REFLECTOR: FUEL FEED & Vents

[Jim Sower]

Kieth,
I have the check valve you allude to.  My sump vent runs to the top of 
the firewall where I patched in a couple inches of a automotive schrader 
valve (less the core) and stop it up with an ordinary valve cap.  I 
remove the cap to fill the sump and then replace it.  The check valve 
evidently does what it does.  I have no evidence that the system ever 
stopped up and needed the check valve, but I think it's a good idea.
With all my problems, I've thought it through pretty well.  I'll stick 
with what I've got ... Jim S.


KeithHallsten wrote:

>Scott,
>
>Your point is well taken, but I think Jim has an excellent point regarding
>the sump tank vent.  It seems that it's much more likely to hurt than to
>help.  I personally (after careful consideration) will install a
>quarter-turn valve in the sump vent line, below the vent manifold.  That
>valve will be closed once the sump tank is full.  That way the fuel pump can
>suck from the strake tanks through the sump tank, should the need ever
>arise.  I have never been able to identify a situation in which the sump
>vent would do anything good for you (once the sump tank is filled the first
>time).  Anyone got a scenario to the contrary?
>
>Having an alternate vent (check valve into the system from the cabin) might
>well be a good idea, provided that the cracking pressure of the check valve
>is low enough that the fuel vaporization problem isn't experienced in the
>case of a plugged main vent.  I have never seen any information on the
>cracking pressure and flow capacity of the check valves that have been
>installed for this purpose - does anyone have such information for their
>check valve?  I suspect that most won't work as the builder imagined.
>Anyone have some test data to share?
>
>Keith Hallsten
>
>
>
>----- Original Message ----- 
>From: "Scott Baker" <sbakr at comcast.net>
>To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
>Sent: Wednesday, February 16, 2005 5:07 PM
>Subject: Re: REFLECTOR: FUEL FEED [heur]
>
>
>  
>
>>The Velocity factory position is that shown in the Construction Manual.
>>Assuming that you are using the recommended fuel tank and vent line
>>locations, the center sump tank should be vented with a check valve (flow
>>towards the sump tank) that is "T'd" into the vent line.  The check valve
>>allows the sump to continue venting if the main vent line were to clog
>>    
>>
>(ice,
>  
>
>>bugs).
>>
>>Short story - When I was in the business of being one of them-there
>>professional "amateur" builder assistants, I toyed with the idea of
>>modifying the Velocity fuel supply/vent system.  While doing so I
>>    
>>
>consulted
>  
>
>>with Duane Swing to ask his opinion - and I vividly remember his reply -
>>which was (paraphrased) "Don't screw around with the fuel system.  There
>>    
>>
>is
>  
>
>>a lot 'there' that does not meet the eye - and it is easy to create a
>>problem".  Following this I "screwed around" with the schematic anyway -
>>    
>>
>and
>  
>
>>discovered (the hard way) that a) venting the sump tank without routing
>>    
>>
>the
>  
>
>>vent line to the uppermost portion of the roof of the aircraft tends to
>>siphon the fish tank dry (full tanks + high pitch angle during take off =
>>beginning of fuel flow through the vent system being pumped overboard) ...
>>"I say ... we're "burning" fuel at an alarming rate.  20-gallons in the
>>    
>>
>last
>  
>
>>10-minutes.  Maybe it would be a good idea to land".  One of friends at
>>    
>>
>the
>  
>
>>airport waved me off from approaching the hangar because of the trail of
>>fuel that continued to be siphoned overboard during taxi.
>>
>>On another occasion I did NOT vent the sump tank - opting to vent the
>>    
>>
>mains
>  
>
>>only (sort of what Jim S. is suggesting).  This was a 260-hp engine
>>installation with 0.375" diameter fuel lines and ... what were we using
>>    
>>
>back
>  
>
>>then ...0.250" vent lines?  This system involved a fuel selector valve
>>    
>>
>(near
>  
>
>>the pilot seat), so fuel was going from the mains, around the gear
>>bulkheads, up the keel, short riser to the fuel-selector valve, back down
>>through the keel, and into the sump tank.  Anyway, before the first flight
>>    
>>
>I
>  
>
>>tested the fuel delivery by installing a flow restrictor after the
>>    
>>
>electric
>  
>
>>fuel pump (a partially crimped aluminum tube) and turning on the electric
>>fuel pump (dumping the fuel into a portable fuel can).  Everything went
>>    
>>
>well
>  
>
>>...at first.  Fuel delivery was somewhere near 25-gph (using stopwatch and
>>measured volume) for about 5-minutes - at which point ... and this is VERY
>>IMPORTANT - PLEASE PAY ATTENTION - because of the small vent lines, and
>>because of the long fuel line runs, the fuel pump was delivering fuel
>>    
>>
>faster
>  
>
>>than the vent and feed lines could supply fuel.  The fuel pressure fell
>>    
>>
>off
>  
>
>>as the system began to pull a vacuum on the fuel tanks - and the fuel
>>    
>>
>lines
>  
>
>>literally filled with air bubbles due to cavitations - I could see the
>>bubbles in the clear tubing I was using during the test.  The fuel flow
>>    
>>
>just
>  
>
>>about stopped!  Just thinking about a loss of fuel flow at the 5-minute
>>    
>>
>mark
>  
>
>>of flight (during takeoff and probably below 1,000' agl) gave me the
>>shivers.  Why I decided to do a ground test to mimic the fuel flow during
>>takeoff is beyond me.  I've never done this before and it has never been
>>suggested by anyone in the Velo organization.  I came to the ongoing
>>realization that I was not running a "one-man shop" ... that "Someone
>>else" - without my thinking about it - was there to look over my shoulder.
>>Sometimes divine intervention plays an important part in our lives!
>>
>>Lessons learned:
>>a) Fix problems as they occur
>>b) Don't "fix" problems if they don't exist
>>c) Don't re-design the fuel system without giving it a lot of careful
>>    
>>
>study!
>  
>
>>Scott B.
>>
>>
>>
>>    
>>
>  
>