[rebel-builders] Fuel Tanks- 1 or 2 ports

[Richard Wampach]

1 port or 2, a good question!

Piper and a few other brand names have only; left-right-off option.
Cessna 150, 152 have; both or off option---not too many choices here.
Cessna 172, 182 have; both, left, right, off system, and 2 ports to pick
up at front and rear of tank.
Most of the Cessna 206 / 210 (about 1981 & earlier), have; left, right,
off, with the fuel valve porting injection system surplus back to the
tank that is selected, & ports at the front & rear of the tank.

The Cessna 120, 140 & 170 only picked up fuel from the front of the
tanks. Thus the red line on the bottom 1/4 of the fuel gage and a
statement "No takeoff below 1/4 fuel---" Because you WILL experience an
engine out on climb out.

The later Cessna 206 / 210 series, (about)1981 and later till the end of
the production run in 1986 had; left, right, both & off. With injector
surplus ported back to the selected tank. & ports at the front & rear
of the tank.
I know for a fact that you can use all but about 2 quarts of this fuel
before the engine quits completely! Please don't ask!

Carbureted engines normally need about one pound of fuel pressure. In a
high wing airplane a 39" fuel drop from the tank to the carb normally
gives you this required 1 LB. and the float is set to this pressure
requirement. A plane with a extreme nose high climb angle will not meet
this required pressure, thus an engine driven fuel pump is normally
required, & if it has an engine driven pump then it must have an
electric (or other) back up pump. Low wing planes require the pumps
because gravity will not give the 39" / 1 LB requirement.

Injected engines all have pumps, (That I know of).

So back to the original thaught here. To pump or not to pump, & 1 port
or 2 Those are all good questions!

What do you feel good with? What did the designer of the plane have in
mind? What did the designer of the engine have in mind? What kind of
flying do you plan to do? And what engine configuration are you using?
What fuel pressure did the plane it was taken from use?

I hope this does not cloud the thinking here!

As for my SR-3500, I am starting out with a high time (experienced) Lyc.
O-540 from a Cessna R-182 and matching Propeller. I do not know what
the next choice will be. With this engine configuration I have
installed the required fuel pumps. Also 2 ports from each tank "Tee'd"
to a P-210 fuel valve. I hope to have all of the bases covered & not
worry about any fuel starvation or vapor-lock.

Good luck on your choice!

Dick Wampach SR-108



-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of
Rick Harper
Sent: Friday, February 02, 2007 2:53 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] Fuel Tanks

G'day Alvin & Welcome to the group !

(I'm down here in OZ BTW - ( that's "by the way" Jean !)

It's "standard practise" in just about everything flying - to take off
on BOTH tanks, and once you are at your cruising altitude you then
switch to the fullest tank ONLY for the first half an hours' flight, ...
then back to the other tank for the next half an hours' flight, and so
on & so on ... and then BOTH tanks back on for landing - (just in case
you need full power / go around )

This means that if "something happens" to the flow rate of one tank (for
whatever reason) - you KNOW that the other side WILL flow - because it
WAS running fine a half an hour ago !)

IE: Here's a "both taps open" scenario ....

Your happily flying along with BOTH valves open ..... and unbeknownst to
you "something" has happened to one of the tanks - call it "tank A" and
you either aren't watching your fuel gauges, OR, they are not indicating
correctly .... or, you just don't notice the lack of change in the tank
level for whatever reason ...

Anyway , ... "Tank A" runs out of fuel - worked up a vacuum in it due
to a blockage in the vent system, dead bug jammed in the feed line, lack
of fossilised fuel deposits - whatever .... BUT, ... the engine still
runs OK - because "tank B" is still feeding it ! ....

BUT - when "tank B" runs out / gets used up / has a separate problem &
the engine starts to splutter through fuel starvation and STOPS ...

You think "WHATTHE$#@& !?!?!?!?!?" - WHY ?!?

a) In your PANIC, you don't know which tank is the problem tank if they
were both open

b) Even if the gauge is telling you "Tank A" has fuel IN IT ... it still
isn't going to give it to you !

c) You now have a FORCED LANDING situation on your hands !

BUT ...

However - If you were running a different tank every half an hour - and
then IF one tank developed a problem - you could switch to the other
tank an KNOW that it IS going to give you what was in it !

That's why we alternate the tank feeds

Hope this helps

Rick "Biggus" Harper
541R

PS : Please excuse my "odd" spelling .... we use English here in
Australia

----- Original Message -----
From: Alvin and Glenys Adams
To: rebel
Sent: Saturday, February 03, 2007 8:57 AM
Subject: [rebel-builders] Fuel Tanks


I read the comments on the fuel system with great interest.Not fond
off the fuel system on the rebel as i have created a bad habit off not
wanting to carry around uncessary pounds off fuel,would rather carry
items that i need.What would be the practical reason to shut off either
off your fuel valves ever while flying.

