fuel tank venting reviseted

Jesse Jenks · Post by **Jesse Jenks** » Sat Feb 18, 2012 10:47 am

I have been reading the archives on tank venting, trying to decide on the
simplest system. Right now I have a few questions that I couldn't find
answers to:
If I installed an inverted J vent in each wing root fairing teed of of the
top sight gauge fitting, then do I really need a cross vent line through the
cabin? I personally want the fewest possible paths for fuel to enter the
cabin.
I am concerned with fuel dumping out in a rollover accident. With the
standard vent, or even the internal line running inside the tank to the high
point as Ken and others have done, you would get fuel coming out the vents
if the airplane is inverted right? Is there a way to prevent this without a
check valve? If you had an internal line to the high point, and bent a U in
it so that it went down to the bottom of the tank, and then back up, it
seems like that would stop fuel from draining when upside down. It might not
work as a vent either though.
If you go to the trouble to put an internal tube to the tank high point in
order to stop fuel dumping or transferring if parked on a slope, then what
about the vented filler caps? It seems like fuel will just leak out of those
anytime the level in the tank reaches them. Same goes for a rollover.
Thanks
Jesse

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Ken · Post by **Ken** » Sat Feb 18, 2012 10:47 am

Jesse Jenks wrote:

I have been reading the archives on tank venting, trying to decide on the
simplest system. Right now I have a few questions that I couldn't find
answers to:
If I installed an inverted J vent in each wing root fairing teed of of the
top sight gauge fitting, then do I really need a cross vent line through the
cabin? I personally want the fewest possible paths for fuel to enter the
cabin.

You can always use aluminum hard lines within the cabin and hope that
the flex lines sever outside the cabin in an accident...

I am concerned with fuel dumping out in a rollover accident. With the
standard vent, or even the internal line running inside the tank to the high
point as Ken and others have done, you would get fuel coming out the vents
if the airplane is inverted right? Is there a way to prevent this without a
check valve? If you had an internal line to the high point, and bent a U in
it so that it went down to the bottom of the tank, and then back up, it
seems like that would stop fuel from draining when upside down. It might not
work as a vent either though.

That also would probably provide a spot for condensation and water to
freeze and won't help if the wing is not reasonably level while upside
down. Even if level, siphoning could get started from sloshing I guess.
I can't think of anything except the Cessna below the wing vent with a
check valve to help this concern much but that is not the simplest or
lowest maintanance system.

If you go to the trouble to put an internal tube to the tank high point in
order to stop fuel dumping or transferring if parked on a slope, then what
about the vented filler caps? It seems like fuel will just leak out of those
anytime the level in the tank reaches them.

Yes but at least the fuel won't transfer out of the upwing tank to the
low tank and then out. Also it will tolerate a lot more slope before it
comes out a filler cap.

Same goes for a rollover.

At some point, I just figure a fuel line will separate or the tank will
rupture anyway so I wouldn't want to increase the risk of fuel
starvation for the sake of possibly reducing leakage after the accident ;)

Ken

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Jesse Jenks · Post by **Jesse Jenks** » Sat Feb 18, 2012 10:47 am

If I do put in a cross vent I will use a hard line, but I just keep thinking
if I have a vent on each tank do I need a cross vent? Not having one would
alleviate the need for the in-tank high point line also. So you get rid of
the cross cabin fuel line, and 2 interior tank lines, and add one short vent
line. Less work, and fewer lines. It seems like the way to go, but I don't
get the feeling many builders do it this way. Is there something I am
missing? I have no real knowledge of how a fuel system should work, just
trying to figure it out.
Jesse

From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Wed, 20 Jul 2005 17:41:50 -0400

Jesse Jenks wrote:

I have been reading the archives on tank venting, trying to decide on the
simplest system. Right now I have a few questions that I couldn't find
answers to:
If I installed an inverted J vent in each wing root fairing teed of of
the
top sight gauge fitting, then do I really need a cross vent line through
the
cabin? I personally want the fewest possible paths for fuel to enter the
cabin.

