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I have read about some helicopters now using a mechanism referred to as a "bearingless rotor". From what I've seen online, the system involves a flapping hinge.
What I struggle to get my head around is how the rotor spins without a bearing. How is this achieved?
First of all a short clarification about the difference between bearingless and hingeless rotors is given followed by the description of the "bearing-part" of a rotor.
Without entering in the detail why the blade has to have the possibility to 1)flap, 2)lead-leg and 3)pitch in respect to the hub, bearingless, hingeless and fully-articolated rotors are different technological and historical ways to get these 3 movement. In the fully-articolated rotors these 3 movements happen around 3 bearings. The use of bearings was the first logical and technological way to let those movements free and virtually all of the oldest helicopters used it. Typical example is the Sikorsky CH-53 and this is also a very nice example about how much the rotor head can become packed:
CH-53 Main-Rotor head. Source: https://i.pinimg.com/originals/21/6c/82/216c8236203d4906840a821063aef3a8.jpg
Here I highlighted the axis of the 3 bearings for flapping (green), lead-lag (blue) and pitch (red). Hopefully they can be recognised despite the perspective. Note that in this particular design yet another hinge is present, i.e. for blade's folding, in yellow. Circled in yellow are the hydraulic actuators for folding.
Unfortunately bearings were delicate and had to be greased and substituted quite often with high costs. The second iteration saw therefore the replacement of the most stressed bearings (the ones for flapping and lead-leg) with a blade whose root was soft enough to let these 2 movements happen. The bearing for pitch changing survived. These were/are the hingeless rotors. The first rotor of this type was the one of the BO105. The use of glass fiber instead of aluminium for the construction of the blade was surely a key technological aspect for the success of this kind of rotors.
BO-105 main-rotor head. Source: https://www.flyingbulls.at/fileadmin/_processed_/6/1/csm_bo-105-19_9ad787a50b.jpg
BO-105 main-rotor head dismantled. Source: https://aviationweek.blob.core.windows.net/aviationweek19670626thumbnails/Media/0x600/1967062683_1.jpg
These 2 pictures clearly show how much cleaner this kind of rotor head was/is. The hinges for flapping and lead-lag are just gone replaced by a softer blade's root (circled in yellow). The hinge for pitching (circled in red) has survived and it just inserts inside the X-shaped one-piece titanium rotor head. The mutual movement is assured by a set of 2 bearings, not visible in the picture.
The last iteration was obviously the elimination of the pitch bearing too, giving a bearingless design. Here the 3 hinges are replaced either by a blade's root soft in all 3 directions or by elastomeric hinge(s). Typical examples are the H135 and the AS350 respectively. Also in this case the switch to CFRP as building material for the blade has been a key enabler together with the technological maturity of elastomeric materials.
Bearingless rotor head. Source:
https://assets.verticalmag.com/wp-content/uploads/2019/03/Blades-cross-section-1024x633.jpg
This picture shows the main rotor head of an H145-D3(?). Here all 3 hinges are gone replaced by a soft CFRP blade's root, in brown in the cutaway.
That's for the connection blades-rotor head. As pointed out by @John K, there are still bearings for the spinning part of the rotor. The rotor head with its shaft is inserted in a gearbox which is used to mechanically connect the rotor to the engine(s) and to the fuselage. A typical gearbox looks like the one in this picture:
Main gearbox of the Sikorsky S-49. Source: https://www.sikorskyarchives.com/S-49.php
Even if this gearbox dates back to the 40s, its layout is still up-to-date. In this cutaway is easy to see that everything which rotates does it thanks to one ore more sets of bearings (I circled a couple in blue). So whatever the connection blades-hub is (hingeless or bearingless), the gearbox, together with any other rotating mechanical parts, is anyway "bearingful"
There are still bearings for the spinning part of the rotor mast coming out of the transmission, and for blade pitch in the hub itself. The bearingless part is that the blade flapping and lead/lag functions are accommodated by "elastomeric" hinges, basically, rubber-type compliant bushings. It's very similar to the control arm attachments on modern cars where the control arm is attached to the frame with a large rubber bushing, bonded into a sleeve that is bolted to the structure, that accommodates the arm's rotation while isolating the control arm from the car.
Some helicopters also achieve this function by making the root section of the blade into a flexible shaft that is free bend in the flapping and lead/lag direction.
Hingeless rotors have been around since at least the late 70s.
Bearingless rotors spin just like any other rotor mast would, whether fully articulated or semi-rigid. It's called bearingless because the angle of attack and flapping is caused by pitch arms rather than a bearing or a hinge that feathers the blade with each circle. They also absorb stress and not move so they can also be called rigid systems, Airbus H-145 and UH-1Y are an example of that.
Refer to (Page 4) of : https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch04.pdf
Further clarification :
A change in pitch angle of the blade helps the lift gets generated, but in order for the helicopter to pitch up and down, roll to both sides and provide cyclic control, not only increase or decrease in lift becomes a matter but also the direction of this lift, with a change in lift direction comes change in center of pressure and blades tend to undergo forces that might cause them to flap up or down, lead and lag in reference to the default radius, although some rotor-craft systems allow that to happen but they have to be limited and compensated only enough to allow for other movements, at the same time these different changes and their controls have to be connected with each other so that the pilot can increase their lift through the governor/throttle but at the same time cycle in a 360 inclination. Some rotor systems don't allow for any movements when cycling happens, such as the rigid, some allow for a few of, and some allow movement but it never effects the change in pitch, meaning you can reduce or increase your thrust effectively no matter what cycling direction you are putting. With different systems comes different pros and cons. But to focus on the movement of the hinges, a bearingless system never allows for hinges to cause flapping or leading unless it's bend by force but not through a hinge. when a pilot rolls to the left e,g, a blade will have to suffer pressure, every time it's on the right side of the rotation. The material allows it to bend and never have damage. The downside of such systems is that the helicopter buffets when flying because there are no hinges that partially absorb that pressure.
