Fuel Flow Problem?

[Walter Klatt]

I had a little situation yesterday that I believe I
know the cause of, but thought I would run it by the
group to see if anyone else has encountered this, and
would be interested in your opinions.

My engine cut out after a rough water take off. I was
about 100 AGL when it happened, and had just started a
steep climb after gaining speed and lowering flaps. I
quickly lowered the nose and was setting up to land,
pulled back the throttle, when the engine started up
again. I pushed in power again, and the same thing
happened. So I pulled back the throttle a bit, and
managed to retain enough power to maintain a slow climb
to about 1500 feet. There, I tried full power again,
but with same result. It seemed like it couldn't get
enough fuel. Anyway, after a couple more tries, I flew
back and landed at my cottage.

My first thoughts were plugged vents, so I checked
them, but they were OK. I checked the gascolator and
fuel drains for water or dirt, but everything was good.
Couldn't see anything wrong under the cowls, but did
not pull them off since I was sitting on the water.

I had extra fuel at the cottage and added 2 cans to
bring my fuel up 30 US gallons. I then flew it again
this morning, and absolutely no problems. I then flew
it back to my hangar at Langley, and started checking
everything out.

I have a fuel flow transducer after the gascolator on
mine, and did fuel flow tests at the line going into
the carb. I thought perhaps I might have a problem with
the transducer, but the fuel flow was fine on both
tanks. To be sure I drained one, and checked the intake
screens, but it was clean as could be. Even if that was
a problem, I couldn't see how both would plug at
exactly the same time.

My fuel at the time of the incident was about 14 US
gallons (7 each tank) and was using regular mogas. It
was very hot, though, about 31C (90F+), and had been
flying about 2 hours earlier. The plane was sitting in
the sun, and it was pretty hot. However, it started
fine, and the run-up, etc, was all normal right up to
take-off.

I have taken off with this low fuel before and have run
it down a lot more than that in flight many times with
no problems. I use mogas most of the time, and the hot
day was not unusual either.

However, it was a very rough water take-off, and with
my low fuel (although again not the first time with
this combination), I am thinking with the bouncing off
the waves that air got into my fuel intake lines
resulting in reduced pressure with the gravity feed
thus reducing fuel flow. My fuel flow transducer does
use .5 PSI, so I know I am a little more susceptible to
this than if I didn't have one.

I know air does take a little while to purge itself
from the lines, and while in flight probably longer. I
have found this out on the ground some time ago, after
adding a little fuel to empty tanks, and trying to
start it up after just opening up the tank valves. It
would run for a minute and then quit. The solution was
to let it sit for 1/2 an hour with the tank valves
open, and then no problem.

Anyway, since I couldn't find anything else wrong, my
theory is the air in the lines at take-off. I am
planning a few long trips this summer, and some over
hostile mountain terrain, so sure hope I am right about
this.

My engine is an 0320 150 hp on a Rebel with 1800
amphibs, and no fuel pumps.

Any opinions and thoughts welcome.

Walter





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[Walter Klatt]

Thanks Drew and Steve for the comments.

I also have forward facing ram vents on my fuel caps,
but they are only 1/4 inch tubes with flared openings.
It would be nice to know just how much PSI they
actually add to a tank at given speeds, say 70 mph and
100 mph. Is there some kind of formula or calculation
for that? I may want to enlarge mine if that helps.

I have never run a tank completely dry in flight, but
would now be a little nervous about trying it. In my
case, it didn't clear up in just a few seconds.

If you can get air into a line at the tank intakes with
just turbulence, then I would say it is more likely to
happen during a rough water take-off as that is
considerably more violent than turbulence. I usually
bounce off 2 swells pretty hard before getting into the
air in rough water. The swells are more of a problem at
my cottage, because the wind has about 8 miles of open
water to build them.

As for vapour lock and mogas, that's what some of my
hangar locals said, too, but again, as I mentioned, I
have been in that type of heat many times before with
mogas and no problems. Also, it started and ran-up
fine, and produced full power at take-off.

I did hear of air in the Rebel lines in the early years
before they separated the fuel sight gauge from the
intakes. However, I don't know if that actually caused
any engine stoppages.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Drew Dalgleish
Sent: Monday, June 30, 2003 2:31 PM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Hi Walter I had a similar problem flying in
bumpy conditions with low fuel
my engine would quit. If you get a slug of
air in the lines it takes
several seconds to clear itself. I was just
using the MAM recommended hole
in the rear of each filler for venting but
I've switched to the Wayne
O'shea recommended pressure vent where you
stick an aluminum tube from the
overhead vent straight into the airstream at
the wingroot fairing. This
gives a little increase in head pressure and
should quickly blow through
any air bubbles. I've flown a tank empty
since doing this without any
misses but I haven't tried it on a rough day yet.
Drew

At 05:08 PM 6/29/2003 -0700, you wrote:

I had a little situation yesterday that I believe I
know the cause of, but thought I would run it by the
group to see if anyone else has encountered

this, and

would be interested in your opinions.

My engine cut out after a rough water take

off. I was

about 100 AGL when it happened, and had

just started a

steep climb after gaining speed and

lowering flaps. I

quickly lowered the nose and was setting up to land,
pulled back the throttle, when the engine started up
again. I pushed in power again, and the same thing
happened. So I pulled back the throttle a bit, and
managed to retain enough power to maintain

a slow climb

to about 1500 feet. There, I tried full power again,
but with same result. It seemed like it couldn't get
enough fuel. Anyway, after a couple more

tries, I flew

back and landed at my cottage.

My first thoughts were plugged vents, so I checked
them, but they were OK. I checked the gascolator and
fuel drains for water or dirt, but

everything was good.

Couldn't see anything wrong under the cowls, but did
not pull them off since I was sitting on the water.