You can always use aluminum hard lines within the cabin and hope that
the flex lines sever outside the cabin in an accident...

I am concerned with fuel dumping out in a rollover accident. With the
standard vent, or even the internal line running inside the tank to the
high
point as Ken and others have done, you would get fuel coming out the
vents
if the airplane is inverted right? Is there a way to prevent this without
a
check valve? If you had an internal line to the high point, and bent a U
in
it so that it went down to the bottom of the tank, and then back up, it
seems like that would stop fuel from draining when upside down. It might
not
work as a vent either though.

That also would probably provide a spot for condensation and water to
freeze and won't help if the wing is not reasonably level while upside
down. Even if level, siphoning could get started from sloshing I guess.
I can't think of anything except the Cessna below the wing vent with a
check valve to help this concern much but that is not the simplest or
lowest maintanance system.

If you go to the trouble to put an internal tube to the tank high point
in
order to stop fuel dumping or transferring if parked on a slope, then
what
about the vented filler caps? It seems like fuel will just leak out of
those
anytime the level in the tank reaches them.

Yes but at least the fuel won't transfer out of the upwing tank to the
low tank and then out. Also it will tolerate a lot more slope before it
comes out a filler cap.

Same goes for a rollover.

At some point, I just figure a fuel line will separate or the tank will
rupture anyway so I wouldn't want to increase the risk of fuel
starvation for the sake of possibly reducing leakage after the accident ;)

Ken

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Reed Britt Civ 309 MXSG/M · Post by **Reed Britt Civ 309 MXSG/M** » Sat Feb 18, 2012 10:47 am

I guess the tank vent is part of the preflight inspection - Is two better
than one (probably not)?? The mud wasps gave me an exciting flight once in a
150 (one vent) - fuel starvation at 200 feet after liftoff - Figured it out
when on the ground looking for water in the gas and the tank drain instead
of draining gas started sucking air --

Also parking on uneven ground - one wing high might get it going out the low
side (probably not enough difference in the height between the two to make a
difference)? Maybe a ball valve between??

Look in the archives - Scott Aldrich has a deluxe system -

Britt -- SR194

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Jesse
Jenks
Sent: Thursday, July 21, 2005 10:07 AM
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted

If I do put in a cross vent I will use a hard line, but I just keep thinking

if I have a vent on each tank do I need a cross vent? Not having one would
alleviate the need for the in-tank high point line also. So you get rid of
the cross cabin fuel line, and 2 interior tank lines, and add one short vent

line. Less work, and fewer lines. It seems like the way to go, but I don't
get the feeling many builders do it this way. Is there something I am
missing? I have no real knowledge of how a fuel system should work, just
trying to figure it out.
Jesse

From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Wed, 20 Jul 2005 17:41:50 -0400

Jesse Jenks wrote:

I have been reading the archives on tank venting, trying to decide on the
simplest system. Right now I have a few questions that I couldn't find
answers to:
If I installed an inverted J vent in each wing root fairing teed of of
the
top sight gauge fitting, then do I really need a cross vent line through
the
cabin? I personally want the fewest possible paths for fuel to enter the
cabin.

You can always use aluminum hard lines within the cabin and hope that
the flex lines sever outside the cabin in an accident...

I am concerned with fuel dumping out in a rollover accident. With the
standard vent, or even the internal line running inside the tank to the
high
point as Ken and others have done, you would get fuel coming out the
vents
if the airplane is inverted right? Is there a way to prevent this without

a
check valve? If you had an internal line to the high point, and bent a U
in
it so that it went down to the bottom of the tank, and then back up, it
seems like that would stop fuel from draining when upside down. It might
not
work as a vent either though.

That also would probably provide a spot for condensation and water to
freeze and won't help if the wing is not reasonably level while upside
down. Even if level, siphoning could get started from sloshing I guess.
I can't think of anything except the Cessna below the wing vent with a
check valve to help this concern much but that is not the simplest or
lowest maintanance system.

