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Fuel Flow Test question...what pitch settings?

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  • Fuel Flow Test question...what pitch settings?

    There is a good discussion going on in another area of the forum on fuel system design that referenced this service bulletin from Glastar. gssb43revb.pdf The service bulletin talks the fuel quantity test and about the degrees of pitch that their aircraft achieves in various realms of flight in order to perform a fuel flow test.

    When the fuel flow test is performed, the pitch attitude of the fuselage must set at the most extreme setting that will be achieved in normal flight to ensure that sufficient fuel flow is available in that configuration and to define amount of unusable fuel that exists in each tank.

    I do not know nor have I ever seen published what pitch setting that is. What is acceptable for us? What pitch setting results in attaining Vx at 100% power while weighing 1400 pounds? In the descent, what pitch setting would result in a power off glide at Vfe with full flaps?
    Brooks Cone
    Southeast Michigan
    Patrol #303, Kit build

  • #2
    Warning, anecdote: I can say that climbing out of Stehekin in Mike Creek's 250hp 4-place with 3 aboard at 65-70 mph was quite steep.

    It seems like in those steeper pitch attitudes, the head pressure would increase as the column of fuel vertically would actually increase in height (at least for the outlet that counts.)
    Last edited by Zzz; 05-08-2018, 03:15 PM.


    • #3
      Hey I plan to go into Stehekin here for the first time when they open it up!

      But wouldn't the max head pressure, at the fuel valve in this case, be in level flight? Thinking when you pitch up you're rotating the fuel valve about the wings and moving it up in relation to the (rear) tank pickups? If we're talking minimum fuel, which I believe is the scenario to be tested for flow rates.
      Dave B.
      Edmonds, WA
      4 Place Quick Build


      • #4
        When I had my RV-7 AW inspection I had to show the results of my fuel flow test. AC 90.89B has a procedure for determining the fuel flow rate starting on page 33. Here is most of the test.

        "Fuel Flow. A fuel flow and unusable fuel check is a field test to ensure the aircraft engine will get enough fuel to run properly, even if the aircraft is in a steep climb or stall attitude, and is accomplished by:
        (1) Place the aircraft’s nose at an angle 5 degrees above the highest anticipated climb angle. The easiest and safest way to do this with a conventional gear aircraft is to dig a hole and place the aircraft’s tail in it. For a nose gear aircraft, build a ramp to raise the nose gear to the proper angle.
        (2) Make sure the aircraft is tied-down and chocked. With minimum fuel in the tanks, disconnect the fuel line to the carburetor. The fuel flow with a gravity flow system should be 150 percent of the fuel consumption of the engine at full throttle. With a fuel system that is pressurized, the fuel flow should be at least 125 percent. When the fuel stops flowing, the remaining fuel is the “unusable fuel” quantity.
        (3) The formula for fuel flow rate for a gravity-feed fuel system is 0.55 times engine horsepower (HP) times 1.50. This gives a fuel flow rate in pounds of fuel per hour. Divide the pounds-per-hour number by 60 to calculate pounds per minute, and divide again by 6 to calculate gallons per minute. To get gallons per hour for Avgas divide pounds per hour by 6; or multiple gallons per minute by 60. For a pressurized system, substitute 1.25 for 1.50 to calculate the fuel flow rate.
        (4) The fuel consumption rate of most modern engines is about 0.55 pounds per hour per brake HP.


        • #5
          Originally posted by Bcone1381 View Post
          What pitch setting results in attaining Vx at 100% power while weighing 1400 pounds? In the descent, what pitch setting would result in a power off glide at Vfe with full flaps?
          Still looking for a pitch setting. I would think a flight path angle during best rate of climb at a light weight, in conjunction with the angle of attack of the wing would give us a pitch setting we could target for this test. Digging a hole for the tail wheel helps, but lacks a scientific, repeatable, valid and accurate answer to my question.

          Maybe the 5 degrees above the climb angle take the angle of attack into account?
          Last edited by Bcone1381; 08-15-2018, 10:16 AM. Reason: added question at the end of my post minutes after original post, plus corrected spelling
          Brooks Cone
          Southeast Michigan
          Patrol #303, Kit build


          • #6
            After writing Post #5, I recalled reading this report of a flight test evaluation of the Prototype Patrol...

            Author Greg King says the prototype Patrol Angle of Climb at Vx at 1300 lbs. was 19 degrees. So, a test in accordance with AC90.89B would require a 24 degrees pitch setting......time for a reality check, will the Patrol fly with a 24 degree nose up attitude in full power climb?

