Pumping fuel backwards through an electric fuel pump

I've installed a 12-gallon fuel cell aux tank in the tailcone of my
4-place conventional low-wing plane -- intended for go-fast trimming
when flying solo more than for its extra fuel capacity. As you know,
a typical 4-place is terribly nose-heavy with only front-seat
passengers and no baggage. The tank would be emptied if flying with a
full load of passengers and baggage.

Now I'm considering how to plumb it. My engine is gasoline, not
fuel-injected.

The simplest approach would be to vent the tank it to the outside, and
run a single 3/8 hard tube between my left main and the aux tank. The
aux tank is positioned towards the ceiling of the turtledeck so
there's a fair amount of gravity-feed available for it in normal
flying attitude to drain back into that same tank. I'd have a single
electric pump to pump from the main uphill to the aux tank, with a
shut-off valve in the line to keep it there. Opening the valve would
let it gravity-feed back into the main tank ... slowly. That is, if
it's OK to allow it to gravity feed "backwards" through the fuel pump.

(I need to consider the possibility of the main tank gravity-feeding
back into the aux tank in a prolonged steep climb. I'll need to
remember to shut off the valve whenever a fuel transfer isn't wanted.)

I'm wondering about installing a second electric pump in series at the
aux tank end, pointing back towards the main tank, to considerably
speed up the draining. Only one pump would be run at a time of course
-- either to fill or to drain the aux tank. But when either pump was
on, it would be pumping fuel "backwards" through the other one. I
understand that the simple Facet pumps have no check-valve so this
should be possible. But is it harmful to such pumps?

I've got a more elegant approach (a little complicated to draw here)
that would use a single pump and a double-stacked selector valve as
used on a fuel-injected engine. The selector valve makes it so that
in the "fill" position, the pump would pump fuel from the main to the
aux, and in the "drain" position, that same pump would pump fuel from
the aux to the main. This is the slick set-up, but is considerably
more complex/expensive/heavy to implement.

The fourth approach would use separate fill and return lines, with
separate pumps, check valves, and shut-off valves. This would avoid
venting overboard. But it's even more complex, and I hope to not have
to go there.

Surely others have solved this simple plumbing challenge before me. I
welcome your ideas. :-)

Greg

Greg,
If you're not planning to put fuel into the aux. tank during flight, and
only possibly drain it from the aux. tank to a main, then why not put a
simple hard line with shutoff between the aux. and main, and a filler
opening on the turtledeck? Put fuel in the aux. by shutting off the
transfer line and filling via the filler. Empty aux. to the main by
opening the transfer shutoff valve. No pumps, no checkvalves.

Russell Kent

Unless you absolutely positvely need the extra fuel why not just
put a water tank back there with a fill up arrangement and drain
spout to the ground.

This way if you need the ballast fill it with water and if you don't
then just drain it to the ground.

A lot less hazardous I would think.

Thanks for the suggestions so far. I had already considered both of
them:

1. Filling on the ground, and gravity draining in flight

For optimum "go fast" trimming, I really need to be able to transfer
fuel rearwards in flight, once at altitude. The plane will fly
fastest and most efficient when trimmed so far back that it's unsafe
for take-off ... or anything but fairly straight-and-level at high
altitudes. I'll want to pump some forward again as I'm decending to
land. (And no, I'm not crazy enough to go WAY back out of the
envelope.)

2. Filling with water instead of fuel

Ya, that's certainly the safer thing to do, and is in fact what one
builder is planning. I'd still have the transfer-in-flight
consideration above, of course. And I'd really like to be able to
have those extra 12 gallons onboard (when rear seats empty) for extra
range. I just hate carrying around dead weight. I'm also planning on
routing another line from the aux tank to the engine, via a shut-off,
so that it could gravity-feed directly as a fail-safe. Tougher to do
with water. ;-) This fuel cell is from the NASCAR circuit and it very
well built, and way back in the tailcone; I'm not terribly worried
about it in a crash.

Meanwhile, I've found a site (misplaced its link at the moment, but
can find it again via Google) that sells Facet pumps with integral
valves specifically intended for fuel transfer. When OFF, no fuel can
flow in either direction. The idea is to use two of them on separate
"fill" and "drain" lines of course, with no extra manual valves
required. They're cheap enough, and eliminate other hardware, so I'll
probably go with them ... even tho' they'll require the second line
plumbing. Unfortunately, they're only 30GPH, and I'd like to find
some with at least twice that capacity. I'm still looking.

Greg

What about some sort of emergency and you have to get down fast. Will you build in a dump valve or other means to bring
the CG back forward so handling is ok for the landing?

--
Dan D.



