REFLECTOR:Low fuel light
John Dibble
reflector@tvbf.org
Sun, 27 Jul 2003 12:26:28 -0500
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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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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>
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