Fuel Vent Line


Mike Perry <dmperry1012@...>
 

I appreciate everyone's feedback on the fuel vent line question. It seems
to me that the best system should have:
-- the fuel fill on top of the header tank
-- the fuel vent built stock as per the Q-200 plans (bent into the
top of the fuel return line)
-- some sort of check-valve or header space in the fuel return line
to prevent fuel from being forced up the return line

The fluid dynamics diagrams posted by Larry Hamm also point to a potential
problem: The standard gas cap location is a very low pressure area in
flight. Thus cracked/broken/missing gas caps can easily put negative
pressure into our fuel systems. The top of the header tank appears to be a
neutral pressure area.

Mike Perry
not currently building :-(
PS: Larry, or anyone, do you have a pressure diagram of the TOP of the
Q2? I would like to see that also if available. Thanks -- Mike Perry

At 10:22 PM 8/6/2006 -0500, Lynn French wrote:

[snip]
Until all the high time Q200 drivers are
willing to loosen their gas cap, put a crack in it, put a small hole in it,
leave it laying at the pump, or any other anomaly, and then hop in without
any stuttering or hesitation and go fly without incident, then the plans
vent system is not good enough.
[snip]
There are a few Q builders that have solved the "low pressure gas cap
location" issue by installing it on the upper fuselage - Patillo, Marstall,
and others. I would think this takes care of many of the vent system issues
that have been reported in the back issues of Q-Talk.
On 8/7/06 Jim Patillo wrote:

I agree with Sam 100%. I've got the stock system with the vent line
turned into the 5/8" return line and it has never caused a problem.
I gave a demo ride Friday and my plane is still running just fine!

[snip]

Jim Patillo N46JP Q200

P.S. I would highly suggest anyone building now consider installing
their non vented fuel cap on top of the header tank. There's no
reason to install the filler in a box behind a door on the side per
plans. [snip]

On 8/6/06 Peter Harris wrote:

The vent 90 deg bend turning down the fuel return line is not a good idea
because fuel is sometimes forced UP the fuel return line. This happens
when the main is full and in turbulence. If the main is full there is
nowhere else for the fuel to go but up the fuel return line. The pilots
seat acts like a bellows also forcing fuel up the return line.
The best fix is a vent for the header pointing up not down.
Peter


peterjfharris <peterjfharris@...>
 

The risk of fuel blown on the canopy from a header vent is zero provided the vent is NOT fitted into the fuel return line. Mine is fitted into the fuel return as per plans amendment and I have seen a small splash of fuel on the canopy twice in the last 3 years. Also it could be mounted clear of view.The point about this type of vent is that it clears immediately under air pressure combined with gravity. In the plans system gravity is working against air pressure so fuel is held up frequently in the excessively long pipe ,restricting flow to the carb fuel bowl and enough to stop a Posa with no bowl. I am pleased I do not need to think about this unnecesary restriction to fuel flow to my Jab.I would think twicw before fitting a check valve in the return line as it is another element that could fail.
Peter
---- Jim Patillo <logistics_engineering@msn.com> wrote:


I agree with Sam 100%. I've got the stock system with the vent line
turned into the 5/8" return line and it has never caused a problem.
I gave a demo ride Friday and my plane is still running just fine!

The only time I had an issue of fuel in the vent line was my own
STUPIDITY which is well documented here). There should be no reason
to have a fuel vent on top of the fuselage in front of the
windscreen. It may have worked for Peter Harris but there are other
issues there!

Jim Patillo N46JP Q200

P.S. I would highly suggest anyone building now consider installing
their non vented fuel cap on top of the header tank. There's no
reason to install the filler in a box behind a door on the side per
plans. It is awkward to use and takes longer to fill. I can fill my
plane as fast as its pumped in. I've flown in snow and rain and
never seen one drop of water in my fuel so that shouldn't be a
concern.

--- In Q-LIST@yahoogroups.com, "Sam Hoskins" <shoskins@...> wrote:

I firmly believe that if you do the vent system, per the Q-200
plans, you
will have no trouble. It is imperative that there are no leaks in
filler
cap.

Sam Hoskins Q-200 1,580 hrs.

Murphysburger, IL



_____

From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com] On
Behalf Of
Mike Perry
Sent: Sunday, August 06, 2006 5:12 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Fuel Vent Line (was Carb ice follow up)



Peter suggests that the fuel can go UP the return line in
turbulence,
blocking the vent tube. Mike Dwyer doesn't like the fuel vent
sticking up
in front of the canopy (makes sense to me). Question:

Could the fuel vent/ram air pressure line run to some other point
in the
system? Could it run into the main tank, or into the fuel fill
tube?

Related questions: Does positive pressure in the main tank
transmit to the
header tank? Also, do you guys with flying Qs ever get gasoline
drips from
the fuel vent line when the plane is left full of fuel?

Mike Perry

At 06:23 PM 8/6/2006 +1000, Peter Harris wrote:

The vent 90 deg bend turning down the fuel return line is not a
good idea
because fuel is sometimes forced UP the fuel return line. This
happens
when the main is full and in turbulence. If the main is full
there is
nowhere else for the fuel to go but up the fuel return line. The
pilots
seat acts like a bellows also forcing fuel up the return line.
The best fix is a vent for the header pointing up not down.
Peter

---- Mike Perry
<<mailto:dmperry1012%40charter.net>dmperry1012@
<mailto:dmperry1012%40charter.net> charter.net> wrote:
The original QAC plans for the Q2 has an opening in the gas
cap for
pressure relief. There is no vent tube as far as I can find.

Plans change Q2PC27 (March 1983) has a vent line oriented
vertically in
the
header tank; the vent tube opening points straight up. The
other end is
routed below and behind the canard, facing forward to provide
ram air
pressure. This change was made to "minimize mixture changes
with speed
and
attitude."

The Q200 addendum has a vent line that bends180 deg in the
header and
points down the fuel return tube.

Mike Perry

At 08:52 AM 8/5/2006 -0400, Mike Dwyer wrote:

I've always been confused by this. The Q200 plans (as I
remember) have
a different header tank design than the Q2. The Q200 header
tank has no
propensity to get fuel in the vent line when built according
to the
plans. Maybe this item should be added to the 6 pack? The
ugly gas cap
spewing gas fumes right in front of the window is a big
negative to me.
Mike Q200 1000+ hours












Jim Patillo
 

Peter,

I know you're convinced of your fuel vent modification and it does
make some sense for the Posa carb without a fuel bowl. First hand
knowledge is critical. If I had a Posa I would have to think about
it.

