REFLECTOR:Low fuel light
Ronnie Brown
reflector@tvbf.org
Sun, 27 Jul 2003 16:11:59 -0400
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John,
I don't have a data point for 150 knots but 174 knots =3D 20.04 inches =
of water or 0.72 psig. =20
I would be concerned about the two different over flow vents being at =
the same static pressure as the vent system. It is difficult to get a =
good, reliable, accurate static pressure at 150 or 200 knots, so you =
have a good chance of having just an inch or two difference between the =
3 vents you have, which CAN cause flow problems between the three =
tanks, especially if the central vent system is made up of 1/4" lines as =
many Velocities have.
By the way, late model Cessnas only have a single forward facing vent, =
because the earlier Cessnas had separate tank vents and fuel levels =
would not stay the same. The single vent is tied to both tanks and =
there are also one way check valves involved.
Ronnie
----- Original Message -----=20
From: John Dibble=20
To: reflector@tvbf.org=20
Sent: Sunday, July 27, 2003 1:26 PM
Subject: Re: REFLECTOR:Low fuel light
Scott,=20
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.=20
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.=20
So we have 2 things to discuss - what to do with my system, and what =
to do with the standard Velocity system.=20
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.=20
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.=20
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?=20
John=20
Scott Baker wrote:=20
John, Something you just said in your previous post has me =
concerned. =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D"I =
want to clarify something. Previously I said my tank vent faced=20
forward, but it is the tank overflow line and not the vent that =
faces=20
forward and should pressurize the tank. My manifold vent points to =
the=20
rear and should pull a little vacuum on the vent system."=20
John=20
=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D 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><FONT size=3D2>John,</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>I don't have a data point for 150 knots but 174 =
knots =3D 20.04=20
inches of water or 0.72 psig. </FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>I would be concerned about the two different over =
flow vents=20
being at the same static pressure as the vent system. It is =
difficult to=20
get a good, reliable, accurate static pressure at 150 or 200 knots, =
so you=20
have a good chance of having just an inch or two difference between the =
3 =20
vents you have, which CAN cause flow problems between the three tanks,=20
especially if the central vent system is made up of 1/4" lines as many=20
Velocities have.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>By the way, late model Cessnas only have a single =
forward=20
facing vent, because the earlier Cessnas had separate tank vents and =
fuel levels=20
would not stay the same. The single vent is tied to both tanks and =
there=20
are also one way check valves involved.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Ronnie</FONT></DIV>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV=20
style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
<A title=3Daminetech@dixie-net.com =
href=3D"mailto:aminetech@dixie-net.com">John=20
Dibble</A> </DIV>
<DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3Dreflector@tvbf.org=20
href=3D"mailto:reflector@tvbf.org">reflector@tvbf.org</A> </DIV>
<DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Sunday, July 27, 2003 =
1:26 PM</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Subject:</B> Re: REFLECTOR:Low fuel =
light</DIV>
<DIV><BR></DIV>Scott,=20
<P>I'm glad we're having this discussion. On my plane, each wing =
has an=20
overflow line exiting the bottom of the wing with a 90 degree bend to =
face=20
forward, exactly like one would see on a Cessna. I know it's an =
overflow=20
line because fuel pours out if I try to fill my tanks to the very top =
and the=20
wings are not completely level. The manifold vent line exits =
vertically=20
through the bottom of the fuselage and is cut at a 45 degree angle =
such that=20
the long end faces forward. <BR>Basically what I have are pitot tubes =
pumping=20
air into the tanks. I believe the pressure condition is dynamic, =
not=20
static, with a constant flow from the tanks to the manifold vent. =
<BR>So we=20
have 2 things to discuss - what to do with my system, and what to do =
with the=20
standard Velocity system. <BR>Concerning the Velocity system, I do not =
agree=20
that pulling a vacuum via the vent line will necessarily reduce fuel =
flow from=20
the tanks to the sump. In a static situation (no overflow lines) =
the=20
vapor space pressure will be the same in the tanks and in the sump and =
there=20
will be no effect on fuel flow no matter if there is a vacuum, =
neutral, or=20
pressure on the vent line. Only if the tank vapor space pressure =
is=20
different from the sump vapor space pressure can there be an effect on =
the=20
flow. The pressure can be temporarily different whenever there =
is a=20
change in the manifold vent pressure. For example during descent =
the=20
vent pressure will increase. The small (if any) vapor space in =
the sump=20
will quickly increase while the much greater air space in the tanks =
will take=20
more time thereby making the tank vapor pressure temporarily less than =
the=20
sump and reducing fuel flow from the tank to the sump. This =
effect will=20
be greatest when the tanks have the least fuel and that is the worst =
time for=20
it because the head of fuel that provides the flow to the sump is the=20
least. I would think this problem can be eliminated by =
eliminating the=20
sump vent to the manifold and adding new vents from the sump to each=20
tank. The key point is that the sump would be vented to the =
tanks. =20
This would guarantee the sump vapor pressure would be close to the =