If you go to the trouble to put an internal tube to the tank high point
in
order to stop fuel dumping or transferring if parked on a slope, then
what
about the vented filler caps? It seems like fuel will just leak out of
those
anytime the level in the tank reaches them.

Yes but at least the fuel won't transfer out of the upwing tank to the
low tank and then out. Also it will tolerate a lot more slope before it
comes out a filler cap.

Same goes for a rollover.

At some point, I just figure a fuel line will separate or the tank will
rupture anyway so I wouldn't want to increase the risk of fuel
starvation for the sake of possibly reducing leakage after the accident ;)

Ken

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Jeff McMurrer · Post by **Jeff McMurrer** » Sat Feb 18, 2012 10:47 am

We have just completed Rebel 743 without crossventing tanks. Tank caps are vented, we have no problems at this point. All other fuel lines are hard lines. No rubber hose within cabin area.

This is what we did with our fuel system routing and venting.

Hope it helps you out, making your decision easier.

Jeff McMurrer
R743

----- Original Message -----
From: Jesse Jenks <jessejenks@hotmail.com>
Date: Thursday, July 21, 2005 9:06 am
Subject: Re: fuel tank venting reviseted

If I do put in a cross vent I will use a hard line, but I just
keep thinking
if I have a vent on each tank do I need a cross vent? Not having
one would
alleviate the need for the in-tank high point line also. So you
get rid of
the cross cabin fuel line, and 2 interior tank lines, and add one
short vent
line. Less work, and fewer lines. It seems like the way to go, but
I don't
get the feeling many builders do it this way. Is there something I
am
missing? I have no real knowledge of how a fuel system should
work, just
trying to figure it out.
Jesse

From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Wed, 20 Jul 2005 17:41:50 -0400

Jesse Jenks wrote:

I have been reading the archives on tank venting, trying to
decide on the
simplest system. Right now I have a few questions that I
couldn't find
answers to:
If I installed an inverted J vent in each wing root fairing
teed of of
the
top sight gauge fitting, then do I really need a cross vent
line through
the
cabin? I personally want the fewest possible paths for fuel to
enter the
cabin.

You can always use aluminum hard lines within the cabin and hope
that>the flex lines sever outside the cabin in an accident...

I am concerned with fuel dumping out in a rollover accident.
With the
standard vent, or even the internal line running inside the
tank to the
high
point as Ken and others have done, you would get fuel coming
out the
vents
if the airplane is inverted right? Is there a way to prevent
this without
a
check valve? If you had an internal line to the high point, and
bent a U
in
it so that it went down to the bottom of the tank, and then
back up, it
seems like that would stop fuel from draining when upside down.
It might
not
work as a vent either though.

That also would probably provide a spot for condensation and
water to
freeze and won't help if the wing is not reasonably level while
upside>down. Even if level, siphoning could get started from
sloshing I guess.
I can't think of anything except the Cessna below the wing vent
with a
check valve to help this concern much but that is not the
simplest or
lowest maintanance system.

If you go to the trouble to put an internal tube to the tank
high point
in
order to stop fuel dumping or transferring if parked on a
slope, then
what
about the vented filler caps? It seems like fuel will just leak
out of
those
anytime the level in the tank reaches them.

Yes but at least the fuel won't transfer out of the upwing tank
to the
low tank and then out. Also it will tolerate a lot more slope
before it
comes out a filler cap.

Same goes for a rollover.

At some point, I just figure a fuel line will separate or the
tank will
rupture anyway so I wouldn't want to increase the risk of fuel
starvation for the sake of possibly reducing leakage after the
accident ;)
Ken

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Ken · Post by **Ken** » Sat Feb 18, 2012 10:47 am

I believe the intent of the cross vent is to make both tanks feed
equally. I gather that one tank feeding faster than the other is usually
attributed to very minor differences in venting. I guess you could
always install the cross vent later if needed. Depending on what fuel
caps you use, I also like the idea that even if one vent plugs the tank
can still vent from the other side through the cross vent.
In my case I will try one inverted J vent plumbed into the cross vent
line but I will not drill the fuel filler necks (MAM recommends drilling
them) as I don't want airflow into the J vent and back out the drilled
filler necks. My caps themselves seem to be vented (sort of) but I
believe MAM says not to trust the caps as the main vent. Safest course
might be to just follow the manual though...
Ken