However, there are a lot of "old timer Q200's" out there with the
stock fuel setup flying just fine. Collectively we have a hell of a
lot more flying time on that setup than you will ever get. How do
you account for all the hours we've flown without fuel starvation?
That is not by luck my friend.
.
I am not making light of your findings. My comments are directed at
new builders and how to make a simpler fuel fill and delivery system
that works.
Regards,
Jim

----- Original Message -----
From: peterjfharris
To: Q-LIST@yahoogroups.com
Cc: Jim Patillo
Sent: Tuesday, August 08, 2006 12:48 AM
Subject: Re: [Q-LIST] Re: Fuel Vent Line


The risk of fuel blown on the canopy from a header vent is zero
provided the vent is NOT fitted into the fuel return line. Mine is
fitted into the fuel return as per plans amendment and I have seen a
small splash of fuel on the canopy twice in the last 3 years. Also
it could be mounted clear of view.The point about this type of vent
is that it clears immediately under air pressure combined with
gravity. In the plans system gravity is working against air pressure
so fuel is held up frequently in the excessively long
pipe ,restricting flow to the carb fuel bowl and enough to stop a
Posa with no bowl. I am pleased I do not need to think about this
unnecesary restriction to fuel flow to my Jab.I would think twicw
before fitting a check valve in the return line as it is another
element that could fail.
Peter


peterjfharris <peterjfharris@...>
 

That is fine Jim and I know that you and Sam have got a big score on the board. I take this view about the plans vent because I do not like the engineering of this design and it bit me with the Posa installation. I think everyone should know it is not safe without a fuel bowl, and although no 0-200 have reported fuel starvation (AFAIK) I personally do not like to know that the supply to my carb is messed up by fuel in a such a loooong vent tube.If you were to witness the effect when there is no fuel bowl I think you would appreciate what this long vent is doing to the fuel supply to any carb. .I know my header vent is shorter and I think it is simpler and safer than the plans vent, but I respect your view even though I do not necessarily agree in this particular case.
I am sitting up here in Devon visiting the latest grand kids. The weather is fine for flying and I am over 22
hrs from my Q by Boeng 777-300. It is about time to check out the nearest pub.
Cheers,
Peter


Peter

---- Jim Patillo <logistics_engineering@msn.com> wrote:

Peter,

I know you're convinced of your fuel vent modification and it does
make some sense for the Posa carb without a fuel bowl. First hand
knowledge is critical. If I had a Posa I would have to think about
it.

However, there are a lot of "old timer Q200's" out there with the
stock fuel setup flying just fine. Collectively we have a hell of a
lot more flying time on that setup than you will ever get. How do
you account for all the hours we've flown without fuel starvation?
That is not by luck my friend.
.
I am not making light of your findings. My comments are directed at
new builders and how to make a simpler fuel fill and delivery system
that works.
Regards,
Jim
----- Original Message -----
From: peterjfharris
To: Q-LIST@yahoogroups.com
Cc: Jim Patillo
Sent: Tuesday, August 08, 2006 12:48 AM
Subject: Re: [Q-LIST] Re: Fuel Vent Line


The risk of fuel blown on the canopy from a header vent is zero
provided the vent is NOT fitted into the fuel return line. Mine is
fitted into the fuel return as per plans amendment and I have seen a
small splash of fuel on the canopy twice in the last 3 years. Also
it could be mounted clear of view.The point about this type of vent
is that it clears immediately under air pressure combined with
gravity. In the plans system gravity is working against air pressure
so fuel is held up frequently in the excessively long
pipe ,restricting flow to the carb fuel bowl and enough to stop a
Posa with no bowl. I am pleased I do not need to think about this
unnecesary restriction to fuel flow to my Jab.I would think twicw
before fitting a check valve in the return line as it is another
element that could fail.
Peter




Bob Farnam <bfarnam@...>
 

Just a couple of points to think about. If you put the fuel filler on top of
the header tank, you increase the possibility of introducing water or
contaminants into the header tank. Therefore, it seems to me that there
should be provision for draining the header tank sump to get rid of these
contaminants. A gascolator would seem to be the best thing, although a lot
of us don't like gascolators much. If the header tank is filled only from
the main tank per the original Q200 plans, the fuel comes from a tank with a
drainable sump and is passed through a filter before it enters the header
tank. There is very little opportunity for water or grit to get into the
header tank. Also, since the header tank is generally full, there is little
likelihood of much condensation in the header tank. My feeling is that the
Q200 plans system is well thought out and, for me at least, has been
reliable. It does have the potential problem of placing the filler in a low
pressure area, but a properly maintained gas cap with NO vent should make
that a non-issue. A band-aid to cover that possiblility would be to get high
pressure air into the filler pocket by bleeding in plenum pressure, or with
a small scoop or vent on the fuel filler cover. Wouldn't do anything if the
cap didn't leak, but would keep a leak from being a problem. Personally, I
reinforced the kit fuel cap with a fiberglas wrap and inspect it when I
refuel.

Bob Farnam
N200QK
EAA Flight Advisor

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]On Behalf Of
Mike Perry
Sent: Monday, August 07, 2006 9:13 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Fuel Vent Line


I appreciate everyone's feedback on the fuel vent line question. It seems
to me that the best system should have:
-- the fuel fill on top of the header tank
-- the fuel vent built stock as per the Q-200 plans (bent into the
top of the fuel return line)
-- some sort of check-valve or header space in the fuel return line
to prevent fuel from being forced up the return line

The fluid dynamics diagrams posted by Larry Hamm also point to a potential
problem: The standard gas cap location is a very low pressure area in
flight. Thus cracked/broken/missing gas caps can easily put negative
pressure into our fuel systems. The top of the header tank appears to be a
neutral pressure area.

Mike Perry
not currently building :-(
PS: Larry, or anyone, do you have a pressure diagram of the TOP of the
Q2? I would like to see that also if available. Thanks -- Mike Perry

At 10:22 PM 8/6/2006 -0500, Lynn French wrote:

>[snip]
> Until all the high time Q200 drivers are
>willing to loosen their gas cap, put a crack in it, put a small hole in
it,
>leave it laying at the pump, or any other anomaly, and then hop in
without
>any stuttering or hesitation and go fly without incident, then the plans
>vent system is not good enough.
> [snip]
>There are a few Q builders that have solved the "low pressure gas cap
>location" issue by installing it on the upper fuselage - Patillo,
Marstall,
>and others. I would think this takes care of many of the vent system
issues
>that have been reported in the back issues of Q-Talk.

On 8/7/06 Jim Patillo wrote:

>I agree with Sam 100%. I've got the stock system with the vent line
>turned into the 5/8" return line and it has never caused a problem.
>I gave a demo ride Friday and my plane is still running just fine!
>
> [snip]
>
>Jim Patillo N46JP Q200
>
>P.S. I would highly suggest anyone building now consider installing
>their non vented fuel cap on top of the header tank. There's no
>reason to install the filler in a box behind a door on the side per
>plans. [snip]

On 8/6/06 Peter Harris wrote:

>The vent 90 deg bend turning down the fuel return line is not a good idea
>because fuel is sometimes forced UP the fuel return line. This happens
>when the main is full and in turbulence. If the main is full there is
>nowhere else for the fuel to go but up the fuel return line. The pilots
>seat acts like a bellows also forcing fuel up the return line.
>The best fix is a vent for the header pointing up not down.
>Peter


French <LJFrench@...>
 

Remember, what really matters - even with a float bowl carburator - is the difference in pressure between the carb bowl and the tank pressure. After all, all the vent pressure in the world coming from ram air will not do any good if the float bowl is seeing the same pressure. Same goes for the head pressure of the fuel supply above the carb. A header tank 10 feet above the carb doesn't help if the float bowl pressure overcomes the added head pressure. If you measure 100 GPH fuel flow from your fuel system on the ground at the carb, it doesn't mean in any way that you have a system capable of delivering any where near that amount of fuel flow in flight.

Anybody know what pressure is in the float bowl under various flight conditions (a clue is in the mixure control...anybody need the MA-3SPA handbook)???

Anybody know what pressure is being provided by the ram air vent under various flight conditions??

Woudn't it be awsome if there was some enhancement to the vent system of the plans Q200 that would avoid fuel starvation if you accidently flooded the vent system by overflowing the main tank from the auxillary tank, or if the gas cap accidently got a crack in it that didn't get detected, or any of the many other "accidents" that have happened with our systems. Now that would really be cool.......a root cause fix.

Lynn French

----- Original Message -----
From: "Jim Patillo" <logistics_engineering@msn.com>
To: <Q-LIST@yahoogroups.com>
Sent: Tuesday, August 08, 2006 12:03 PM
Subject: [Q-LIST] Re: Fuel Vent Line


Peter,

it does
make some sense for the Posa carb without a fuel bowl. First hand
knowledge is critical.
How do
you account for all the hours we've flown without fuel starvation?
That is not by luck my friend.

