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C. Beaty

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JC, this youtube film, “Birth of the Bell Helicopter,” part 3, shows some of the early problems of Arthur Young’s seesaw rotor. At 3:10 on the film, severe 2/rev vibration is first encountered during the first free flight at speeds above 20 mph. At 4:43 on the film, the first solution is shown; a device called the ‘Swedish Yoke” that increases the inplane stiffness of the rotor.
https://www.youtube.com/watch?v=JOh09JJwoWM

Unfortunately, the film is English only; perhaps you can watch it with someone that can do the English/French translation for you.
Cierva had experimented extensively with 2-blade rotors during jump takeoff development but was never able to solve the 2/rev vibration problem.
Arthur Young invented the underslung seesaw rotor but he also experienced 2/rev vibration problems which were solved by the use of a soft mast and inplane rigidity of the rotor.
 

Jean Claude

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Chuck,
Perhaps another possible explanation:
The very low value of the cyclic drag is probably correct, but it is with the asomption that the hub bar is perfectly rigid. And we know that perfection is not of this world.
Hovever, my hypotheses seem to show me that even if the hub bar is totally "soft", the rotation frequency remains far below the free frequency of the drag hinged blades. In other words, there is no resonance possibility.
However, I had completely forgotten that there no need resonance to produce a great vibration in the plane, due to the cyclic misalignment of the drag hinged blades, and also their centrifugal forces.
So, if the hub bar too soft keep the possibilité of a mesalignement cyclique, perhaps this is the key to the mystery? Sans titre.png
 
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C. Beaty

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JC, I wonder if anyone truly understands the functioning of the Young rotor. I have most of the available textbooks on rotorcraft and all analyze Cierva rotors in excruciating detail but lightly skip over the underslung, teetering rotor.
Anyhow, a soft inplane Young rotor offsets the rotor CG from the rotational axis during hub flexure and can cause strange behavior that depends upon mast stiffness. And it’'s all Bensen’s fault for necking the hub down to 2 ½ inches and using a square mast.
My first gyro used a round, 2024 mast and no combination of undersling and hub stiffness that I tried caused severe vibration.
As an example, the first Hughes 269 blades I flew were obtained from a man that had sawn a taper at the root ends of the blades to mimic Bensen blades and when I used these blades, I thought the root ends were not strong enough for use on a Young type rotor. That was the reason for the drag hinges mentioned previously. Even so, the drag hinged rotor was relatively free of vibration on my round tube gyro.
 
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Jean Claude

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About 500 h (FW + ultra light)
C. Beaty;n1133516 said:
it’'s all Bensen’s fault for necking the hub down to 2 ½ inches and using a square mast.
It seems to me, however, that the quadratic moment relative to the diagonal axis does not change. I'm wrong?
 

C. Beaty

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My knowledge of round vs square masts is primarily empirical, JC.

My first gyro, built to Bensen B-8 dimensions except for use of a round 2024 mast did not have 2/rev vibration problems whatever the rotor; Bensen wood blades, Bensen metal blades or Hughes helicopter rotor blades. The first Hughes blades used a hub with drag hinges and no undersling but no vibration problems.

During the same time period, my flying partner, R. Carbonell had a Bensen B-8 with double 2x1 mast tubes structurally bonded together with horrible 2/rev problems. My rotor blades on his gyro vibrated. We even installed my rotorhead on his gyro; no improvement. Many years later the film, Birth of the Bell Helicopter on TV showing Arthur Young’s struggles with 2/rev vibration gave some clues about 2/vibration problems of see saw rotors.

This film clip shows my round rube B-8 gyro, first with 3-blade hingeless rotor and at 1:28 with see saw rotor using Hughes OH-6 rotor blades. No vibration.

https://www.youtube.com/watch?v=YnLXL3gmESM

Another friend, Gary Yanson who frequently flew my B-8 gyro said it was so free of vibration, it frightened him because it felt like the rotor must have departed; to reassure himself, he’d have to look up at the rotor occasionally to make sure it was still attached.
 
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Jean Claude

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I piloted gliders C800, Bijave, C 310, airplanes Piper J3 , PA 28, Jodel D117, DR 220, Cessna 150, C
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About 500 h (FW + ultra light)
Mike told me that "quadratic moment" is not known. The correct term is "second moment of inertia"
Forgive my bad English and thanks to Mike for correcting me.

Sans titre.png
 

C. Beaty

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[h=5][FONT=Liberation Sans, sans-serif]JC, I don’t have a rational explanation for my empirical experience with 2/rev vibration. I selected round tube for my airframe because it was available and I chose tube size to approximately match the stiffness of the Bensen square tube airframe.[/FONT][/h]
 
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