I had extra fuel at the cottage and added 2 cans to
bring my fuel up 30 US gallons. I then flew it again
this morning, and absolutely no problems. I

then flew

it back to my hangar at Langley, and

started checking

everything out.

I have a fuel flow transducer after the

gascolator on

mine, and did fuel flow tests at the line going into
the carb. I thought perhaps I might have a

problem with

the transducer, but the fuel flow was fine on both
tanks. To be sure I drained one, and

checked the intake

screens, but it was clean as could be. Even

if that was

a problem, I couldn't see how both would plug at
exactly the same time.

My fuel at the time of the incident was about 14 US
gallons (7 each tank) and was using regular

mogas. It

was very hot, though, about 31C (90F+), and had been
flying about 2 hours earlier. The plane was

sitting in

the sun, and it was pretty hot. However, it started
fine, and the run-up, etc, was all normal

right up to

take-off.

I have taken off with this low fuel before

and have run

it down a lot more than that in flight many

times with

no problems. I use mogas most of the time,

and the hot

day was not unusual either.

However, it was a very rough water

take-off, and with

my low fuel (although again not the first time with
this combination), I am thinking with the

bouncing off

the waves that air got into my fuel intake lines
resulting in reduced pressure with the gravity feed
thus reducing fuel flow. My fuel flow

transducer does

use .5 PSI, so I know I am a little more

susceptible to

this than if I didn't have one.

I know air does take a little while to purge itself
from the lines, and while in flight

probably longer. I

have found this out on the ground some time

ago, after

adding a little fuel to empty tanks, and trying to
start it up after just opening up the tank

valves. It

would run for a minute and then quit. The

solution was

to let it sit for 1/2 an hour with the tank valves
open, and then no problem.

Anyway, since I couldn't find anything else

wrong, my

theory is the air in the lines at take-off. I am
planning a few long trips this summer, and some over
hostile mountain terrain, so sure hope I am

right about

this.

My engine is an 0320 150 hp on a Rebel with 1800
amphibs, and no fuel pumps.

Any opinions and thoughts welcome.

Walter





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Drew Dalgleish




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[Drew Dalgleish]

Hi Walter I had a similar problem flying in bumpy conditions with low fuel
my engine would quit. If you get a slug of air in the lines it takes
several seconds to clear itself. I was just using the MAM recommended hole
in the rear of each filler for venting but I've switched to the Wayne
O'shea recommended pressure vent where you stick an aluminum tube from the
overhead vent straight into the airstream at the wingroot fairing. This
gives a little increase in head pressure and should quickly blow through
any air bubbles. I've flown a tank empty since doing this without any
misses but I haven't tried it on a rough day yet.
Drew

At 05:08 PM 6/29/2003 -0700, you wrote:

I had a little situation yesterday that I believe I
know the cause of, but thought I would run it by the
group to see if anyone else has encountered this, and
would be interested in your opinions.

My engine cut out after a rough water take off. I was
about 100 AGL when it happened, and had just started a
steep climb after gaining speed and lowering flaps. I
quickly lowered the nose and was setting up to land,
pulled back the throttle, when the engine started up
again. I pushed in power again, and the same thing
happened. So I pulled back the throttle a bit, and
managed to retain enough power to maintain a slow climb
to about 1500 feet. There, I tried full power again,
but with same result. It seemed like it couldn't get
enough fuel. Anyway, after a couple more tries, I flew
back and landed at my cottage.

My first thoughts were plugged vents, so I checked
them, but they were OK. I checked the gascolator and
fuel drains for water or dirt, but everything was good.
Couldn't see anything wrong under the cowls, but did
not pull them off since I was sitting on the water.

I had extra fuel at the cottage and added 2 cans to
bring my fuel up 30 US gallons. I then flew it again
this morning, and absolutely no problems. I then flew
it back to my hangar at Langley, and started checking
everything out.

I have a fuel flow transducer after the gascolator on
mine, and did fuel flow tests at the line going into
the carb. I thought perhaps I might have a problem with
the transducer, but the fuel flow was fine on both
tanks. To be sure I drained one, and checked the intake
screens, but it was clean as could be. Even if that was
a problem, I couldn't see how both would plug at
exactly the same time.

My fuel at the time of the incident was about 14 US
gallons (7 each tank) and was using regular mogas. It
was very hot, though, about 31C (90F+), and had been
flying about 2 hours earlier. The plane was sitting in
the sun, and it was pretty hot. However, it started
fine, and the run-up, etc, was all normal right up to
take-off.

I have taken off with this low fuel before and have run
it down a lot more than that in flight many times with
no problems. I use mogas most of the time, and the hot
day was not unusual either.

However, it was a very rough water take-off, and with
my low fuel (although again not the first time with
this combination), I am thinking with the bouncing off
the waves that air got into my fuel intake lines
resulting in reduced pressure with the gravity feed
thus reducing fuel flow. My fuel flow transducer does
use .5 PSI, so I know I am a little more susceptible to
this than if I didn't have one.

I know air does take a little while to purge itself
from the lines, and while in flight probably longer. I
have found this out on the ground some time ago, after
adding a little fuel to empty tanks, and trying to
start it up after just opening up the tank valves. It
would run for a minute and then quit. The solution was
to let it sit for 1/2 an hour with the tank valves
open, and then no problem.

Anyway, since I couldn't find anything else wrong, my
theory is the air in the lines at take-off. I am
planning a few long trips this summer, and some over
hostile mountain terrain, so sure hope I am right about
this.

My engine is an 0320 150 hp on a Rebel with 1800
amphibs, and no fuel pumps.

Any opinions and thoughts welcome.

