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Mike Goodrich Visit to FD92

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  • #16
    Attached to a bulldozer the rotary drag looks 65N for a load of 2050 N at 65 mph
    How can he slide with a coefficient of friction of 0.19?
    It would take at least 390 N is not it?


    • #17
      Originally posted by Doug Riley View Post
      The slider on my Dominator tandem showed a definite wear pattern, indicating that "sliding" was, in fact, occurring. The tandem Dominator has a 3-tube, bolted pylon structure that is rather rigid in the fore-aft direction.

      It's more limber side-to-side and vibratory movement of the rotor head was very obvious any time I looked up (or saw the shadow of the head on my dashboard).

      Allowing aluminum to flex to isolate vibrations makes me uneasy. Aluminum has very poor fatigue qualities. Steel is better in that regard, but, in my opinion, rubber isolation is better still.
      This assumes that the theory of sliding actually alleviates 2/rev or reduces it significantly which is something to be shown by data, not just theory and paper analysis. In the end if there is an implementation of any mechanism within the rotor-head to significantly reduce 2/rev, its efficacy can only be proven by measurement right at the rotor. An accelerometer above and below the slider would satisfy the requirement of isolating measurement to slider's effectiveness. If it is doing its job the theory that says that something like a slider can help is correct and the implementation of the slider steel spring method is also correct. If its not then one or both these things are on the wrong path.
      Measuring vibrations in the cockpit is nice but there are many other reasons you may get good results in the cockpit even while your rotor-head even in 1/rev is not ideal not to speak of 2/rev.
      More testing and validation is needed to verify results IMO.


      • #18
        Originally posted by Jean Claude View Post
        Attached to a bulldozer the rotary drag looks 65N for a load of 2050 N at 65 mph
        How can he slide with a coefficient of friction of 0.19?
        It would take at least 390 N is not it?
        I think you’ve underestimated the periodic aerodynamic drag variation of the rotor, JC.
        In any case, my 2050 N gyro shows a polished area on the roll axle of 3-4 mm.
        Last edited by C. Beaty; 05-14-2018, 05:57 PM.


        • #19
          My calculation by 11 slices of blade gives:
          Rpm: 372, drag 279 N, radial vibration: 65 N Sin (2wt)

          The analytical method of Naca programmed by Jean Fourcade (GyroRotor) gives with the same Cdmin of blade :
          Rpm: 380, drag 247 N, radial vibration: 109 N Sin (2wt)

          This remains too far from the 390 N required to begin the expected slip.

          Perhaps area polishing is makes only during the run, when the mu transitorily reaches values greater, while that the load is still low (???)
          Last edited by Jean Claude; 05-14-2018, 11:25 PM.


          • #20
            JC, when I first built this gyro with rigid rotor pylon, 2/rev vibration was so violent that it was dangerous to fly. In searching for a solution, I tried a drag hinged rotor with a coning hinge at the center of rotation and no undersling.
            The coning hinge looked like a door hinge with the hinge pin also serving as the teeter bolt.

            Such an arrangement had worked well on my first gyro; built to Bensen B-8 dimensions, except using round 2024 aluminum tube instead of square tube. But a drag hinged rotor was no solution with a rigid rotor pylon. I think 2/rev vibration was worse with drag hinges.
            The solution came when I watched a television series on the Discovery Channel showing the film, “Birth of the Bell Helicopter” and saw Arthur Young’s solution to 2/rev vibration. Remember, this was at a time before the Internet was ubiquitous; I bought my first computer in 1997 when my only available Internet connection was via a slow telephone line.
            In any case, Arthur Young’s solution for the Bell Helicopter, stiff inplane rotor mounted on a soft mast solved my 2/rev problem; the slider completely eliminated 2/rev vibration.


            • #21
              JC, this youtube film, “Birth of the Bell Helicopter,” part 3, shows some of the early problems of Arthur Young’s seesaw rotor. At 3:10 on the film, severe 2/rev vibration is first encountered during the first free flight at speeds above 20 mph. At 4:43 on the film, the first solution is shown; a device called the ‘Swedish Yoke” that increases the inplane stiffness of the rotor.