This is agreat site.


regards: alvin rebel 776 40 Hrs 1000hrs on PA-11



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[Walter Klatt]

Dick, if it were me, I would not use the front tank port, for the reasons
just described here in the last couple of days. You are running the risk of
allowing air to enter the fuel lines during a steep climb and low fuel. And
if you have a fuel pump, it will suck from the point of least resistance
which is the port exposed to air.

Walter

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of
Richard Wampach
Sent: Friday, February 02, 2007 4:22 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Fuel Tanks- 1 or 2 ports



Also 2 ports from each tank "Tee'd"
to a P-210 fuel valve. I hope to have all of the bases covered & not
worry about any fuel starvation or vapor-lock.

Good luck on your choice!

Dick Wampach SR-108

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[Tim Hickey]

Guys:
I am at a grave disadvantage when it comes to contributing to this
discussion. I do not know what the parts even look like, let alone how they
go together. But I do want to learn as much as I can, and past experience
is certainly a good teacher. However, I want to understand what is
happening, if only for my own satisfaction.

To Recap a bit:
What we are trying to pin down here is the question one outlet port or two
in a Rebel tank. Charlie Eubanks has made a scale drawing of the Rebel wing
showing the fuel level in a two bay tank at various volumes. I wish I could
show everyone the drawing, as I think it would help the conversation.
His drawing show that when the wing is at a 10 degree nose down position, a
two bay tank will un-port completely with 3 gallons or less in the tank. Now
I know that there are several things to say about the drawing, one of them
is that the WING my never see a 10 degree nose down condition. I do not know
how much nose down a Rebel will produce in what one might call a normal
descent.
But geometrically, at three gallons in a two bay tank, the fuel outlet is in
free air with a 10 degree nose down position. In a three bay tank (not shown
in Charlie's drawing) I can make a extension that at 4,5 gallons the port
will uncover as described above.

Some mention was made on one post about a "siphon" effect. Is there a small
dip tube that bends down toward the bottom of the tank from the outlet port
on the root rib?
If there is, that causes more confusion for me, because now I can see the
pick up tube acting like a p trap in a sink, only in reverse. This
configuration might trap a bubble of air, and if the head in the tank was
low, very low, it might not have enough pressure to push the bubble out and
restart the flow. This may explain why Wayne had to add 10 gallons of fuel
to the tank to get flow to the carb.

With two outlet ports in the tank, one fore and one aft, I can not see how
uncovering one port will result in fuel starvation UNLESS the line is under
suction. If a fuel pump is trying to pump more fuel per minute that the fuel
line can deliver, then that is the answer. Guys with O-320's may be sucking
up to 12 gallons per hour at full throttle. That may be enough to cause the
pressure in the line to go negative.

Walter hints at it below, but it seems to me that this is true only when the
line goes from a pressure state positive to atmospheric, to negative.

We all need to sit around the table with a pad and paper so we can make
drawings of what the fuel system looks like, and what might be happing in
different flight conditions.

And... let me add that while I do enjoy the theoretical discussion about the
fuel system and what problems might or might not occur, I understand what
Bob P and other have said. I just want to develop an understanding.

My own experience is from a Zenith CH-300 that I have been flying for 18
years. It has two tanks, a left and a right. Sometimes it has three tanks.
That is when I put the 17 gallon aux tank on the back seat. My fuel valve
has four positions. Off. Left. Right. Aux. This valve feeds an electric
fuel pump that is located at the lowest point in the fuel system. I have run
tanks dry on long cross country trips, and have no problem promptly
switching tanks when one goes empty. Actually, if you watch the fuel
pressure gauge, you will get about a 10 second warning as the fuel pressure
begins to drop. Switch tanks at this clue, and you will never miss a beat.
But I have missed the clue several times, and when the engine stops making
power, just switch to a tank with fuel and it fires right back up. I should
add that when I switch tanks in this mode I turn the boost pump on to hurry
the fuel along. The engine will fire right back up in about 5 seconds.
(Which can seem like a very long time)

Thanks for listening.

Tim Hickey.
----- Original Message -----
From: "Walter Klatt" <Walter.Klatt@shaw.ca>
To: <rebel-builders@dcsol.com>
Sent: Friday, February 02, 2007 8:08 PM
Subject: RE: [rebel-builders] Fuel Tanks- 1 or 2 ports

Dick, if it were me, I would not use the front tank port, for the reasons
just described here in the last couple of days. You are running the risk
of
allowing air to enter the fuel lines during a steep climb and low fuel.
And
if you have a fuel pump, it will suck from the point of least resistance
which is the port exposed to air.

Walter

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[Rick Harper]

G'day Tim ...

