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<< Lancair Builders' Mail List >>
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To Bill Gradwohl:
Why won't this work?
What have I forgotten? Which one of my assumptions is incorrect? <<
What you are missing in your basic system (two wing tanks plumbed into a
small plenum which in turn feeds the boost pump and engine) is that a pump
will suck air uphill better than fuel. If air is available at the pickup
in the plenum, then that is what the pump will draw.
Your system will be the functional equivalent of a single tank with a
single sump and pickup if you make sure the top of the pickup is lower than
the bottom feed point of BOTH wing tanks. This will work as well as the
single tank running tip to tip featured in my Glasair. Extra features such
as redundant pumps, check valves, and cutoff valves may or may not add
safety or utility, but the basic design will work as long as the pickup
point is below all feed points in any reasonable flight attitude.
This will not work without the plenum tank. Using only 3/8" lines
(or whatever) will undoubtedly get air started in the lines and that is
what will continue to feed.
A very similar solution would be to put in a crossover line and feed from
only one or the other wing sump. This will very likely increase the amount
of unusable fuel, however.
Watch out using check valves and be sure to vent the system properly.
Remember, air and fuel must be able to enter and exit each space to allow
for fuel consumption, pressure changes from temperature variations on the
ground and changing altitude. The top of your plenum will need to be
vented into one (maybe both) wing tanks to prevent an air pocket from
expanding and interrupting fuel flow and also prevent head pressure of full
wing tanks from over-filling the plenum. This is getting more complicated
isn't it?
I still believe that the best solution would be to just have a
left/right/off valve (like Piper and Mooney) which feeds the pump.
!^NavFont02F05FB000B61GVHGWHLFD8E28
- Doug Dodson
Glasair II-S FT
Flight Test Engineer, CFI-A,G
Baby Dragon IF1 Race Team
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