My comments are directed at
new builders and how to make a simpler fuel fill and delivery system
that works.
Regards,
Jim
----- Original Message -----
From: peterjfharris
To: Q-LIST@yahoogroups.com
Cc: Jim Patillo
Sent: Tuesday, August 08, 2006 12:48 AM
Subject: Re: [Q-LIST] Re: Fuel Vent Line


The risk of fuel blown on the canopy from a header vent is zero
provided the vent is NOT fitted into the fuel return line. Mine is
fitted into the fuel return as per plans amendment and I have seen a
small splash of fuel on the canopy twice in the last 3 years. Also
it could be mounted clear of view.The point about this type of vent
is that it clears immediately under air pressure combined with
gravity. In the plans system gravity is working against air pressure
so fuel is held up frequently in the excessively long
pipe ,restricting flow to the carb fuel bowl and enough to stop a
Posa with no bowl. I am pleased I do not need to think about this
unnecesary restriction to fuel flow to my Jab.I would think twicw
before fitting a check valve in the return line as it is another
element that could fail.
Peter


David J. Gall
 

Good points, Lynn

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If we stack all those cubic inches atop one another, we get all 231 cubic inches pressing down on one square inch of surface area at the bottom of the container. Therefore, there is 6 lbs per square inch of fuel pressure at the bottom of a column of fuel 231 inches tall. Dividing 6 lbs by 231 inches yields a usable value of head pressure of 0.026 lbs per inch of height. So if your header tank bottom (figuring a nearly empty tank) is 20 inches above the carb needle valve, then the fuel pressure at the carb is .52 psi or about ONE inch of mercury. Not a lot, and easily overcome by misguided ram air pressure. At 60 mph the dynamic air pressure is about 9 psi or about 18 inches of mercury. Anybody remember the excellent manometer tests that the late Larry Koutz did on his cowl outlets? It might be beneficial if someone did similar tests on the pressure inside the cowl near the carb vs. near the various fuel tank vent locations. In fact, the pressure in the float bowl could be measured against the pressure in the fuel tank directly, on the ground with the engine running to give some indication of the expected in-flight pressure performance of the fuel system....

Back to lurking (and hacking foam),


David J. Gall
P.S. Who was looking for Onan engine mount parts? I might have some in the garage; email me off list.

----- Original Message -----
From: French <LJFrench@FBNetUSA.com>
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Tue, 8 Aug 2006 20:04:17 -0500


Remember, what really matters - even with a float bowl carburator - is the
difference in pressure between the carb bowl and the tank pressure. After
all, all the vent pressure in the world coming from ram air will not do any
good if the float bowl is seeing the same pressure. Same goes for the head
pressure of the fuel supply above the carb. A header tank 10 feet above the
carb doesn't help if the float bowl pressure overcomes the added head
pressure. If you measure 100 GPH fuel flow from your fuel system on the
ground at the carb, it doesn't mean in any way that you have a system
capable of delivering any where near that amount of fuel flow in flight.

Anybody know what pressure is in the float bowl under various flight
conditions (a clue is in the mixure control...anybody need the MA-3SPA
handbook)???

Anybody know what pressure is being provided by the ram air vent under
various flight conditions??

Woudn't it be awsome if there was some enhancement to the vent system of the
plans Q200 that would avoid fuel starvation if you accidently flooded the
vent system by overflowing the main tank from the auxillary tank, or if the
gas cap accidently got a crack in it that didn't get detected, or any of the
many other "accidents" that have happened with our systems. Now that would
really be cool.......a root cause fix.

Lynn French


Patrick Panzera <panzera@...>
 

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If
we stack all those cubic inches atop one another, we get all 231
cubic inches pressing down on one square inch of surface area at the
bottom of the container. Therefore, there is 6 lbs per square inch of
fuel pressure at the bottom of a column of fuel 231 inches tall.
Dividing 6 lbs by 231 inches yields a usable value of head pressure
of 0.026 lbs per inch of height. So if your header tank bottom
(figuring a nearly empty tank) is 20 inches above the carb needle
valve, then the fuel pressure at the carb is .52 psi or about ONE
inch of mercury. Not a lot, and easily overcome by misguided ram air
pressure.
Wouldn't that only be true if your fuel line was equal to one square
inch? 3/8" line is quite a bit smaller that one square inch.

Pat


Bob Farnam <bfarnam@...>
 

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]On Behalf Of
David Gall
Sent: Tuesday, August 08, 2006 7:15 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Good points, Lynn

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If we
stack all those cubic inches atop one another, we get all 231 cubic inches
pressing down on one square inch of surface area at the bottom of the
container. Therefore, there is 6 lbs per square inch of fuel pressure at the
bottom of a column of fuel 231 inches tall. Dividing 6 lbs by 231 inches
yields a usable value of head pressure of 0.026 lbs per inch of height. So
if your header tank bottom (figuring a nearly empty tank) is 20 inches above
the carb needle valve, then the fuel pressure at the carb is .52 psi or
about ONE inch of mercury. Not a lot, and easily overcome by misguided ram
air pressure. At 60 mph the dynamic air pressure is about 9 psi or about 18
inches of mercury.
[Bob Farnam] Actually, at 60 mph the dynamic pressure of air at standard
conditions is about 1.5 inches of water. At typical Q200 cruise speed, it's
about 10 inches of water.


Anybody remember the excellent manometer tests that the late Larry Koutz
did on his cowl outlets? It might be beneficial if someone did similar tests
on the pressure inside the cowl near the carb vs. near the various fuel tank
vent locations. In fact, the pressure in the float bowl could be measured
against the pressure in the fuel tank directly, on the ground with the
engine running to give some indication of the expected in-flight pressure
performance of the fuel system....

Back to lurking (and hacking foam),

David J. Gall
P.S. Who was looking for Onan engine mount parts? I might have some in the
garage; email me off list.

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David J. Gall
 

Bob,

Dang it, I hate it when I mess up like that! Yes, you're right. I was calculating in psf and writing psi. Then I went from there to inches of mercury and really screwed it up. So much for rushed emails. So much for conclusions, too. Anyway, if water is 8.35 lbs per gallon and fuel is 6 lbs per gallon, then 10 inches of water is about 14 inches of fuel... check my work for me please... so if I get something like 14 inches of fuel into the vertical part of the vent line while in cruising flight (due to turbulence or something(?)), there's a chance that the airspeed (ram pressure) will be enough to keep it from draining out and the weight of the fuel will be enough to prevent it from being pushed back up into the tank by ram pressure, and it'll act like a plug in the vent line....

David J. Gall

----- Original Message -----
From: "Bob Farnam" <bfarnam@pacbell.net>
To: Q-LIST@yahoogroups.com
Subject: RE: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 11:01:00 -0700



-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]On Behalf Of
David Gall
Sent: Tuesday, August 08, 2006 7:15 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Good points, Lynn

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If we
stack all those cubic inches atop one another, we get all 231 cubic inches
pressing down on one square inch of surface area at the bottom of the
container. Therefore, there is 6 lbs per square inch of fuel pressure at the
bottom of a column of fuel 231 inches tall. Dividing 6 lbs by 231 inches
yields a usable value of head pressure of 0.026 lbs per inch of height. So
if your header tank bottom (figuring a nearly empty tank) is 20 inches above
the carb needle valve, then the fuel pressure at the carb is .52 psi or
about ONE inch of mercury. Not a lot, and easily overcome by misguided ram
air pressure. At 60 mph the dynamic air pressure is about 9 psi or about 18
inches of mercury.

