REFLECTOR: FUEL FEED - One New Idea

Al Gietzen ALVentures at cox.net
Tue Feb 22 09:20:09 CST 2005


Subject: Re: REFLECTOR: FUEL FEED - One New Idea

 

Al,

 

I have a concern with your concept of "picking up another 8" - 10" of
pressure head - in the event that the level in the sump is for dropping for
some reason".  If the sump is vented, the pressure head in the sump must be
measured from its free surface.  This free surface may be up in the vent
line.  However, if the level in the sump is dropping, the free surface has
been drawn down.  The only way such a condition could exist is if there were
some resistance to flow between the strake and the sump.

Keith;

 

Yes, I agree, under normal circumstances lowering the feed to the sump will
make no difference. I think what we are trying to do here is provide
additional pressure head from the mains to the sump to overcome abnormal
conditions; where there is resistance to flow from the main to the sump. In
abnormal conditions, like leaking cap, where the sump level is drawn down
the available pressure head from the main to the sump is increased by the
amount that you have lowered the feed point to the sump.

 

The bottom line is that the location of the entries into the sump will not
affect the head.  The location will affect whether bubbles can escape up the
fuel supply lines, however.  A little wing rock can easily result in air
travelling up one supply line to one strake while fuel passes from the other
strake to the sump through the opposite supply line.

True; it does not change the pressure head from sump to pump. That remains
positive and is not the issue. When the level in the sump is being drawn
down; the max available pressure head is from the surface of the fuel in the
main to the exit into the sump; and is increased by the amount that you
lower the feed point to the sump.  If this increased pressure head from main
to sump is sufficient to overcome the negative pressure above the fuel cap,
you have eliminated potential fuel exhaustion due to leaky cap.

 

Air bubbles getting into the sump will go out the vent rather than back up
the line to the main in either case.

 

Does anyone know the pressure difference between ambient and that which
exists in the area of the fuel cap at cruise?

 

Jim's approach to "managing" the sump vent is a way to overcome lack of flow
from the mains, but I think are potential issues with that as well; but that
is another discussion.

 

Al

 

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