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Cabin Hop - GyroTech Blaeds

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  • Cabin Hop - GyroTech Blaeds

    This question is to all of you rotor balance geniuses. We need some additional brain power to think through an issue. We installed new rotor blades (GyroTech) on the Cavalon today. They were dynamically and statically balanced at the factory with the teeter towers included. We installed the teeter tower and rotor system, all seems good.

    On spin up, itís very solid with no vibration, but on test runs / short flights down the runway we begin to get a strong cabin hop. Rotor RPM is about 330-350 and the hop is about 5-6 hops per second (which would point to a blade balance issue I think).

    The dilemma is that the stick is really smooth, but the cabin hop is pretty extreme.

    Thoughts?

  • #2
    soon someone will chime in
    Last edited by FRANK'S; 07-05-2018, 12:23 PM.
    I'm a gyro Dude now! Bensen FTW

    FRANK

    Comment


    • #3
      Just thinking out loud here...

      Define what you mean by "Cabin Hop" a bit more specifically. ???

      If you mean the airframe is moving up and down at 1/revolution, that would not be a balance issue.
      Out of Balance would be side to side with the stick shaking to match.

      Have you checked the tracking?
      If one blade has more pitch it will fly higher than the other even though it is balanced.
      You would not notice the dissymmetry of lift until the blades were carrying the weight of flight.
      When the higher lift blade is advancing it would lift the airframe.
      When it is on the retreating side, the lower lift, advancing blade, would allow the airframe to descend.

      Comment


      • #4
        When I installed my gyro-tech blades the cordwise adjustment had to be tweaked just a little,but it sounds like it wasn't anything like

        your experiencing. just take a good look that everything is nice and tight,blade bolts,teeterbolt,gimbal bolts etc.you might also try turning the teeter tower 180 degrees

        in relationship to the hubbar.check the rotor blades and hubbar numbers for for proper matching. give the factory a call if all fails.

        My blades run smoother than anything I have used, and I believe they tried the blades on a cavalon in Europe with good success.

        keep us informed of your efforts.
        Best Regards,
        Eddie Sigman,Polvadera,nm
        (575) 835-4921

        Comment


        • #5
          20 years ago, or so? When I was learning and adjusting my Bensen blades on my Bensen gyro. I discovered a correlation between blade pitch relationship to the hubbar. I fabricated a set of tracking bars, approx. 48" long. When I pitched the blade to 1* the rotor spin at 410 RRPM and had a 2/rev stick shake. Re-adjusted to 1 1/2* and the RRPM was 375 and smooth as silk. Re-adjusted to 2* pitch, and the RRPM dropped to about 330 and there was a very pronounced up & down hop in the airframe, and the stick was pretty smooth. I concluded that at the lower RRPM the coning angle increased and the teeter bolt was above the vertical CG of the flying rotor; and this creates the vibration of a hopping motion.
          Point is; each rotor system has a desirable RRPM for smooth operation. Bensens' like around 375; and so did mine. I think you need to find out a true "all up weight" for your machine and I suspect you will find the rotor is pitched too much for that weight. Decrease the pitch and increase the RRPM and see if this solves the vibration. Call the manufacture and ask them what is the desired RRPM for their rotors? I suspect you are spinning too slow.
          Just a guess.
          David McCutchen
          615-390-2228
          Bensen B7m, 90 hp Mac
          Dominator Tandem, 100 hp Hirth
          Kolb Mark III Classic, 80 hp Verner
          Certified - Advanced Master Beef Producer
          EAA Member #0511805
          PRA Member #28866
          PRA Chapter 16 Member
          Secretary & Treasure - PRA Chapter 16
          President / Sylvia - Yellow Creek Volunteer Fire Dept.
          Chairmen - Dickson County Veteran's Day Committee
          Volunteer - Dickson County Airport Aviation Day Committee
          2 busy 2 No!

          Comment


          • #6
            If your blades are not pitch adjustable; then you will have to get shorter blades to spin faster. \
            CB put a spreadsheet on here several years ago. In this spreadsheet, you can place your actual "all up flying weight, rotor cord width, and rotor diameter. This created a fairly accurate estimated rotor rpm. You can play with the rotor diameter to see what will create the desired RRPM based on Manufactures recommendations.
            David McCutchen
            615-390-2228
            Bensen B7m, 90 hp Mac
            Dominator Tandem, 100 hp Hirth
            Kolb Mark III Classic, 80 hp Verner
            Certified - Advanced Master Beef Producer
            EAA Member #0511805
            PRA Member #28866
            PRA Chapter 16 Member
            Secretary & Treasure - PRA Chapter 16
            President / Sylvia - Yellow Creek Volunteer Fire Dept.
            Chairmen - Dickson County Veteran's Day Committee
            Volunteer - Dickson County Airport Aviation Day Committee
            2 busy 2 No!

