REFLECTOR: FUEL FEED - One New Idea

Mon Feb 21 13:56:45 CST 2005

Jim,

While I certainly agree that a 1/2" line would be less subject to anything that might cause resistance to flow, a 3/8" line is large enough not to be particularly affected by surface tension.  Besides, the phenomena I am talking about is mass flow, not just bubbles working their way up against the prevailing flow.  A "slug" of air headed to one strake is balanced by a "slug" of fuel coming in from the opposite strake.

Upsizing fuel lines can be taken to absurdity - if a 1/2" is better than a 3/8", why not a 1" line?  As has been proven experimentally, a single 3/8" line from one strake will happily provide fuel (by gravity alone) faster that a 300 HP engine can drink it.

Keith

  ----- Original Message ----- 
  From: Jim Sower 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Monday, February 21, 2005 11:11 AM
  Subject: Re: REFLECTOR: FUEL FEED - One New Idea

  Keith,
  KeithHallsten wrote:

    Al,

    I have a concern with your concept of "picking up another 8" - 10" of pressure head - in the event that the level in the sump is for dropping for some reason".  If the sump is vented, the pressure head in the sump must be measured from its free surface.  This free surface may be up in the vent line.  However, if the level in the sump is dropping, the free surface has been drawn down.  The only way such a condition could exist is if there were some resistance to flow between the strake and the sump.
  Agreed!

    The bottom line is that the location of the entries into the sump will not affect the head.  The location will affect whether bubbles can escape up the fuel supply lines, however.  A little wing rock can easily result in air travelling up one supply line to one strake while fuel passes from the other strake to the sump through the opposite supply line.
  Provided surface tension allowed bubble(s) to [easily] pass up the 3/8" line.  I think (key word) a 1/2" line from strake to sump would be a slam dunk in that regard and make virtually all assymmetric transfer problems cease to exist or go away very simply.
  Jim S.

    Keith

    ----- Original Message ----- 
      From: Al Gietzen 
      To: 'Velocity Aircraft Owners and Builders list' 
      Sent: Monday, February 21, 2005 8:37 AM
      Subject: RE: REFLECTOR: FUEL FEED - One New Idea

      Jim wrote:

      I wold bet that the highest point in the strake during climb out would be within 6" - 8" from the most forward point in the fuel tank.  When you cock the airplane up to climb attitude, you will have a very considerable elevation difference between the "bubble" in the forward portion of the strake and the vent outlet at the spar.  The vent air has to overcome that head pressure to replace fuel being consumed with air.  

      The 6" - 8" may be a reasonable bet for the full tank, steep climb; but what ever it is; keep in mind that when the tank is full you have a 12" head to the sump tank at level, and with the nose tilted up 10 degrees, that goes up another 8" - 9", so in that condition you have about 20" of head.  Again, more than needed to overcome the 6-8" of back up in the vent line.  No pump suction needed.

      So I will postulate once again; unless the vents are plugged, or the fuel caps are leaking, adequate fuel will flow.

      NEW IDEA

      In all this ruminating; it occurred to me that there is an improvement can easily be made.  The maximum driving head available for flow from the main to the sump is the difference in elevation between the surface of the fuel in the tank, and the exit of the feed line from the tank to the sump.  Normally the fuel level in the sump is above that line exit, and the effective head is the difference between the two levels. 

      But in the event of some anti-gravity force in the main tank (cap leak), and the sump level drops, we'd like to have more head available to drive the flow.  This can be achieved by lowering the point at which the feed line enters the sump.  There is no advantage, that I can see, for the feed line to enter at the top of the sump, as long as the sump is vented so you never need air to flow back up the line to the main.  By lowering the point that the feed line enters the sump you can pick up another 8" - 10" of pressure head - in the event that the level in the sump is for dropping for some reason.  

      This additional driving head may be sufficient to overcome the negative pressure above the strake, and prevent fuel exhaustion in the event of a fuel cap leak that is larger than the vent lines can handle.

      Tell me if this is all wrong.

      Al

      <>About the sump tank vent - 1.) the tank must be vented to allow it to fill,
      2.) because of the low vapor pressure of fuel (particularly auto fuel) you do not want to depend on "sucking" fuel anywhere because of the possibility of forming vapor at the pump (vapor lock); 3.) if you have fuel injection with recycle flow back to the sump, you want a sump vent for air or vapor bubbles from the return line.

        I respectfully disagree.  There is about 6" head pressure from a half full strake to sump at cruise AoA..  That is about 0.16 psi.  Damned small pressure.  A damned small aberration (like a damned small leak in a fuel cap) will make it go away.  Gravity feed can be stopped entirely by a problem (cap leak again?) that you can't even measure!  That's mainly what drove me nuts for so long.

        I agree that it is a small difference, but it takes very little pressure head to drive 20 gph through a 3/8" tube.  But I respectfully disagree with disagreement (are we carrying diplomacy too far J) that it only takes a damned small leak because a ¼" (or 3/8") vent line provides enough air flow to offset what I would consider a fairly large leak; one that could be easily measured; maybe difficult to overlook.  I can't think of many aberrations that stop gravity in the tank beyond a serious cap leak.  But then, small is relative.

      I was in enthusiastic with all of this until it happened to me.  My vents are 3/8" (min) all the way to within 3" of the tank where it necks down to 1/4" (I didn't think it was worth chopping a hole from the cabin to the tank to install 4" of 3/8" line).  I taped up my fuel caps to preclude the possibility of even a molecular sized lea,.  Still I got assymmetric flow.  Perhaps I should have said "damned small aberration" or something.