Jesse Jenks wrote:

If I do put in a cross vent I will use a hard line, but I just keep thinking
if I have a vent on each tank do I need a cross vent? Not having one would
alleviate the need for the in-tank high point line also. So you get rid of
the cross cabin fuel line, and 2 interior tank lines, and add one short vent
line. Less work, and fewer lines. It seems like the way to go, but I don't
get the feeling many builders do it this way. Is there something I am
missing? I have no real knowledge of how a fuel system should work, just
trying to figure it out.
Jesse

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Walter Klatt · Post by **Walter Klatt** » Sat Feb 18, 2012 10:47 am

I have forward ram air vents on my caps which are sealed to the
tanks, and a cross vent through the cabin connecting the two
tanks. One backs up the other in case a wasp decides to plug one
cap vent, and the cross vent tube also can be handy sometimes if
you overfill one tank, and the excess drains across to the other
one.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of Ken
Sent: Thursday, July 21, 2005 8:31 PM
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted

I believe the intent of the cross vent is to make both
tanks feed
equally. I gather that one tank feeding faster than
the other is usually
attributed to very minor differences in venting. I
guess you could
always install the cross vent later if needed.
Depending on what fuel
caps you use, I also like the idea that even if one
vent plugs the tank
can still vent from the other side through the cross vent.
In my case I will try one inverted J vent plumbed into
the cross vent
line but I will not drill the fuel filler necks (MAM
recommends drilling
them) as I don't want airflow into the J vent and back
out the drilled
filler necks. My caps themselves seem to be vented
(sort of) but I
believe MAM says not to trust the caps as the main
vent. Safest course
might be to just follow the manual though...
Ken

Jesse Jenks wrote:

If I do put in a cross vent I will use a hard line,
but I just keep thinking
if I have a vent on each tank do I need a cross vent?
Not having one would
alleviate the need for the in-tank high point line
also. So you get rid of
the cross cabin fuel line, and 2 interior tank lines,
and add one short vent
line. Less work, and fewer lines. It seems like the
way to go, but I don't
get the feeling many builders do it this way. Is
there something I am
missing? I have no real knowledge of how a fuel
system should work, just
trying to figure it out.
Jesse

-------------------------------------------------------
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Jesse Jenks · Post by **Jesse Jenks** » Sat Feb 18, 2012 10:47 am

Ken,
Yes, good point about equal feeding. As you say one could add the Xvent
later if imbalance problems became a factor. I also don't want to have the
caps vented if there is another vent source already. My caps have some small
holes already drilled in them. I was thinking of filling them with proseal,
actually I think I would have to put a bunch of proseal inside the cap to
seal them up again. If I add the internal tank high point vent lines as you
did I will need extra flanges for the inside of the end rib. Where did you
get them? ACS has "welding flanges" but they appear to have a raised area
around the flange that might not be good for riveting.
Jesse

From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Thu, 21 Jul 2005 23:30:32 -0400

I believe the intent of the cross vent is to make both tanks feed
equally. I gather that one tank feeding faster than the other is usually
attributed to very minor differences in venting. I guess you could
always install the cross vent later if needed. Depending on what fuel
caps you use, I also like the idea that even if one vent plugs the tank
can still vent from the other side through the cross vent.
In my case I will try one inverted J vent plumbed into the cross vent
line but I will not drill the fuel filler necks (MAM recommends drilling
them) as I don't want airflow into the J vent and back out the drilled
filler necks. My caps themselves seem to be vented (sort of) but I
believe MAM says not to trust the caps as the main vent. Safest course
might be to just follow the manual though...
Ken

Jesse Jenks wrote:

If I do put in a cross vent I will use a hard line, but I just keep
thinking
if I have a vent on each tank do I need a cross vent? Not having one
would
alleviate the need for the in-tank high point line also. So you get rid
of
the cross cabin fuel line, and 2 interior tank lines, and add one short
vent
line. Less work, and fewer lines. It seems like the way to go, but I
don't
get the feeling many builders do it this way. Is there something I am
missing? I have no real knowledge of how a fuel system should work, just
trying to figure it out.
Jesse

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Drew Dalgleish · Post by **Drew Dalgleish** » Sat Feb 18, 2012 10:47 am

Hi Ken and Jesse
The early Rebels didn't have a cross-vent. Just a 1/8" hole in the back of
each filler neck. This worked well most of the time. Then the Couglin
brothers crashed on the way to Oshkosh. It was determined that they ran out
of fuel because it siphoned out of one tank into the other and leaked out
the vent hole in the filler neck. The factory reportedly flew all kinds of
test flights at all kinds of different attitudes trying to replicate what
happened but was unable to. They still decided in the interest of safety to
publish a service bulletin calling for a cross vent between the tanks. It's
main purpose is to break any siphon action.
I found with my rebel with the vents at the back of each filler neck that
I could starve the engine of fuel with low fuel levels if I was doing
anything other than flying straight and level in smooth air. I blocked off
the 2 filler neck vents and installed 1 ram air vent at the wing root that
joins into the cross vent. Since making that change I've had no problems at
all. Probably with 2 ram air vents you'd have no problems at all but with
all the fuel lines already in the cabin eliminating one won't likely make
much difference in surviving a roll over. The cross vent is well protected
and if it ruptures I suspect the crash wouldn't have been survivable anyway.

Drew Dalgleish Driving from centralia to thunder bay tommorrow because my
planes not big enough :(

At 11:30 PM 7/21/2005 -0400, you wrote:

I believe the intent of the cross vent is to make both tanks feed
equally. I gather that one tank feeding faster than the other is usually
attributed to very minor differences in venting. I guess you could
always install the cross vent later if needed. Depending on what fuel
caps you use, I also like the idea that even if one vent plugs the tank
can still vent from the other side through the cross vent.
In my case I will try one inverted J vent plumbed into the cross vent
line but I will not drill the fuel filler necks (MAM recommends drilling
them) as I don't want airflow into the J vent and back out the drilled
filler necks. My caps themselves seem to be vented (sort of) but I
believe MAM says not to trust the caps as the main vent. Safest course
might be to just follow the manual though...
Ken

Jesse Jenks wrote:

If I do put in a cross vent I will use a hard line, but I just keep

thinking

if I have a vent on each tank do I need a cross vent? Not having one would
alleviate the need for the in-tank high point line also. So you get rid of
the cross cabin fuel line, and 2 interior tank lines, and add one short

vent

line. Less work, and fewer lines. It seems like the way to go, but I don't
get the feeling many builders do it this way. Is there something I am
missing? I have no real knowledge of how a fuel system should work, just
trying to figure it out.
Jesse

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Drew

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Ken · Post by **Ken** » Sat Feb 18, 2012 10:47 am

thanks Drew
I had forgotton about the syphoning incident.
Ken

Drew Dalgleish wrote:

snipHi Ken and Jesse
The early Rebels didn't have a cross-vent. Just a 1/8" hole in the back of
each filler neck. This worked well most of the time. Then the Couglin
brothers crashed on the way to Oshkosh. It was determined that they ran out
of fuel because it siphoned out of one tank into the other and leaked out
the vent hole in the filler neck. The factory reportedly flew all kinds of
test flights at all kinds of different attitudes trying to replicate what
happened but was unable to. They still decided in the interest of safety to
publish a service bulletin calling for a cross vent between the tanks. It's
main purpose is to break any siphon action.
I found with my rebel with the vents at the back of each filler neck that
I could starve the engine of fuel with low fuel levels if I was doing
anything other than flying straight and level in smooth air. I blocked off
the 2 filler neck vents and installed 1 ram air vent at the wing root that
joins into the cross vent. Since making that change I've had no problems at
all. Probably with 2 ram air vents you'd have no problems at all but with
all the fuel lines already in the cabin eliminating one won't likely make
much difference in surviving a roll over. The cross vent is well protected
and if it ruptures I suspect the crash wouldn't have been survivable anyway.

