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A second problem where the tanks are connected
together as shown is that as fuel is removed and returned to the tank(s) the
fuel flow back to the tanks may surprise you as to where it goes. It is very
difficult to get it to feed equally from both tanks, and return equally to both
tanks. any difference in pressure drop due to vagaries of plumbing can cause one
of the tanks to preferentially lose fuel, and the other to fill enough to
overflow, and if I understand your drawing correctly, there is no way to force
the fuel to come from a *specific* tank, therefore if one draws down
sufficiently, you will suck air and have a fuel starvation incident with gas on
board.
Bill Schertz
KIS Cruiser # 4045
----- Original Message ----- From: Finn Lassen Sent: 5/27/2004 7:36:59 AM To: flyrotary@lancaironline.net Subject: [FlyRotary] Re: Fuel Pump Design
2nd fuel filter should be before the T ?
If this is a low wing aircraft, you do NOT want direct connection
between left and right tanks: you could be sucking air when one wing is
lower than the other (with no fuel). Not an issue if the common pickup
point is substantially lower then the lowest point of each tank, as in
a high wing aircraft.
Finn
Bob Perkinson wrote:
The attached drawing is my idea of a simple fuel
system for the 9a that I am building, with Aux fuel tanks akin to Berne
Kerr?s Project. I would like some feed back as
to what problems this arrangement might pose, and what the remedy would
be for such problems. The plan is to use large
diameter pipe between the main fuel tanks so as to allow the fuel to
equalize
Bob Perkinson.
Fuel system design hasn't come up in quite a while. But since a large
portion of accidents are fuel related, I thought this would be worthy
of discussion.
I have experienced a failure mode during ground runs on my Lancair ES
with 20B that seems to be a problem with other types of installations
as well. I recently read a first flight report by a professional test
pilot, David Allen, that almost had to set a Lancair ES, N711RG, down
on I-70 due to the inability to restart the engine after running a tank
dry. (See www.geocities.com/daveandjj
for the full story.) This was a certified fuel-injected engine
installation. Also, the Subaru guys (I have been lurking on the
Eggenfelder Subie site today) had a similar situation, resulting in
some glider time and a dead-stick landing.
The problem is that after exhausting the fuel in one tank an airlock
forms between the fuel pump(s) and the fuel pressure regulator that the
efi pump cannot overcome due to lack of fuel in the supply line. This
is what I have experienced with my installation on ground runs. I
can't get the efi pumps to re-prime unless I momentarily break a line
loose between the pump outlet and the pressure regulator. As soon as I
do that the pump will re-prime and all is well.
For the record, I have two of Tracy's efi pumps, with two efi filters,
hooked in parallel. They are mounted low on the fire wall. Upstream
is a gascolator and an Andair 6-port selector valve. Downstream of the
pumps/filters is a map sensing pressure regulator. There is a -6AN
going to the fuel rail, and a -4 return line back to the Andair
valve/tank.
I was curious if anyone else has had this happen and what they did to
resolve the issue. I have come up with two possible solutions.
1. Tee off the pump output and put an electric solenoid valve in the
line and tee the outlet of the solenoid to the fuel return line, past
the regulator. Push-button switch on panel would activate the
solenoid. If needed, a second or two of activation should relieve the
air-lock and allow the pumps to re-prime.
2. Install a bleed line around the pressure regulator with a small
metering orifice (.020-.030") that would bleed off any air that might
get trapped. (This appears to be the solution the Subaru group is
focusing on). This would be a full time bypass. This seems to be the
simpler solution.
3. Install a low pressure, high volume fuel pump in one, or both, of
the wing tanks. Procedure would be to always keep reserve fuel in this
tank.
Any comments from seasoned flyers would be welcomed.
Mark S.
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