[Bob Farnam] Actually, at 60 mph the dynamic pressure of air at standard
conditions is about 1.5 inches of water. At typical Q200 cruise speed, it's
about 10 inches of water.


bfarnam@...
 

Dave,
I wonder if the pressure differential on the vent tube with a fuel plug would allow bubbles to get through into the tank. Mark Summers and I were discussing that last evening. What do you think?
Bob F.

----- Original Message ----
From: David Gall <David@Gall.com>
To: Q-LIST@yahoogroups.com
Sent: Wednesday, August 9, 2006 7:45:30 PM
Subject: RE: [Q-LIST] Re: Fuel Vent Line

Bob,

Dang it, I hate it when I mess up like that! Yes, you're right. I was calculating in psf and writing psi. Then I went from there to inches of mercury and really screwed it up. So much for rushed emails. So much for conclusions, too. Anyway, if water is 8.35 lbs per gallon and fuel is 6 lbs per gallon, then 10 inches of water is about 14 inches of fuel... check my work for me please... so if I get something like 14 inches of fuel into the vertical part of the vent line while in cruising flight (due to turbulence or something(?) ), there's a chance that the airspeed (ram pressure) will be enough to keep it from draining out and the weight of the fuel will be enough to prevent it from being pushed back up into the tank by ram pressure, and it'll act like a plug in the vent line....

David J. Gall

----- Original Message -----
From: "Bob Farnam" <bfarnam@pacbell. net>
To: Q-LIST@yahoogroups. com
Subject: RE: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 11:01:00 -0700



-----Original Message-----
From: Q-LIST@yahoogroups. com [mailto:Q-LIST@yahoogroups. com]On Behalf Of
David Gall
Sent: Tuesday, August 08, 2006 7:15 PM
To: Q-LIST@yahoogroups. com
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Good points, Lynn

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If we
stack all those cubic inches atop one another, we get all 231 cubic inches
pressing down on one square inch of surface area at the bottom of the
container. Therefore, there is 6 lbs per square inch of fuel pressure at the
bottom of a column of fuel 231 inches tall. Dividing 6 lbs by 231 inches
yields a usable value of head pressure of 0.026 lbs per inch of height. So
if your header tank bottom (figuring a nearly empty tank) is 20 inches above
the carb needle valve, then the fuel pressure at the carb is .52 psi or
about ONE inch of mercury. Not a lot, and easily overcome by misguided ram
air pressure. At 60 mph the dynamic air pressure is about 9 psi or about 18
inches of mercury.

[Bob Farnam] Actually, at 60 mph the dynamic pressure of air at standard
conditions is about 1.5 inches of water. At typical Q200 cruise speed, it's
about 10 inches of water.


peterjfharris <peterjfharris@...>
 

That is exactly right David. True in theory and true in practice. According to my experience an airspeed of very approx 80KTS is enough to hold the fuel in place but not enough to clear it back into the header, then soon fuel supply begins to suffer as the header airspace begins to stretch and pressure is reduced in there.
Peter
---- David Gall <David@Gall.com> wrote:

Bob,

Dang it, I hate it when I mess up like that! Yes, you're right. I was calculating in psf and writing psi. Then I went from there to inches of mercury and really screwed it up. So much for rushed emails. So much for conclusions, too. Anyway, if water is 8.35 lbs per gallon and fuel is 6 lbs per gallon, then 10 inches of water is about 14 inches of fuel... check my work for me please... so if I get something like 14 inches of fuel into the vertical part of the vent line while in cruising flight (due to turbulence or something(?)), there's a chance that the airspeed (ram pressure) will be enough to keep it from draining out and the weight of the fuel will be enough to prevent it from being pushed back up into the tank by ram pressure, and it'll act like a plug in the vent line....

David J. Gall

----- Original Message -----
From: "Bob Farnam" <bfarnam@pacbell.net>
To: Q-LIST@yahoogroups.com
Subject: RE: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 11:01:00 -0700



-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]On Behalf Of
David Gall
Sent: Tuesday, August 08, 2006 7:15 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Good points, Lynn

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If we
stack all those cubic inches atop one another, we get all 231 cubic inches
pressing down on one square inch of surface area at the bottom of the
container. Therefore, there is 6 lbs per square inch of fuel pressure at the
bottom of a column of fuel 231 inches tall. Dividing 6 lbs by 231 inches
yields a usable value of head pressure of 0.026 lbs per inch of height. So
if your header tank bottom (figuring a nearly empty tank) is 20 inches above
the carb needle valve, then the fuel pressure at the carb is .52 psi or
about ONE inch of mercury. Not a lot, and easily overcome by misguided ram
air pressure. At 60 mph the dynamic air pressure is about 9 psi or about 18
inches of mercury.

[Bob Farnam] Actually, at 60 mph the dynamic pressure of air at standard
conditions is about 1.5 inches of water. At typical Q200 cruise speed, it's
about 10 inches of water.


David J. Gall
 

Bob,

I think you and Mark are teasing me! :)

Well, OK, I'll bite.

If the vent tube were big enough to be able to pass bubbles through a slug of
trapped fuel, I don't think you'd be able to get enough bubbles per hour to
replace the five to six gallons per hour of fuel volume your engine wants to
consume during climb.

You can't get bubbles through a soda straw, but you can through a garden hose.
The diameter matters a lot. The tube from the main tank to the header tank is
designed to do essentially what you suggest, but sized appropriately to allow
fuel and air to flow simultaneously in opposite directions. Dumb design, but
QAC apparently made it work.

The design parameter for the fuel tank vent line is that it either be
impervious to fuel entering the line (ever - not realistic) or else be capable
of performing its vent function when fuel gets into it. To do that requires
that it either be self-purging when it gets fuel in it, or that it be able to
flow air (in sufficient volume) even when there's fuel in it. A short vent
line extending up from the top of the tank or filler cap has the self-purging
capability with the added advantage that gravity works with ram pressure to
purge the line in normal flight.

A long vent line extending down through the bottom of the fuselage has the
distinct disadvantage of having gravity working against ram pressure as
previously discussed. The pressure differential needed to purge the line by
forcing fuel up into the tank is apparently not available, so the line must
either be self-draining against ram pressure or else have a shunt to capture
the fuel (up to some assumed reasonable amount) in a secondary tank of some
kind for later return to the main tank. That is a bad idea for lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect on its ability to
pass air and fuel in opposite directions simultaneously. The length of the
tube and the relative heights of its ends have an effect on its ability to
trap and retain a slug of fuel (siphon effect). A short length of small
diameter tubing at the airframe exit is tolerable because ram pressure could
force a small slug of fuel up against the action of gravity; likewise, a short
length of small diameter tubing is tolerable at the top of the tank to "turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line that is the problem
in the QAC design. This section of the vent line can hold a slug of fuel just
like when you cap off the top of your soda straw and pull a slug of your
favorite beverage up out of the cup. Try the same stunt with a piece of 5/8"
tubing and you'll find all the soda (fuel) draining back into the cup as you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent line with a larger
diameter line and you should have a self-draining line that is capable of
passing air even when there's a bit of fuel in it.

That's my opinion.


David J. Gall

----- Original Message -----
From: bfarnam@pacbell.net
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)


Dave,
I wonder if the pressure differential on the vent tube with a fuel plug
would
allow bubbles to get through into the tank. Mark Summers and I were
discussing
that last evening. What do you think?
Bob F.


French <LJFrench@...>
 

David,
Thank you for your opinion.
I appreciate the willingness of you and others to discuss this issue.
This thought would seem to make some sense to me.

