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4-place Unusable Fuel

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  • #16

    Hi Whee,
    Here is another ''real world'' results for your consideration.
    But before, Let me describe my set-up:
    O-540 carb engine, with mechanical fuel pump + Facet back-up pump
    Gascolator , Flowscan Transducer.
    Newton fuel Selector (OFF/Both/Left/Right)
    Fuel line: standard routing, with NO up- slope from the tank (all downhill)

    One of my Test Run (during the first 25 hrs) was to run one tank dry. I did that test while circling over my local airfield. The test was conducted Feb.10-2019
    I initially took off with about 20 gal in each tank, with the selector on both. I went away and did some other tests and came back over the airfield. At that point, I had about 8 Gal remaining in each tank.
    I then select the Left tank, and start circling around, at 75% power, with the Facet pump ON.
    I kept an eye on the Fuel Pressure all the time. After what seem way too much time, the FP start varying quite a bit between 6 psi to 0, and then went down to 0 psi for about 5 seconds...at that point the engine start backfiring. I switch the tank to BOTH, and it took 4 seconds to restore normal power. After that, I switch the tank to RIGHT and proceed for landing.
    After landing, I filled the LEFT tank. It took 93.5L (24.7 Gal US) to fill....So my Un-usable fuel is negligible.

    Mike

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    • #17
      Mike, where is the facet pump mounted?
      I'm a Tapatalk user so I can't see your "comment"

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      • #18
        Mike, where is the facet pump mounted?
        I'm a Tapatalk user so I can't see your "comment"

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        • #19
          Whee,
          The Facet pump is downstream from the Gascolator.
          Fuel system starting from the Fuel selector, and going downstream is : Fuel Selector - Gascolator - Facet pump (with an Andair check valve plumbed in parallel) - Flowscan transducer - (F/wall) - engine driven pump - Carb.

          Mike

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          • #20
            Okay, so I purposely went and ran out of gas tonight. Well... one tank anyway. Even knowing that it's coming, it still makes your heart jump! As a side note, in case anyone cares, with the engine shutdown and windmilling the descent rate was 1100 fpm at best glide.

            I circled the airport up high and ran the left tank dry. Switched to the right tank and landed. I went to the pumps and filled up. I chained the wings down so it would stay completely level as I filled it up. I also kept the fuel selector in the off position so there would be no fuel transfer during fueling.

            I topped off the left tank and it took exactly 25 gallons. Assuming the pump is accurate, I have no unusable fuel in the left tank. I also topped off the right tank. That brought the total to 43.2 gallons. My totalizer said I had 7.8 remaining. So, that theoretically means I have 51 gallons in my plane now.... Although after a full top off, my totalizer is usually off by 1.0 gallons. So that checks.

            So for what it's worth... those are my results.

            In a few days when I need gas again, I'll repeat with the right tank.

            IMG_1353.jpeg
            IMG_1354.jpeg
            IMG_1355.jpeg
            Attached Files
            Bobby Stokes
            4-Place Kit Builder
            Litchfield Park, AZ
            http://azbearhawk.com

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            • #21
              Thanks for taking the time to do that! I’m very glad to hear that you have zero unusable.

              I talked to another BHer and he has basically zero unusable fuel in his plane also. That’s 3 solid data points indicating zero unusable in level flight.

              I have an electric boost pump installed after my gascolator so Friday I’ll go up and see if running it will reduce my unusable fuel.

              I'm a Tapatalk user so I can't see your "comment"

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              • #22
                Thanks for taking the time to do that! I’m very glad to hear that you have zero unusable.

                I talked to another BHer and he has basically zero unusable fuel in his plane also. That’s 3 solid data points indicating zero unusable in level flight.

                I have an electric boost pump installed after my gascolator so Friday I’ll go up and see if running it will reduce my unusable fuel.

                I'm a Tapatalk user so I can't see your "comment"

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                • #23
                  Let us know how it goes!!! My set up is with a carb, no fuel pumps at all, just gravity and with a gold cube (FT-90) fuel flow transducer.
                  Bobby Stokes
                  4-Place Kit Builder
                  Litchfield Park, AZ
                  http://azbearhawk.com

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                  • #24
                    Finally got back in the air. With the boost pump on there is zero unusable fuel. I can live with that.
                    I'm a Tapatalk user so I can't see your "comment"

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                    • #25
                      Are you implying that with the boost bump on your fuel supply is unlimited? I would think that at some point the unusable fuel would all be used up, then quiet time..........

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                      • JimParker256
                        JimParker256 commented
                        Editing a comment
                        Not sure if you're kidding or not... Hard to tell in print. But on the off-chance that you are not kidding...

                        As I understand it, "unusable fuel" is the fuel remaining in the fuel tank when the engine experiences fuel starvation in flight attitudes... In other words, your engine cannot use the last few gallons remaining in the tank. I'm pretty sure that's what Whee experienced while flying with the boost pump turned OFF. That would appear to argue that the "usable" fuel was substantially less than the tank's total capacity.

                        But with the fuel pump turned ON, he was able to use virtually ALL of the fuel in the tank, so the "unusable fuel" is actually very close to 'zero' for his plane. (Albeit requiring the fuel pump to be ON to achieve that.)

                        IMHO, this is something we should all check on our personal planes, since every build is slightly different.

                    • #26
                      Originally posted by Bearhwk272 View Post
                      Are you implying that with the boost bump on your fuel supply is unlimited? I would think that at some point the unusable fuel would all be used up, then quiet time..........
                      I’m pretty sure your kidding at the way I worded my post but just to be sure, all 25 gallons in each tank are usable if I run my boost pump at ‘low’ boost. I will be investigating the cause but I’m pretty sure it’s because the Continental engine driven pump has very little capacity to “suck” and little tolerance for air bubbles in the fuel. Once either case happens the pump cavitates and stops pumping. Running the boost pump at low boost (9psi) gives the engine driven pump with a good supply a fuel so it is basically acting as a lift pump.
                      I'm a Tapatalk user so I can't see your "comment"

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                      • #27
                        Sorry to hear of this issue Whee,

                        I checked and our's was a level flight measurement after all. I found 3.5L unusable, which is almost 1gal - same as others are reporting.

                        We discovered this in the safest way possible - by blocking up the tail (I found the photos) into level flight attitude, while we had dead empty tanks (first fill ever). I took exactly 20L of fuel and poured it into one tank with the selector on that side. I caught all the fuel which flowed out of the fuel servo supply hose, and measured the difference. Then multiplied by 2 for the total unusable capacity, roughly, noting a very small portion of the system is a single fuel line instead of a mirror image of the other side.

                        The benefit of this approach was, we knew exactly how much fuel was in the system, as it started totally dry.

                        There cannot be truly *zero* unusable fuel, as the tank pickups are significantly higher than the lowest point of the tank - this is a deliberate move by Bob, as we discussed it years ago.
                        Last edited by Battson; 05-15-2019, 11:55 PM.

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                        • #28
                          Yes, I was a bit tongue in cheek on the wording.
                          I am interested in your findings. I am using Continental
                          injection also. What is type / mfg of your current electric boost pump?
                          I suspect the pump bypass check might be too restrictive.

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