REFLECTOR:Low fuel light
Chuck Jensen
reflector@tvbf.org
Sun, 27 Jul 2003 16:52:29 -0400
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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
-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On Behalf Of
John Dibble
Sent: Sunday, July 27, 2003 1:26 PM
To: reflector@tvbf.org
Subject: Re: REFLECTOR:Low fuel light
Scott,
I'm glad we're having this discussion. On my plane, each wing has an
overflow line exiting the bottom of the wing with a 90 degree bend to face
forward, exactly like one would see on a Cessna. I know it's an overflow
line because fuel pours out if I try to fill my tanks to the very top and
the wings are not completely level. The manifold vent line exits vertically
through the bottom of the fuselage and is cut at a 45 degree angle such that
the long end faces forward.
Basically what I have are pitot tubes pumping air into the tanks. I believe
the pressure condition is dynamic, not static, with a constant flow from the
tanks to the manifold vent.
So we have 2 things to discuss - what to do with my system, and what to do
with the standard Velocity system.
Concerning the Velocity system, I do not agree that pulling a vacuum via the
vent line will necessarily reduce fuel flow from the tanks to the sump. In
a static situation (no overflow lines) the vapor space pressure will be the
same in the tanks and in the sump and there will be no effect on fuel flow
no matter if there is a vacuum, neutral, or pressure on the vent line. Only
if the tank vapor space pressure is different from the sump vapor space
pressure can there be an effect on the flow. The pressure can be
temporarily different whenever there is a change in the manifold vent
pressure. For example during descent the vent pressure will increase. The
small (if any) vapor space in the sump will quickly increase while the much
greater air space in the tanks will take more time thereby making the tank
vapor pressure temporarily less than the sump and reducing fuel flow from
the tank to the sump. This effect will be greatest when the tanks have the
least fuel and that is the worst time for it because the head of fuel that
provides the flow to the sump is the least. I would think this problem can
be eliminated by eliminating the sump vent to the manifold and adding new
vents from the sump to each tank. The key point is that the sump would be
vented to the tanks. This would guarantee the sump vapor pressure would be
close to the tank vapor pressure at all times.
If the Velocity system has the vent line pointing into the wind, then both
the sump and tanks are being pressurized. I would think that the max
pressure would occur at max IAS, say about 150 kn for the standard Velocity.
Can someone tell me what pitot line pressure corresponds to 150 kn? This is
the max pressure that will be put on the tanks and I want to know if that is
acceptable. Also, is there any concern about the effect of repeated
pressurization and depressurization of the tanks? As I said before the
pressurization will not enhance flow from the tanks, however it will enhance
flow from the sump to the engine. However, I believe both fuel pumps can
achieve several psi of suction which dwarfs the pressurization effect.
My system should balance the tank and sump pressures dynamically. There
should be a constant influx of air through the overflow vents, throught the
tanks, to the manifold and out the vent. This flow should dominate the
pressure in the vent lines and keep the sump at the same pressure as the
tanks without pressurizing the tanks much, just whatever the small pressure
drop is as the air flows from the tanks to the vent. However, I need to
understand what's going on with the sensor. I haven't checked the sensor
yet. Shouldn't a sensor problem occur all the time and not just at high
altitude?
John
Scott Baker wrote:
John, Something you just said in your previous post has me concerned.
==============================================="I want to clarify something.
Previously I said my tank vent faced
forward, but it is the tank overflow line and not the vent that faces
forward and should pressurize the tank. My manifold vent points to the
rear and should pull a little vacuum on the vent system."