Lynn French

----- Original Message -----
From: "David Gall" <David@Gall.com>
To: <Q-LIST@yahoogroups.com>
Sent: Friday, August 11, 2006 3:03 PM
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Bob,

I think you and Mark are teasing me! :)

Well, OK, I'll bite.

If the vent tube were big enough to be able to pass bubbles through a slug of
trapped fuel, I don't think you'd be able to get enough bubbles per hour to
replace the five to six gallons per hour of fuel volume your engine wants to
consume during climb.

You can't get bubbles through a soda straw, but you can through a garden hose.
The diameter matters a lot. The tube from the main tank to the header tank is
designed to do essentially what you suggest, but sized appropriately to allow
fuel and air to flow simultaneously in opposite directions. Dumb design, but
QAC apparently made it work.

The design parameter for the fuel tank vent line is that it either be
impervious to fuel entering the line (ever - not realistic) or else be capable
of performing its vent function when fuel gets into it. To do that requires
that it either be self-purging when it gets fuel in it, or that it be able to
flow air (in sufficient volume) even when there's fuel in it. A short vent
line extending up from the top of the tank or filler cap has the self-purging
capability with the added advantage that gravity works with ram pressure to
purge the line in normal flight.

A long vent line extending down through the bottom of the fuselage has the
distinct disadvantage of having gravity working against ram pressure as
previously discussed. The pressure differential needed to purge the line by
forcing fuel up into the tank is apparently not available, so the line must
either be self-draining against ram pressure or else have a shunt to capture
the fuel (up to some assumed reasonable amount) in a secondary tank of some
kind for later return to the main tank. That is a bad idea for lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect on its ability to
pass air and fuel in opposite directions simultaneously. The length of the
tube and the relative heights of its ends have an effect on its ability to
trap and retain a slug of fuel (siphon effect). A short length of small
diameter tubing at the airframe exit is tolerable because ram pressure could
force a small slug of fuel up against the action of gravity; likewise, a short
length of small diameter tubing is tolerable at the top of the tank to "turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line that is the problem
in the QAC design. This section of the vent line can hold a slug of fuel just
like when you cap off the top of your soda straw and pull a slug of your
favorite beverage up out of the cup. Try the same stunt with a piece of 5/8"
tubing and you'll find all the soda (fuel) draining back into the cup as you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent line with a larger
diameter line and you should have a self-draining line that is capable of
passing air even when there's a bit of fuel in it.

That's my opinion.


David J. Gall


----- Original Message -----
From: bfarnam@pacbell.net
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)


Dave,
I wonder if the pressure differential on the vent tube with a fuel plug
would
allow bubbles to get through into the tank. Mark Summers and I were
discussing
that last evening. What do you think?
Bob F.


Quickie Builders Association WEB site
http://www.quickiebuilders.org


Yahoo! Groups Links








Bob Farnam <bfarnam@...>
 

David,
No, not teasing! I agree as far as it goes. But there is another factor to
consider. As an unvented tank supplies fuel through gravity flow, it will
begin to pull a negative pressure on the tank system. I know about this
because a Pacer of which I was part owner partially collapsed a wing tank
due to negative pressure because of a blocked (top) vent. Now the question
is: Does the stagnation pressure due to ram air plus the suction generated
in the tank constitute enough pressure differential to purge the fuel plug
from the vent line? I may see if I remember enough manometry to put some
numbers to the problem.
Bob

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]On Behalf Of
David Gall
Sent: Friday, August 11, 2006 1:04 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Bob,

I think you and Mark are teasing me! :)

Well, OK, I'll bite.

If the vent tube were big enough to be able to pass bubbles through a slug
of
trapped fuel, I don't think you'd be able to get enough bubbles per hour
to
replace the five to six gallons per hour of fuel volume your engine wants
to
consume during climb.

You can't get bubbles through a soda straw, but you can through a garden
hose.
The diameter matters a lot. The tube from the main tank to the header tank
is
designed to do essentially what you suggest, but sized appropriately to
allow
fuel and air to flow simultaneously in opposite directions. Dumb design,
but
QAC apparently made it work.

The design parameter for the fuel tank vent line is that it either be
impervious to fuel entering the line (ever - not realistic) or else be
capable
of performing its vent function when fuel gets into it. To do that
requires
that it either be self-purging when it gets fuel in it, or that it be able
to
flow air (in sufficient volume) even when there's fuel in it. A short vent
line extending up from the top of the tank or filler cap has the
self-purging
capability with the added advantage that gravity works with ram pressure
to
purge the line in normal flight.

A long vent line extending down through the bottom of the fuselage has the
distinct disadvantage of having gravity working against ram pressure as
previously discussed. The pressure differential needed to purge the line
by
forcing fuel up into the tank is apparently not available, so the line
must
either be self-draining against ram pressure or else have a shunt to
capture
the fuel (up to some assumed reasonable amount) in a secondary tank of
some
kind for later return to the main tank. That is a bad idea for lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect on its ability
to
pass air and fuel in opposite directions simultaneously. The length of the
tube and the relative heights of its ends have an effect on its ability to
trap and retain a slug of fuel (siphon effect). A short length of small
diameter tubing at the airframe exit is tolerable because ram pressure
could
force a small slug of fuel up against the action of gravity; likewise, a
short
length of small diameter tubing is tolerable at the top of the tank to
"turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line that is the
problem
in the QAC design. This section of the vent line can hold a slug of fuel
just
like when you cap off the top of your soda straw and pull a slug of your
favorite beverage up out of the cup. Try the same stunt with a piece of
5/8"
tubing and you'll find all the soda (fuel) draining back into the cup as
you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent line with a
larger
diameter line and you should have a self-draining line that is capable of
passing air even when there's a bit of fuel in it.

That's my opinion.

David J. Gall

> ----- Original Message -----
> From: bfarnam@pacbell.net
> To: Q-LIST@yahoogroups.com
> Subject: Re: [Q-LIST] Re: Fuel Vent Line
> Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)
>
>
> Dave,
> I wonder if the pressure differential on the vent tube with a fuel plug
> would
> allow bubbles to get through into the tank. Mark Summers and I were
> discussing
> that last evening. What do you think?
> Bob F.


Mike Perry <dmperry1012@...>
 

First, thanks to everyone who keeps writing on this issue. Fuel starvation
is one of the classic killers in early flights of experimental aircraft and
well worth a lot of attention.

Second, Dave Gall, you seem to be advocating a large diameter down pipe
narrowing to a small diameter tube as it exits the airframe. I have
concern we could still end up with a fluid column trapped above the narrow
vent tube. Perhaps some experiments are in order.

(Lets see, mock up of fuel tank, 2 or 3 vent tube designs, mount on top of
the pickup truck, drive down the freeway at 100 mph, see if ram air
pressure will clear the vent tube . . . "Gee officer, I wasn't speeding . .
. this is a scientific experiment . . . yes, I'm working on an airplane
design . . . No, I'm not crazy. Well I don't think I am . . . sure, I'll
talk to the nice psych tech . . . " NOT GONNA HAPPEN!)

Also, Dave you wrote: "or else have a shunt to capture the fuel . . . in a
secondary tank . . . That is a bad idea for lots of reasons . . ." Here is
an idea that might work:

Run a fuel vent line from the header to the fuel fill tube, then a second
vent line from the fuel fill to below the canard. In essence, I am using
the main fuel tank as the "secondary tank" in your earlier
suggestion. Drawback: Complexity, weight. Advantages: short vertical in
each of two separate tubes, unlikely to develop a significant siphon effect
in either. Should develop positive pressure in main and header tank in all
situations.