            Blindly following AC90.89B advice and digging a little hole for the tail wheel will be easiest and a photo might convince the DAR that your test was valid. But if you are a thinking person, and want to know a factual answer for 5 degrees above the highest anticipated climb angel, you will need a deep hole......a 35 inch deep hole will put the level datum at 24 degrees ( Vx of 19 degrees plus AC90.98B recommendation of 5 degrees)

            Seems to me a real world Vx flight test evaluation/demonstration during the Test Flight phase is a reasonable and prudent course of action. Also a prudent builder might impose a Pitch attitude limitation to limit extreme attitudes with small amounts of fuel to ensure no spectacular events with a Patrol get recorded on video during your favorite STOL event.

            Brooks Cone
            Southeast Michigan
            Patrol #303, Kit build


            • #7
              Refer to Russ Erb's "Bearhawk #164 “Three Sigma” Fuel flow Report " . Very detailed report.
              According to Russ, the fuselage need to be raised to a pitch angle of 19.0 degrees.

              For some reason, the link doesn't work properly. Copy and paste this link to your browser:
              Last edited by Aero_tango; 08-15-2018, 11:32 AM.


              • Bcone1381
                Bcone1381 commented
                Editing a comment
                Thanks for the Link to Russ' writeup. Russ' 19* pitch indeed is based on a very detailed report....good research there. That test is very applicable to the A model Four place Bearhawk and was based on a 1500ft/min rate of climb, at 60 kts. But I intended this thread to provide data the applies to the Patrol.

                Greg King report was done on the Patrol recorded Vx at 1300 lbs. of 2000 ft/min rate of climb at 52.4 kts. The Patrol uses the Riblett airfoil or course.

            • #8
              I don't classify digging holes as recreation, so I used two cinder blocks under each wheel. If that isn't enough, one could remove the tailwheel and get a few more degrees. Once you get it all figured out, please consider doing a Beartracks article or a write-up for the KBM. There is an entry for the test on the map, but it does not yet have an instructional post.
              Last edited by jaredyates; 08-15-2018, 11:49 AM.


              • #9

                I didn’t dig a hole or lift the front wheel. I wasn’t really confortable in raising the plane to such an unstable condition, like describes in Russ’ report.

                Here is what I did:
                According to Russ’ fuel flow test, the max pitch angle of 19 deg equal to a pressure head (height of the fuel above the carburetor) of 10.7 in.
                Keeping the plane on the ground, I hooked up a temporarily aluminum fuel line to the fuel line disconnected from the carburetor. The aluminum fuel line was bent upward, so that the fuel travel up, to a gas container sitting on a ladder. The outlet of the aluminum line was about 6 in below the fuel level in the (airplane) gas tank. As such, I was simulating a pressure head of 6 in, which is a much more severe condition than Russ’ 10.7 in.
                (I have an engine-driven pump and an electrical backup pump, so no issue in meeting the 125% requirement with 6 in pressure head, but someone can accurately simulate the 10.7 in, if you have marginal results)
                A photo is worth a thousand words...


                • Collin Campbell
                  Collin Campbell commented
                  Editing a comment
                  I am not into digging a hole or jacking up the aircraft to an unstable condition either. I was fortunate enough to have a steep bank at the end of my airstrip to do the fuel test. My concerns with doing it with the gas container method you describe is that it does not address one of the main concerns of doing the test, that being to test the system itself. There are many things that can affect/restrict fuel flow...fuel valve, fittings, tube size, fuel flow sender, fuel line routing/design, etc. Many builders have found that by removing/replacing certain components (fuel flow transducer/ fuel valve, etc) of the system flow was improved. Just my thoughts...Collin

                • Aero_tango
                  Aero_tango commented
                  Editing a comment
                  ""My concerns with doing it with the gas container method you describe is that it does not address one of the main concerns of doing the test, that being to test the system itself""

                  Yes it does. The fuel traveled thru all my fuel system + the additional temporarily fuel line, which was hooked to the firesleved hose at the carburetor....

              • #10
                Here is what I did with my 4-place Bearhawk:


                Eric Newton
                Eric Newton - Long Beach, MS
                Bearhawk Tailwheels and Builder's Manuals


                • #11
                  Welcome back Eric!
                  ​Christopher Owens, EAA #808438
                  Project "Expedition"
                  Bearhawk 4-Place Scratch Built, Plans #991
                  Bearhawk Patrol Scratch Built, Plans #P313
                  Germantown, Wisconsin, USA