John
=============================================== a) If your vent system is
pulling a little vacuum, then you are "unpressurizing" the fuel tanks -
which slows the fuel flow from the main tanks to the header tank. The fuel
vent line should be installed so that it introduces pressure into the fuel
tanks. The construction manual clearly shows the vent line pointing into
the wind.b) What tank overflow line? The Velocity schematic does not
feature a "tank overflow line" (other than the vent line itself). It sounds
like your system has more tubes - which may be just fine - however it is
different from what is shown in the construction manual. Short story - For a
few years I owned a business to help composite aircraft owners build their
aircraft. While I was working on the fuel system for a Velocity customer I
had the "brainstorm" to redesign the fuel and vent system for a XL which
would allow for a fuel selector valve. I consulted with Duane Swing at the
factory - who was kind enough to review the facsimiles that I sent which
outlined my new and improved fuel system. Duane patiently pointed out some
possible pitfalls to the design. He offered a stern warning to be very
careful when changing something like this (meaning - the fuel system needs
to be 100% reliable if the engine is going to stay running - if you're going
to screw around (i.e.. alter the schematic or the installation) you need to
be pretty sure of what you are doing - and to think through every possible
situation that the change might affect. I went ahead with my improved fuel
line/vent system - and later - when mimicking a ground fuel flow test to see
if the system would deliver 25 gph (take off power) - I found that the fuel
lines had too many curves and had too long a run to sustain this flow rate -
which would have meant a partial loss of power about 2-minutes following
takeoff. What a sobering discovery! Following this I removed the "new and
improved" plumbing and reinstalled the system that was "in the book" - and
the aircraft is operating without problem. What I'm leading to is this - it
looks like someone has changed your fuel system. If you have more than one
vent line sticking out under the aircraft, then it is different from the
Velocity schematic. Someone obviously felt a need to "improve" the
fuel/vent system - what we don't know is this really an improvement - or is
it the source of your problem (as in my true story above)? As to your
question as to why the low sump fuel light might come on in aircraft with
the Velocity fuel schematic - in an earlier post I offered an opinion that
the float switch might be culprit - causing a false signal. Have you had a
chance to remove the float switch in order to test it? Is the float buoyant
in avgas? Best regards,Scott B
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<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2>John,</FONT></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff size=2>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></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff size=2>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></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff size=2>After
all, how could a pocket of air exist in a vented sump???(that'll start
another round!) </FONT></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff size=2>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></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff size=2>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></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff size=2>And,
this configuration </FONT></SPAN><SPAN class=436085419-27072003><FONT
face=Arial color=#0000ff size=2>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></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2>Simple, no?</FONT></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2>Chuck</FONT></SPAN></DIV>
<DIV><SPAN class=436085419-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT face=Tahoma
size=2>-----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 1:26 PM<BR><B>To:</B>
reflector@tvbf.org<BR><B>Subject:</B> Re: REFLECTOR:Low fuel
light<BR><BR></FONT></DIV>Scott,
<P>I'm glad we're having this discussion. On my plane, each wing has an
overflow line exiting the bottom of the wing with a 90 degree bend to face
forward, exactly like one would see on a Cessna. I know it's an overflow
line because fuel pours out if I try to fill my tanks to the very top and the
wings are not completely level. The manifold vent line exits vertically
through the bottom of the fuselage and is cut at a 45 degree angle such that
the long end faces forward. <BR>Basically what I have are pitot tubes pumping
air into the tanks. I believe the pressure condition is dynamic, not
static, with a constant flow from the tanks to the manifold vent. <BR>So we
have 2 things to discuss - what to do with my system, and what to do with the
standard Velocity system. <BR>Concerning the Velocity system, I do not agree
that pulling a vacuum via the vent line will necessarily reduce fuel flow from
the tanks to the sump. In a static situation (no overflow lines) the
vapor space pressure will be the same in the tanks and in the sump and there
will be no effect on fuel flow no matter if there is a vacuum, neutral, or
pressure on the vent line. Only if the tank vapor space pressure is
different from the sump vapor space pressure can there be an effect on the
flow. The pressure can be temporarily different whenever there is a
change in the manifold vent pressure. For example during descent the
vent pressure will increase. The small (if any) vapor space in the sump
will quickly increase while the much greater air space in the tanks will take
more time thereby making the tank vapor pressure temporarily less than the
sump and reducing fuel flow from the tank to the sump. This effect will
be greatest when the tanks have the least fuel and that is the worst time for
it because the head of fuel that provides the flow to the sump is the
least. I would think this problem can be eliminated by eliminating the
sump vent to the manifold and adding new vents from the sump to each
tank. The key point is that the sump would be vented to the tanks.
This would guarantee the sump vapor pressure would be close to the tank vapor
pressure at all times. <BR>If the Velocity system has the vent line pointing
into the wind, then both the sump and tanks are being pressurized. I
would think that the max pressure would occur at max IAS, say about 150 kn for
the standard Velocity. Can someone tell me what pitot line pressure
corresponds to 150 kn? This is the max pressure that will be put on the
tanks and I want to know if that is acceptable. Also, is there any
concern about the effect of repeated pressurization and depressurization of
the tanks? As I said before the pressurization will not enhance flow
from the tanks, however it will enhance flow from the sump to the
engine. However, I believe both fuel pumps can achieve several psi of
suction which dwarfs the pressurization effect.