In other words, why not replace a long vent line that can generate
significant negative pressure thru siphon effects with two short vent lines
much less vulnerable to siphon effect negative pressure?

I will be off line for 36-48 hours (work) but look forward to any comments
when I get home.

Mike Perry
(18 months away from needing this question answered but still very
interested in a safe fuel system when I get there)

At 12:03 PM 8/11/2006 -0800, Dave Gall wrote:

[snip]

The design parameter for the fuel tank vent line is that it either be
impervious to fuel entering the line (ever - not realistic) or else be
capable
of performing its vent function when fuel gets into it. To do that requires
that it either be self-purging when it gets fuel in it, or that it be able to
flow air (in sufficient volume) even when there's fuel in it. A short vent
line extending up from the top of the tank or filler cap has the self-purging
capability with the added advantage that gravity works with ram pressure to
purge the line in normal flight.

A long vent line extending down through the bottom of the fuselage has the
distinct disadvantage of having gravity working against ram pressure as
previously discussed. The pressure differential needed to purge the line by
forcing fuel up into the tank is apparently not available, so the line must
either be self-draining against ram pressure or else have a shunt to capture
the fuel (up to some assumed reasonable amount) in a secondary tank of some
kind for later return to the main tank. That is a bad idea for lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect on its ability to
pass air and fuel in opposite directions simultaneously. The length of the
tube and the relative heights of its ends have an effect on its ability to
trap and retain a slug of fuel (siphon effect). A short length of small
diameter tubing at the airframe exit is tolerable because ram pressure could
force a small slug of fuel up against the action of gravity; likewise, a
short
length of small diameter tubing is tolerable at the top of the tank to "turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line that is the
problem
in the QAC design. This section of the vent line can hold a slug of fuel just
like when you cap off the top of your soda straw and pull a slug of your
favorite beverage up out of the cup. Try the same stunt with a piece of 5/8"
tubing and you'll find all the soda (fuel) draining back into the cup as you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent line with a larger
diameter line and you should have a self-draining line that is capable of
passing air even when there's a bit of fuel in it.

That's my opinion.

David J. Gall

----- Original Message -----
From: <mailto:bfarnam%40pacbell.net>bfarnam@pacbell.net
To: <mailto:Q-LIST%40yahoogroups.com>Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)


Dave,
I wonder if the pressure differential on the vent tube with a fuel plug
would
allow bubbles to get through into the tank. Mark Summers and I were
discussing
that last evening. What do you think?
Bob F.


chrishazlitt <chrishazlitt@...>
 

I have been cut and paste'ing the posts in that thread myself,
thanks for sharing on this subject, I had no idea there was that
much to know.

Chris

--- In Q-LIST@yahoogroups.com, Mike Perry <dmperry1012@...> wrote:

First, thanks to everyone who keeps writing on this issue. Fuel
starvation
is one of the classic killers in early flights of experimental
aircraft and
well worth a lot of attention.

Second, Dave Gall, you seem to be advocating a large diameter down
pipe
narrowing to a small diameter tube as it exits the airframe. I
have
concern we could still end up with a fluid column trapped above
the narrow
vent tube. Perhaps some experiments are in order.

(Lets see, mock up of fuel tank, 2 or 3 vent tube designs, mount
on top of
the pickup truck, drive down the freeway at 100 mph, see if ram
air
pressure will clear the vent tube . . . "Gee officer, I wasn't
speeding . .
. this is a scientific experiment . . . yes, I'm working on an
airplane
design . . . No, I'm not crazy. Well I don't think I am . . .
sure, I'll
talk to the nice psych tech . . . " NOT GONNA HAPPEN!)

Also, Dave you wrote: "or else have a shunt to capture the
fuel . . . in a
secondary tank . . . That is a bad idea for lots of
reasons . . ." Here is
an idea that might work:

Run a fuel vent line from the header to the fuel fill tube, then a
second
vent line from the fuel fill to below the canard. In essence, I
am using
the main fuel tank as the "secondary tank" in your earlier
suggestion. Drawback: Complexity, weight. Advantages: short
vertical in
each of two separate tubes, unlikely to develop a significant
siphon effect
in either. Should develop positive pressure in main and header
tank in all
situations.

In other words, why not replace a long vent line that can generate
significant negative pressure thru siphon effects with two short
vent lines
much less vulnerable to siphon effect negative pressure?

I will be off line for 36-48 hours (work) but look forward to any
comments
when I get home.

Mike Perry
(18 months away from needing this question answered but still very
interested in a safe fuel system when I get there)


At 12:03 PM 8/11/2006 -0800, Dave Gall wrote:

[snip]

The design parameter for the fuel tank vent line is that it
either be
impervious to fuel entering the line (ever - not realistic) or
else be
capable
of performing its vent function when fuel gets into it. To do
that requires
that it either be self-purging when it gets fuel in it, or that
it be able to
flow air (in sufficient volume) even when there's fuel in it. A
short vent
line extending up from the top of the tank or filler cap has the
self-purging
capability with the added advantage that gravity works with ram
pressure to
purge the line in normal flight.

A long vent line extending down through the bottom of the
fuselage has the
distinct disadvantage of having gravity working against ram
pressure as
previously discussed. The pressure differential needed to purge
the line by
forcing fuel up into the tank is apparently not available, so the
line must
either be self-draining against ram pressure or else have a shunt
to capture
the fuel (up to some assumed reasonable amount) in a secondary
tank of some
kind for later return to the main tank. That is a bad idea for
lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect on its
ability to
pass air and fuel in opposite directions simultaneously. The
length of the
tube and the relative heights of its ends have an effect on its
ability to
trap and retain a slug of fuel (siphon effect). A short length of
small
diameter tubing at the airframe exit is tolerable because ram
pressure could
force a small slug of fuel up against the action of gravity;
likewise, a
short
length of small diameter tubing is tolerable at the top of the
tank to "turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line that
is the
problem
in the QAC design. This section of the vent line can hold a slug
of fuel just
like when you cap off the top of your soda straw and pull a slug
of your
favorite beverage up out of the cup. Try the same stunt with a
piece of 5/8"
tubing and you'll find all the soda (fuel) draining back into the
cup as you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent line
with a larger
diameter line and you should have a self-draining line that is
capable of
passing air even when there's a bit of fuel in it.

That's my opinion.

David J. Gall

----- Original Message -----
From: <mailto:bfarnam%40pacbell.net>bfarnam@...
To: <mailto:Q-LIST%40yahoogroups.com>Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)


Dave,
I wonder if the pressure differential on the vent tube with a
fuel plug
would
allow bubbles to get through into the tank. Mark Summers and I
were
discussing
that last evening. What do you think?
Bob F.




David J. Gall
 

Mike,

I'm not advocating this installation at all: I'm trying to get people to see
the difficulties of it and to opt for the simpler vent going out the top of
the fuselage. We're all so used to seeing a fuel tank vent under the wing of
the Cessna we rent that we may not be putting much thought into the "simple"
act of "copying" that vent. Indeed, that vent is very different from the QAC
vent. And you're right, the transition from large diameter to small in my
suggestion does pose a potential problem.

Let me jump to the conclusion regarding your suggestion to use the filler
pipe as a catch tank. In essence, the Q2/200 fuel tank(s) are actually (in a
fluids sense) just one tank. The header tank is connected to the main tank
via the 5/8" standpipe down the middle, through which the "two" tanks
communicate both fuel and air.