Walter





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Drew Dalgleish




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[Walter Klatt]

Yes, I was thinking that, too. If air can't purge out
of the lines in a few seconds, a pump may be necessary.
I sometimes run my tanks pretty low (about 6 or 7
gallons US total) on my return to my home airport, and
would hate the thought of my engine quitting because of
some turbulence during the last few miles when I'm at
low altitude and not over water. Or trying to clear
some trees getting out of a small mountain lake. The
latter is less likely, because first of all I don't
like taking off without someplace to land if my engine
did quit, and secondly small lakes don't have rough
water.

I still haven't decided if I'm going to the Nimpo Lake
fly-in or Arlington. It depends on the weather, too. I
would need perfect weather for the Nimpo Lake trip, as
the route takes you through a pass right between the
highest mountains in BC, and there is no place to land
for about 2 hours of that leg.

Right after that weekend, ie Monday, I am also planning
a trip to Manitoba, which is about 1500 miles away and
again having to cross a few mountain passes along the
way.

So my first priority is getting confidence back in my
plane, and knowing exactly what its limitations are.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
legeorgen@aol.com
Sent: Monday, June 30, 2003 9:03 PM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Hi Walter,

You might consider installing an electric
fuel pump and turn it on for take
off ... my 2 cents.

Are you doing Arlington or the float fly in?

Bruce 357R



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[steve whitenect]

Walter,
the only thing I think of is vapour lock. Fresh fuel probably reducing the
overall temp of the gas thus flowing without boiling in the lines in the
engine compartment. Sounds like everythnig was heat soaked at that point. I
went with 1/2 " lines after the door posts forward to the gasolator for that
reason. I did it on my last homebuilt with a nose tank only and never
(touch wood) had any restriction problems or vapour lock. Mogas will boil
at a lower temp than 100LL. Line agatation or shaking of the fuel
ines/rough water should not be a problem. I wouldn't think any further
restrictions in the line in a gravety feed system plus high temps would be
very beneficial. Try draining the gasolator for a lengthy time right away
if it happens again to see if there is any interuptions in the flow. Just my
2 cents worth. Worked with Rotax products in the early to mid 70's and they
had vapour lock problems galore if the outside temp was above 0 C. Solution
was to isolate heat and improve air circulation near the fuel systems.

Steve W.

From: Walter Klatt <walter.klatt@shaw.ca>
Reply-To: <murphy-rebel@dcsol.com>
To: murphy-rebel@dcsol.com
Subject: Fuel Flow Problem?
Date: Sun, 29 Jun 2003 17:08:06 -0700

I had a little situation yesterday that I believe I
know the cause of, but thought I would run it by the
group to see if anyone else has encountered this, and
would be interested in your opinions.

My engine cut out after a rough water take off. I was
about 100 AGL when it happened, and had just started a
steep climb after gaining speed and lowering flaps. I
quickly lowered the nose and was setting up to land,
pulled back the throttle, when the engine started up
again. I pushed in power again, and the same thing
happened. So I pulled back the throttle a bit, and
managed to retain enough power to maintain a slow climb
to about 1500 feet. There, I tried full power again,
but with same result. It seemed like it couldn't get
enough fuel. Anyway, after a couple more tries, I flew
back and landed at my cottage.

My first thoughts were plugged vents, so I checked
them, but they were OK. I checked the gascolator and
fuel drains for water or dirt, but everything was good.
Couldn't see anything wrong under the cowls, but did
not pull them off since I was sitting on the water.

I had extra fuel at the cottage and added 2 cans to
bring my fuel up 30 US gallons. I then flew it again
this morning, and absolutely no problems. I then flew
it back to my hangar at Langley, and started checking
everything out.

I have a fuel flow transducer after the gascolator on
mine, and did fuel flow tests at the line going into
the carb. I thought perhaps I might have a problem with
the transducer, but the fuel flow was fine on both
tanks. To be sure I drained one, and checked the intake
screens, but it was clean as could be. Even if that was
a problem, I couldn't see how both would plug at
exactly the same time.

My fuel at the time of the incident was about 14 US
gallons (7 each tank) and was using regular mogas. It
was very hot, though, about 31C (90F+), and had been
flying about 2 hours earlier. The plane was sitting in
the sun, and it was pretty hot. However, it started
fine, and the run-up, etc, was all normal right up to
take-off.

I have taken off with this low fuel before and have run
it down a lot more than that in flight many times with
no problems. I use mogas most of the time, and the hot
day was not unusual either.

However, it was a very rough water take-off, and with
my low fuel (although again not the first time with
this combination), I am thinking with the bouncing off
the waves that air got into my fuel intake lines
resulting in reduced pressure with the gravity feed
thus reducing fuel flow. My fuel flow transducer does
use .5 PSI, so I know I am a little more susceptible to
this than if I didn't have one.

I know air does take a little while to purge itself
from the lines, and while in flight probably longer. I
have found this out on the ground some time ago, after
adding a little fuel to empty tanks, and trying to
start it up after just opening up the tank valves. It
would run for a minute and then quit. The solution was
to let it sit for 1/2 an hour with the tank valves
open, and then no problem.

Anyway, since I couldn't find anything else wrong, my
theory is the air in the lines at take-off. I am
planning a few long trips this summer, and some over
hostile mountain terrain, so sure hope I am right about
this.

My engine is an 0320 150 hp on a Rebel with 1800
amphibs, and no fuel pumps.

Any opinions and thoughts welcome.

Walter





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[Lloydstoops]

consider the possibility of vapor lock



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[Legeorgen]

Hi Walter,

You might consider installing an electric fuel pump and turn it on for take
off ... my 2 cents.

Are you doing Arlington or the float fly in?

Bruce 357R



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[Walter Klatt]

Thanks, Ken. I vaguely recall some discussion about the
ram vents some years ago. I will have a look.