tank vapor=20
pressure at all times. <BR>If the Velocity system has the vent line =
pointing=20
into the wind, then both the sump and tanks are being =
pressurized. I=20
would think that the max pressure would occur at max IAS, say about =
150 kn for=20
the standard Velocity. Can someone tell me what pitot line =
pressure=20
corresponds to 150 kn? This is the max pressure that will be put =
on the=20
tanks and I want to know if that is acceptable. Also, is there =
any=20
concern about the effect of repeated pressurization and =
depressurization of=20
the tanks? As I said before the pressurization will not enhance =
flow=20
from the tanks, however it will enhance flow from the sump to the=20
engine. However, I believe both fuel pumps can achieve several =
psi of=20
suction which dwarfs the pressurization effect.=20
<P>My system should balance the tank and sump pressures =
dynamically. =20
There should be a constant influx of air through the overflow vents, =
throught=20
the tanks, to the manifold and out the vent. This flow should =
dominate=20
the pressure in the vent lines and keep the sump at the same pressure =
as the=20
tanks without pressurizing the tanks much, just whatever the small =
pressure=20
drop is as the air flows from the tanks to the vent. However, I =
need to=20
understand what's going on with the sensor. I haven't checked =
the sensor=20
yet. Shouldn't a sensor problem occur all the time and not just =
at high=20
altitude?=20
<P>John=20
<P>Scott Baker wrote:=20
<BLOCKQUOTE TYPE=3D"CITE">
<STYLE></STYLE>
<FONT face=3DArial><FONT size=3D-1>John,</FONT></FONT> <FONT=20
face=3DArial><FONT size=3D-1>Something you just said in your =
previous post has=20
me concerned.</FONT></FONT> <FONT face=3DArial><FONT=20
=
size=3D-1>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D</FONT></FONT><FONT=20
face=3DArial><FONT size=3D-1>"I want to clarify something. =
Previously I=20
said my tank vent faced</FONT></FONT> <BR><FONT face=3DArial><FONT=20
size=3D-1>forward, but it is the tank overflow line and not the vent =
that=20
faces</FONT></FONT> <BR><FONT face=3DArial><FONT size=3D-1>forward =
and should=20
pressurize the tank. <FONT color=3D#ff0000>My manifold vent =
points to=20
the</FONT></FONT></FONT> <BR><FONT face=3DArial><FONT =
size=3D-1><FONT=20
color=3D#ff0000>rear and should pull a little vacuum on the vent=20
system</FONT>."</FONT></FONT> <BR><FONT face=3DArial><FONT=20
size=3D-1>John</FONT></FONT> <BR><FONT face=3DArial><FONT=20
=
size=3D-1>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D</FONT></FONT> <FONT=20
face=3DArial><FONT size=3D-1>a) If your vent system is pulling =
a little=20
vacuum, then you are "unpressurizing" the fuel tanks - which slows =
the fuel=20
flow from the main tanks to the header tank. <U>The fuel vent =
line=20
should be installed so that it introduces pressure into the fuel=20
tanks</U>. The construction manual clearly shows the vent line =
pointing into the wind.</FONT></FONT><FONT face=3DArial><FONT =
size=3D-1>b) What=20
tank overflow line? The Velocity schematic does not feature a =
"tank=20
overflow line" (other than the vent line itself). It sounds =
like your=20
system has more tubes - which may be just fine - however it is=20
<I>different</I> from what is shown in the construction=20
manual.</FONT></FONT> <FONT face=3DArial><FONT size=3D-1>Short =
story - For=20
a few years I owned a business to help composite aircraft owners =
build their=20
aircraft. While I was working on the fuel system for a =
Velocity=20
customer I had the "brainstorm" to redesign the fuel and vent system =
for a=20
XL which would allow for a fuel selector valve. I consulted =
with Duane=20
Swing at the factory - who was kind enough to review the facsimiles =
that I=20
sent which outlined my new and improved fuel system. Duane =
patiently=20
pointed out some possible pitfalls to the design. He offered a =
stern=20
warning to be <I>very</I> careful when changing something like this =
(meaning=20
- the fuel system needs to be 100% reliable if the engine is going =
to stay=20
running - if you're going to screw around (i.e.. alter the schematic =
or the=20
installation) you need to be pretty sure of what you are doing - and =
to=20
<U>think through every possible situation that the change might=20
affect</U>. I went ahead with my improved fuel line/vent =
system - and=20
later - when mimicking a ground fuel flow test to see if the system =
would=20
deliver 25 gph (take off power) - I found that the fuel lines had =
too many=20
curves and had too long a run to sustain this flow rate - which =
would have=20
meant a partial loss of power about 2-minutes following =
takeoff. What=20
a sobering discovery! Following this I removed the "new and =
improved"=20
plumbing and reinstalled the system that was "in the book" - and the =
aircraft is operating without problem.</FONT></FONT> <FONT=20
face=3DArial><FONT size=3D-1>What I'm leading to is this - it looks =
like someone=20
has changed your fuel system. If you have more than one vent =
line=20
sticking out under the aircraft, then it is different from the =
Velocity=20
schematic. Someone obviously felt a need to "improve" the =
fuel/vent=20
system - what we don't know is this really an improvement - or is it =
the=20
source of your problem (as in my true story =
above)?</FONT></FONT> <FONT=20
face=3DArial><FONT size=3D-1>As to your question as to why the low =
sump fuel=20
light might come on in aircraft with the Velocity fuel schematic - =
in an=20
earlier post I offered an opinion that the float switch might be =
culprit -=20
causing a false signal. Have you had a chance to remove the =
float=20
switch in order to test it? Is the float buoyant in=20
avgas?</FONT></FONT> <FONT face=3DArial><FONT size=3D-1>Best=20
regards,</FONT></FONT><FONT face=3DArial><FONT size=3D-1>Scott=20
B</FONT></FONT></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>
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