The Murphy Builders Site

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.

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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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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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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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

-----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

-------------------------------------------------------
----------
List archives located at: https://www.dcsol.com/login
username "rebel" password "builder"
Subscription services located at:
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List administrator: mike.davis@dcsol.com
-------------------------------------------------------
----------

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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username "rebel" password "builder"
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-----------------------------------------------------------------

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

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

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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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 :(

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

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

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

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.

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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-----------------------------------------------------------------

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

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