I am not sure I understand your point about the size of
the ram vent or having multiple vents not increasing
pressure. Isn't that like saying the cowl inlet size or
having 2 instead of 1 doesn't affect cowl air flow?
Isn't it the pressure under the cowl that forces more
air through the cylinders? I may be missing something
fundamental here, so please educate me.

I do agree with you, though, that my problem is likely
more to do with air bubbles in my line than vapour
lock. And my fuel flow transducer uses up head to the
tune of .5 PSI according to the specs. The engine ran
fine under partial power when I throttled back, just
full power caused the problem, which I presume would
happen with reduced flow because of less head pressure.

I do like my fuel monitor, though, and would really
hate to have to disconnect that.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
klehman@albedo.net
Sent: Tuesday, July 01, 2003 7:12 AM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


The exact ram pressure is in the archives in
a few places. It is small
but real. Since there should be no
significant flow restriction to the
air in a 1/4" vent there will be no real
advantage to a larger vent or
multiple vents from a ram pressure point of view.

All my experiences with vapour lock (not on
a Rebel) typically result in
a larger loss of power when throttling back
which is not what has
happened here. The fuel lines etc. are being
cooled by fuel and a
reduction in flow usually makes it worse. If
it took awhile for power
recovery after throttling back it could mean
that bubbles are indeed
working their way back to the tank but I
have my doubts in this case.

From the exact description in this
particular case I'd guess that
losing the flow sensor or adding a pump
would make the problem go away.
Fuel pressure drop through a restriction
increases rapidly with flow and
that sure sounds like what is happening
here. The system with the flow
sensor probably can't tolerate a reduction
in head due to air bubbles in
the fuel line. Bubbles are a restriction and
they reduce head even
without fuel flowing until they clear.

FWIW
Ken

Walter Klatt wrote:

Thanks Drew and Steve for the comments.

I also have forward facing ram vents on my

fuel caps,

but they are only 1/4 inch tubes with

flared openings.

It would be nice to know just how much PSI they
actually add to a tank at given speeds,

say 70 mph and

100 mph. Is there some kind of formula or

calculation

for that? I may want to enlarge mine if that helps.

I have never run a tank completely dry in

flight, but

would now be a little nervous about trying

it. In my

case, it didn't clear up in just a few seconds.

If you can get air into a line at the tank

intakes with

just turbulence, then I would say it is

more likely to

happen during a rough water take-off as that is
considerably more violent than turbulence.

I usually

bounce off 2 swells pretty hard before

getting into the

air in rough water. The swells are more of

a problem at

my cottage, because the wind has about 8

miles of open

water to build them.

As for vapour lock and mogas, that's what

some of my

hangar locals said, too, but again, as I

mentioned, I

have been in that type of heat many times

before with

mogas and no problems. Also, it started and ran-up
fine, and produced full power at take-off.

I did hear of air in the Rebel lines in

the early years

before they separated the fuel sight gauge from the
intakes. However, I don't know if that

actually caused

any engine stoppages.

Walter

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[Walter Klatt]

Thanks Ken, I guess I learned something. Still seems
counter intuitive to me, but then I still remember as a
kid learning that a heavy rock didn't fall any faster
than a light one (drag aside), so everything isn't
always as it seems.

Just one more question, though. I do have flared
openings on my ram vents. Wouldn't that funnel shape
accelerate the air flow into the tube, and with the
higher velocity then increase the tank pressure? Or am
I out to lunch here again?

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
klehman@albedo.net
Sent: Tuesday, July 01, 2003 9:56 AM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Walter Klatt wrote:

I am not sure I understand your point

about the size of

the ram vent or having multiple vents not

increasing

pressure. Isn't that like saying the cowl

inlet size or

having 2 instead of 1 doesn't affect cowl air flow?
Isn't it the pressure under the cowl that

forces more

air through the cylinders? I may be

missing something

fundamental here, so please educate me.

Yes flow is the key word.

If the vent has no flow (like an airspeed
pitot head) then a small 1/8"
port reads the same as a large 1" port if
the pitot port is aligned with
airflow. (A very small pitot doesn't work
well at high angle of attack
but we can ignore that here.) And if there
is no flow in the plumbing
lines then there is no pressure drop in the lines.

I'm postulating that the air flow through a
1/4" vent into the fuel
tanks is low enough that there is not a
significant pressure loss in the
vent line. Therefore a larger vent line
won't help. A larger vent and
line won't recover more pressure. It would
reduce the loss between the
vent and the tank if you had high flow like
the cowl situation but with
low flow it won't help.

For fun say 15gph of fuel flow
=15gh*230 in3 pergal /60min per hour /60 sec per min
= roughly 1 cubic inch per second through a 1/4" vent

With a 0.18" id (for 1/4" tubing) that gives
about (0.18/2) squared * pi
= 0.025 sq. in. of internal tube cross section area.

1 cubic inch per sec through 0.025 are gives
about 39" per sec or 3.2
feet per sec or maybe 2 mph flow through the
vent which is not going to
give a significant pressure drop. Hmm that's
even lower than I'd have
guessed but I don't see any obvious math error...

I do agree with you, though, that my

problem is likely

more to do with air bubbles in my line than vapour
lock. And my fuel flow transducer uses up

head to the

tune of .5 PSI according to the specs. The

engine ran

fine under partial power when I throttled

back, just

full power caused the problem, which I

presume would

happen with reduced flow because of less

head pressure.

I do like my fuel monitor, though, and would really
hate to have to disconnect that.

I never liked the idea of using flow sensors
in a gravity feed system.
0.5 psi would be roughly 2 feet of head
loss. Instead of a pump which
leads to other complications, could you
easilly add a bypass valve to
that sensor. Just a crazy idea but I like
the idea of being able to open
a valve to route fuel around the sensor in
case of a problem...

