Pumping fuel backwards through an electric fuel pump

[Greg Reid]

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

[Russell Kent]

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

[Bernie the Bunion]

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.

[Greg Reid]

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

[Blueskies]

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

[Russell Kent]

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

[Ray Toews]

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

[Big John]

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

[Kevin Horton]

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

[Bruce A. Frank]

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"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|

[sleepy6]

Check the cutaway picture and info at

http://www.ppavionics.com/FacSolidState.htm

Notice that there is only one model that has a positive shutoff and a
built in check valve to prevent backflow. The others depend on the
valve built into the inlet. Ordinarily I would say that the inlet
valve would prevent backflow but I have seen two cases where it did
not. One resulted in a fuel starvation crash. Apparently the inlet
valve can hang open too easily to bet your life on it. I recommend
that a seperate check valve be used for safety on the models that do
not have one built in.

I also would not consider an application that depends on reverse flow
through the pump.

Bob




In article >,
says...
>
>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 the
>se
>pumps, but I remember that the design did not allow flow in the opposi
>te
>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]

I have two of the 40108 pumps listed at that web site. I just hooked
them up opposed to each other. Neither could force gasoline backwards
through the other. They may not be leak free when exposed to reverse
pressure...the design may not be a full check valve design, but they
certainly do not flow backwards enough to allow filling a tank.

sleepy6 wrote:
>
> Check the cutaway picture and info at
>
> http://www.ppavionics.com/FacSolidState.htm
>
> Notice that there is only one model that has a positive shutoff and a
> built in check valve to prevent backflow. The others depend on the
> valve built into the inlet. Ordinarily I would say that the inlet
> valve would prevent backflow but I have seen two cases where it did
> not. One resulted in a fuel starvation crash. Apparently the inlet
> valve can hang open too easily to bet your life on it. I recommend
> that a seperate check valve be used for safety on the models that do
> not have one built in.
>
> I also would not consider an application that depends on reverse flow
> through the pump.
>
> Bob
>
> In article >,
> says...
> >
> >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 the
> >se
> >pumps, but I remember that the design did not allow flow in the opposi
> >te
> >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"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|
*------------------------------**----*
\(-o-)/ AIRCRAFT PROJECTS CO.
\___/ Manufacturing parts & pieces
/ \ for homebuilt aircraft,
0 0 TIG welding

While trying to find the time to finish mine.

[Bill Daniels]

How about a reversible positive displacement pump that won't allow fuel to
seep past it while stopped? A gear pump will do that if closely fitted.
That way you could pump fuel in either direction until you were trimmed and
just shut it off.

Bill Daniels

"Greg Reid" > wrote in message
om...
> 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

[Scott Correa]

"Greg Reid" > wrote in message
om...
> 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.

Greg,
Thats a bunch of weight and complexity to add for trim fiddling. I can
see the benefits but think that a fixed/removable ballast bar bolted to
somewhere
around the rudder post weighs less, is less complex, gets you 90% of what
you are looking
to achieve and has no failure modes that would be critical to flight. Make
a bar for solo,
one for 2 up. Some certificated gliders and helicopters do it this way.
But I'm afraid it
might be too simple..........

Scott

[Greg Reid]

Hey guys, thanks for all of the commentary/concern ... and sorry I've
been remiss in responding until now.

Thanks in particular to Bruce for going the extra mile and a half in
actually hooking up and testing to see if you can pump backwards thru
a typical Facet pump (nope).

A simple "gear motor" mentioned elsewhere is what I need, I suppose,
so that the single pump could pump both ways (with polarity reversed).
I could add an on/off valve (manual or electric solenoid) to complete
the single-pump-and-single-tube hookup. Such gear motors are
typically found in cheap hardware-store siphoning pumps that let you
drain or fill your auto crankcase via the dipstick tube (turned by an
electric hand drill). I wonder if I can find one of aircraft quality
somewhere.

I did find the ppavionics site mentioned. That's the link I mentioned
having found (and then temporarily misplaced) earlier. They've got a
Facet pump model that contains internal valving to prevent flow in
either direction when off. So they could be used for a two-line and
two-pump setup for "fill" and "drain". I found some other stuff on
their site that seems "interesting", and wrote them (no response as
yet). (And no, I have no financial or other interest.)