Yep - the "two ports" are about forward & aft fuel outlets IN EACH FUEL TANK / Wing

The general discussion is about the "2 port system" - which popular consensus says will let air get introduced into the fuel line / feed when the Rebel is at a significant nose down attitude on decent (obviously a rather undesirable effect !)

I have just recalibrated my fuel sight tubes - and also done my minimum -unusable sump levels for each tank

with the plane in a straight and level flying attitude , the port wing gets down to three litres before it stops feeding - and the starboard gets down to five litres before it stops feeding the ... ( and personally I will be setting ten litres in each tank as my "LETS FIND A LANDING SPOT NOW !" level

obviously, with a little side slip happening, you could get those last few litres out in an emergency - but if you plan your flying to need to do that - then I suggest that one day chance is going to catch you !

NOTE: Yes ... I HAVE actually got the tanks down to that level once ! - by accident .... and the side slip technique and reduced throttle got us the last 20miles to our destination Airport - And the only in jury was to my pride ! :o(

stick with the normal one port outlet per wing - just like the plan shows !

(BTW - I used to run the fuel sight tubes vertically - As per the plans ...
but they give very poor lower level fuel indications :o(
(personally I don't care how inaccurate the gauges are when the tanks a full, but I want them accurate when the tanks are low !!!)

Now I run the fuel tube from the top sight tube outlet - diagonally , down to the outlet of each tank ( which quite a few builders have warned not to do - ala letting bubbles enter the line) - BUT, I blocked off the nipple at the top sight tube outlet and then drilled a 0.030" hole in the plug and similar at the bottom .
this gives the sight tube a very delayed reaction - which is great when flying in turbulent conditions , as the fuel his an "average" reading of levels ...
AND, ... I have never had any troubles with bubbles entering the line !

hope this helps

Biggus


----- Original Message -----
From: Tim Hickey
To: rebel-builders@dcsol.com
Sent: Saturday, February 03, 2007 3:24 PM
Subject: Re: [rebel-builders] Fuel Tanks- 1 or 2 ports


Guys:
I am at a grave disadvantage when it comes to contributing to this
discussion. I do not know what the parts even look like, let alone how they
go together. But I do want to learn as much as I can, and past experience
is certainly a good teacher. However, I want to understand what is
happening, if only for my own satisfaction.

To Recap a bit:
What we are trying to pin down here is the question one outlet port or two
in a Rebel tank. Charlie Eubanks has made a scale drawing of the Rebel wing
showing the fuel level in a two bay tank at various volumes. I wish I could
show everyone the drawing, as I think it would help the conversation.
His drawing show that when the wing is at a 10 degree nose down position, a
two bay tank will un-port completely with 3 gallons or less in the tank. Now
I know that there are several things to say about the drawing, one of them
is that the WING my never see a 10 degree nose down condition. I do not know
how much nose down a Rebel will produce in what one might call a normal
descent.
But geometrically, at three gallons in a two bay tank, the fuel outlet is in
free air with a 10 degree nose down position. In a three bay tank (not shown
in Charlie's drawing) I can make a extension that at 4,5 gallons the port
will uncover as described above.

Some mention was made on one post about a "siphon" effect. Is there a small
dip tube that bends down toward the bottom of the tank from the outlet port
on the root rib?
If there is, that causes more confusion for me, because now I can see the
pick up tube acting like a p trap in a sink, only in reverse. This
configuration might trap a bubble of air, and if the head in the tank was
low, very low, it might not have enough pressure to push the bubble out and
restart the flow. This may explain why Wayne had to add 10 gallons of fuel
to the tank to get flow to the carb.

With two outlet ports in the tank, one fore and one aft, I can not see how
uncovering one port will result in fuel starvation UNLESS the line is under
suction. If a fuel pump is trying to pump more fuel per minute that the fuel
line can deliver, then that is the answer. Guys with O-320's may be sucking
up to 12 gallons per hour at full throttle. That may be enough to cause the
pressure in the line to go negative.

Walter hints at it below, but it seems to me that this is true only when the
line goes from a pressure state positive to atmospheric, to negative.

We all need to sit around the table with a pad and paper so we can make
drawings of what the fuel system looks like, and what might be happing in
different flight conditions.

And... let me add that while I do enjoy the theoretical discussion about the
fuel system and what problems might or might not occur, I understand what
Bob P and other have said. I just want to develop an understanding.

My own experience is from a Zenith CH-300 that I have been flying for 18
years. It has two tanks, a left and a right. Sometimes it has three tanks.
That is when I put the 17 gallon aux tank on the back seat. My fuel valve
has four positions. Off. Left. Right. Aux. This valve feeds an electric
fuel pump that is located at the lowest point in the fuel system. I have run
tanks dry on long cross country trips, and have no problem promptly
switching tanks when one goes empty. Actually, if you watch the fuel
pressure gauge, you will get about a 10 second warning as the fuel pressure
begins to drop. Switch tanks at this clue, and you will never miss a beat.
But I have missed the clue several times, and when the engine stops making
power, just switch to a tank with fuel and it fires right back up. I should
add that when I switch tanks in this mode I turn the boost pump on to hurry
the fuel along. The engine will fire right back up in about 5 seconds.
(Which can seem like a very long time)

Thanks for listening.