Ken




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[klehman]

The exact ram pressure is in the archives in a few places. It is small
but real. Since there should be no significant flow restriction to the
air in a 1/4" vent there will be no real advantage to a larger vent or
multiple vents from a ram pressure point of view.

All my experiences with vapour lock (not on a Rebel) typically result in
a larger loss of power when throttling back which is not what has
happened here. The fuel lines etc. are being cooled by fuel and a
reduction in flow usually makes it worse. If it took awhile for power
recovery after throttling back it could mean that bubbles are indeed
working their way back to the tank but I have my doubts in this case.

From the exact description in this particular case I'd guess that
losing the flow sensor or adding a pump would make the problem go away.
Fuel pressure drop through a restriction increases rapidly with flow and
that sure sounds like what is happening here. The system with the flow
sensor probably can't tolerate a reduction in head due to air bubbles in
the fuel line. Bubbles are a restriction and they reduce head even
without fuel flowing until they clear.

FWIW
Ken

Walter Klatt wrote:

Thanks Drew and Steve for the comments.

I also have forward facing ram vents on my fuel caps,
but they are only 1/4 inch tubes with flared openings.
It would be nice to know just how much PSI they
actually add to a tank at given speeds, say 70 mph and
100 mph. Is there some kind of formula or calculation
for that? I may want to enlarge mine if that helps.

I have never run a tank completely dry in flight, but
would now be a little nervous about trying it. In my
case, it didn't clear up in just a few seconds.

If you can get air into a line at the tank intakes with
just turbulence, then I would say it is more likely to
happen during a rough water take-off as that is
considerably more violent than turbulence. I usually
bounce off 2 swells pretty hard before getting into the
air in rough water. The swells are more of a problem at
my cottage, because the wind has about 8 miles of open
water to build them.

As for vapour lock and mogas, that's what some of my
hangar locals said, too, but again, as I mentioned, I
have been in that type of heat many times before with
mogas and no problems. Also, it started and ran-up
fine, and produced full power at take-off.

I did hear of air in the Rebel lines in the early years
before they separated the fuel sight gauge from the
intakes. However, I don't know if that actually caused
any engine stoppages.

Walter

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[klehman]

Walter Klatt wrote:

I am not sure I understand your point about the size of
the ram vent or having multiple vents not increasing
pressure. Isn't that like saying the cowl inlet size or
having 2 instead of 1 doesn't affect cowl air flow?
Isn't it the pressure under the cowl that forces more
air through the cylinders? I may be missing something
fundamental here, so please educate me.

Yes flow is the key word.

If the vent has no flow (like an airspeed pitot head) then a small 1/8"
port reads the same as a large 1" port if the pitot port is aligned with
airflow. (A very small pitot doesn't work well at high angle of attack
but we can ignore that here.) And if there is no flow in the plumbing
lines then there is no pressure drop in the lines.

I'm postulating that the air flow through a 1/4" vent into the fuel
tanks is low enough that there is not a significant pressure loss in the
vent line. Therefore a larger vent line won't help. A larger vent and
line won't recover more pressure. It would reduce the loss between the
vent and the tank if you had high flow like the cowl situation but with
low flow it won't help.

For fun say 15gph of fuel flow
=15gh*230 in3 pergal /60min per hour /60 sec per min
= roughly 1 cubic inch per second through a 1/4" vent

With a 0.18" id (for 1/4" tubing) that gives about (0.18/2) squared * pi
= 0.025 sq. in. of internal tube cross section area.

1 cubic inch per sec through 0.025 are gives about 39" per sec or 3.2
feet per sec or maybe 2 mph flow through the vent which is not going to
give a significant pressure drop. Hmm that's even lower than I'd have
guessed but I don't see any obvious math error...

I do agree with you, though, that my problem is likely
more to do with air bubbles in my line than vapour
lock. And my fuel flow transducer uses up head to the
tune of .5 PSI according to the specs. The engine ran
fine under partial power when I throttled back, just
full power caused the problem, which I presume would
happen with reduced flow because of less head pressure.

I do like my fuel monitor, though, and would really
hate to have to disconnect that.

I never liked the idea of using flow sensors in a gravity feed system.
0.5 psi would be roughly 2 feet of head loss. Instead of a pump which
leads to other complications, could you easilly add a bypass valve to
that sensor. Just a crazy idea but I like the idea of being able to open
a valve to route fuel around the sensor in case of a problem...

Ken




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[Alan Hepburn]

If nobody else told you, the formula for dynamic pressure is 1/2 roh V^2.
Same thing you have in the lift equation. roh is the air density, and V the
true airspeed. You need to use a consistent set of units. e.g. V in ft/sec,
and roh in slugs/cubic foot gives dynamic pressure in lbf/square foot (roh =
0.002378 slugs/cubic ft under standard conditions). If my calculations are
correct, that gives about 0.17 psi at 100 mph.

It is, as somebody else pointed out, independent of the size of the vent,
since the air flow through the vent is insignificant.

Al.
----- Original Message -----
From: "Walter Klatt" <walter.klatt@shaw.ca>
To: <murphy-rebel@dcsol.com>
Sent: Monday, June 30, 2003 9:16 PM
Subject: RE: Fuel Flow Problem?

Thanks Drew and Steve for the comments.

I also have forward facing ram vents on my fuel caps,
but they are only 1/4 inch tubes with flared openings.
It would be nice to know just how much PSI they
actually add to a tank at given speeds, say 70 mph and
100 mph. Is there some kind of formula or calculation
for that? I may want to enlarge mine if that helps.

I have never run a tank completely dry in flight, but
would now be a little nervous about trying it. In my
case, it didn't clear up in just a few seconds.

