From: CHRISTOPHER_ZAVATSON@udlp.com
Date: Tue, 20 Oct 1998 17:35:28 -0500
Message-ID: <0014301E.C21254@udlp.com>
Subject: Fuel tank vents
To: Lancair.list@olsusa.com
Mime-Version: 1.0

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     Marv,
     
     Regarding the two fuel vent options you mentioned;
     
     Running the vent back into the fuselage and up the seat back will trap 
     some fuel in the vent line at its lowest point.  The fuel system will 
     have to pull a bit of a vacuum to overcome pulling this trapped fuel 
     up hill out to the wing tip back into the tank.  Depending on the 
     diameter of the this line and given its length and the vibration it 
     will see, the line may never clear itself of fuel.  Unless the line is 
     very small in diameter, the vacuum created will start fuel on its way 
     out to the tip, the fluid column will break down flowing back to the 
     low point while allowing an air bubble to get past the fuel into the 
     tank, for the moment eliminating the vacuum.  If vibration were not 
     present the line would most likely clear itself, as soon as, fuel was 
     drawn from the tank.  Also, if the diameter is very small it will 
     clear itself.  Is this a bad situation?  Probably not, but I would 
     prefer using the vents to slightly positively pressurize the tanks 
     instead of potentially applying a small vacuum and forever trapping 
     fuel in the tubing.
     
     The concept of venting back to the tank could create a problem 
     depending on the details of the plumbing. If the vent line to the fuel 
     tank is submerged in fuel and the transfer pump is off, the engine 
     will create a vacuum inside the header tank.  Fuel will then be drawn 
     into the header tank, continuously, through either the fuel supply 
     line or vent, whichever offers the least resistance.  (This assumes 
     the wing tank is still vented otherwise everything locks up)  If you 
     route the header vent line to the highest point in the wing tank 
     everything works great until you get fuel in the vent line.  Now your 
     looking at the same situation as described in the first paragraph.  
     
     I have fuel out to the tips and find I rarely have to worry about 
     dumping fuel overboard.  With completely topped off tanks, turns on 
     the ground and uncoordinated flight will dump fuel, but that just 
     doesn't occur very often.  The vent line is as critical as the fuel 
     supply line in that, if it gets blocked, your fuel supply is useless. 
      I would suggest keeping it simple, inspectable and removable.  
     
     Chris Zavatson
     N91CZ