Message-ID: <3B3E63BA.700E7576@regandesigns.com>
Date: Sat, 30 Jun 2001 18:41:46 -0500
From: Brent Regan <Brent@regandesigns.com>
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To: Lancair List <lancair.list@olsusa.com>
Subject: Re: LNC4 Fuel Return
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The primary reason for having a fuel return on Bendix injection systems is
to allow air and gasoline vapor to be purged out of the system without
having to go through the injectors. Typically there is fitting with a
restrictor at the fuel servo which allows a continuos but small flow of fuel
back to the tank. Once the metered fuel leaves the fuel servo on it's way to
the injectors it's opportunity to escape back to the tank has been
eliminated. Most turbine (Flowscan) are therefore installed between the fuel
servo and the flow divider (aka spider). If you look at the flow divider you
may see a port labeled "Gage". This is not for a conventional fuel pressure
gage. It is for a fuel flow pressure gage. HUH????

It works like this. Bendix servos regulate flow, Continental servos regulate
pressure  but because the injectors are fixed orifices pressure and flow are
proportional to one another. Flow varies roughly as the square of pressure.
If you measure the pressure at the flow divider and calibrate your pressure
gage with a logarithmic scale you will get a passable measurement of GPH.
Many Bonanzas have such a flow(pressure)gage.

Regarding the question of a cabin tank, making the cabin tank part of the
active fuel system in a pressurized airplane would be difficult at best. A
simple solution would be to make the cabin tank passive and use it to fill
the wing tank in flight. The 4+- PSI differential between the cabin and the
wing eliminates the need for a transfer pump. I would add a T fitting to one
of the suction lines at the wing root inside the cabin. Connect a ball valve
to the tank at a handy location and then on to the cabin tank outlet. A
second ball valve can be added between the T fitting and the wing. Closing
this valve would cause the fuel pump to suck out of the cabin tank
exclusively in the event that the cabin was not/ could not be pressurized. A
vent line from the cabin tank runs down to near the outflow valve. Tie a
three foot nylon lanyard to the handle of the valve and a 3/4 inch hex nut
to the other end. During the flight run the top 10 gallons off of each wing
tank then an additional 10 gallons from the wing with the T fitting. Take
the plane off of autopilot, open the transfer valve and place the 3/4" nut
in the pilots mouth to remind him to watch the gauge and turn off the valve
when that wing gets heavy. The Copilot should make rude and unflattering
comments about the pilot having hardware in his mouth. This will ensure that
the valve is closed and the nut removed as soon as the transfer is complete.

An alternate monitoring method would be for the pilot to sing "I'm a little
tea pot" in a falsetto voice during transfer, helping redefine the term
"Ferry Pilot". This method should not be used late in the flight as it may
result in uncontrolled urination from members of the flight crew.

Regards
Brent Regan

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