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