..
"Greg Reid" wrote in message
om...
Thanks for the suggestions so far. I had already considered both of
them:

1. Filling on the ground, and gravity draining in flight

For optimum "go fast" trimming, I really need to be able to transfer
fuel rearwards in flight, once at altitude. The plane will fly
fastest and most efficient when trimmed so far back that it's unsafe
for take-off ... or anything but fairly straight-and-level at high
altitudes. I'll want to pump some forward again as I'm decending to
land. (And no, I'm not crazy enough to go WAY back out of the
envelope.)

2. Filling with water instead of fuel

Ya, that's certainly the safer thing to do, and is in fact what one
builder is planning. I'd still have the transfer-in-flight
consideration above, of course. And I'd really like to be able to
have those extra 12 gallons onboard (when rear seats empty) for extra
range. I just hate carrying around dead weight. I'm also planning on
routing another line from the aux tank to the engine, via a shut-off,
so that it could gravity-feed directly as a fail-safe. Tougher to do
with water. ;-) This fuel cell is from the NASCAR circuit and it very
well built, and way back in the tailcone; I'm not terribly worried
about it in a crash.

Meanwhile, I've found a site (misplaced its link at the moment, but
can find it again via Google) that sells Facet pumps with integral
valves specifically intended for fuel transfer. When OFF, no fuel can
flow in either direction. The idea is to use two of them on separate
"fill" and "drain" lines of course, with no extra manual valves
required. They're cheap enough, and eliminate other hardware, so I'll
probably go with them ... even tho' they'll require the second line
plumbing. Unfortunately, they're only 30GPH, and I'd like to find
some with at least twice that capacity. I'm still looking.

Greg

Greg Reid wrote:

Thanks for the suggestions so far. I had already considered both of
them:

1. Filling on the ground, and gravity draining in flight

For optimum "go fast" trimming, I really need to be able to transfer
fuel rearwards in flight, once at altitude. The plane will fly
fastest and most efficient when trimmed so far back that it's unsafe
for take-off ... or anything but fairly straight-and-level at high
altitudes. I'll want to pump some forward again as I'm decending to
land. (And no, I'm not crazy enough to go WAY back out of the
envelope.)

OK, what you're planning to do sounds fairly dangerous. It sounds like
you're trying to shift the W&B during flight so that the horizontal
stabilizer is generating upforce instead of neutral or downforce (that's
why the plane goes faster: less thrust goes into making induced negative
lift on the horizontal stabilizer). Stalling the upforce-producing
horizontal stabilizer before the main on a conventionally rigged aircraft
would be about as much fun as stalling the main before the horizontal
stabilizer on a canard. Normally, stalling the horizontal stabilizer on a
conventionally rigged aircraft is no biggie. Since the horizontal
stabilizer normally produces downforce, stalling it makes the nose pitch
down, which causes an increase in airspeed, which unstalls the
stabilizer. But if you make the stabilizer produce upforce and then stall
it, you're in a deep stall and a world of hurt.

But to propose another solution to your original question: plumb a manual
on/off valve in parallel to the checkvalve-equipped pump from main to aux.
tanks. Want fuel in the aux? Turn off the valve and turn on the pump
until you reach the desired fullness, then turn off pump. Want to take
fuel out of the aux? Open the valve until you reach the desired
emptiness, then turn off the valve.

Russell Kent

I have a coffee table book which describes "some" technical detail of
the Concorde and it uses fuel transfer to trim the aircraft in flight.

Ray Toews

On 29 Sep 2003 11:32:02 -0700,
(Greg Reid) wrote:

I've installed a 12-gallon fuel cell aux tank in the tailcone of my
4-place conventional low-wing plane -- intended for go-fast trimming
when flying solo more than for its extra fuel capacity. As you know,
a typical 4-place is terribly nose-heavy with only front-seat
passengers and no baggage. The tank would be emptied if flying with a
full load of passengers and baggage.

Now I'm considering how to plumb it. My engine is gasoline, not
fuel-injected.

The simplest approach would be to vent the tank it to the outside, and
run a single 3/8 hard tube between my left main and the aux tank. The
aux tank is positioned towards the ceiling of the turtledeck so
there's a fair amount of gravity-feed available for it in normal
flying attitude to drain back into that same tank. I'd have a single
electric pump to pump from the main uphill to the aux tank, with a
shut-off valve in the line to keep it there. Opening the valve would
let it gravity-feed back into the main tank ... slowly. That is, if
it's OK to allow it to gravity feed "backwards" through the fuel pump.

(I need to consider the possibility of the main tank gravity-feeding
back into the aux tank in a prolonged steep climb. I'll need to
remember to shut off the valve whenever a fuel transfer isn't wanted.)

I'm wondering about installing a second electric pump in series at the
aux tank end, pointing back towards the main tank, to considerably
speed up the draining. Only one pump would be run at a time of course
-- either to fill or to drain the aux tank. But when either pump was
on, it would be pumping fuel "backwards" through the other one. I
understand that the simple Facet pumps have no check-valve so this
should be possible. But is it harmful to such pumps?

