Message-Id: <3.0.3.32.19981022000129.00d489d8@olsusa.com>
Date: Thu, 22 Oct 1998 00:01:29 -0400
To: lancair.list@olsusa.com
From: "Douglas L. Dodson, Jr." <73773.1546@compuserve.com> (by way of Marvin Kaye <marvkaye@olsusa.com>)
Subject: Fuel tank vents
Mime-Version: 1.0

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To: Marvin Kaye
>> venting his wing tanks from the outboard high point back into the
fuselage and then into a semi-circular loop up his seat backs... the actual
vent drain line was plumbed in via a tee fitting at the apex of the loop on
the seat backs and out the bottom of the fuselage.  It seems to me that
this arrangement would nicely address the issue of draining fuel overboard
when parked on a non-level ramp.  Other than the extensive distance from
the outboard end of the tanks back to the fuselage, what problems do any of
you see with this venting scheme?<<

All true, but you will need a drain fitting at the low point of the
vent lines.  They will trap some fuel/water/sediment and need to be kept
clear.

>>Along a similar vein, rather than venting the header tank directly to
the outside, why can't you vent it back to the wing tanks? <<

Your connected selector valve setup is mechanically complicated but
not impossible to build.  You must account for the possibility of the
header tank sucking/blowing air and fuel into and out of each tank as
temperature and altitude changes.  It will probably work just fine, but
carefully check for conditions where vent ports are submerged on either end
and make sure each chamber has a way to the atmosphere with minimal
restriction under all fueling combinations and flight attitudes.

In my Glasair, the wing tank and fuselage tank are both selectable for
feed.  Wing tanks (really one long tank but each tip is vented) are plumbed
together (downhill) then to the fuselage tank (uphill).  The fuselage tank
vents overboard through a separate line.  I put a valve at the low point of
the wing vent lines that is controllable in the cockpit. When closed, fuel
can not vent overboard from the wings, it will try to go uphill into the
fuselage tank. Drawing fuel from the wing tank will require some suction in
the tank dead air space to pull trapped fuel up hill in the vent line and
back into the tank.  The valve allows me to vent the wing tanks overboard
in the event I suspect fuel flow problems from the wing tank as well as
allowing me to drain the trapped fuel/water/sediment periodically.  I
intend to fly with the valve open except perhaps when the wing tank is very
nearly full and will probably keep it open for takeoff and landing
regardless of wing tank fuel quantity.  Its main purpose is to prevent
excessive fuel loss through the vents while parked and taxiing.


 - Doug Dodson
   Glasair II-S FT
   Flight Test Engineer, CFI-A,G
   Baby Dragon IF1 Race Team