If you can get air into a line at the tank intakes with
just turbulence, then I would say it is more likely to
happen during a rough water take-off as that is
considerably more violent than turbulence. I usually
bounce off 2 swells pretty hard before getting into the
air in rough water. The swells are more of a problem at
my cottage, because the wind has about 8 miles of open
water to build them.

As for vapour lock and mogas, that's what some of my
hangar locals said, too, but again, as I mentioned, I
have been in that type of heat many times before with
mogas and no problems. Also, it started and ran-up
fine, and produced full power at take-off.

I did hear of air in the Rebel lines in the early years
before they separated the fuel sight gauge from the
intakes. However, I don't know if that actually caused
any engine stoppages.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Drew Dalgleish
Sent: Monday, June 30, 2003 2:31 PM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Hi Walter I had a similar problem flying in
bumpy conditions with low fuel
my engine would quit. If you get a slug of
air in the lines it takes
several seconds to clear itself. I was just
using the MAM recommended hole
in the rear of each filler for venting but
I've switched to the Wayne
O'shea recommended pressure vent where you
stick an aluminum tube from the
overhead vent straight into the airstream at
the wingroot fairing. This
gives a little increase in head pressure and
should quickly blow through
any air bubbles. I've flown a tank empty
since doing this without any
misses but I haven't tried it on a rough day yet.
Drew

At 05:08 PM 6/29/2003 -0700, you wrote:

I had a little situation yesterday that I believe I
know the cause of, but thought I would run it by the
group to see if anyone else has encountered

this, and

would be interested in your opinions.

My engine cut out after a rough water take

off. I was

about 100 AGL when it happened, and had

just started a

steep climb after gaining speed and

lowering flaps. I

quickly lowered the nose and was setting up to land,
pulled back the throttle, when the engine started up
again. I pushed in power again, and the same thing
happened. So I pulled back the throttle a bit, and
managed to retain enough power to maintain

a slow climb

to about 1500 feet. There, I tried full power again,
but with same result. It seemed like it couldn't get
enough fuel. Anyway, after a couple more

tries, I flew

back and landed at my cottage.

My first thoughts were plugged vents, so I checked
them, but they were OK. I checked the gascolator and
fuel drains for water or dirt, but

everything was good.

Couldn't see anything wrong under the cowls, but did
not pull them off since I was sitting on the water.

I had extra fuel at the cottage and added 2 cans to
bring my fuel up 30 US gallons. I then flew it again
this morning, and absolutely no problems. I

then flew

it back to my hangar at Langley, and

started checking

everything out.

I have a fuel flow transducer after the

gascolator on

mine, and did fuel flow tests at the line going into
the carb. I thought perhaps I might have a

problem with

the transducer, but the fuel flow was fine on both
tanks. To be sure I drained one, and

checked the intake

screens, but it was clean as could be. Even

if that was

a problem, I couldn't see how both would plug at
exactly the same time.

My fuel at the time of the incident was about 14 US
gallons (7 each tank) and was using regular

mogas. It

was very hot, though, about 31C (90F+), and had been
flying about 2 hours earlier. The plane was

sitting in

the sun, and it was pretty hot. However, it started
fine, and the run-up, etc, was all normal

right up to

take-off.

I have taken off with this low fuel before

and have run

it down a lot more than that in flight many

times with

no problems. I use mogas most of the time,

and the hot

day was not unusual either.

However, it was a very rough water

take-off, and with

my low fuel (although again not the first time with
this combination), I am thinking with the

bouncing off

the waves that air got into my fuel intake lines
resulting in reduced pressure with the gravity feed
thus reducing fuel flow. My fuel flow

transducer does

use .5 PSI, so I know I am a little more

susceptible to

this than if I didn't have one.

I know air does take a little while to purge itself
from the lines, and while in flight

probably longer. I

have found this out on the ground some time

ago, after

adding a little fuel to empty tanks, and trying to
start it up after just opening up the tank

valves. It

would run for a minute and then quit. The

solution was

to let it sit for 1/2 an hour with the tank valves
open, and then no problem.

Anyway, since I couldn't find anything else

wrong, my

theory is the air in the lines at take-off. I am
planning a few long trips this summer, and some over
hostile mountain terrain, so sure hope I am

right about

this.

My engine is an 0320 150 hp on a Rebel with 1800
amphibs, and no fuel pumps.

Any opinions and thoughts welcome.

Walter





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[Walter Klatt]

Yeah, I think I get it. With no flow, there is nothing
to accelerate with a flared opening. Thanks, Ken. This
was a good thing to learn for me. My next step now will
be to figure out how to apply that formula. Thanks to
Al for that, too.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
klehman@albedo.net
Sent: Wednesday, July 02, 2003 6:01 AM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Air is flowing so slowly in that vent that
you can think of it as a
static situation with no flow in the vent.
With a static situation you
can vary the size of the line or the opening
but the pressure will be
the same everywhere. With no flow to confuse
things, the static pressure
has to be the same everywhere. As Alan
calculated, 1/2 rho V squared is
all you get.

Similar analogy would be an elevated water
tank. If there is no flow
then the pressure on the ground will be the
same no matter how large or
small the pipe from the tank is. Similary
the fuel tank vent is flowing
so little that there is essentially no
pressure drop or dynamic effects
in the line. Flow enough water to get a
pressure drop in the pipe and
everything changes but that is not the case here.

Things get interesting in a hurry when there
is flow though. I thought I
knew the basics, but I spent a looong time
learning and deciding how to
shape and make the ducting for my radiator.
Hope it works :)

Ken

Walter Klatt wrote:

Thanks Ken, I guess I learned something.

Still seems

counter intuitive to me, but then I still

remember as a

kid learning that a heavy rock didn't fall

any faster

than a light one (drag aside), so everything isn't
always as it seems.