I've got a more elegant approach (a little complicated to draw here)
that would use a single pump and a double-stacked selector valve as
used on a fuel-injected engine. The selector valve makes it so that
in the "fill" position, the pump would pump fuel from the main to the
aux, and in the "drain" position, that same pump would pump fuel from
the aux to the main. This is the slick set-up, but is considerably
more complex/expensive/heavy to implement.

The fourth approach would use separate fill and return lines, with
separate pumps, check valves, and shut-off valves. This would avoid
venting overboard. But it's even more complex, and I hope to not have
to go there.

Surely others have solved this simple plumbing challenge before me. I
welcome your ideas. :-)

Greg

Ray

The B-1 uses fuel to trim. Probably also the B-2.

Big John

On Sat, 04 Oct 2003 15:55:16 GMT, (Ray Toews)
wrote:

I have a coffee table book which describes "some" technical detail of
the Concorde and it uses fuel transfer to trim the aircraft in flight.

Ray Toews

On 29 Sep 2003 11:32:02 -0700,
(Greg Reid) wrote:

I've installed a 12-gallon fuel cell aux tank in the tailcone of my
4-place conventional low-wing plane -- intended for go-fast trimming
when flying solo more than for its extra fuel capacity. As you know,
a typical 4-place is terribly nose-heavy with only front-seat
passengers and no baggage. The tank would be emptied if flying with a
full load of passengers and baggage.

Now I'm considering how to plumb it. My engine is gasoline, not
fuel-injected.

The simplest approach would be to vent the tank it to the outside, and
run a single 3/8 hard tube between my left main and the aux tank. The
aux tank is positioned towards the ceiling of the turtledeck so
there's a fair amount of gravity-feed available for it in normal
flying attitude to drain back into that same tank. I'd have a single
electric pump to pump from the main uphill to the aux tank, with a
shut-off valve in the line to keep it there. Opening the valve would
let it gravity-feed back into the main tank ... slowly. That is, if
it's OK to allow it to gravity feed "backwards" through the fuel pump.

(I need to consider the possibility of the main tank gravity-feeding
back into the aux tank in a prolonged steep climb. I'll need to
remember to shut off the valve whenever a fuel transfer isn't wanted.)

I'm wondering about installing a second electric pump in series at the
aux tank end, pointing back towards the main tank, to considerably
speed up the draining. Only one pump would be run at a time of course
-- either to fill or to drain the aux tank. But when either pump was
on, it would be pumping fuel "backwards" through the other one. I
understand that the simple Facet pumps have no check-valve so this
should be possible. But is it harmful to such pumps?

I've got a more elegant approach (a little complicated to draw here)
that would use a single pump and a double-stacked selector valve as
used on a fuel-injected engine. The selector valve makes it so that
in the "fill" position, the pump would pump fuel from the main to the
aux, and in the "drain" position, that same pump would pump fuel from
the aux to the main. This is the slick set-up, but is considerably
more complex/expensive/heavy to implement.

The fourth approach would use separate fill and return lines, with
separate pumps, check valves, and shut-off valves. This would avoid
venting overboard. But it's even more complex, and I hope to not have
to go there.

Surely others have solved this simple plumbing challenge before me. I
welcome your ideas. :-)

Greg

On Sat, 04 Oct 2003 16:55:16 +0000, Ray Toews wrote:

I have a coffee table book which describes "some" technical detail of
the Concorde and it uses fuel transfer to trim the aircraft in flight.


The centre of pressure moves aft quite a bit in supersonic flight. If the
CG was kept in the same place it was in subsonic flight the elevons would
have to be deflected up at a significant angle to trim the aircraft, and
the drag from the deflected elevons would be very large. So, in
supersonic flight they pump fuel aft to move the CG aft and reduce the
trim drag from the elevons. But I wouldn't say they use it to trim the
aircraft in the aviation sense of the word, because you can be sure that a
movement of the trim switch makes something happen in the flight control
system rather than pump fuel to reduce the stick force.

Quite a few modern airliners have fuel tanks in the horizontal stablizer,
and they pump fuel aft in cruise to reduce the trim drag.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

Are you really sure fuel will flow backwards through the Facet pump? I
may be wrong as it has been a long time since I worked with one of these
pumps, but I remember that the design did not allow flow in the opposite
direction. I have seen a number of fuel setups that used two pumps in
parallel. With one pump "on" the other pump, the pump that was not
energized, did not allow any back flow. There was no need for a check
valve in the system as the pump acted as its own.

Greg Reid wrote:

I understand that the simple Facet pumps have no check-valve so this
should be possible. But is it harmful to such pumps?


Greg

--
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
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