake during climb out would be
within 6" - 8" from the most forward point in the fuel tank.  When you cock
the airplane up to climb attitude, you will have a very considerable
elevation difference between the "bubble" in the forward portion of the
strake and the vent outlet at the spar.  The vent air has to overcome that
head pressure to replace fuel being consumed with air.  

 

The 6” – 8” may be a reasonable bet for the full tank, steep climb; but what
ever it is; keep in mind that when the tank is full you have a 12” head to
the sump tank at level, and with the nose tilted up 10 degrees, that goes up
another 8” – 9”, so in that condition you have about 20” of head.  Again,
more than needed to overcome the 6-8” of back up in the vent line.  No pump
suction needed.

 

So I will postulate once again; unless the vents are plugged, or the fuel
caps are leaking, adequate fuel will flow.

 

NEW IDEA

In all this ruminating; it occurred to me that there is an improvement can
easily be made.  The maximum driving head available for flow from the main
to the sump is the difference in elevation between the surface of the fuel
in the tank, and the exit of the feed line from the tank to the sump.
Normally the fuel level in the sump is above that line exit, and the
effective head is the difference between the two levels. 

 

But in the event of some anti-gravity force in the main tank (cap leak), and
the sump level drops, we’d like to have more head available to drive the
flow.  This can be achieved by lowering the point at which the feed line
enters the sump.  There is no advantage, that I can see, for the feed line
to enter at the top of the sump, as long as the sump is vented so you never
need air to flow back up the line to the main.  By lowering the point that
the feed line enters the sump you can pick up another 8” – 10” of pressure
head – in the event that the level in the sump is for dropping for some
reason.  

 

This additional driving head may be sufficient to overcome the negative
pressure above the strake, and prevent fuel exhaustion in the event of a
fuel cap leak that is larger than the vent lines can handle.

 

Tell me if this is all wrong.

 

Al

 

 





   

<>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.

I was in enthusiastic with all of this until it happened to me.  My vents
are 3/8" (min) all the way to within 3" of the tank where it necks down to
1/4" (I didn't think it was worth chopping a hole from the cabin to the tank
to install 4" of 3/8" line).  I taped up my fuel caps to preclude the
possibility of even a molecular sized lea,.  Still I got assymmetric flow.
Perhaps I should have said "damned small aberration" or something.



 

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.

ONLY if it is flowing as advertised (which mind did not, and it is the
exceptions that we are discussing).  Consider:  If the gravity feed from one
strake is "underperforming", who is to say the other is not underperforming,
but to a lesser extent?  In any event, if you have an open system, you are
projecting that the two components (strake and sump) will work in concert.
If they do not, you are limited to sump head pressure.  If you cap off the
sump vent, they will work in concert whether the strakes want to or not.



 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. 

Far from it.  The fuel level in the sump is NEVER as high as the strake.  If
they are a single system (sump vent closed) they MUST act in concert whether
the strakes want to or not, and head pressure to the pump is total head
pressure.  If the sump vent is open, they CAN act in concert, but if the
strakes (including the plumbing to the sump) get balky, they need not, and
with an open sump vent the pump CANNOT overcome the balkiness of the
strakes, and you can end up with nearly full strakes and only the sump
available to the engine.





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.  

</> 

Which brings us back to the bumblebee ...



<>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.  

</> 

Damned if I know.  With the numbers as small as they are, I reasoned that
invisible line losses could upset the delicate balance between the tanks.
Half inch lines would be the mallet with which I obliterate those losses.
Like I said, I didn't so it because I had nothing scientific to support it,
and it required chopping holes from the cabin into the wing tank.  It would
result in greatly reduced line losses, but would only solve the transfer
issue if line losses were the problem.



<>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

</> 

I tend to agree.  But that is small comfort :o)  I would hasten to add that
the PROBLEM is not at all specific to my case, only its SEVERITY and
tenacity.  Getting back to the basic issue, the problem is too common to be
acceptable and if the easy fixes don't solve it, we are obliged to question
the design (face it, LOTS of stuff works in spite of itself).  
I had a flameout in Canada and had to dead stick back to Port Huron because
of assymmetric transfer.  I had noticed it early on and was monitoring it
and trying to get some fuel from my left strake by kicking rudders, etc.
The right tank went dry and sometime thereafter the sump light came on.
That interval was long enough that I thought is was because the left tank
was doing better (how much better was impossible to tell).  Left tank fuel
seemed to be going down, and I was kind of reluctant to land unannounced in
Canada so soon after 9/11.  The sump fuel got me further than I could have
expected out of five gallons, but when it quit, I was ready for it.
My concern is for the guy who gets a "blinking" sump light and has learned
to ignore it as a false positive.  Some day when it casually goes from
blinking to steady,  he may very well not give it the attention it deserves.
I have a workaround (not to be confused with a fix) that [very nearly] moots
this whole issue. 
Jim S.



<> </> 

Al
 
 
 
 





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