REFLECTOR:Low fuel light

John Dibble reflector@tvbf.org
Sun, 27 Jul 2003 21:36:27 -0500


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Yes, I agree that the  pressure will be equal, under static conditions.
The problem is keeping the pressure equal under dynamic conditions like
climb, descent, and leaking caps. If connected directly to the manifold,
the sump pressure will change faster than the tanks because there is
very little air space in the sump.

John

Chuck Jensen wrote:

>  John,At the risk of interpreting what you wrote, I believe you are
> concuring with my earlier post; that being, if the tanks and sump were
> all connected to the same vent-line (a manifold if you like), then the
> pressure in both tanks and sump, by definition, must be equal.   In
> your case, you were proposing to vent the sump to each of the tanks,
> though this might allow water/air traps, depending on configuration.
> I would be more comfortable with each being vented to a collection
> manifold.With a common vent manifold, it really matters little whether
> the atmosphere above all the tanks/sump is slightly positive or
> negative; the draw of the fuel pump will overwhelm any slight negative
> pressure that could hypothetically be present.  And, since fuel
> responds to gravity, I would think it would dependably flow to the
> sump from both tanks and the sump would always be 100% full. After
> all, how could a pocket of air exist in a vented sump???(that'll start
> another round!) There would be a vertical column of fuel in the sump
> vent line that rises up to the equal height of the fuel in the
> strakes.  This fuel column would need to let any air introduced to the
> sump (ie. sharp descent which uncovers the tank(s) outlet) ,pass out
> the vent.  Of course, if the strake outlets are uncovered and fuel is
> drawn by the engine from the sump, the fuel column in the vent line
> would empty into the sump.  When level flight resumed, the air in the
> sump would be pushed out the evacuated sump vent line by the ample
> fuel flowing from both tanks. Just to be sure, if there was any
> question of the ability of air to vent through a fuel filled 3/8" vent
> line (and Al's earlier posting indicates there probably isn't), a 1/2"
> vent line could be used instead to rise vertically from the sump to
> the common vent manifold, which is connected to the other two tank
> vents.And, this configuration certainly couldn't siphon from the sump
> tank, as experienced by SB during his test flight (when the backward
> facing sump vent drew a vacuum and started a siphon from the sump
> overboard), since siphoning depends on a hi/lo differential and there
> is none because all of the tanks are the same pressure.  No more sump
> sloshing.  No more concern about a leaky fuel tank cap.  No more
> unbalanced tank feeds from different pressures in the tank strakes
> from variation in pressurization from the vent line or leaky fuel
> cap......unless the plane is unbalanced in cruised, but that's a
> different problem and different post. Simple, no?Chuck

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Yes, I agree that the&nbsp; pressure will be equal, under static conditions.&nbsp;
The problem is keeping the pressure equal under dynamic conditions like
climb, descent, and leaking caps. If connected directly to the manifold,
the sump pressure will change faster than the tanks because there is very
little air space in the sump.
<p>John
<p>Chuck Jensen wrote:
<blockquote TYPE=CITE>&nbsp;<span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>John,</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>At
the risk of interpreting what you wrote, I believe you are concuring with
my earlier post; that being, if the tanks and sump were all connected to
the same vent-line (a manifold if you like), then the pressure in both
tanks and sump, by definition, must be equal.&nbsp;&nbsp; In your case,
you were proposing to vent the sump to each of the tanks, though this might
allow water/air traps, depending on configuration.&nbsp; I would be more
comfortable with each being vented to a collection manifold.</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>With
a common vent manifold, it really matters little whether the atmosphere
above all the tanks/sump is slightly positive or negative; the draw of
the fuel pump will overwhelm any slight negative pressure that could hypothetically
be present.&nbsp; And, since fuel responds to gravity, I would think it
would dependably flow to the sump from both tanks and the sump would always
be 100% full.&nbsp;</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>After
all, how could a pocket of air exist in a vented sump???(that'll start
another round!)&nbsp;</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>There
would be a vertical column of fuel in the sump vent line that rises up
to the equal height of the fuel in the strakes.&nbsp; This fuel column
would need to let any air introduced to the sump (ie. sharp descent which
uncovers the tank(s) outlet) ,pass out the vent.&nbsp; Of course, if the
strake outlets are uncovered and fuel is drawn by the engine from the sump,
the fuel column in the vent line would empty into the sump.&nbsp; When
level flight resumed, the air in the sump would be pushed out the evacuated
sump vent line by the ample fuel flowing from both tanks.&nbsp;</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>Just
to be sure, if there was any question of the ability of air to vent through
a fuel filled 3/8" vent line (and Al's earlier posting indicates there
probably isn't), a 1/2" vent line could be used instead to rise vertically
from the sump to the common vent manifold, which is connected to the other
two tank vents.</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>And,
this configuration&nbsp;</span><span class=436085419-27072003>certainly
couldn't siphon from the sump tank, as experienced by SB during his test
flight (when the backward facing sump vent drew a vacuum and started a
siphon from the sump overboard), since siphoning depends on a hi/lo differential
and there is none because all of the tanks are the same pressure.&nbsp;
No more sump sloshing.&nbsp; No more concern about a leaky fuel tank cap.&nbsp;
No more unbalanced tank feeds from different pressures in the tank strakes
from variation in pressurization from the vent line or leaky fuel cap......unless
the plane is unbalanced in cruised, but that's a different problem and
different post.&nbsp;</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>Simple,
no?</font></font></font></span><span class=436085419-27072003></span><span class=436085419-27072003><font face="Arial"><font color="#0000FF"><font size=-1>Chuck</font></font></font></span><span class=436085419-27072003></span></blockquote>
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