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Date: Fri, 06 Apr 2007 21:30:34 -0400
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Subject: Re: Testing for fuel leaks and fuel probes to use
From: "bob mackey" <n103md@yahoo.com>
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> Couldn't a pressure sensor in the wing tank outlet line sense fuel
> pressure/level?  Who is using this method?

Most washing machine manufacturers. Those parts have found also
some applications in improvised munitions that blow up when
the pressure drops. It triggers a blasting cap instead of a
water fill valve. But that's not the version we want on our Lancairs.

A pressure sensor makes sense when the the tank is tall, and the
force of gravity is fairly constant. If using a pressure sensor
in a LNC2 wing tank, The total pressure head between full and empty
is (I'm guessing) about 8 inches for the 235 or 6 inches for the
360 style tanks. An absolute sensor would sense

P = rho*g*h + atmospheric pressure + dynamic pressure at the vent
The latter two terms can be eliminated by using a differential
pressure sensor connected to the top and bottom of the tank.
Connecting to the vent line and the fuel delivery line should suffice.
In this case it is convenient that the vent line is routed inboard.

The first term, rho * g * h, is fuel density, acceleration (G), and height
of the fuel column. rho is fairly constant. G varies with aircraft
maneuvering, and height varies with fuel level and lateral sloshing.

The sloshing variation is averaged out with a filter, perhaps a
30 sec - 1 min time constant would do.

The G variation could be averaged out the same way, but sustained
turns would cause the fuel level to indicate high. A better way is
to include an accelerometer, and divide the pressure signal rho*g*h
by the measured g. Incidentally, and easy way to measure g is by using
a tube containing liquid connected to a differential pressure sensor.

BTW, that's exactly what the sight tube on the nose tank is
a pressure sensor. The pressure diffence between the top and
the bottom of the fuel tank determines the fluid level in the
sight tube. Because the sight tube has the same density fluid,
g forces on the sight tube and tank cancel each other.
However, because there is fluid in the horizontal portions of the
tube, and because the sight tube is  behind the tank, the level
in the sight tube indicates pitch angle as well as fuel level.
It is very disconcerting to some passengers to see the last
five gallons of fuel disappear from view in a steep descent.


> Also, the original builder ran the wing tank vents form the
> filler area inboard to the wing root.  Any suggestions as to
> where to position this vent -- higher than the wingtips' level?

I would run the tube up to the ceiling or at least the top of the
seat bank, just behind the rollover frame, then back down to the floor to
a forward facing pitot vent between or aft of the landing gear doors.
If an enlarged tube (1/2") is used on the tank-side of the inverted
U-tube, then there will be less risk of siphoning from an overfull
tank.

Or you can do what I do... fly without any fuel level indication
in the wings and make sure that there is enough fuel in the header
and air under the belly to ensure a safe landing if the available
fuel in the wings is less than expected.

Most of the fuel in a 235 is not visible from the fuel filler.
That's less of a concern for the 320/360. On mine, if the fuel
is visible below the filler, that means >5 gal. If I can hear the
fuel slosh when I rock the wing that is "some" fuel. Above 5 gal,
the level is visible.