Drew Dalgleish Driving from centralia to thunder bay tommorrow because my
planes not big enough :(

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Jesse Jenks · Post by **Jesse Jenks** » Sat Feb 18, 2012 10:47 am

Thanks Ken,
I don't want to be the one who thinks he can do it better and ends up sorry.
I will plan for the cross vent with internal tank lines, and leave the caps
as they are.
Jesse

From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Fri, 22 Jul 2005 09:33:57 -0400

Jesse
I probably should not have mentioned that I did not drill extra holes in
the filler necks but I DO indeed want some kind of small alternate vent
in the caps.
From what I've heard, I don't think you will get even fuel feed without
the crossvent?? I didn't want to have to watch or manage left/ right
fuel and I didn't want one side to go lower than the other and suck air
during turns to final if I'm not paying attention. Perhaps that is a
minor concern but it seems that there are always complications when we
deviate from the plans.
Extra flanges can be turned on a lathe but MAM has them or you can sand
off the raised lip on the ACS ones if you don't like the lip.
Ken

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Phil Stubley · Post by **Phil Stubley** » Sat Feb 18, 2012 10:47 am

Subject: Re: fuel tank venting reviseted

Hi Jesse,

Sounds like you're on the right track. Here's my 2c worth to help you set
yoiur mind at ease about sucking air, but give you another worry about an
air bubble.

I have (in the third incarnation of at least 10 designs) a hole in the
filler neck as per MAM, no cross vent, (but will put one back in) and in
addition I have a 'bubble separator' in the line just after the elbow, but
before the valve. This is a little chamber vented from the top back to the
upper tank fitting. Essentially a large Y fitting.(The fitting is the one
originally for the top of the fuel site gauge as per MAM.).

Here is my rational:

First the venting, which can cause a reduced or imbalanced pressure over the
fuel in the tank. How much?
Fuel flow by volume is less than 1/1000 cfs. To get that VOLUME of air
into the tank through a small ( 1/16 in) hole will cause a pressure loss
measured in small (less than 1/32 in) fractions. Insignificant!!
If the pressure at the two different tank vents is different, then a
difference in levels between the tanks will occur, causing one tank to run
lower than the other. How much? And how could this happen? The dynamic
pressure due to 110 mph air flow (the maximum that could occur I think) is
about 7 in (by my crude calcs) so some fraction of that could appear due to
differences in flow patterns around the holes. I think half that is the
most I can envisage. So one tank would run 3.5 in below the other, ie one
3/4 and one less than 1/4. This is what I see on my setup now. So I will
replace the crossvent and see what happens.
Sucking air?? If one tank level drops a foot, ie dry and down the tube,
then the flow from that tank WILL STOP, same as closing the valve, (assuming
the other tank will supply full flow. If not you really have a problem)

Now the bubble problem.
When (not if) you uncover one of the ports air will flow into the line
and try to go down the tube (the bubble, not you) It will stop after it
passes the corner to the vertical run, going as far as the flow will drive
it. Fuel will flow past this bubble, enough to run the engine at idle and
at part throttle, for an O320 enough for a runup to ckeck the engine!!
Probably enough to run a 912 at full throttle. Why the problem of 'loss of
fuel flow' didn't appear until we started to get the larger engines in
serrvice. How do I know this? After an 'incident' running on one tank I
duplicated the setup in the backyard using plastic tubing and water, so I
could see what happens. So now I recommend ALWAYS run with both tanks open,
even if one appears empty!! The full tank will fill the empty line,
hopefully pushing the bubble back into the tank..But this is hard for it to
do, so that is why I built the bubble separators.

Hope this helps.

Phil

Thanks Ken,
I don't want to be the one who thinks he can do it better and ends up

sorry.