      Further, it seems to me; if you vent the sump for it to fill, and then close the vent, you eliminate gravity flow from the mains because you have fixed the level in the sump (somewhere up the vent line) at whatever the level was in the mains at that time, and you now are depending on the slight negative pressure created by the fuel pump to draw the fuel from the tank. Not the best situation.

      Beg to differ.  Assuming you mount your boost pump as low in the bilges as you can, you are much better off.  You've made the strake(s) and the sump a single system.  Head pressure to the pump inlet is the distance from the pump inlet to the surface of the fuel in the strake.  That's (depending on aircraft attitude and how full the strakes are) maybe a foot (and possible 18") higher than if the sump were vented.  Sump being maximum a foot from surface to pump inlet, you've at least doubled the head pressure at the pump face by capping off the sump vent.

      You are correct, but perhaps I am also correct because we are looking at two different aspects.  The head at the bottom of the sump (barring any other difficulties) is the same in either case.  The level in the vented sump is the same as in the strake; the fuel will go up the sump vent line until it reaches the level in the strake. You have to include that as "head" as well.

      ONLY if it is flowing as advertised (which mind did not, and it is the exceptions that we are discussing).  Consider:  If the gravity feed from one strake is "underperforming", who is to say the other is not underperforming, but to a lesser extent?  In any event, if you have an open system, you are projecting that the two components (strake and sump) will work in concert.  If they do not, you are limited to sump head pressure.  If you cap off the sump vent, they will work in concert whether the strakes want to or not.

       I was considering only the driving force from the tank to the entrance of the sump.  If the fuel level in the sump is the same or higher than in the tank (and it will be including up the sump vent line, and the vent line is closed), what is the driving force from the tank to the sump?  But, OK; taken by itself it is probably irrelevant. 

      Far from it.  The fuel level in the sump is NEVER as high as the strake.  If they are a single system (sump vent closed) they MUST act in concert whether the strakes want to or not, and head pressure to the pump is total head pressure.  If the sump vent is open, they CAN act in concert, but if the strakes (including the plumbing to the sump) get balky, they need not, and with an open sump vent the pump CANNOT overcome the balkiness of the strakes, and you can end up with nearly full strakes and only the sump available to the engine.

      FURTHERMORE, try a little experiment for me.  Can we assume that the airplane deck angle (angle between fuselage reference line and the deck - climb angle + AoA - is 10*?  If not, figure out a deck angle of your own for say 2000 fpm climb at say 90-100 kts and use it.  Let's further assume that the tanks are full.

      NOW:  A)  Cap off your sump vent;  B)  Fill your strakes with fuel;  C)  Jack the nose to a 10* deck angle;  D)  Open your fuel line at the engine driven pump and let it drain into a bucket.  E)  Observe (keeping time) what happens.  I think your strakes will drain, but slowly.

      NEXT: A)  Open your sump vent;  Repeat steps B thru E above and measure what happens.
       I will hazard a guess that since your vent line is flooded, and there's maybe 6" - 8" of fuel in the vent line, that only your sump will vent.  Absent suction from your fuel pump, your strakes cannot drain.  

      Like I said above; the 6 or 8" of fuel in the main tank vent line only represents a ½" or so of head (the line is basically horizontal, no?) and is easily drawn out for the tank to vent and the fuel to flow.  The only way it may not is if the main vent is plugged. In the case you present here, unless I misunderstand, it makes no difference whether the sump is vented or not.

      I can think of no explanation for the problems the Jim S. experienced because he tried every fix, but there is something wrong somewhere.  Possibly different flow characteristics from the two lines; and/or less than ambient pressure at the vent exit.

      Neither can I.  I fancy myself a pretty good engineer.  I have looked at this problem from every angle I can think of and tried every fix (except switching from 3/8" to 1/2" strake-to-sump lines - which I suspect may have solved the problem).  

 <>Like I said; I can think of no explanation for the problems the Jim S. experienced.  

      Which brings us back to the bumblebee ...

      <>But if I can get 20 gph feed from one tank through a 3/8" line how does it help to go the ½" line for 4-6 gph; unless there is the real problem that you haven't found yet.  

      Damned if I know.  With the numbers as small as they are, I reasoned that invisible line losses could upset the delicate balance between the tanks.  Half inch lines would be the mallet with which I obliterate those losses.  Like I said, I didn't so it because I had nothing scientific to support it, and it required chopping holes from the cabin into the wing tank.  It would result in greatly reduced line losses, but would only solve the transfer issue if line losses were the problem.

      <>I have to think that the problem is specific to your case, not to the system design.  But I appreciate that you have been very thorough about this issue, so I'm not able to be of any help.  You have a solution that works; go with it

      I tend to agree.  But that is small comfort :o)  I would hasten to add that the PROBLEM is not at all specific to my case, only its SEVERITY and tenacity.  Getting back to the basic issue, the problem is too common to be acceptable and if the easy fixes don't solve it, we are obliged to question the design (face it, LOTS of stuff works in spite of itself).  
      I had a flameout in Canada and had to dead stick back to Port Huron because of assymmetric transfer.  I had noticed it early on and was monitoring it and trying to get some fuel from my left strake by kicking rudders, etc.  The right tank went dry and sometime thereafter the sump light came on.  That interval was long enough that I thought is was because the left tank was doing better (how much better was impossible to tell).  Left tank fuel seemed to be going down, and I was kind of reluctant to land unannounced in Canada so soon after 9/11.  The sump fuel got me further than I could have expected out of five gallons, but when it quit, I was ready for it.
      My concern is for the guy who gets a "blinking" sump light and has learned to ignore it as a false positive.  Some day when it casually goes from blinking to steady,  he may very well not give it the attention it deserves.
      I have a workaround (not to be confused with a fix) that [very nearly] moots this whole issue. 
      Jim S.

      <>  

Al    

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