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Fuel Flow Discussion, Moved from Float Mounting

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  • Originally posted by MattS View Post
    On Murphy Rebels, they used to have the interconnecting vent connected through the sight gauges. After some problems, they issued a bulletin to move it to a separate port.
    It would be good to know what the issue was.

    Comment


    • MattS
      MattS commented
      Editing a comment
      Below is the bulletin. There is a previous one that talks about the tanks needing to be interconnected. I can't find that one.

    • Battson
      Battson commented
      Editing a comment
      That is interesting, thanks for sharing!
      Again, they have been unable to come up with hard test results to prove what was happening, and were forced to assume it was the sight gauge causing the issue. I have not thought about the problem hard enough to understand how interconnected sight gauges could cause the problem they were having - air bubbles in the main fuel lines. Seems very counter-intuitive, and I am getting a sore head already!

    • Nev
      Nev commented
      Editing a comment
      This Safety Bulletin shows surprisingly similar symptoms to the Cessna AD from 1969, although they prescribe different fixes.
      Murphy term it an “air locked” fuel line and suspect air somehow entering the fuel lines. Cessna suspected vapor lock from fuel essentially boiling in the lines at bends under specific atmospheric conditions.
      Last edited by Nev; 02-10-2020, 06:35 PM.

  • That's not the same problem as we've been talking about, either. They think their problem was from having the fuel feed at the bottom of the sight gauges, and their solution is to move the fuel feed to its own fitting. Bearhawk tanks are already that way.
    Last edited by JJohnston; 02-10-2020, 04:22 PM.

    Comment


    • Battson
      Battson commented
      Editing a comment
      In which case, there is no known problem from Murphy Aircraft with using fuel sight gauges to connect the cross vent system.

  • Spoke with my mechanic friend again about the vent. Tentative plan is to weld bungs on the top/fwd/outside of each fuel tank. Put in a barbed fitting and connect the two tanks with clear tubing and watch how that goes. There is plenty of room for the fitting. The only potential negative is the tubing would have to go down a bit to fit through the front spar lightening hole. After that it's a straight shot to the other side. If it doesn't work just plug both tank ports and call it good.

    Comment


    • svyolo
      svyolo commented
      Editing a comment
      There is only a degree of dihedral. I doubt it would be possible to have the line run slightly uphill to the center of the fuselage. There will probably be a slug of fuel most of the time in the middle, but I think it still works. I wonder what Cessna's implementation looks like.

  • If this is to be a permanent install I would use aluminum tube over most of the install. If you want to observe what is happening, put the clear tube in an area where you can observe the activity but where the tube will be easily replaced. The clear tubes I have found become hard, shrink and become discolored over time.

    Comment


    • alaskabearhawk
      alaskabearhawk commented
      Editing a comment
      Yep, I agree. The front rib lightening holes are relatively easy to access so that shouldn’t be a problem to retrofit later.

  • My latest offering: https://www.youtube.com/watch?v=yrpklAFL3ZI

    Comment


    • Really nice video. I already had a couple of bungs welded to the inboard side of the tanks to use as fuel return ports. I added a header tank and want to use them for vents for the header tank, and to cross vent the tanks.. After looking at a bunch of fuel system designs I noticed Cessna had a preference for venting the tanks from the outboard side, vs the inboard. With only 1 degree of dihedral, this is only a very slight difference in height on the outboard. Obviously, higher is better, but in the case it isn't much.

      Is there any other advantage to using the outboard location for the vents? I wonder if I could run a vent line internally in the tank, to the outboard edge of the tank, to the high point.

      Comment


      • Cessna has a pretty elaborate venting system with check valves in the tanks, Vents going from outboard tank to opposite tank outboard side, etc, etc. The one issue I can see with interconnecting the tanks inboard is if you parked on a slope with full tanks. The tanks would be in essence one tank. With full tanks the downslope tank has a hole in it, namely the fuel cap vent. Mean ol' Mr. Gravity would have his way and the fuel would flow out of the downslope cap. The only way to mitigate that with the tanks interconnected like that is to install an inline valve in the interconnecting vent line and turn it off when parked. However, one more thing to remember to turn on and a potential point of failure.

