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  • Useable Fuel Capacity

    What is the useable capacity of the 55 gallon fuel tanks?

  • #2
    On my 4-place B model, I measured only about 1 liter/quart each side to be unusable. However it's probably prudent to treat it as about 10 liters (2,5 gallons) each side particularly if you're in the habit of selecting an individual tank at lower fuel levels, and to account for an out-of-balance flight condition.
    Nev Bailey
    Christchurch, NZ

    BearhawkBlog.com - Safety & Maintenance Notes
    YouTube - Build and flying channel
    Builders Log - We build planes

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    • #3
      My tanks are home built, but built according to the original 4-place plans. On the B model they may be larger. I determined the unusable fuel to be 2 gallons for the main tanks and 1 gal for the aux. Before I filled them I put 4 gallons in each main and 3 gallons in the aux. I leveled the plane and pumped each aux tank into the main until the pumps lost suction. Could clearly tell by the sound of the pumps when that happened. Then I disconnected the fuel line at the servo and used the boost pump to drain each main tank into containers until the pump lost suction. Then I put the plane back in three point, drained each tank through the rear drain plugs and weighed the result. It was about 1.6, 1.7 gallons for the main, and .8, .9 for the aux tanks. I just round it to two and one gallons as unusable fuel.

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      • #4
        Thanks guys. Sounds like estimating 2.0 to 2.5 gallons/main tank as unusable fuel will be safe assumption.

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        • #5
          I think the safest plan is to test and find out?

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          • #6
            Yes, understood. I was looking for expected range of unusable fuel as benchmark to check against my own results.

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            • #7
              My actual unusable fuel is extremely low, but I would never want to be flying when I got close. I did my fuel flow testing with <2 gallons per side.

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              • #8
                On the ferry flight to my location, the pilot noted rather large amounts of unusable fuel (maybe 3-4 gallons) in each tank of my B-model 4pl when in level flight. I have a spare main fuel tank from another BH project, and wondered about modifying that tank by cutting the bottom aluminum, and rewelding it 1/2" to 1" higher on the outboard edge, to provide a larger "dihedral" to the tank.

                Any comments from BHawkers??

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                • #9
                  Did he run a tank dry to determine this? I cant see how else he would have known what the unuseable quantity is while in flight. I tested mine on the ground the first time I filled the tanks. I cant work out how to determine this without filling a completely dry system to a known quantity and then draining to compare what comes back out.

                  For a data point, my A model has virtually no measurable amount of unuseable tested in level flight attitude. Something less than 0.5lbs. Of course that's sitting perfectly level and static.
                  4-Place QB kit #111. First flight May 2022.
                  IO-470 - 260hp

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                  • #10
                    This comes up for discussion periodically. My thoughts are that the BH's tend to be more sensitive in yaw than other aircraft, and therefore it's very easy for a new-to-type pilot to fly out of balance for long periods of time. This can result in an engine stoppage with fuel in the opposite end of the tank, and may be exasperated if the fuel selector is not in the BOTH position.

                    I did my own fuel testing HERE running one tank completely dry, and restarted multiple times by changing the fuel selector to the tank containing fuel - when in balance. The unusual fuel when in balance as Tim mentioned above is "bugger all". When out of balance, it can be significant.

                    There is a discussion HERE of how the fuel imbalance can develop and lead to an engine stoppage.

                    During my initial test flying, I always flew with 100 liters fuel or more at all times, and kept a fuel log of how much fuel was required to fill each tank. It was usually different amounts because I was flying out of balance a lot. After a while, when I was able to keep it in balance most of the time, this was reflected in the fuel tanks requiring similar amounts to refill. Now I'm quite happy to run down to 30 liters (15 each side) remaining.
                    Last edited by Nev; 09-17-2025, 04:23 PM. Reason: to align with Battsons good minimum fuel quantity advice below
                    Nev Bailey
                    Christchurch, NZ

                    BearhawkBlog.com - Safety & Maintenance Notes
                    YouTube - Build and flying channel
                    Builders Log - We build planes

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                    • #11
                      Originally posted by Proi View Post
                      On the ferry flight to my location, the pilot noted rather large amounts of unusable fuel (maybe 3-4 gallons) in each tank of my B-model 4pl when in level flight. I have a spare main fuel tank from another BH project, and wondered about modifying that tank by cutting the bottom aluminum, and rewelding it 1/2" to 1" higher on the outboard edge, to provide a larger "dihedral" to the tank.