Just one more question, though. I do have flared
openings on my ram vents. Wouldn't that

funnel shape

accelerate the air flow into the tube, and with the
higher velocity then increase the tank

pressure? Or am

I out to lunch here again?

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
klehman@albedo.net
Sent: Tuesday, July 01, 2003 9:56 AM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Walter Klatt wrote:
about the size of
increasing
inlet size or

air flow?

forces more
missing something
Yes flow is the key word.

If the vent has no flow (like an airspeed
pitot head) then a small 1/8"
port reads the same as a large 1" port if
the pitot port is aligned with
airflow. (A very small pitot doesn't work
well at high angle of attack
but we can ignore that here.) And if there
is no flow in the plumbing
lines then there is no pressure drop in the lines.

I'm postulating that the air flow through a
1/4" vent into the fuel
tanks is low enough that there is not a
significant pressure loss in the
vent line. Therefore a larger vent line
won't help. A larger vent and
line won't recover more pressure. It would
reduce the loss between the
vent and the tank if you had high flow like
the cowl situation but with
low flow it won't help.

For fun say 15gph of fuel flow
=15gh*230 in3 pergal /60min per hour /60

sec per min

= roughly 1 cubic inch per second through

a 1/4" vent

With a 0.18" id (for 1/4" tubing) that gives
about (0.18/2) squared * pi
= 0.025 sq. in. of internal tube cross

section area.

1 cubic inch per sec through 0.025 are gives
about 39" per sec or 3.2
feet per sec or maybe 2 mph flow through the
vent which is not going to
give a significant pressure drop. Hmm that's
even lower than I'd have
guessed but I don't see any obvious math error...

problem is likely
head to the
engine ran
back, just
presume would
head pressure.

would really

I never liked the idea of using flow sensors
in a gravity feed system.
0.5 psi would be roughly 2 feet of head
loss. Instead of a pump which
leads to other complications, could you
easilly add a bypass valve to
that sensor. Just a crazy idea but I like
the idea of being able to open
a valve to route fuel around the sensor in
case of a problem...

Ken




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[klehman]

Air is flowing so slowly in that vent that you can think of it as a
static situation with no flow in the vent. With a static situation you
can vary the size of the line or the opening but the pressure will be
the same everywhere. With no flow to confuse things, the static pressure
has to be the same everywhere. As Alan calculated, 1/2 rho V squared is
all you get.

Similar analogy would be an elevated water tank. If there is no flow
then the pressure on the ground will be the same no matter how large or
small the pipe from the tank is. Similary the fuel tank vent is flowing
so little that there is essentially no pressure drop or dynamic effects
in the line. Flow enough water to get a pressure drop in the pipe and
everything changes but that is not the case here.

Things get interesting in a hurry when there is flow though. I thought I
knew the basics, but I spent a looong time learning and deciding how to
shape and make the ducting for my radiator. Hope it works :)

Ken

Walter Klatt wrote:

Thanks Ken, I guess I learned something. Still seems
counter intuitive to me, but then I still remember as a
kid learning that a heavy rock didn't fall any faster
than a light one (drag aside), so everything isn't
always as it seems.

Just one more question, though. I do have flared
openings on my ram vents. Wouldn't that funnel shape
accelerate the air flow into the tube, and with the
higher velocity then increase the tank pressure? Or am
I out to lunch here again?

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
klehman@albedo.net
Sent: Tuesday, July 01, 2003 9:56 AM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?


Walter Klatt wrote:

I am not sure I understand your point

about the size of

the ram vent or having multiple vents not

increasing

pressure. Isn't that like saying the cowl

inlet size or

having 2 instead of 1 doesn't affect cowl air flow?
Isn't it the pressure under the cowl that

forces more

air through the cylinders? I may be

missing something

fundamental here, so please educate me.

Yes flow is the key word.

If the vent has no flow (like an airspeed
pitot head) then a small 1/8"
port reads the same as a large 1" port if
the pitot port is aligned with
airflow. (A very small pitot doesn't work
well at high angle of attack
but we can ignore that here.) And if there
is no flow in the plumbing
lines then there is no pressure drop in the lines.

I'm postulating that the air flow through a
1/4" vent into the fuel
tanks is low enough that there is not a
significant pressure loss in the
vent line. Therefore a larger vent line
won't help. A larger vent and
line won't recover more pressure. It would
reduce the loss between the
vent and the tank if you had high flow like
the cowl situation but with
low flow it won't help.

For fun say 15gph of fuel flow
=15gh*230 in3 pergal /60min per hour /60 sec per min
= roughly 1 cubic inch per second through a 1/4" vent

With a 0.18" id (for 1/4" tubing) that gives
about (0.18/2) squared * pi
= 0.025 sq. in. of internal tube cross section area.

1 cubic inch per sec through 0.025 are gives
about 39" per sec or 3.2
feet per sec or maybe 2 mph flow through the
vent which is not going to
give a significant pressure drop. Hmm that's
even lower than I'd have
guessed but I don't see any obvious math error...

I do agree with you, though, that my

problem is likely

more to do with air bubbles in my line than vapour
lock. And my fuel flow transducer uses up

head to the

tune of .5 PSI according to the specs. The

engine ran

fine under partial power when I throttled

back, just

full power caused the problem, which I

presume would

happen with reduced flow because of less

head pressure.

I do like my fuel monitor, though, and would really
hate to have to disconnect that.

I never liked the idea of using flow sensors
in a gravity feed system.
0.5 psi would be roughly 2 feet of head
loss. Instead of a pump which
leads to other complications, could you
easilly add a bypass valve to
that sensor. Just a crazy idea but I like
the idea of being able to open
a valve to route fuel around the sensor in
case of a problem...

Ken




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[Walter Klatt]

Thanks Phil. It's good to know I am not the only one
that has experienced this.