The advent of the header tank is that it provides a reliable head pressure
to feed the carb. The header tank is kept full by the fuel transfer pump.
You can think of this transfer pump as a part of a "live" tank as opposed to
a passive tank. In a passive tank like on a Cessna, when the tank is nearly
empty the head pressure is still good because the bottom of the tank is
above the carb. In the Q2/200, the "bottom" of the tank is artificially
moved up to the level of the bottom of the header tank, because as the main
tank empties we transfer the fuel to the header. This "live" transfer makes
the tank seem to the carb (in a fluids sense) to be mounted higher in the
airframe.

Consider the act of filling the tank: If you fill the main tank until the
filler neck is full, doesn't the fuel also rise within the 5/8" standpipe to
the same level as in the filler neck? Now, mind you, I haven't got
operational experience with a Q2, but it seems to me that you either have to
run the electric fuel pump during refueling in order to fill the header
tank, or else the header tank fills when the fuel level reaches the top of
the 5/8" standpipe. Either way, when the header tank is full so is the 5/8
stand pipe and so is the filler pipe. Therefore, ANY vent connected to this
tank system anywhere had better be above the fuel level or it will have fuel
in it. Ergo, your proposed vent from the filler pipe would need to be just
as long (in the vertical dimension) as the existing vent from the header
tank. Moreover, the filler pipe vent would be just as susceptible to fuel
getting into it as the existing vent line is.

Keep up the good work,


David J. Gall

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Mike Perry
Sent: Saturday, August 12, 2006 9:20 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line

First, thanks to everyone who keeps writing on this issue.
Fuel starvation is one of the classic killers in early
flights of experimental aircraft and well worth a lot of attention.

Second, Dave Gall, you seem to be advocating a large diameter
down pipe narrowing to a small diameter tube as it exits the
airframe. I have concern we could still end up with a fluid
column trapped above the narrow vent tube. Perhaps some
experiments are in order.

(Lets see, mock up of fuel tank, 2 or 3 vent tube designs,
mount on top of the pickup truck, drive down the freeway at
100 mph, see if ram air pressure will clear the vent tube . .
. "Gee officer, I wasn't speeding . .
. this is a scientific experiment . . . yes, I'm working on
an airplane design . . . No, I'm not crazy. Well I don't
think I am . . . sure, I'll talk to the nice psych tech . . .
" NOT GONNA HAPPEN!)

Also, Dave you wrote: "or else have a shunt to capture the
fuel . . . in a secondary tank . . . That is a bad idea for
lots of reasons . . ." Here is an idea that might work:

Run a fuel vent line from the header to the fuel fill tube,
then a second vent line from the fuel fill to below the
canard. In essence, I am using the main fuel tank as the
"secondary tank" in your earlier suggestion. Drawback:
Complexity, weight. Advantages: short vertical in each of
two separate tubes, unlikely to develop a significant siphon
effect in either. Should develop positive pressure in main
and header tank in all situations.

In other words, why not replace a long vent line that can
generate significant negative pressure thru siphon effects
with two short vent lines much less vulnerable to siphon
effect negative pressure?

I will be off line for 36-48 hours (work) but look forward to
any comments when I get home.

Mike Perry
(18 months away from needing this question answered but still
very interested in a safe fuel system when I get there)


David J. Gall
 

Ya know, Bob, I thought about that too. Here's the way I see it: the lowest
pressure the carb can pull on the fuel line will be at high rpm with the
throttle closed. The highest ram pressure will be at max airspeed. Dive with
the throttle closed to purge the vent line. Not a good plan.

So there you are on climbout and the engine sags due to fuel starvation. The
rpm drops off, you push the throttle open, and the airspeed decays. What
good will it do to know that you would be able to clear the vent line at Vne
in a power-off dive if the powerlines ahead and every nerve fiber in your
body are screaming that you'd better add power and get the nose up?

I already know that the correct response to a power loss in climb is to push
the nose down to retain flying speed and that is already contrary to my
instincts. Please don't make me learn to hope for power restoration by
diving at the ground and chopping the throttle, too! I don't care if the
theory says there "ought to be" some combination of airspeed and throttle
setting (closed!!?) and rpm that will clear that slug of splashed fuel from
the vent line, I won't have time to diagnose that THAT particular problem
is, indeed, THE problem.

I would much prefer a fool-proof system that works in an intuitive way that
I can count on time and again to be reliable, predictable, and repeatable.
("Gee, would zero-gee help the ram pressure to purge the vent line," Robin
queried Batman as the Bat-plane plummeted perilously earthward.) If my fuel
tank vent line does NOT require me to perform acrobatics in order to restore
proper engine operation then I'm more likely to have time to apply carb heat
and squeeze the manual fuel pump bulb -- oh, yeah, we took that out of the
system last month, didn't we.... :)

Not to make too much light of the situation, your Pacer wing tank collapse
was caused by gravity feed PLUS pressure differential, working together, and
there's a good bit of head pressure from a Pacer's high wings to the
low-mounted carb. And there are other factors to consider. Even if the fuel
flow from that tank to the engine were shut off, simple contraction of the
fuel with decreasing temperature could have collapsed that tank. Top off an
already almost full tank in the morning, leave it setting in the sun all
day, then the mud dauber builds his nest in the fuel vent in the
afternoon... And you may never even have needed to start the engine!

As opposed to your Pacer's gravity feed PLUS pressure differential, purging
a slug of fuel from the QAC fuel tank vent requires determining the
difference between the action of gravity and the pressure differential,
working against each other, in other words, gravity MINUS pressure
differential.

In fact, the "gravity" portion of the equation might just be zero, or even a
little bit negative, if the slug of fuel in the vent line is taller than the
height of the fuel level in the header tank is above the carb inlet. So we'd
be left with just the pressure differential to move fuel through several
feet of small diameter (0.22") fuel line at approximately six gallons per
hour, or 0.85 ft/sec. Given that ram pressure at 60 mph is 1.5 inches of
water, and assuming that the carb is in the high pressure ram recovery
region of the cowl, the pressure difference available to drive the fuel flow
in climb might be quite small indeed. Certainly not equivalent to the head
pressure of approximately 14 inches of fuel (10 inches of water) that the
header tank normally gives....

I think you get my point by now. Thanks for helping me to have to think
through this problem. Now I know that there's another thing I want to do
differently on my airplane than what the plans call for.... :)


David J. Gall

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Bob Farnam
Sent: Friday, August 11, 2006 3:01 PM
To: Q-LIST@yahoogroups.com
Subject: RE: [Q-LIST] Re: Fuel Vent Line

David,
No, not teasing! I agree as far as it goes. But there is
another factor to consider. As an unvented tank supplies fuel
through gravity flow, it will begin to pull a negative
pressure on the tank system. I know about this because a
Pacer of which I was part owner partially collapsed a wing
tank due to negative pressure because of a blocked (top)
vent. Now the question
is: Does the stagnation pressure due to ram air plus the
suction generated in the tank constitute enough pressure
differential to purge the fuel plug from the vent line? I may
see if I remember enough manometry to put some numbers to the problem.
Bob
-----Original Message-----
From: Q-LIST@yahoogroups.com
[mailto:Q-LIST@yahoogroups.com]On Behalf Of David Gall
Sent: Friday, August 11, 2006 1:04 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line


Bob,

I think you and Mark are teasing me! :)

Well, OK, I'll bite.

If the vent tube were big enough to be able to pass bubbles
through a slug of
trapped fuel, I don't think you'd be able to get enough
bubbles per hour to
replace the five to six gallons per hour of fuel volume
your engine wants to
consume during climb.

You can't get bubbles through a soda straw, but you can
through a garden hose.
The diameter matters a lot. The tube from the main tank to
the header tank is
designed to do essentially what you suggest, but sized
appropriately to allow
fuel and air to flow simultaneously in opposite directions.
Dumb design, but
QAC apparently made it work.