              Unfortunately, the film is English only; perhaps you can watch it with someone that can do the English/French translation for you.
              Cierva had experimented extensively with 2-blade rotors during jump takeoff development but was never able to solve the 2/rev vibration problem.
              Arthur Young invented the underslung seesaw rotor but he also experienced 2/rev vibration problems which were solved by the use of a soft mast and inplane rigidity of the rotor.


              • #22
                Perhaps another possible explanation:
                The very low value of the cyclic drag is probably correct, but it is with the asomption that the hub bar is perfectly rigid. And we know that perfection is not of this world.
                Hovever, my hypotheses seem to show me that even if the hub bar is totally "soft", the rotation frequency remains far below the free frequency of the drag hinged blades. In other words, there is no resonance possibility.
                However, I had completely forgotten that there no need resonance to produce a great vibration in the plane, due to the cyclic misalignment of the drag hinged blades, and also their centrifugal forces.
                So, if the hub bar too soft keep the possibilité of a mesalignement cyclique, perhaps this is the key to the mystery?
                Click image for larger version  Name:	Sans titre.png Views:	2 Size:	10.0 KB ID:	1133513

                Last edited by Jean Claude; 05-15-2018, 01:29 PM.


                • #23
                  JC, I wonder if anyone truly understands the functioning of the Young rotor. I have most of the available textbooks on rotorcraft and all analyze Cierva rotors in excruciating detail but lightly skip over the underslung, teetering rotor.
                  Anyhow, a soft inplane Young rotor offsets the rotor CG from the rotational axis during hub flexure and can cause strange behavior that depends upon mast stiffness. And it’'s all Bensen’s fault for necking the hub down to 2 ½ inches and using a square mast.
                  My first gyro used a round, 2024 mast and no combination of undersling and hub stiffness that I tried caused severe vibration.
                  As an example, the first Hughes 269 blades I flew were obtained from a man that had sawn a taper at the root ends of the blades to mimic Bensen blades and when I used these blades, I thought the root ends were not strong enough for use on a Young type rotor. That was the reason for the drag hinges mentioned previously. Even so, the drag hinged rotor was relatively free of vibration on my round tube gyro.
                  Last edited by C. Beaty; 05-15-2018, 06:12 PM.


                  • #24
                    Originally posted by C. Beaty View Post
                    it’'s all Bensen’s fault for necking the hub down to 2 ½ inches and using a square mast.
                    It seems to me, however, that the quadratic moment relative to the diagonal axis does not change. I'm wrong?


                    • #25
                      My knowledge of round vs square masts is primarily empirical, JC.

                      My first gyro, built to Bensen B-8 dimensions except for use of a round 2024 mast did not have 2/rev vibration problems whatever the rotor; Bensen wood blades, Bensen metal blades or Hughes helicopter rotor blades. The first Hughes blades used a hub with drag hinges and no undersling but no vibration problems.

                      During the same time period, my flying partner, R. Carbonell had a Bensen B-8 with double 2x1 mast tubes structurally bonded together with horrible 2/rev problems. My rotor blades on his gyro vibrated. We even installed my rotorhead on his gyro; no improvement. Many years later the film, Birth of the Bell Helicopter on TV showing Arthur Young’s struggles with 2/rev vibration gave some clues about 2/vibration problems of see saw rotors.

                      This film clip shows my round rube B-8 gyro, first with 3-blade hingeless rotor and at 1:28 with see saw rotor using Hughes OH-6 rotor blades. No vibration.


                      Another friend, Gary Yanson who frequently flew my B-8 gyro said it was so free of vibration, it frightened him because it felt like the rotor must have departed; to reassure himself, he’d have to look up at the rotor occasionally to make sure it was still attached.
                      Last edited by C. Beaty; 05-21-2018, 11:07 AM.


                      • #26
                        Mike told me that "quadratic moment" is not known. The correct term is "second moment of inertia"
                        Forgive my bad English and thanks to Mike for correcting me.

                        Click image for larger version

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                        • #27
                          JC, I don’t have a rational explanation for my empirical experience with 2/rev vibration. I selected round tube for my airframe because it was available and I chose tube size to approximately match the stiffness of the Bensen square tube airframe.