                      Any comments from BHawkers??
                      There really is no need to modify the fuel tanks.
                      Like others have, I also tested mine in level flight attitude. I kept on getting a steady stream of fuel down to just 1.75 L remaining on each side, in my machine.
                      Assuming you were flying perfectly in balance, you can use almost all the fuel. You will see others' results are similar.

                      In real life, it's much too risky to run the tanks that low.

                      Once I am below 15 L per side (roughly 4 US Gal), I am starting to get quite worried about fuel level. I only do that in the pattern for STOL training.
                      When I get to that level of fuel remaining, I focus on staying in balance and have a plan in mind for an unplanned engine power loss. Strictly no "dragging it in" under engine power, low over the fence.

                      Considering all the different reasons which have been mentioned, I don't think you need to modify the tanks.
                      Last edited by Battson; 09-17-2025, 04:20 PM.

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                      • #12
                        Anytime I am flying an airplane that I am not very familiar with, I go very conservative on the fuel planning. I had a rental 172 teach me a lesson I will never forget. I was fat, dumb, and happy thinking I had an hour of fuel left. The fuel pump receipt showed I had less than a gallon. To make matters worse, I was in mountainous terrain, and landed right at sunset. I was 1986, and I was 17 at the time. It scared the hell out of me.

                        So, when my 5 is done, I will consider at least 5 gallons per side as my 'zero fuel' for planning purposes, and add my reserve on top of that. I will gradually step that down over time, but only when I am comfortable.

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                        • #13
                          On the ground in level flight you will find a very small amount of unusable fuel left in the main tanks. If you drain through the gascolator you will find about 6-8 oz per side in level flight. Theoretically this is flight attitude with perfect coordination. The tanks are large and any amount of yaw can send some fuel outboard but it will be a very small amount. The aux tanks hold 9.5 gallons, in practice We find 4-8 OZ will come out if sumped on the ground after use. We plan for getting 9 Gallons out of the aux tanks when transferring.

                          In short on a four or five with 74 gallons there is roughly 72-73 usable, if the fuel system was assembled properly. Each system should be tested as everyone builds their fuel system slightly different when bending fuel lines.

                          In an IO580 Five the fuel burn can be 10-20 gallons/hour depending on power settings. 32 on takeoff. Best practice is to land with min fuel of 5 gallons per side. This is a healthy 30 minute reserve as legally required and you will not experience any flow issues at these fuel levels.

                          In testing minimum fuel in the IO580 Five we have held sustained climbs of 25 degrees with less than 5 gallons per side and no boost pump engaged no adverse effects have been noted.

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                          • #14
                            Originally posted by Proi View Post
                            On the ferry flight to my location, the pilot noted rather large amounts of unusable fuel (maybe 3-4 gallons) in each tank of my B-model 4pl when in level flight. I have a spare main fuel tank from another BH project, and wondered about modifying that tank by cutting the bottom aluminum, and rewelding it 1/2" to 1" higher on the outboard edge, to provide a larger "dihedral" to the tank.

                            Any comments from BHawkers??
                            Do you have any datalog information from the flight? If so, you could upload it to an analysis platform and see how the flight looks. If coordination/inclinometer values are recorded, that could narrow things down.

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                            • #15
                              I have a 2 liter header tank under the right front seat. With a few wing wags, I think my unusable fuel is pretty close to zero. When I say wing wag, I did it on the ground during fuel flow testing by moving the wing tip up and down with by hand. This also works without a header tank, but the 2 liter tank makes it easier. I don't ever want to try it for real, but I think the BH has extremely low unuseable fuel.

                              Carburated with no pump mine would be less than a gallon. With any kind of pump, it will scavenge even almost all of the 2 liter tank.

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