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Fuel Flow Discussion, Moved from Float Mounting

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  • On my installation, the port for the cross vent it at the top and outboard side of each tank.


    • Originally posted by Nev View Post

      More likely when only feeding from one tank when lower on fuel. I would think the 6/4 is more likely to unport if feeding from the L tank and the ball well out to the Left. Problem is that eventually the individual tank selected will contain LESS fuel because it's been burn off, and unporting is even more likely if flying uncoordinated.

      Using the 9/1 as a more extreme scenario to illustrate, with only the R tank selected and the ball out to the right indicating a skid/slip situation, the fuel in the right tank will be at the right hand (outboard) end of the tank away from the ports so unporting is very likely. Fuel from the left tank in this situation is not available because only the right tank is selected. If the BOTH position was selected, the fuel in the left tank would be at the right hand (inboard) end of the left tank covering the ports, and available to feed the engine.

      This effect can be observed inflight by putting the aircraft out of balance. If you look at the sight gauges, one gauge will read very full (it has fuel at the inboard end of the tank), and the other gauge will read very low (fuel is at the outboard end of the tank). Both tanks might actually contain the same amount of fuel. What is happening is that the fuel in both tanks is moving in the same direction as the ball and occupying that end of the respective tanks. Hence the recommendation to normally run with BOTH tanks selected.
      I think a few more distinctions are needed here. I was talking about the likely hood of unporting any port which would probably cause issues when using a fast moving pump, and I think you are talking about the likely hood of having every port unported which is the mode of failure with a gravity fed system.

      That leads to the next distinction, what is the source of imbalance? Your example seemed to consider the source being fuel flowing through the valve from one side to the other due to gravity effects of imbalanced flying, when I was more talking about fuel drawing more from one side than the other due to the venting differences between one wing flying more than other with the vent on top of said wings.

      From the perspective of pumps, you really don't want anything unported, thus having both selected may make the system more likely to unport because there are more ports, and less fuel over those ports.

      I used an extreme 9/1 example in an effort to point out that if you only have 10 gallons in the airplane and a pump pumping fuel, I'd rather it be over two ports that are more likely covered because there is nearly double the fuel over them. Yes, in a slip the wrong way that can lead to fuel starvation, but with 9 gallons in a flat 25 gallon tank, I suspect you would have to be in a very significant slip with the wrong tank selected before it matters.

      At the end of the day there are a lot of things to consider, and as I'm not flying yet I'm going to call it good here on this thread, but I do hope my comments helped organize and explore the numerous theories and possibilities as there are a lot of things to consider.


      • Nev
        Nev commented
        Editing a comment
        Absolutely Schu - your comments all help to explore possibilities - and I think a few of those thoughts are probably close to hitting the nail on the head. The fact that this thread is on it's 12th page shows just how interested we all are in it !

    • Originally posted by schu View Post
      Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.
      The Bearhawk fuel system can supply well over 140 L/hr from one tank to the selector. The IO-540 engine needs less than 90 to run at full power.

      For an engine cruising consuming 60 L/hr or less, how can the other tank draw more than 80 L/hr to starve the engine - all while fighting against gravity, against the suction from the engine, and when it's specifically designed to be under slight positive pressure? Remember this suction condition would have to occur gradually, it can't just happen instantaneously.... so the engine would splutter and give plenty of warning signs, before giving up. So we would be seeing partial stoppages more commonly than full stoppages, if this was a real thing.

      I doubt this is a risk for the normal Bearhawk fuel system.
      Last edited by Battson; 04-25-2022, 08:18 PM.


      • For anyone new to Bearhawk and reading this thread:

        I think it's important to keep sight of the fundamentals:

        Fuel systems should be designed by experts.
        The system designed by the expert is safe, changes to the approved design should be consulted on with the expert designer. We are allowed to experiment, but generally we are not experts, and don't fully understand what we are doing.

        The best fuel system is simple.
        We know that complex fuel systems have killed competent pilots on many occasions. The less you touch your fuel system in flight, the better.

        Having a system which can always draw from both tanks is desirable.
        If there is fuel in the plane, the engine keeps running.


        • schu
          schu commented
          Editing a comment
          Ed.Meyer, did you do this with less than 10 gallons in the airplane?

        • Sir Newton
          Sir Newton commented
          Editing a comment
          Battson, This entire thread has struck a nerve with me. I 100% agree with all your statements.

        • Ed.Meyer
          Ed.Meyer commented
          Editing a comment
          schu I'm sure there was more than 10 gallons but was able to confirm un-porting by selecting the un-ported tank and watch fuel pressure to zero.

      • Battson makes some very good points above.

        Other than during flight testing, it's a very good idea to always fly in balance (ball centered), and keep fuel in both tanks. (One possible exception being a brief side slip to land).

        However, a situation can occur the other way around, where unintentionally flying slightly out of balance over time causes a tank to run dry as a result. It's illustrated here starting with half fuel and the ball out significantly for clarity. In reality the ball only needs to be out by a small amount.

        If I got airborne and flew around for a couple of hours unintentionally slightly out of balance here's how the flight might proceed (I saw this beginning to develop on several flights during my own flight testing) :




        There are a number of reasons why the aircraft could be unintentionally out of balance more to one side than the other. The most common one for me was flying at an airspeed above or below the speed at which the vertical stab and rudder are set for a trimmed condition, and not being used to flying with the ball centered.

        This is just to show the importance of maintaining balanced (coordinated) flight - the same for any aircraft type, not to suggest that any changes need to be made.
        Last edited by Nev; 04-28-2022, 05:53 AM.
        Nev Bailey
        Christchurch, NZ
        YouTube Bearhawk Blog


        • Nev,
          I enjoy your commentary and graphics spot on on the issue!

          Clearly stated, KISS is best.
          systems often become complex when political solutions are applied to fix a technical non problem.

          ​​​​​​​Enjoyed the comprehensive discussion

          Kevin D
          # 272