REFLECTOR: Pressurized Tanks

Wed Nov 3 22:46:21 CST 2004

Larry,

The "pressurization" we are talking about amounts to a few inches of water and has no effect whatsoever on the fuel/air mixture in the tank.

What you are talking about is venting.  With low-volitility fuels like diesel or kerosene, it's safer to have flow-through ventilation to keep the mixture in the gas volume above the fuel pool too lean to support combustion.  The situation is opposite for high-volitility fuels like gasoline, where the idea is to keep the mixture in the tank so rich that it can't burn.  Therefore, the vent systems are designed differently depending on what type of fuel is to be used in the tank.  

The standard Velocity fuel system vent is fine for gasoline, with only a single point open to the outside.  If you plan to use a diesel, you should really consider adding more vents for a "flow-through" venting system.  You want at least one vent per tank to be in a relatively high pressure area, and another to be in a relatively low pressure area.  This can be done by facing one vent forward and the other aft, or by selecting locations where the wing creates a pressure difference. 

Keith Hallsten

  ----- Original Message ----- 
  From: Lawrence Epstein,MD 
  To: 'Velocity Aircraft Owners and Builders list' 
  Sent: Thursday, November 04, 2004 4:45 AM
  Subject: RE: REFLECTOR: Pressurized Tanks

  Does "pressurizing" the tank change the environment of the tank so as to make it a more or less (or no change) explosive environment?  

  I do know that there is a difference between av-gas & Jet-A/Diesel.  I believe that with AV-gas the intent is to maintain a high fuel-vapor content in the tank.  My hope has been to use a Diesel and with diesel tanks you need to pressurize the tanks as described in order to keep the vapor content low.

  Larry Epstein

  -----Original Message-----
  From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Lynn Gallup
  Sent: Wednesday, November 03, 2004 6:34 PM
  To: Velocity Aircraft Owners and Builders list
  Subject: Re: REFLECTOR: Pressurized Tanks

  John,

  I was exaggerating with the word "pressure", I'm sure it is positive but it must be so slight as to be almost unmeasurable. The key thing is that with the change the tanks are for sure not under any suction from the vent line which was why the engine wouldn't draw from them evenly.

  Lynn

  /a negative pressuresitvit must be al.  Technicallyx 

    ----- Original Message ----- 

    From: John Dibble 

    To: Velocity Aircraft Owners and Builders list 

    Sent: Saturday, October 30, 2004 9:32 AM

    Subject: REFLECTOR: Pressurized Tanks

    Not sure I understand your setup, but I like the idea of pressurized tanks.  I have tank overflow lines that face forward and I could pressurize my tanks by simply blocking the vent at the manifold (tanks and sump would still be vented to each other).  Is there any concern of fatiguing the tanks with repeated pressure cycling? 

    John 

    Lynn Gallup wrote: 

      I had very uneven fuel flow, so uneven, in fact, that one tank or the other would drain about 3/8 down before the other tank would even start to flow. I traced this to the mini-naca "scoop" on the bottom of airplane acting like a venturi and putting suction on the tanks through the vent line. I laid up a tiny traditionally shaped airscoop, sticking up just 1/4 inch above the fusilage surface, and glued it on over the naca scoop. With my now "pressurized" tanks I have not had even the slightest indication of uneven flow ever since.Lynn 

      ----- Original Message -----

      From: PVTPIL8 at aol.com

      To: reflector at tvbf.org

      Sent: Thursday, October 28, 2004 11:43 AM

      Subject: REFLECTOR: Uneven fuel draw

       I am having a very uneven fuel draw in my XL.  My right tank draws substantially faster than my left.  I understand others have had this problem.  I can transfer fuel with pumps.  I would like to know if anyone has been able to do something to get the fuel to draw more evenly? 
      Steve 
      XL-RG 
      N787SB 

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