The design parameter for the fuel tank vent line is that it
either be
impervious to fuel entering the line (ever - not realistic)
or else be capable
of performing its vent function when fuel gets into it. To
do that requires
that it either be self-purging when it gets fuel in it, or
that it be able to
flow air (in sufficient volume) even when there's fuel in
it. A short vent
line extending up from the top of the tank or filler cap
has the self-purging
capability with the added advantage that gravity works with
ram pressure to
purge the line in normal flight.

A long vent line extending down through the bottom of the
fuselage has the
distinct disadvantage of having gravity working against ram
pressure as
previously discussed. The pressure differential needed to
purge the line by
forcing fuel up into the tank is apparently not available,
so the line must
either be self-draining against ram pressure or else have a
shunt to capture
the fuel (up to some assumed reasonable amount) in a
secondary tank of some
kind for later return to the main tank. That is a bad idea
for lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect
on its ability to
pass air and fuel in opposite directions simultaneously.
The length of the
tube and the relative heights of its ends have an effect on
its ability to
trap and retain a slug of fuel (siphon effect). A short
length of small
diameter tubing at the airframe exit is tolerable because
ram pressure could
force a small slug of fuel up against the action of
gravity; likewise, a short
length of small diameter tubing is tolerable at the top of
the tank to "turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line
that is the problem
in the QAC design. This section of the vent line can hold a
slug of fuel just
like when you cap off the top of your soda straw and pull a
slug of your
favorite beverage up out of the cup. Try the same stunt
with a piece of 5/8"
tubing and you'll find all the soda (fuel) draining back
into the cup as you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent
line with a larger
diameter line and you should have a self-draining line that
is capable of
passing air even when there's a bit of fuel in it.

That's my opinion.

David J. Gall

> ----- Original Message -----
> From: bfarnam@pacbell.net
> To: Q-LIST@yahoogroups.com
> Subject: Re: [Q-LIST] Re: Fuel Vent Line
> Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)
>
>
> Dave,
> I wonder if the pressure differential on the vent tube
with a fuel plug
> would
> allow bubbles to get through into the tank. Mark Summers
and I were
> discussing
> that last evening. What do you think?
> Bob F.


Bob Farnam <bfarnam@...>
 

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]On Behalf Of
David J. Gall
Sent: Monday, August 14, 2006 1:34 AM
To: Q-LIST@yahoogroups.com
Subject: RE: [Q-LIST] Re: Fuel Vent Line


Ya know, Bob, I thought about that too. Here's the way I see it: the
lowest
pressure the carb can pull on the fuel line will be at high rpm with the
throttle closed. The highest ram pressure will be at max airspeed. Dive
with
the throttle closed to purge the vent line. Not a good plan.

So there you are on climbout and the engine sags due to fuel starvation.
The
rpm drops off, you push the throttle open, and the airspeed decays. What
good will it do to know that you would be able to clear the vent line at
Vne
in a power-off dive if the powerlines ahead and every nerve fiber in your
body are screaming that you'd better add power and get the nose up?

I already know that the correct response to a power loss in climb is to
push
the nose down to retain flying speed and that is already contrary to my
instincts. Please don't make me learn to hope for power restoration by
diving at the ground and chopping the throttle, too! I don't care if the
theory says there "ought to be" some combination of airspeed and throttle
setting (closed!!?) and rpm that will clear that slug of splashed fuel
from
the vent line, I won't have time to diagnose that THAT particular problem
is, indeed, THE problem.

I would much prefer a fool-proof system that works in an intuitive way
that
I can count on time and again to be reliable, predictable, and repeatable.
("Gee, would zero-gee help the ram pressure to purge the vent line," Robin
queried Batman as the Bat-plane plummeted perilously earthward.) If my
fuel
tank vent line does NOT require me to perform acrobatics in order to
restore
proper engine operation then I'm more likely to have time to apply carb
heat
and squeeze the manual fuel pump bulb -- oh, yeah, we took that out of the
system last month, didn't we.... :)
[Bob Farnam] Not me! Just like one good remaining engine on a twin above
its single engine ceiling, it may not be a perfect solution, but it sure
extends your glide.

Not to make too much light of the situation, your Pacer wing tank collapse
was caused by gravity feed PLUS pressure differential, working together,
and
there's a good bit of head pressure from a Pacer's high wings to the
low-mounted carb. And there are other factors to consider. Even if the
fuel
flow from that tank to the engine were shut off, simple contraction of the
fuel with decreasing temperature could have collapsed that tank. Top off
an
already almost full tank in the morning, leave it setting in the sun all
day, then the mud dauber builds his nest in the fuel vent in the
afternoon... And you may never even have needed to start the engine!

As opposed to your Pacer's gravity feed PLUS pressure differential,
purging
a slug of fuel from the QAC fuel tank vent requires determining the
difference between the action of gravity and the pressure differential,
working against each other, in other words, gravity MINUS pressure
differential.
[Bob Farnam] I think you miss my point, David. Forget any possible help
from engine suction, just think about the float bowl itself as the lower
termination of one line from the fuel system. It has a certain head pressure
on it which is the height of the fuel column above it. We'll assume for the
moment that the fuel system is completely full, that is, above the stand
pipe. So the vent line has a head pressure on it which is the height of the
fuel above its lowest point. There is a net head that is the difference in
height of the float bowl and the height of the lowest point on the vent
line. The ram pressure only needs to overcome that head pressure which is
the difference in heights. This implies that the ram pressure at cruise of
around 10" water, or 13.3" gasoline would clear the vent AS LONG AS the vent
is less than 13.3" below the float bowl level. I agree that at lower
speeds, lower ram pressure reduces that margin. Maybe one fix would be to
place the ram vent no lower than the float bowl, but not necessarily in
front of the windshield. Possible a location on the side of the fuselage
would be workable.

In fact, the "gravity" portion of the equation might just be zero, or even
a
little bit negative, if the slug of fuel in the vent line is taller than
the
height of the fuel level in the header tank is above the carb inlet. So
we'd
be left with just the pressure differential to move fuel through several
feet of small diameter (0.22") fuel line at approximately six gallons per
hour, or 0.85 ft/sec. Given that ram pressure at 60 mph is 1.5 inches of
water, and assuming that the carb is in the high pressure ram recovery
region of the cowl, the pressure difference available to drive the fuel
flow
in climb might be quite small indeed.

[Bob Farnam] Actually, I climb away at a speed which results in about 5"
water pressure. And my carb is in the lower pressure bottom of the cowl.
Other engines may differ. Bottom line, for me at least, is that I stick with
my existing system. My fuse side fill location ensures that I can't fill up
to the top of the overflow pipe. If I have run into the header tank during
the previous flight, I do run the transfer pump to refill it as part of the
refueling process. The design of the tank with arched top and vertical
plate baffles makes the main tank very stiff with minimal volume change when
I sit on it, so that it isn't much of a pump itself. I think it very
unlikely that turbulence could put enough fuel into the opening of a 1/4"
aluminum tube vent line to fill it, especially with ram pressure acting on
it to help keep it clear. The only way I can see to fill the vent tube is
the way that Jim Patillo described when he transfered fuel from his aux.
tank into the header without providing room in the main for the overflow. My
system has worked flawlessly for 8 years and 500 hours. Come to the
Livermore flyin this weekend and you can see (and ride in) two very
successful airplanes.

Certainly not equivalent to the head
pressure of approximately 14 inches of fuel (10 inches of water) that the
header tank normally gives....

I think you get my point by now. Thanks for helping me to have to think
through this problem. Now I know that there's another thing I want to do
differently on my airplane than what the plans call for.... :)

David J. Gall

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