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From: Dale Rogers <dale.r@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: Vapor lock
Date: Sat, 12 Feb 2005 9:55:35 -0500
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Message-Id: <20050212145535.KQXU8811.fed1rmmtao09.cox.net@smtp.west.cox.net>

Paul,

   There a piece missing from this puzzle.  Help me understand.

> If I were returning fuel to one of the main tanks, I would think that venting the sump would be appropriate, but since I am sending fuel undere pressure back into the sump tank, I am concerned that once the sump tank is full, the additional fuel pumped back into the sump tank would take the path of least resistance....out the vent instead of back up the fuel line into the tank?   
>

   When you first power-up your fuel system, you have a 
full header tank (or at whatever level remaining in the 
mains, if there's overlap).  You have an empty return line 
from the regulator to the header tank.  As soon as your 
fuel pressure exceeds the regulator setting, fuel begins 
pushing air out of the return line, into the header tank, 
creating a "head" pressure against the mains.  At some 
point, fuel pumped out of the tank equals what's coming 
back - until you start the engine.  Now less fuel is 
coming back than is being pumped out, and the difference 
should be made up from the mains.  

   However ... you have bubble of air and vaporized fuel 
in the header tank - only as long as the head pressure of 
the mains exceeds the pressure in the header tank, will 
fuel flow from the mains to the header tank.  Heating the 
fuel will increase the pressure of the bubble.

   I suspect that the header tank should be vented back to 
the mains.

   What did I miss?

Dale R.
COZY MkIV #1254