            Comment


            • #7
              Thanks David that's good info about the rrpm Vs vibrations.
              Best Regards,
              Eddie Sigman,Polvadera,nm
              (575) 835-4921

              Comment


              • #8
                Finally made my first flight this morning with my GT blades that I have had since January. Only flew 20 minutes but so far I am pleased. There was no cabin hop and stick shake was there but less than old RAF blades which were not terrible anyway. With a 5 kt headwind they seemed to spin up faster than RAF blades and I was pleasantly surprised that the landing was much easier than I had anticipated. I had not flown in about 3 months but just seemed to hang there till the tail wheel touched and never needed help with power. This is from an airport at 6800 ft and a DA of 9200 ft. Rotor rpm was about 360. Need to fly more to make a better assessment.

                David Craft

                Comment


                • #9
                  Originally posted by lindandavid View Post
                  Finally made my first flight this morning with my GT blades that I have had since January. Only flew 20 minutes but so far I am pleased. There was no cabin hop and stick shake was there but less than old RAF blades which were not terrible anyway. With a 5 kt headwind they seemed to spin up faster than RAF blades and I was pleasantly surprised that the landing was much easier than I had anticipated. I had not flown in about 3 months but just seemed to hang there till the tail wheel touched and never needed help with power. This is from an airport at 6800 ft and a DA of 9200 ft. Rotor rpm was about 360. Need to fly more to make a better assessment.
                  What are the stick forces like in pitch and roll on your new GT rotors versus your old RAF blades?

                  Thanks,
                  Eric

                  Comment


                  • #10
                    I would have to say there wasn't much difference with the GT blades vs RAF in pitch and roll once I got it trimmed. I have had it down for maintenance for a couple of months changing head gaskets, all the pushpull tube end bearings , oil pickup, radiator hoses, in addition to the new head and blades , and the list goes on. I was only up 20 minutes because about the time that I got it trimmed, the crosswind on the runway was more than when I took off so I went to another runway, which was worse. About that time I noticed the coolant temp was higher than I liked so I went back to the original runway and put it down. I am now changing out the temp sending unit and the gauge
                    . Sorry for all the dialog, but the point being I really was not able to get more of a feel for the blades in that amount of time, although I sure was happy with the way they performed in the takeoff and landing. One side note is that the oem head gaskets on this 2001 Subaru 2.5 were pitiful. I'm surprised they didn't fail before now. The engine actually quit momentarily when it overheated and coolant was sucked in the intake, but that is another story.
                    David Craft

                    Comment


                    • #11
                      This is from an airport at 6800 ft and a DA of 9200 ft. Rotor rpm was about 360. Need to fly more to make a better assessment.
                      Once again, I think speaking to the rotor manufacture about the desired RRPM is a key component in figuring this out. Because of the DA the rotor has to spin faster to compensate. You very well may have to increase the disk size to slow the rotor down to compensate for the DA.
                      First: Define the flight mission. Then build the aircraft for the mission. Your machine may be built for a much lower DA mission and would perform well at 1500'.
                      My 100hp Tandem Dominator is a real hotrod when I go to Bensen Days, not so much at home right now where the DA is hitting 4000' each day.
                      David McCutchen
                      615-390-2228
                      Bensen B7m, 90 hp Mac
                      Dominator Tandem, 100 hp Hirth
                      Kolb Mark III Classic, 80 hp Verner
                      Certified - Advanced Master Beef Producer
                      EAA Member #0511805
                      PRA Member #28866
                      PRA Chapter 16 Member
                      Secretary & Treasure - PRA Chapter 16
                      President / Sylvia - Yellow Creek Volunteer Fire Dept.
                      Chairmen - Dickson County Veteran's Day Committee
                      Volunteer - Dickson County Airport Aviation Day Committee
                      2 busy 2 No!

                      Comment


                      • #12
                        You can also increase the pitch to compensate for the alt/high rrpm, We did talk to the GT manufactors and they provided us with 2 sets of blade shims one for lower elevations and one for the higher altutide

                        we live at.In the winter my rrpm is about 15-20 rpm lower than in the summer. I do think you are right about the various rrpm's having something to do with the different types of vibration.
                        Best Regards,
                        Eddie Sigman,Polvadera,nm
                        (575) 835-4921

                        Comment


                        • #13
                          If your Cabin Hop is an up and down oscillation it is most likely the tracking, since one blade has more pitch than the other.
                          You feel the dissymmetry of lift as a hop.
                          Life,The leading cause of Death

                          Live and Learn--OR--Die and be an example

                          321.252.7705

                          Comment


                          • #14
                            . . . changing . . . all the pushpull tube end bearings . . .
                            That was wise of you.