<P>My system should balance the tank and sump pressures dynamically.
There should be a constant influx of air through the overflow vents, throught
the tanks, to the manifold and out the vent. This flow should dominate
the pressure in the vent lines and keep the sump at the same pressure as the
tanks without pressurizing the tanks much, just whatever the small pressure
drop is as the air flows from the tanks to the vent. However, I need to
understand what's going on with the sensor. I haven't checked the sensor
yet. Shouldn't a sensor problem occur all the time and not just at high
altitude?
<P>John
<P>Scott Baker wrote:
<BLOCKQUOTE TYPE="CITE">
<STYLE></STYLE>
<FONT face=Arial><FONT size=-1>John,</FONT></FONT> <FONT
face=Arial><FONT size=-1>Something you just said in your previous post has
me concerned.</FONT></FONT> <FONT face=Arial><FONT
size=-1>===============================================</FONT></FONT><FONT
face=Arial><FONT size=-1>"I want to clarify something. Previously I
said my tank vent faced</FONT></FONT> <BR><FONT face=Arial><FONT
size=-1>forward, but it is the tank overflow line and not the vent that
faces</FONT></FONT> <BR><FONT face=Arial><FONT size=-1>forward and should
pressurize the tank. <FONT color=#ff0000>My manifold vent points to
the</FONT></FONT></FONT> <BR><FONT face=Arial><FONT size=-1><FONT
color=#ff0000>rear and should pull a little vacuum on the vent
system</FONT>."</FONT></FONT> <BR><FONT face=Arial><FONT
size=-1>John</FONT></FONT> <BR><FONT face=Arial><FONT
size=-1>===============================================</FONT></FONT> <FONT
face=Arial><FONT size=-1>a) If your vent system is pulling a little
vacuum, then you are "unpressurizing" the fuel tanks - which slows the fuel
flow from the main tanks to the header tank. <U>The fuel vent line
should be installed so that it introduces pressure into the fuel
tanks</U>. The construction manual clearly shows the vent line
pointing into the wind.</FONT></FONT><FONT face=Arial><FONT size=-1>b) What
tank overflow line? The Velocity schematic does not feature a "tank
overflow line" (other than the vent line itself). It sounds like your
system has more tubes - which may be just fine - however it is
<I>different</I> from what is shown in the construction
manual.</FONT></FONT> <FONT face=Arial><FONT size=-1>Short story - For
a few years I owned a business to help composite aircraft owners build their
aircraft. While I was working on the fuel system for a Velocity
customer I had the "brainstorm" to redesign the fuel and vent system for a
XL which would allow for a fuel selector valve. I consulted with Duane
Swing at the factory - who was kind enough to review the facsimiles that I
sent which outlined my new and improved fuel system. Duane patiently
pointed out some possible pitfalls to the design. He offered a stern
warning to be <I>very</I> careful when changing something like this (meaning
- the fuel system needs to be 100% reliable if the engine is going to stay
running - if you're going to screw around (i.e.. alter the schematic or the
installation) you need to be pretty sure of what you are doing - and to
<U>think through every possible situation that the change might
affect</U>. I went ahead with my improved fuel line/vent system - and
later - when mimicking a ground fuel flow test to see if the system would
deliver 25 gph (take off power) - I found that the fuel lines had too many
curves and had too long a run to sustain this flow rate - which would have
meant a partial loss of power about 2-minutes following takeoff. What
a sobering discovery! Following this I removed the "new and improved"
plumbing and reinstalled the system that was "in the book" - and the
aircraft is operating without problem.</FONT></FONT> <FONT
face=Arial><FONT size=-1>What I'm leading to is this - it looks like someone
has changed your fuel system. If you have more than one vent line
sticking out under the aircraft, then it is different from the Velocity
schematic. Someone obviously felt a need to "improve" the fuel/vent
system - what we don't know is this really an improvement - or is it the
source of your problem (as in my true story above)?</FONT></FONT> <FONT
face=Arial><FONT size=-1>As to your question as to why the low sump fuel
light might come on in aircraft with the Velocity fuel schematic - in an
earlier post I offered an opinion that the float switch might be culprit -
causing a false signal. Have you had a chance to remove the float
switch in order to test it? Is the float buoyant in
avgas?</FONT></FONT> <FONT face=Arial><FONT size=-1>Best
regards,</FONT></FONT><FONT face=Arial><FONT size=-1>Scott
B</FONT></FONT></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>
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