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I wonder why is this type of helicopter rotor hub so rare... It is a very simple construction, having no flap or lead-lag hinges. A universal, constant-speed joint leaves the rotor free to find his position in space, and the only complication is the swashplate, essential in order to control cyclic and collective pitch. This type of hingeless rotor hub needs little maintenance and is relatively easy to build with standard materials... But it's nevertheless nowhere to be seen...

Doman floating hub helicopter

Floating hub

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    $\begingroup$ According to wiki, that's a "Doman LZ-1A (Sikorsky R-6 conversion)", for reference. $\endgroup$ – AEhere Sep 1 '18 at 12:15
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It's basically the teetering Bell/Robinson 2 blade rotor concept applied to a 4 blade rotor. Like a teetering system, the gimballed rotor head accounts for most of the lead/lag/flapping, with whatever isn't accounted for absorbed by flexing of the long skinny root shafts.

With a 2 blade teetering system you have the weight of the machine hanging from a big cross bolt - pretty simple. This system appears to be a bit complicated in the manner that the load of the machine is suspended from the rotor head, and that may have worked against it when other designers considered it.

The designer may have seen it as the advantages of a teetering system and an articulating multi blade system combined, whereas other designers saw mostly the disadvantages of the two. An example: a teetering rotor, unlike an articulated one, can't tolerate hitting the teetering limits in flight because it puts huge bending loads on the mast (mast bumping) and makes the mast come apart. This is why you never let a 2 blade machine go zero G. I would expect this system could have a similar limitation, which a conventional articulated hub doesn't have.

Doman Hub

enter image description here

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  • $\begingroup$ Ha -- I found the same article about it. That would be the only thing that I could think of as well. That said, when you say that a 2-bladed teetering rotor is relatively simple (hanging from a bolt) and that this would be more complex...it would depend on the style of CV joint, since that's what the aircraft is hanging off of. Some CV joints are as simple as a really well greased bolt. My other thought for an answer to the OP would include a potential sensitivity to a localized hydraulic failure because of this. $\endgroup$ – Marius Sep 2 '18 at 12:11
  • $\begingroup$ Hard to say for sure but I think the weight of the machine is suspended by that wishbone shaped doohickey, a yoke with 4 arms creating a gimbal, that connects to the centre ring, not the CV joint, which I think transmits torque only. That would make it a fairly complicated load bearing mechanism. The control inputs are the same as any other helicopter, with a swashplate controlling blade rotation, so I would think that any hydraulics are to boost the mechanical control rods coming from the cyclic and a hydraulic failure would be the same as on any other machine with boosted controls. $\endgroup$ – John K Sep 2 '18 at 13:25
  • $\begingroup$ Yeah, it is difficult to make it out...but I wouldn't think that the wishbone-thing (which seems to correspond to the portion of the tiltable ring that does not rotate) would react vertical loads. It looks like it's part of some sort of radial bearing, not a thrust bearing that would react vertical loads, whereas the CV joint seems to connect to that dome structure that you can see in the picture of the LZ-1. Going along with how the R22 rotor works, it'd make sense to me that torque and thrust would be transmitted at the same point. Also, I misremembered how hydraulics were used here. $\endgroup$ – Marius Sep 2 '18 at 13:46
  • $\begingroup$ Well I'm convinced the CV joint is torque only because otherwise you wouldn't need any of that other gimbal/yoke stuff attached to the structure. Remember that the teetering system just has the rotor mast which doubles as a drive shaft, and no other structural members. A Robinson/Bell fuselage is effectively hanging by its transmission. This thing has a whole structure that wouldn't be necessary if the CV joint was taking the weight of the machine. CV joints on cars don't have to take tension loads. I imagine one that can take several thousand pounds in tension would be pretty massive. $\endgroup$ – John K Sep 2 '18 at 20:49
  • $\begingroup$ I was convinced that the CV joint was, effectively, the gimbal for the rotor head. Where are the gimbal/yoke for the rotor head otherwise? Could you label the drawings? What I see are 4 blades on a teetering hub that tilts via the CV joint currently labeled in the drawing, which, similar to an R22, would transmit thrust forces from the rotor blades to the mast via a teetering hinge of some sort (sourced from the R22 maint. manual: robinsonheli.com/wp-content/uploads/2017/01/r22_mm_9.pdf). $\endgroup$ – Marius Sep 3 '18 at 2:18
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This type of rotor is not common because flapping blades make the helicopter fly better. Flapping vertically allows the blades to reduce angle of attack as they advance/rise and increase angle of attack as they retreat/fall, evening out lift in concert with cyclic feathering. Flapping horizontally damps vibration and eases acceleration/deceleration.

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  • $\begingroup$ Flapping and lead-lag hinges just allow the tip-path plane to orient itself in space in response to aerodynamic forces, working as a big universal joint. The same effect, with simpler components, may be achieved with the 'floating hub' used by the Doman choppers (and also by the pioneering Doblhoff gyro-helicopter). $\endgroup$ – xxavier Sep 1 '18 at 14:31
  • $\begingroup$ there is in fact a german-made helo that uses a "rigid rotor" like this, but I can't remember its name... $\endgroup$ – niels nielsen Sep 1 '18 at 17:26
  • $\begingroup$ @niels nielsen You probably mean the Bo105. It's true that it uses a hingeless rotorhead, but it isn't a 'floating hub' rotor. There are several hingeless designs around, based upon elastic materials, but it isn't the same thing... $\endgroup$ – xxavier Sep 1 '18 at 18:18
  • $\begingroup$ that's the one. -NN $\endgroup$ – niels nielsen Sep 1 '18 at 22:32

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