REFLECTOR: FUEL FEED & Vents

Thu Feb 17 00:56:55 CST 2005

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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