I was talking today to a person that was telling me
that Super Cubs on floats with the bigger engines also
used to have this problem. Their solution was to go to
a header tank.

Not sure yet what I will do to rectify it, but if I
know what the problem was, at least I can take steps
now to avoid and be prepared for it.

BTW, we're having Canada Day all over again today here
in Vancouver, as we just won the 2010 Winter Olympics
bid.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Phil Stubley
Sent: Wednesday, July 02, 2003 3:13 PM
To: murphy-rebel@dcsol.com
Subject: Re: Fuel Flow Problem?



----- Original Message -----
From: "Walter Klatt" <walter.klatt@shaw.ca>
To: <murphy-rebel@dcsol.com>
Sent: Sunday, June 29, 2003 8:08 PM
Subject: Fuel Flow Problem?

I had a little situation yesterday that I believe I
know the cause of, but thought I would run

it by the

group to see if anyone else has

encountered this, and

would be interested in your opinions.

My engine cut out after a rough water take

off. I was

about 100 AGL when it happened, and had

just started a

steep climb after gaining speed and

lowering flaps. I

quickly lowered the nose and was setting

up to land,

pulled back the throttle, when the engine

started up

again. I pushed in power again, and the same thing
happened. So I pulled back the throttle a bit, and
managed to retain enough power to maintain

a slow climb

to about 1500 feet. There, I tried full

power again,

but with same result. It seemed like it

couldn't get

enough fuel. Anyway, after a couple more

tries, I flew

back and landed at my cottage.

My first thoughts were plugged vents, so I checked
them, but they were OK. I checked the

gascolator and

fuel drains for water or dirt, but

everything was good.

Couldn't see anything wrong under the

cowls, but did

not pull them off since I was sitting on the water.

I had extra fuel at the cottage and added 2 cans to
bring my fuel up 30 US gallons. I then

flew it again

this morning, and absolutely no problems.

I then flew

it back to my hangar at Langley, and

started checking

everything out.

I have a fuel flow transducer after the

gascolator on

mine, and did fuel flow tests at the line

going into

the carb. I thought perhaps I might have a

problem with

the transducer, but the fuel flow was fine on both
tanks. To be sure I drained one, and

checked the intake

screens, but it was clean as could be.

Even if that was

a problem, I couldn't see how both would plug at
exactly the same time.

My fuel at the time of the incident was about 14 US
gallons (7 each tank) and was using

regular mogas. It

was very hot, though, about 31C (90F+),

and had been

flying about 2 hours earlier. The plane

was sitting in

the sun, and it was pretty hot. However, it started
fine, and the run-up, etc, was all normal

right up to

take-off.

I have taken off with this low fuel before

and have run

it down a lot more than that in flight

many times with

no problems. I use mogas most of the time,

and the hot

day was not unusual either.

However, it was a very rough water

take-off, and with

my low fuel (although again not the first time with
this combination), I am thinking with the

bouncing off

the waves that air got into my fuel intake lines
resulting in reduced pressure with the gravity feed
thus reducing fuel flow. My fuel flow

transducer does

use .5 PSI, so I know I am a little more

susceptible to

this than if I didn't have one.

I know air does take a little while to purge itself
from the lines, and while in flight

probably longer. I

have found this out on the ground some

time ago, after

adding a little fuel to empty tanks, and trying to
start it up after just opening up the tank

valves. It

would run for a minute and then quit. The

solution was

to let it sit for 1/2 an hour with the tank valves
open, and then no problem.

Anyway, since I couldn't find anything

else wrong, my

theory is the air in the lines at take-off. I am
planning a few long trips this summer, and

some over

hostile mountain terrain, so sure hope I

am right about

this.

My engine is an 0320 150 hp on a Rebel with 1800
amphibs, and no fuel pumps.

Any opinions and thoughts welcome.

Hi Walter,

I had a very similar problem, took off with
about half tanks, but on ONE
tank only. Lost power but didn't quit.
Throttled back and it pulled OK for
a slow climb. Got back and checked it out.
Here's my conclusion:

an air bubble in the line will not move up
or down unless there is a
considerable slope or hi flow. It will
however let flow pass by it. Enough
to do a runup, and to run a small
engine,(912?) so no problems appeared
until we started with the big Lycs. (Mine is
an O320 150 hp also). I did
the testing on a mockup using plastic hose
and water so I could see what was
going on. I was amazed to see the fluid
flowing pretty well with this huge
bubble in the line!!! The numbers fit with
the experience.

So what to do? First, always have both
tanks on. Lets a bubble get back to
a tank easier. Second, make sure your lines
NEAR THE TANK are big, and
slope down as much as possible. I have an
oversize screen and fitting to
install there,( but haven't done it yet.)
Third, always keep the tanks
full!!! (yeh.)

During the early days of Rebel flying, some
problems started to appear when
ARRIVING, (easiest time to get air in the
tanks) and resulted in no power
for an engine clearance or go-around. Scary
isn't it?

To get my confidence back I duplicated the
event on the ground. I tipped
the plane (one wheel in the ditch) and with
one tank off, I drained fuel
from the gasacolator the ensure I had a
bubble in the line. Then I
relevelled, and started the engine. Ran
fine for a long time. When I
opened to full throttle it gave about 40
secs full, then lost power for
several seconds, and then cleared itself.
DUPLICATING THE ACTUAL SITUATION.
(Only problem is that I burned my speed
cowl. Maybe I should trash it
anyway according to Wayneand Bob)

Note that I knew of a problem during
construction and we all tried our
fixes. I added a reservoir ABOVE the
gascolator, vented back to the tanks,
and this gave me several seconds running
time while this littel tank
swallowed the air bubble.

Hope this helps tou get a fix and confidence.

Phil. R302 and 5 years but not many hours.




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