REFLECTOR: FUEL FEED & Vents

Scott Derrick scott at tnstaafl.net
Thu Feb 17 08:16:52 CST 2005


A better question is who is running an engine with out a fuel pump??

Scott

KeithHallsten wrote:

>The real question is, "Will that check valve open with a small enough
>resistance to flow that the engine will be able to get fuel?"
>
>I suspect that it will, as long as the fuel system is arranged so that the
>fuel pump suction is applied through the strake fuel tank(s) and on to the
>check valve through the vent system.  However, if only the gravity head of
>the fuel tank is applied, I think that the odds of getting any flow into the
>check valve (and therefore fuel out of the tank) is small indeed.  Has
>anyone actually tested what pressure difference across the check valve is
>necessary to open it?
>
>
>----- Original Message ----- 
>From: "Jim Sower" <canarder at frontiernet.net>
>To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
>Sent: Wednesday, February 16, 2005 10:17 PM
>Subject: Re: REFLECTOR: FUEL FEED & Vents
>
>
>  
>
>>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.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>        
>>>>
>>>      
>>>
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