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Strange rotorhead

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  • Strange rotorhead

    I was going through my computer junk and came across this photo of a collective pitch rotorhead on an old hard drive that I’d completely forgotten about.
    The main rotorhead bearing was a spherical roller bearing that allowed pitch and roll inputs much like a Bensen spindle head except the teeter bolt axis passed through the center of the bearing, eliminating rotor thrust feedback into the cyclic control system. Seemed like a good idea at the time but wasn’t.
    The Bensen offset gimbal rotorhead feeds back a component of rotor thrust into the cyclic control system in a stabilizing direction that provides enough stability to permit some very unstable gyros to be flown.
    Anyhow, I installed this rotorhead on a very unstable gyro and Ernie Boyette, David Seace and I managed to fly it, although very sedately,

    but it was a good learning experience about stability. This must have been 35 years ago.
    The rotorblade thrust bearings were Timken tapered roller thrust bearings intended for use as king pin thrust bearings for heavy equipment.

  • #2
    The picture didn't show up Chuck.
    Mike Gaspard
    Forum Administrator
    Kaplan, Louisiana
    Bensen B8MG, NX36MG

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    • #3
      Strange things happen with strange rotorheads, Mike. The picture shows up just fine on my Linux OS but not on my old WXP OS.
      Here it is again, posted by WXP system:
      Now what? As soon as I posted the picture below, it also shows up on the previous post whereas it didn't before. As Churchill said, a mystery wrapped in an enigma.
      Click image for larger version  Name:	DSCF0113.JPG Views:	1 Size:	250.2 KB ID:	1133962
      Last edited by C. Beaty; 05-28-2018, 09:22 PM.

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      • #4
        Strange indeed. I'm running windows 7 and can't see the first pic, but the second one shows up fine.
        Same thing on my android tablet.

        Anyhow, I didn't think tapered roller bearings were suitable for feathering bearings? Or was this just a short term experiment?
        Mike Gaspard
        Forum Administrator
        Kaplan, Louisiana
        Bensen B8MG, NX36MG

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        • #5
          Take a look here, Mike:
          https://www.timken.com/products/timken-engineered-bearings/thrust/



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          • #6
            Chuck, the idea of eliminating the nose-up bias caused by the geometry of the Bensen spindle head is interesting. Bill Piper out West followed a similar idea when he designed the spindle head for his sheet-metal gyro.

            Apparently, the weight of an overhead stick was about right to eliminate the bias at one G and at the flapping (blowback) angle associated with airspeed X (maybe 45?). At higher speeds, the flapping angle was greater, and there was then a nose-up force feedback into the stick. Bensen had to install springs to counteract this force when using a joystick with the spindle head.

            Gyro oldtimers may recall the stir caused by the crash of Ernie Brooks in England in the late 60's. It was probably a garden-variety precession stall, aided and abetted by an aggressive maneuver (a kick-the-rudder hammerhead), short tail and high thrustline. At the time, however, various imaginative explanations circulated -- he was intentionally looping the gyro, or the mysterious force feedback of the spindle head suddenly forced the stick back, causing a pseudo-loop.

            I flew my original Bensen, both towed and powered, using a spindle head, and never ran into any undue stick forces. When first transitioning to the gimbal head, I rather preferred the very "direct" feel of the spindle head. The mind-of-its own pressures of the gimbal head were annoying at first. I did not throw the gyro around, though, and I did use an overhead stick exclusively at the time.

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            • #7
              Dave Praeter had some interesting comments about Bensen spindle heads, Doug.

              Dave, way back, had a spindle head on his gyro while his flying partner, Bill Parsons had a gimbal head. Dave said that he was all over the sky while Bill was as steady as a rock and couldn’t figure it out until he got his own gimbal head.

              The feedback from a spindle head is destabilizing.



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              • #8
                Chuck, I believe that the spindle head is stable with respect to airspeed (assuming continuous one G) but unstable with respect to disk angle of attack.

                The airspeed stability, such as it is, isn't attributable to the spindle design, but arises from the effects of flapping/blowback. The trouble with this picture is that flapping causes the airspeed to trend back to zero. Part of what the offset gimbal head does, I think, is to move the flapping-induced stable airspeed off zero and up to some selected forward speed. We can fine-tune the stable airspeed with the trim spring.

                The Bensen offset gimbal head (with extra offset plus a trim spring) also can provide some degree of stability with respect to disk AOA, something that's entirely lacking in the Bensen spindle head. Disk AOA, in the short run, controls G load, so disk-AOA stability provides some resistance to unintentional low-G flight -- as long as the pilot lets the head move on its own by "floating" the stick. This is what enables the gimbal head to mask underlying instability in a gyro.

                If the pilot locks the stick in a death grip, the gimbal head is effectively disabled.

                An AOA-stable airframe turns the whole gyro into a big offset gimbal head, eliminating the need for the float-the-stick technique. Or so my experiences suggest.

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                • #9
                  Doug, as you know, the offset gimbal rotorhead with rotor thrust vector passing aft of the pitch pivot, applies a nosedown force to the cyclic control system upon encountering an upward gust and tends to keep the gyro headed into the relative wind, providing angle of attack stability. The spindle head, with the rotor thrust vector passing forward of the pitch pivot, does just the opposite.
                  I don’t think Bensen ever supplied plans for the gimbal rotorhead and back in the Stone Age when I built my first gyro, I borrowed the only gumbal head available and built a copy.
                  BTW: Bensen came up with the slogan: ‘"So stable it flies hands off”" after development of the gimbal head.
                  Last edited by C. Beaty; 05-30-2018, 08:56 AM.

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                  • #10
                    Yup, the offset gimbal head does just that. My old lowrider Air Command was pretty unstable in turbulence, if you held the stick normally. It'd nose down in a downdraft and up in an updraft, thanks to the high thrustline and no HS.

                    If you "floated" the stick gently enough, though, the stick would move visibly forward when you hit an updraft (increase in AOA). This is a stable response, though it wasn't enough to smooth out the G's altogether. It also helped to throttle back and fly at the minimum-power-required airspeed (35-40), since that meant less pushover moment to add to instability. Awful damn slow. I got left behind a lot when flying with a group of ultralights in rough air.

                    Later, when flying the Dominator, I had to get re-accustomed to holding the stick firmly in rough conditions -- the old "float" trick wasn't necessary or even optimal. The whole gyro responded in a stable direction if you just held the stick normally.

                    Interestingly, one of the old Bensen sales brochures shows Igor or somebody holding hands off the stick of a spindle-headed B-8M gyro. Not for long, I bet.

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                    • #11
                      The Bensen wood blades with external noseweights were a bit overbalanced which also enhances angle of attack stability; the advancing blade, encountering an upward gust, twists nosedown more than the retreating blade, tilts the rotor disc nosedown and
                      tends to keep the machine headed into the relative wind.

                      Going to metal blades probably drove Bensen to explore the gimbal head.

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