Tim Hickey.
----- Original Message -----
From: "Walter Klatt" <Walter.Klatt@shaw.ca>
To: <rebel-builders@dcsol.com>
Sent: Friday, February 02, 2007 8:08 PM
Subject: RE: [rebel-builders] Fuel Tanks- 1 or 2 ports

Dick, if it were me, I would not use the front tank port, for the reasons
just described here in the last couple of days. You are running the risk
of
allowing air to enter the fuel lines during a steep climb and low fuel.
And
if you have a fuel pump, it will suck from the point of least resistance
which is the port exposed to air.

Walter

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[Drew Dalgleish]

At 10:24 PM 2/2/2007 -0600, you wrote:

Guys:
I am at a grave disadvantage when it comes to contributing to this
discussion. I do not know what the parts even look like, let alone how they
go together. But I do want to learn as much as I can, and past experience
is certainly a good teacher. However, I want to understand what is
happening, if only for my own satisfaction.

To Recap a bit:
What we are trying to pin down here is the question one outlet port or two
in a Rebel tank. Charlie Eubanks has made a scale drawing of the Rebel wing
showing the fuel level in a two bay tank at various volumes. I wish I could
show everyone the drawing, as I think it would help the conversation.
His drawing show that when the wing is at a 10 degree nose down position, a
two bay tank will un-port completely with 3 gallons or less in the tank. Now
I know that there are several things to say about the drawing, one of them
is that the WING my never see a 10 degree nose down condition. I do not know
how much nose down a Rebel will produce in what one might call a normal
descent.

If you're in a 10 degree descent you'll have lots of speed to bring the
nose up

But geometrically, at three gallons in a two bay tank, the fuel outlet is in
free air with a 10 degree nose down position. In a three bay tank (not shown
in Charlie's drawing) I can make a extension that at 4,5 gallons the port
will uncover as described above.

Not much more than 3 due to the wing dihedral

Some mention was made on one post about a "siphon" effect. Is there a small
dip tube that bends down toward the bottom of the tank from the outlet port
on the root rib?

No the siphon comes from the carb drawing in fuel faster than it will flow
through the line gravity feed systems operate at very low pressure and
every bend and fitting effects the flow rate.

If there is, that causes more confusion for me, because now I can see the
pick up tube acting like a p trap in a sink, only in reverse. This
configuration might trap a bubble of air, and if the head in the tank was
low, very low, it might not have enough pressure to push the bubble out and
restart the flow. This may explain why Wayne had to add 10 gallons of fuel
to the tank to get flow to the carb.

With two outlet ports in the tank, one fore and one aft, I can not see how
uncovering one port will result in fuel starvation UNLESS the line is under
suction. If a fuel pump is trying to pump more fuel per minute that the fuel
line can deliver, then that is the answer. Guys with O-320's may be sucking
up to 12 gallons per hour at full throttle. That may be enough to cause the
pressure in the line to go negative.

Because of the low pressures involved the line can sometimes be under
suction due to many different forces like turbulance or uncoordinated turns.

Walter hints at it below, but it seems to me that this is true only when the
line goes from a pressure state positive to atmospheric, to negative.

We all need to sit around the table with a pad and paper so we can make
drawings of what the fuel system looks like, and what might be happing in
different flight conditions.

And... let me add that while I do enjoy the theoretical discussion about the
fuel system and what problems might or might not occur, I understand what
Bob P and other have said. I just want to develop an understanding.

There's lots of Rebels flying and the fuel system is well worked out. One
outlet is all that is needed and as others have explained adding a second
one would introduce other problems

My own experience is from a Zenith CH-300 that I have been flying for 18
years. It has two tanks, a left and a right. Sometimes it has three tanks.
That is when I put the 17 gallon aux tank on the back seat. My fuel valve
has four positions. Off. Left. Right. Aux. This valve feeds an electric
fuel pump that is located at the lowest point in the fuel system. I have run
tanks dry on long cross country trips, and have no problem promptly
switching tanks when one goes empty. Actually, if you watch the fuel
pressure gauge, you will get about a 10 second warning as the fuel pressure
begins to drop. Switch tanks at this clue, and you will never miss a beat.
But I have missed the clue several times, and when the engine stops making
power, just switch to a tank with fuel and it fires right back up. I should
add that when I switch tanks in this mode I turn the boost pump on to hurry
the fuel along. The engine will fire right back up in about 5 seconds.
(Which can seem like a very long time)

Thanks for listening.

Tim Hickey.

Drew



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