                            I noticed the coolant temp was higher than I liked so I went back to the original runway and put it down. I am now changing out the temp sending unit and the gauge
                            You may want to also R&R your thermostat as well. Mine recently began to fail on me, after 160 hours and three years.

                            Regards,
                            Kolibri
                            PP - ASEL complex (Piper 180, C206, RV-7A), SP - Gyro (Calidus, RAF, SC2), soloed in gliders; checkride soon

                            Wasn't happy with my RAF's pitch instability, so I installed a Boyer H-Stab to my great satisfaction!

                            "
                            When an honest but mistaken man learns of his error, he either ceases to be mistaken -- or he ceases to be honest."

                            Comment


                            • #15
                              Originally posted by magknight View Post
                              This question is to all of you rotor balance geniuses. We need some additional brain power to think through an issue. We installed new rotor blades (GyroTech) on the Cavalon today. They were dynamically and statically balanced at the factory with the teeter towers included. We installed the teeter tower and rotor system, all seems good.

                              On spin up, itís very solid with no vibration, but on test runs / short flights down the runway we begin to get a strong cabin hop. Rotor RPM is about 330-350 and the hop is about 5-6 hops per second (which would point to a blade balance issue I think).

                              The dilemma is that the stick is really smooth, but the cabin hop is pretty extreme.

                              Thoughts?
                              Some thoughts I collected from from a while ago when building our Hornet.

                              Blade shake is cause by many things. Cone set, static balance of the blades, chordwise balance dynamic balance, mast stiffness. These can be tackled one by one. Start with undersling or height under the teeter bolt, then spanwise balance, weight either side, check chordwise balance, achieved by stringing the blades with the string passing through the centre of the teeter bolt, then check dynamic balancing or blade tracking. Make sure your hub bar is in the middle of the rotor head. Use feeler guages to centre hub bar then shim teeter bearing race.

                              Stick shake is fore and aft probably teeter height/undersling needing adjustment, or chord balance/stringing. The machine hoping up and down is tracking or dynamic balance. A round symetrical shake possibly static balance.

                              The COM of a coned rotor should lie at the height of the teeter bolt, which is controlled by the undersling of the rotor. This does not affect stability per se, but affects 2 per rev shake. Undersling can be found in the rotor peformance sheet and will vary with the weight of the gyro and diameter of the rotors.The best teeter height depends on lengths of the blades, mass of the blades, CG locations of the blades, the blades' lifting qualities and other variables. But a given rotor at a given pitch should have the same steady-state coning angle despite varying loads.

                              Moving the teeter bolt position in relation to the center of lift of the blades or cone set, and you will get shake. One way to find out if your cone set is causing blade shake is to load and unload the rotors. This moves the COL or centre of lift above and below the teeter bolt. If it shakes more in the high g situation, the teeter bolt is too high. If it smooths out under high g's, the teeter bolt is too low. Holding the machine in a constant high g approximates straight and level because it gives the blades time to catch up and straighten out to normal. It is the response in the stick as one loads and unloads the blades and they are spinning up or down that gives you the information you need to know.

                              If you have correct cone set but after some time start to get get blade shake begin by checking if there there any friction in the cyclic set up. If so remove and tighten it up. Check your balance springs, have you recently adjusted them? If neither of these remove the rotor. Remove the blades from the hub. Keeping all the bolts nuts and washers from each side separate, weight them to ensure each side is equal. If not adjust to achieve good static ballance. Reassemble the blades then string them to achieve good chord balance. Put them back on and track them to dynamicaly balance them. If tracking looks to be out check bushings and hardware on the teeter bolt. Is there a shim or shims inserted between teeter block and hub bar if so this was from tracking adjustments. Add a piece cut from a thin piece of aluminium coke can then see if tracking gets worse or better. If worse remove the shim you added and the one there before then check again to see if it improves. If needed start adding to the other side. If when you added it improved but still not quite there add some more until tracking is as good as you can achieve. A tendency to roll can be caused by not enough side to side play play in the teeter bearing. Movement should be between teeter bushings and teeter towers. With teeter bolt tight up to .010 side to side
                              Leigh.

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