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Posted for Ken Harwood <kenharw@flash.net>:
FUEL SYSTEMS REVISITED.
Hi Gang,
I've been reading about the different fuel systems in the Lancair
320-360-235 with some interest. The point is well taken that with fuel
selector to BOTH mains has its drawbacks. This condition will result in the
engine driven pump or the fuel boost pump located on the firewall sucking
air instead of fuel as soon as one of the main tanks empties or otherwise
allows air to replace fuel in the lines. This problem can be remedied if
both facet pumps are allowed to run when fuel gets low in both of the tanks.
The facet pumps pump gasoline much better than they do air, so the tank that
still has fuel in it will still feed and overcome the small amount of air
the opposite pump is trying to pump. Another point, if your gas gauges
show one tank is feeding faster than the other, just turn off the facet pump
for the low reading tank.
My Lancair 360 has three fuel tanks. I use a selector switch that selects
either header tank or both wing tanks. I do not have the capability of
feeding from just one wing tank using only the selector switch; however, I
can choose which main tank I feed from by use of the facet pumps. If I
turn off the right facet pump the engine gets its fuel from only the left
tank, and vice versa. I do not burn out of the header tank normally so as
to keep it full and in reserve and to keep the weight and balance more
forward. As long as that tank is full I know I have at least 40 minutes of
fuel to find a place to land in case of total electrical failure. I
actually have what is in the header tank and what is in the lowest wing tank
available at all times. If the electrical system is OK I can burn every
last drop of useable fuel. I tested this system thoroughly during flight
testing in 1992, by burning ALL useable fuel out of one tank by turning off
one facet pump and leaving the other on. The tank feeding the facet pump
that was on emptied totally and the engine started sputtering. All I had
to do was turn on the other facet pump and everything was fine again, except
I had one heck of a lot of aileron trimming to do to account for all that
fuel on one side and none on the other.
Another point. Always leave the facet pumps in the lines from the wing
tanks should you decide to stray from the company layout. They serve three
purposes:
1. They act as one-way valves so that fuel will not drain to the low side of
the airplane and thus spill out the vent in case you are parked on a slope
with one wing lower than the other.
2. They are positive pressure boost pumps and will not cavitate until all
fuel in that line is gone.
3. They are required at higher altitudes to prevent cavitation in the fuel
lines on warm days when the fuel under suction wants to vaporize. I run my
facet pumps continually above 7,000 ft. In the summer and above 9,000 ft. In
winter. The fuel injected engine does poorly under low fuel pressures.
One final thought: Consider putting one-way valves in the wing tank vent
lines to prevent fuel spillage during sharp turns and full tanks. Just
remember, there has to be some way to allow for fuel expansion when the
airplane is left out in the sun, so drill #50 or #60 holes in the diaphragm
of the one-way valves to allow air to escape (and a little fuel too if the
tanks are full). This modification is almost imperative if your wing fuel
goes all the way to the end of the wings.
I now have 570 hours on my 360 and fuel management is a dream, not a chore.
Ken
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