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My 1970s helicopter

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  • #16
    Originally posted by C. Beaty View Post
    My Kohler engine was 2-stroke as were most outboard boat engines at the time which were designed for vertical crankshaft operation.

    For vertical engines, the weight of the connecting rod is supported by rubbing surfaces between it and the crank cheeks so some provision must be made for forming an oil film. This is normally provided by grooves or serrations on the big end which creates a wedging action for the oil. The Kohler engine had no grooves on the connecting rod.

    The connecting rod roller bearing on my engne overheated and the rollers looked like pieces of bread dough that had been rolled between the hands.

    But the actual cause of failure is all a matter of conjecture on my part.
    The only reason I think something happened to your engine to cause this other than being mounted vertically is that these engines and others that are similar ran hundreds of hours in hovercraft mounted vertically without having a problem.


    • #17
      Norm, I base my conjecture on the fact that a rod bearing seized up and the rod broke in half. But it could have been any number of things.

      I purchased this engine direct from Kohler as an OEM and it came without carburetors or exhaust system. I used OMC outboard carburetors that could have been too lean. I should have taken more time without rushing to get it in the air in time for a Bensen Days flyin.

      Whatever the case, the Kohler was one of the nicest engines, including Rotax that I'íve ever seen; hard chrome plating on cylinders and very lightweight.

      But without exhaust system, it probably wasn'ít over 25 hp and the poor thing was running all out.

      For those who don'ít know, Norm lives at the outer edge on the habitable world with polar bears and perhaps an Eskimo or two. So stay warm, Norm.
      Last edited by C. Beaty; 01-08-2018, 05:16 PM.


      • #18
        Chuck, I have not find my video of the experience of the rotating bar. Sorry.


        • #19
          Thatís OK JC. The copy of your video clip on Youtube works fine.

          Alan, did you save a copy of JCís original and repost it on Youtube?

          Whatever the case, thanks to both of you.


          • #20
            I found the video, Chuck.
            It clearly shows that the centrifugal forces does not axifugal. It not bring back the blades in a plane perpendicular to the bearing.


            • #21
              Sketch showing that if JC had used a 2x6 plank rather than a square one, aerodynamic lift force would have brought plank axis into alignment with rope axis. Click image for larger version

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              • #22
                Chuck, I've copied the plank-on-a-rope experiment and, of course, had the same results. Fun and cheap entertainment.

                Another interesting trick to try while you have the plank and rope assembled is a demonstration of gyroscopic precession. While the plank is spinning, tap one end from above or below and watch which way the "disk" tilts.


                • #23
                  Much can be learned from simple models, Doug.

                  My favorites, in addition to JCís plank, are scale model rotors made up from welding rod and aluminum strips and run by a portable electric drill. With such models, itís easy to demonstrate delta-3 coupling, pitching moments and the effects of chordwise balance.

                  A rotor made from a 1Ē x 12Ē strip of aluminum flashing will flutter violently if run without noseweights.


                  • #24
                    Hereís another simple training aid that will help in the understanding of autorotation; a 6Ē long slice from an 1.25Ē dia wood dowel (Loweís stocks 1.25Ē wood dowels in 6í lengths for $8 Ėeach member of the family can have his own windmill).

                    Give it a flip in either direction, hold it out the window of a moving car and it spins merrily away. Once a person understands how this can be, heís well on his way to understanding autorotation,

                    Click image for larger version

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                    • #25
                      Hey Chuck,

                      I just read the Bell Engineering/NTSB final report on the cause of the 525 crash that killed 2 Bell test pilots in 2016. The finding identified "excessive blade flapping" during a high-speed test, which struck the tailboom causing it to fail, followed by complete in-flight breakup.

                      I have to interject, flapping is not imaginary. If the rotor behaves like a rock on a string, it would not have "flapped" low enough as the blades passed the tailboom to chop it off.
                      Bryan Cobb, Helicopter Enthusiast
                      Mfg.Engineer., Composites, Meggitt Aerospace, Rockmart, GA


                      • #26
                        Flapping is a common term, used when viewing a rotor relative to its drive shaft.

                        A long range artillery shell when fired north-south and viewed from the Earthís surface appears to follow a curved path as a result of the Earthís rotation but when viewed from space, moves in a straight line just as Newton said it must.

                        Thatís where Coriolis comes into the picture: when using the Earth reference we must apply an imaginary force to explain the imaginary curvature.

                        Same with rotors; when viewed from the shaft axis, we must apply an imaginary force to explain imaginary lead-lag and to avoid violating Newton.

                        But viewing a rotor along its real axis of rotation, the tip plane axis, there is no flapping nor lead-lag.

                        JCís plank on a rope is flapping in the exact same sense as a see-saw rotor in forward flight; is it flapping? It is if you were rotating with the plank and viewing it along the rope axis.

                        Flap and drag hinges constitute a universal joint which permits a rotor to rotate about its own axis rather than the shaft axis. A teeter hinge is all that is required in the case of a see-saw rotor.

                        Last edited by C. Beaty; 01-18-2018, 09:17 AM.


                        • #27
                          Chuck, back when I started experimenting with rotors I had never seen a two blade machine up close but I had seem three bladed machines and I did know that they had a hinge that allowed them to flap or cone, I built a rotor on a fixed test stand that only had the flap hinge and would it up in a breeze and let it do itís thing and in a few hours it failed and tore up everything, I built another one stronger and kept checking it and it was starting to crack, a helicopter pilot told me that I needed a second hinge, I didnít understand how it helped but it did, I knew that stress canít cross a hinge but I didnít understand where this movement was coming from, I built a small scale model and walked around with the blade and I finally saw what was happening and why it was cracking without the drag hinge, with the blades coned up the blades were swinging the tips through an arc, the higher the cone angle the greater the swing and I now understood the purpose, it simply allowed the hub to change the pitch of the blades as they went around without swinging the tips and over stressing the attaching points. I never could find a lead leg motion even though everyone said that it was there.


                          • #28
                            Click image for larger version

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ID:	1129171 Without a drag hinge, it looks like this, Norm. Has nothing to do with speeding up and slowing down, leaving poor Mr. Coriolis out in the cold.
                            Angles are exaggerated for illustration.


                            • #29
                              Well, with only a flap hinge, the mechanism WANTS the blade to speed up and slow down.

                              The blade would NEED to speed up in order to arrive on time at the spot that Chuck has labelled "flap hinge only forces blade here."

                              This is the old familiar "snap" that occurs in U-joints when their input and output shafts aren't parallel. It's why U-joints in drive trains are used in pairs -- with the U-joint there really IS a speedup-slowdown 2/rev.

                              Seesaw rotors accomplish all this with a minimum of mechanical complexity -- but at a cost. The seesaw arrangement becomes aerodynamically unstable if too much of the retreating blade stalls. In effect, the mechanism adds more and more angle of attack to the retreating blade until the whole blade stalls. That leaves the rotor with lots of lift on the advancing blade and a sudden lack of lift on the (stalled) retreating blade -- like a seesaw when a kid on one side jumps off. It's also not much different, conceptually, from a spin entry in a FW plane (a pair of fixed wings is rather like a semi-rigid rotor). This runaway stall, with lots of lift on the advancing side, is what forces the retreating blade into the tail, prop, etc, whether in a Bell, Robbie or gyro.

                              You can call this phenomenon "flapping" (and gyro folks do) but that does confuse the issue. "Catastrophic flapping" or "divergent retreating blade stall" would be more accurate, but quite a mouthful.