REFLECTOR: fuel feed - my solutions

Mon Feb 28 16:51:12 CST 2005

Ahhhh - I don't buy it.  I've seen too many instances of "It looks good on paper" - and have "it" behave differently in the field.  Install the vent per the recommended schematic - run the engine at take off power - block the primary vent ... and then tell me the engine won't run properly because of an alternate vent check-valve with a 0.5 psi cracking pressure.  Tit-for-Tat ... "I would welcome less sweeping, unsupported scientific theories ... unless backed by field studies proving they are correct".  Engineering analysis often involves field trials to support theories on paper.  I don't understand what is so "magical" or critical of that "momentary" moment where the fuel vent system is blocked to the point where the alternate vent with the in-line check valve is caused to open.  0.5 psi is nothing.  The engine fuel pump(s) often pull fuel at less than zero psi head pressure - such as when the aircraft is pointing downhill (in a Velocity) or uphill in a Cherokee.
I'm reminded of my Calculus professor who stated unequivocally, "I can prove to you, on paper, that it is impossible to walk from one side of the room to the other.  To reach any destination, one must walk half-way ... and than half of remaining distance ... and then half of that remaining distance ... and so on.  And since there is always one-half of a remaining distance, it is impossible to reach your destination."  Mathematically, he's right.  It looks good on paper!
My thoughts.
SB

  ----- Original Message ----- 
  From: Jim Sower 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Monday, February 28, 2005 2:16 PM
  Subject: Re: REFLECTOR: fuel feed - my solutions

  Scott Baker wrote:

    Wait a minute ... I don't think the factory has made any such statements, implied or otherwise.  Brett Ferrell has it figured out exactly right.  The alternate fuel vent check valve will open the moment fuel is delivered to the engine via the fuel pump.  Theorize all you want, the alternate fuel vent check valve works.
    SB
  Scott,
  "... open the moment ..." is, IMO a poor choice of words.  That's true only if the fuel vent check valve has zero cracking pressure (which does not occur in nature).  Per my earlier post, the cracking pressure of the check valve (whatever amount it might be) will inhibit flow from the strakes to the sump.  If it is sufficiently small, it will not have a great effect, but if it is sufficiently large, it will stop flow to the sump.  In my previous post, I believe I demonstrated that a 0.5 psi cracking pressure will totally stop flow.

  I would welcome less sweeping, unsupported generalities and more of a point by point rebuttal of my math, my physics or my analytical methods.

  I hope we can abandon rhetoric in favor of rational engineering analysis ... Jim S.

    ----- Original Message ----- 
    From: "Jim Sower" <canarder at frontiernet.net>
    To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
    Sent: Sunday, February 27, 2005 10:16 PM
    Subject: Re: REFLECTOR: fuel feed - my solutions

    > <... 1 atmosphere versus vacuum will open nearly any check valve ...>
    > *Of course* a vacuum will open a check valve.  Nobody's arguing that.  
    > The (false) implication from the factory, etc. is that the head pressure 
    > of the fuel gravity feeding the fuel pump would open it, which it 
    > clearly would *not*.  The fuel pump would clearly *suck* it open, but 
    > that would put an underpressure at the pump inlet. 
    > 
    > I guess it was on another list that we went into so much detail around 
    > vapor lock which is not a real issue here (unless somebody is interested 
    > in a recirculating MPFI engine (auto conversion)).  However, if you have 
    > a vented sump tank, pretty much any check valve alive would prevent 
    > gravity flow to the sump, and you would be left with < 5 gal usable 
    > fuel.  Actually, you'd probably be under 3 gal and clueless as to the 
    > actual amount you had by the time you figured out that the low-sump 
    > indication was for real).
    > 
    > I think it's a recipe for disaster and you won't see it coming ... Jim S.
    > 
    > Brett Ferrell wrote:
    > 
    >> As soon as the pump pulls some fuel out of the sump with a blocked 
    >> vent line, the check valve will open and vent some air into the 
    >> tanks.  1 atmosphere versus vacuum will open nearly any check valve.  
    >> I've done some process safety work and seen industrial tanks collapsed 
    >> from under-pressure, and I think it would be foolish not to install 
    >> the factory-recommended check valve and allow your tanks to routinely 
    >> run under vacuum.
    >>
    >> Brett
    >>
    >> ----- Original Message ----- From: "Jim Sower" <canarder at frontiernet.net>
    >> To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
    >> Sent: Sunday, February 27, 2005 3:53 PM
    >> Subject: Re: REFLECTOR: fuel feed - my solutions
    >>
    >>
    >> My figures indicate that .5 psi is the equivalent of about 165 kias. Bad
    >> news: they aren't going to vent a damned thing if the vent ices over.
    >> Good news: the fuel pump will generate plenty enough suction to overcome
    >> that (of course that underpressure would add to your chances of vapor
    >> forming in the vent line and/or cavitation at the pump face).
    >> Check valves are not the panacea some folks would have you believe ...
    >> Jim S.
    >>
    >> Wayne wrote:
    >>
    >>> I'm using two of the .5 jobs for an alternate air source in the top 
    >>> of the loop before the vent line goes to the bottom of the fuselage. 
    >>> If the fuel vent iced over I hope they will provide cabin air to the 
    >>> fuel tanks. If the bird is upside down they shouldn't allow fuel to 
    >>> leak inside the cabin.
    >>> Wayne
    >>>
    >>
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