Message-Id: <3.0.3.32.19981008115948.00ce632c@olsusa.com>
Date: Thu, 08 Oct 1998 11:59:48 -0400
To: lancair.list@olsusa.com
From: ReganRanch@aol.com (by way of Marvin Kaye <marvkaye@olsusa.com>)
Subject: Fuel Tank Plumbing
Mime-Version: 1.0

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In a message dated 98-10-08 01:12:45 EDT, you write:

<< The wings are mounted at an angle to the horizontal plane. Gravity would
take
 any fuel and attempt to empty the tanks if given an opportunity. If both
tanks
 emptied into a tiny 1 pint reservoir and that reservoir was considered the
 "tank" in the first paragraph, wouldn't that get rid of the manual
 switching of
 tanks, and reduce the amount of plumbing, connections and weight?  >>

What you are describing is essentially a "BOTH" position on a fuel valve. It
is OK to have both tanks connected on a high-wing but a bad, and sometimes
fatal, idea on a low wing. On a spam can high wing the fuel tanks occupy a
short percentage of the wingspan and they are significantly higher than the
fuel pump. These two factors combine to make it very difficult for the right
fuel system to be completely above the left or visa versa. If one tank runs
dry, gravity supplies fuel from the other tank to ensure a prime on the fuel
pump even with low fuel levels.

On a low wing the fuel is below the engine and the fuel pump, particularly at
the end of a long Xcountry. It is relatively easy to get in the situation
where you un-port (fuel sloshing away from the fuel strainer) one tank while
the other tank line head pressure is below ambient. The immediate result is
instant "Quiet flight" and of course this is most likely to happen low and
slow in the pattern.

One way out is to consider the last 10 gallons or so to be "Unusable" but then
you are trading 5 pounds of lines and valves for 60 pounds of fuel. 

Another complication is evenly dividing the flow from the fuel return line
back to the two tanks.

The short answer is "BOTH" works fine on a low wing UNTIL you need the last
few gallons, then it is a crapshoot.


Brent