I will plan for the cross vent with internal tank lines, and leave the

caps

as they are.
Jesse
From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Fri, 22 Jul 2005 09:33:57 -0400

Jesse
I probably should not have mentioned that I did not drill extra holes in
the filler necks but I DO indeed want some kind of small alternate vent
in the caps.
From what I've heard, I don't think you will get even fuel feed without
the crossvent?? I didn't want to have to watch or manage left/ right
fuel and I didn't want one side to go lower than the other and suck air
during turns to final if I'm not paying attention.

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Jesse Jenks · Post by **Jesse Jenks** » Sat Feb 18, 2012 10:47 am

Phil,
Thank you for the detailed analysis. Regarding the bubble problem: If you
always leave both tank valves open as you say then would it not be nearly
impossible for a bubble to get sucked into the carb?
Jesse

From: "Phil Stubley" <phil.stubley@sympatico.ca>
Reply-To: <rebel-builders@dcsol.com>
To: <rebel-builders@dcsol.com>
Subject: Re: fuel tank venting reviseted
Date: Fri, 29 Jul 2005 12:47:51 -0400

Subject: Re: fuel tank venting reviseted

Hi Jesse,

Sounds like you're on the right track. Here's my 2c worth to help you set
yoiur mind at ease about sucking air, but give you another worry about an
air bubble.

I have (in the third incarnation of at least 10 designs) a hole in the
filler neck as per MAM, no cross vent, (but will put one back in) and in
addition I have a 'bubble separator' in the line just after the elbow, but
before the valve. This is a little chamber vented from the top back to the
upper tank fitting. Essentially a large Y fitting.(The fitting is the one
originally for the top of the fuel site gauge as per MAM.).

Here is my rational:

First the venting, which can cause a reduced or imbalanced pressure over
the
fuel in the tank. How much?
Fuel flow by volume is less than 1/1000 cfs. To get that VOLUME of
air
into the tank through a small ( 1/16 in) hole will cause a pressure loss
measured in small (less than 1/32 in) fractions. Insignificant!!
If the pressure at the two different tank vents is different, then a
difference in levels between the tanks will occur, causing one tank to run
lower than the other. How much? And how could this happen? The dynamic
pressure due to 110 mph air flow (the maximum that could occur I think) is
about 7 in (by my crude calcs) so some fraction of that could appear due to
differences in flow patterns around the holes. I think half that is the
most I can envisage. So one tank would run 3.5 in below the other, ie one
3/4 and one less than 1/4. This is what I see on my setup now. So I will
replace the crossvent and see what happens.
Sucking air?? If one tank level drops a foot, ie dry and down the
tube,
then the flow from that tank WILL STOP, same as closing the valve,
(assuming
the other tank will supply full flow. If not you really have a problem)

Now the bubble problem.
When (not if) you uncover one of the ports air will flow into the line
and try to go down the tube (the bubble, not you) It will stop after it
passes the corner to the vertical run, going as far as the flow will drive
it. Fuel will flow past this bubble, enough to run the engine at idle and
at part throttle, for an O320 enough for a runup to ckeck the engine!!
Probably enough to run a 912 at full throttle. Why the problem of 'loss of
fuel flow' didn't appear until we started to get the larger engines in
serrvice. How do I know this? After an 'incident' running on one tank I
duplicated the setup in the backyard using plastic tubing and water, so I
could see what happens. So now I recommend ALWAYS run with both tanks
open,
even if one appears empty!! The full tank will fill the empty line,
hopefully pushing the bubble back into the tank..But this is hard for it to
do, so that is why I built the bubble separators.

Hope this helps.

Phil

Thanks Ken,
I don't want to be the one who thinks he can do it better and ends up
sorry.
I will plan for the cross vent with internal tank lines, and leave the
caps
as they are.
Jesse
From: Ken <klehman@albedo.net>
Reply-To: <rebel-builders@dcsol.com>
To: rebel-builders@dcsol.com
Subject: Re: fuel tank venting reviseted
Date: Fri, 22 Jul 2005 09:33:57 -0400

Jesse
I probably should not have mentioned that I did not drill extra holes
in
the filler necks but I DO indeed want some kind of small alternate vent
in the caps.
From what I've heard, I don't think you will get even fuel feed
without
the crossvent?? I didn't want to have to watch or manage left/ right
fuel and I didn't want one side to go lower than the other and suck air
during turns to final if I'm not paying attention.

