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>&nbsp;</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.&nbsp; </FONT></DIV>
<DIV><FONT size=3D2></FONT>&nbsp;</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.&nbsp; It is =
difficult to=20
get a good, reliable, accurate&nbsp;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&nbsp;=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>&nbsp;</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.&nbsp; 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>&nbsp;</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.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; The pressure can be temporarily different whenever there =
is a=20
  change in the manifold vent pressure.&nbsp; For example during descent =
the=20
  vent pressure will increase.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; The key point is that the sump would be vented to the =
tanks.&nbsp;=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.&nbsp; I=20
  would think that the max pressure would occur at max IAS, say about =
150 kn for=20
  the standard Velocity.&nbsp; Can someone tell me what pitot line =
pressure=20
  corresponds to 150 kn?&nbsp; This is the max pressure that will be put =
on the=20
  tanks and I want to know if that is acceptable.&nbsp; Also, is there =
any=20
  concern about the effect of repeated pressurization and =
depressurization of=20
  the tanks?&nbsp; 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.&nbsp; 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.&nbsp;=20
  There should be a constant influx of air through the overflow vents, =
throught=20
  the tanks, to the manifold and out the vent.&nbsp; 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.&nbsp; However, I =
need to=20
  understand what's going on with the sensor.&nbsp; I haven't checked =
the sensor=20
  yet.&nbsp; 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>&nbsp;<FONT=20
    face=3DArial><FONT size=3D-1>Something you just said in your =
previous post has=20
    me concerned.</FONT></FONT>&nbsp;<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.&nbsp; =
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.&nbsp; <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>&nbsp;<FONT=20
    face=3DArial><FONT size=3D-1>a)&nbsp; 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.&nbsp; <U>The fuel vent =
line=20
    should be installed so that it introduces pressure into the fuel=20
    tanks</U>.&nbsp; 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?&nbsp; The Velocity schematic does not feature a =
"tank=20
    overflow line" (other than the vent line itself).&nbsp; 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>&nbsp;<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.&nbsp; 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.&nbsp; 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.&nbsp; Duane =
patiently=20
    pointed out some possible pitfalls to the design.&nbsp; 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>.&nbsp; 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.&nbsp; What=20
    a sobering discovery!&nbsp; 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>&nbsp;<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.&nbsp; If you have more than one vent =
line=20
    sticking out under the aircraft, then it is different from the =
Velocity=20
    schematic.&nbsp; 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>&nbsp;<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.&nbsp; Have you had a chance to remove the =
float=20
    switch in order to test it?&nbsp; Is the float buoyant in=20
    avgas?</FONT></FONT>&nbsp;<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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