        Comment


        • svyolo
          svyolo commented
          Editing a comment
          Yeah, more complex, but sounds like Cessna's is as well. They have been doing this for a long time and have discovered all kinds of Gremlins over the years.
          The other option might be to try to figure out how to put a vent pipe internally in the tank to the upper/outboard end, from the inboard side. Lots of tanks of have internal piping for many different purposes, including venting. "Standpipes" are a pretty common feature.

      • Originally posted by alaskabearhawk View Post
        Cessna has a pretty elaborate venting system with check valves in the tanks, Vents going from outboard tank to opposite tank outboard side, etc, etc. The one issue I can see with interconnecting the tanks inboard is if you parked on a slope with full tanks. The tanks would be in essence one tank. With full tanks the downslope tank has a hole in it, namely the fuel cap vent. Mean ol' Mr. Gravity would have his way and the fuel would flow out of the downslope cap. The only way to mitigate that with the tanks interconnected like that is to install an inline valve in the interconnecting vent line and turn it off when parked. However, one more thing to remember to turn on and a potential point of failure.
        It's a risk today - if we leave our fuel selector on "both" parked on a slope, then we still lose most of the fuel. Just part of the design.
        I think parking on a slope is a strange enough event, that I always think "I better close that valve", and I have never lost fuel yet. I do lose fuel to freshly filled tanks on hot days though.

        I can imagine, were I under a lot of stress or very distracted, I could forget to select the fuel to "off". But for normal ops, it has proven to be an insignificant risk for me personally.

        Comment


        • Today I put 10 gallons in each tank for a total of 20.
          at the pump inlet on Both, I had 23 GPH. Switch to left tank only, 23 GPH. Right tank only, 23GPH. I then went back to Both, the remaining 5 gallons was 15 GPH. Not sure why, head pressure I would guess. Previously I had only checked right and left, not both or both with near empty fuel.

          Comment


          • Mark Goldberg
            Mark Goldberg commented
            Editing a comment
            Normally more fuel flow is seen with a BOTH setting. That has been my experience in all my planes. Not sure why you would not see that also Elden. Mark

          • yateselden
            yateselden commented
            Editing a comment
            I was also expecting more flow on Both. Possibly something after the fuel valve restricts more flow on Both. One Pump on, I have 43 gph. I ran the motor, all four injectors working and WOT, tied to my truck (brakes won't hold it) with no issues other than some fine tuning. It really barks now.

          • Archer39J
            Archer39J commented
            Editing a comment
            Are you using a gascolator? Looking at how restrictive they are I've been wondering if that's why folks aren't seeing a difference between single tank and BOTH fuel flow rates.

        • Please clarify......I think you were doing a fuel flow test. And I think your GPH figures is a calculation of volume of fuel flowing into a bucket divided by time.

          When you got the 15 GPH result, is it possible that you had uneven fuel levels in the two tanks?

          I wonder........
          1) When the tank levels are not even the fullest tank has the highest fuel pressure at the TEE of the fuel selector.
          2) When the two tanks are interconnected when you select BOTH, then the differential pressure between the two tanks will want to equalize and fuel will cross feed through the fuel valve.
          3) It seems to me that any flow across the TEE will reduce flow to the engine.
          Brooks Cone
          Southeast Michigan
          Patrol #303, Kit build

          Comment


          • yateselden
            yateselden commented
            Editing a comment
            The 15 gph was the last 5 gal. that emptied the tanks. I have no intentions of letting the tanks get that low. It was just interesting that the flow was less.

        • Yes, I think it's the gaskelater. I'll be looking into the one Mark sells and checking port size

          Comment


          • zkelley2
            zkelley2 commented
            Editing a comment
            The one that Mark sells has 1/4 NPT ports on it. 1/4 NPT is what about every fuel selector I've seen is to.
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