I just want to assure you all that even when the aux tank is full,
I'll still be within rear CG limits ... barely. I wouldn't really
CHOOSE to take-off or land (or do intentional stalls) at that rear
limit, but it WILL be possible to do so safely.

I'm currently leaning towards the obvious two-line and two-pump
arrangement, but am still on the lookout for a lighter/simpler
alternative using a single line and single pump.

As I mentioned earlier, it's easily possible to use a single line and
a single pump to fill and drain (with the pump always running in its
normal mode) by using a stacked selector valve as used with a
fuel-injected engine, and a bit of fancy cross-plumbing of the valve
ports. This is basically trading the expensive selector valve for the
second pump and line. It might be a little lighter and somewhat less
complicated.

Someone else suggested much earlier using a single pump to fill and a
"bypass valve" to drain by gravity. That's an excellent and simple
solution -- if I could trust that gravity alone will drain the tank
within a reasonable length of time. I need to do a simple experiment
to find out.

Thanks again for all the comments,
Greg

[Big John]

Greg

Have you thought about using ram air to empty the tank?

In the P-51, I've seen a 'L' shaped piece of tubing mounted in a
rubber fitting on the top front of the drop tanks, facing forward. The
ram air put pressure in the tanks and caused flow. System had a single
valve that you opened to let fuel transfer to engine.

The 'ram air' tubing was about 7/8 to 1 inch in diameter and cruise
speed was 220-250 mph depending on mission. One mission I remember we
were cruising at 180 mph escorting bombers on a very long range
mission (and drop tanks fed ok). We didn't like to fly that slow
escorting but using a higher airspeed and weaving used more fuel and
we could not escort full range of mission.The fuel transfer was rapid
enough to run the Merlin.

The exhaust of the vacuum pump was also used to pressurize the drop
tanks at times. More complicated to set up than ram air system.

Flew missions at 20K using the ram air pressure to feed the drop
tanks. Understood at higher altitude, with the lower IAS, they might
not always feed ok (or feed very slow)?

This is just an out of the box bit of data that you might use to make
a simple (reliable) feed system.

Big John


On 6 Oct 2003 14:31:30 -0700,
(Greg Reid) wrote:

>Hey guys, thanks for all of the commentary/concern ... and sorry I've
>been remiss in responding until now.
>
>Thanks in particular to Bruce for going the extra mile and a half in
>actually hooking up and testing to see if you can pump backwards thru
>a typical Facet pump (nope).
>
>A simple "gear motor" mentioned elsewhere is what I need, I suppose,
>so that the single pump could pump both ways (with polarity reversed).
> I could add an on/off valve (manual or electric solenoid) to complete
>the single-pump-and-single-tube hookup. Such gear motors are
>typically found in cheap hardware-store siphoning pumps that let you
>drain or fill your auto crankcase via the dipstick tube (turned by an
>electric hand drill). I wonder if I can find one of aircraft quality
>somewhere.
>
>I did find the ppavionics site mentioned. That's the link I mentioned
>having found (and then temporarily misplaced) earlier. They've got a
>Facet pump model that contains internal valving to prevent flow in
>either direction when off. So they could be used for a two-line and
>two-pump setup for "fill" and "drain". I found some other stuff on
>their site that seems "interesting", and wrote them (no response as
>yet). (And no, I have no financial or other interest.)
>
>I just want to assure you all that even when the aux tank is full,
>I'll still be within rear CG limits ... barely. I wouldn't really
>CHOOSE to take-off or land (or do intentional stalls) at that rear
>limit, but it WILL be possible to do so safely.
>
>I'm currently leaning towards the obvious two-line and two-pump
>arrangement, but am still on the lookout for a lighter/simpler
>alternative using a single line and single pump.
>
>As I mentioned earlier, it's easily possible to use a single line and
>a single pump to fill and drain (with the pump always running in its
>normal mode) by using a stacked selector valve as used with a
>fuel-injected engine, and a bit of fancy cross-plumbing of the valve
>ports. This is basically trading the expensive selector valve for the
>second pump and line. It might be a little lighter and somewhat less
>complicated.
>
>Someone else suggested much earlier using a single pump to fill and a
>"bypass valve" to drain by gravity. That's an excellent and simple
>solution -- if I could trust that gravity alone will drain the tank
>within a reasonable length of time. I need to do a simple experiment
>to find out.
>
>Thanks again for all the comments,
> Greg