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Fuel Flow Discussion, Moved from Float Mounting

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  • Ed.Meyer
    commented on 's reply
    Well said. As posted earlier, I have flown an EFI system, with return lines, for several minutes uncoordinated intentionally to un-port one tank while fuel valve was on both. I did this to determine if the fuel delivery to the pump via the other tank would provide enough fuel to avoid "sucking air". Fuel pressure stayed up...

  • schu
    commented on 's reply
    Operation Notice The Bearhawk Fuel System as shown in the Bearhawk Book is designed for use without a fuel pump. If a fuel pump is used, extra care in flying is required so that neither main tanks become unported, as a fuel pump would rather suck air than fuel. If one tank is very low and the other is not very low, set the

  • Battson
    replied
    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.

    Leave a comment:


  • Battson
    replied
    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.

    Leave a comment:


  • schu
    commented on 's reply
    I have a 170 with a cross vent, if the fuel selector is set to isolate the tanks nothing happens because the vent is on top of the tank where there is just air.

  • Nev
    commented on 's reply
    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 !

  • schu
    replied
    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.


    Leave a comment:


  • RatherBFlying
    commented on 's reply
    This was also my impression, after comparing the Newton valve and an Andair valve side by side I decided to use the Andair due to the concerns you've noted.

  • zkelley2
    commented on 's reply
    Generally nothing.

  • Nev
    commented on 's reply
    Haha better memory than me!

  • alaskabearhawk
    replied
    On my installation, the port for the cross vent it at the top and outboard side of each tank. http://www.mykitlog.com/users/displa...=280695&row=19

    Leave a comment:


  • Bcone1381
    commented on 's reply
    "4-8-1-3 green - flight idle - low - beacons on - parallel- starting engine check - clear capped flat on the right - clear capped flat on the left."

  • Nev
    commented on 's reply
    Jared, that's exactly what we think was happening. Eventually after a lot of head scratching, we were able to repeat it consistently on both sides. Specifically we think a bubble was being trapped between the electric pump and the Gold Cube transducer right behind the firewall (we were able to release it by untorquing the AN fittings). Changing the fuel selector to another position cleared it every time and it then flowed from the original position as well. We tested with the opposite tank empty which then probably acted as a vent. When it later contained fuel it probably sufficiently increased the head pressure.
    Last edited by Nev; 04-24-2022, 09:18 PM.

  • jaredyates
    replied
    I have a question about cross venting. If the plane is parked on a slope and the fuel selector is set to isolate the tanks, what is to keep the high side from draining to the low side through the cross vent? A ball valve?

    Leave a comment:


  • jaredyates
    commented on 's reply
    Nev, in this case, imagine if an air bubble was trapped in the left line near the fuel selector. It was trying to rise up the line and there was enough head pressure from the 3 gallons to keep it trapped from rising, but not enough pressure to purge the air out through the firewall end. Selecting both allowed the head pressure from the right tank to overpower the small pressure on the left, thus burping the bubble back up into the left tank. I haven't experienced this type of thing with the fuel system but have with gravity-fed water systems as we collect lots of rainwater here on the farm.
    Last edited by jaredyates; 04-24-2022, 09:04 PM.
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