REFLECTOR:Low fuel light
Chuck Jensen
reflector@tvbf.org
Mon, 28 Jul 2003 09:02:59 -0400
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John,
It's agreed that the sump would react with more gain than the tanks but
there are probably a couple factors going on here that would mitigate that
factor. 1st, any change is so slow, that if the vent system has decent
sized lines (1/4" may be too small) that our rate of descent/climb is really
pretty subtle change rate.
Unless, of course, you're going to rolling over on your back and going into
a vertical dive for a strafing or high speed bombing run. You know, the
more a person flies, the more that I'm astounded at the piloting skills of
our Air Force pilots. Mind-boggling, and some of them are just past being
teenagers; but a little off subject.
And, 2nd, there really shouldn't be any air space in the sump, if it's
properly vent to a static condition with the tanks, and so it may not make a
difference that the change-rate for the sump is slightly different than the
mains.
All these nice things people were posting about mutual respect,
understanding and sensitivity, heck, it made me want to go hug somebody!!
Chuck
-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On Behalf Of
John Dibble
Sent: Sunday, July 27, 2003 10:36 PM
To: reflector@tvbf.org
Subject: Re: REFLECTOR:Low fuel light
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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<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2>John,</FONT></SPAN></DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff size=2>It's
agreed that the sump would react with more gain than the tanks but there are
probably a couple factors going on here that would mitigate that factor.
1st, any change is so slow, that if the vent system has decent sized lines (1/4"
may be too small) that our rate of descent/climb is really pretty subtle change
rate. </FONT></SPAN></DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2>Unless, of course, you're going to rolling over on your back and
going into a vertical dive for a strafing or high speed bombing run.
You know, the more a person flies, the more that I'm astounded at the piloting
skills of our Air Force pilots. Mind-boggling, and some of them
are just past being teenagers; but a little off
subject.</FONT></SPAN></DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff size=2>And,
2nd, there really shouldn't be any air space in the sump, if it's properly vent
to a static condition with the tanks, and so it may not make a difference that
the change-rate for the sump is slightly different than the
mains.</FONT></SPAN></DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff size=2>All
these nice things people were posting about mutual respect, understanding and
sensitivity, heck, it made me want to go hug somebody!!</FONT></SPAN></DIV>
<DIV><SPAN class=868021712-28072003></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2>Chuck</FONT></SPAN></DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=868021712-28072003></SPAN><FONT face=Tahoma><FONT face=Arial
color=#0000ff size=2></FONT><FONT face=Arial color=#0000ff size=2></FONT><FONT
face=Arial color=#0000ff size=2></FONT><BR><FONT size=2><SPAN
class=868021712-28072003><FONT face=Arial
color=#0000ff> </FONT></SPAN>-----Original Message-----<BR><B>From:</B>
reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]<B>On Behalf Of
</B>John Dibble<BR><B>Sent:</B> Sunday, July 27, 2003 10:36 PM<BR><B>To:</B>
reflector@tvbf.org<BR><B>Subject:</B> Re: REFLECTOR:Low fuel
light<BR><BR></FONT></DIV></FONT>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">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.
<P>John
<P>Chuck Jensen wrote:
<BLOCKQUOTE TYPE="CITE"> <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. 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.</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. 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. </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!) </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. 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. </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 </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. 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. </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></BLOCKQUOTE></BODY></HTML>
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