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Ken · Post by **Ken** » Sat Feb 18, 2012 10:47 am

I know I have a bulletin that calls for a hole in the filler neck and
the crossvent but when Jesse got me looking at the CD manual it seems
that mine does not say to drill the neck anymore and neither does it
have that bulletin included. Makes me think perhaps they don't recommend
drilling the filler neck anymore??
Ken
119R

Phil Stubley wrote:

Subject: Re: fuel tank venting reviseted

Hi Jesse,

Sounds like you're on the right track. Here's my 2c worth to help you set
yoiur mind at ease about sucking air, but give you another worry about an
air bubble.

I have (in the third incarnation of at least 10 designs) a hole in the
filler neck as per MAM, no cross vent, (but will put one back in) and in
addition I have a 'bubble separator' in the line just after the elbow, but
before the valve. This is a little chamber vented from the top back to the
upper tank fitting. Essentially a large Y fitting.(The fitting is the one
originally for the top of the fuel site gauge as per MAM.).

Here is my rational:

First the venting, which can cause a reduced or imbalanced pressure over the
fuel in the tank. How much?
Fuel flow by volume is less than 1/1000 cfs. To get that VOLUME of air
into the tank through a small ( 1/16 in) hole will cause a pressure loss
measured in small (less than 1/32 in) fractions. Insignificant!!
If the pressure at the two different tank vents is different, then a
difference in levels between the tanks will occur, causing one tank to run
lower than the other. How much? And how could this happen? The dynamic
pressure due to 110 mph air flow (the maximum that could occur I think) is
about 7 in (by my crude calcs) so some fraction of that could appear due to
differences in flow patterns around the holes. I think half that is the
most I can envisage. So one tank would run 3.5 in below the other, ie one
3/4 and one less than 1/4. This is what I see on my setup now. So I will
replace the crossvent and see what happens.
Sucking air?? If one tank level drops a foot, ie dry and down the tube,
then the flow from that tank WILL STOP, same as closing the valve, (assuming
the other tank will supply full flow. If not you really have a problem)

Now the bubble problem.
When (not if) you uncover one of the ports air will flow into the line
and try to go down the tube (the bubble, not you) It will stop after it
passes the corner to the vertical run, going as far as the flow will drive
it. Fuel will flow past this bubble, enough to run the engine at idle and
at part throttle, for an O320 enough for a runup to ckeck the engine!!
Probably enough to run a 912 at full throttle. Why the problem of 'loss of
fuel flow' didn't appear until we started to get the larger engines in
serrvice. How do I know this? After an 'incident' running on one tank I
duplicated the setup in the backyard using plastic tubing and water, so I
could see what happens. So now I recommend ALWAYS run with both tanks open,
even if one appears empty!! The full tank will fill the empty line,
hopefully pushing the bubble back into the tank..But this is hard for it to
do, so that is why I built the bubble separators.

Hope this helps.

Phil

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Phil Stubley · Post by **Phil Stubley** » Sat Feb 18, 2012 10:47 am

Hi Jesse,

You're right, with both tanks open the bubble won'tet to the tank. But
getting to the tank is not the big problem, it only gives you a slight heart
murmur for a couple of seconds.

I fthe bubble gets stuck at the bend then the flow is reduced but not
stopped. It may cause enough starvastion to slow the engine enough to
prevent it from swallowing the bubble when it (maybe two of them?) finally
stop the engine.

Just conjecture, but I feel a lot happier with both open AND a bubble
separator feature.

Phil
----> Phil,

Thank you for the detailed analysis. Regarding the bubble problem: If you
always leave both tank valves open as you say then would it not be nearly
impossible for a bubble to get sucked into the carb?
Jesse

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