3 or more rotor blades ???

C. Beaty

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Rotor blades don’t flap in the same sense that a bird’s wings flap; they rotate in a uniform circle. Flapping is only an illusion.
But when the rotor’s hub is bolted to the rotorhead and the axis of rotation of the rotor differs from the rotorhead’s axis of rotation, an accommodation is required. Flap/drag hinges are simply a universal joint that permits the rotor axis of rotation to differ from the rotorhead axis of rotation.
Floating hub rotors where the rotor’s hub is permitted to be aligned with the rotor tip plane axis don’t require flap/drag hinges.
It ain’t complicated.
 
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Thank you. Think I got it.

Edit
Another question. When I saw Chuck's rotor head design, the blades can feather. Is this feature necessary for floating hub to work?
 
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C. Beaty

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With fixed hub and flap/drag hinges, tilting the rotorhead applies cyclic pitch variation to the rotor so that it follows the rotorhead. With a floating hub, cyclic pitch variation requires feathering bearings and a swashplate or its equivalent to make the rotor follow.
 

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XXavier

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Or with a spider... The feathering hinge can be seen in these two pictures of Doblhoff's helicopter/gyro hub:

Captura de pantalla 2021-01-15 a las 19.22.46.png

Captura de pantalla 2021-01-15 a las 19.24.09.png

... and also in this photograph of Chuck Beaty's construction, where a degree of tilt was achieved with elastic rubber bushings:

Captura de pantalla 2021-01-15 a las 19.30.01.png

Captura de pantalla 2021-01-15 a las 19.28.15.png
 

XXavier

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Javier, I’m impressed with your English ability; have you spent time in the UK or US?

Very little. When very young (8-10) I went to a English school in a Latin American country. Very English... But I learned most of my English much later, working for a Spanish printing and publishing company, where I was responsible for writing letters and telex messages for the export department...
 

Martin W.

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I think some folks misunderstand the term FLAP as it is used for rotorblades

When a blade moves upward or downward (on its hinge etc) it is called FLAP (Flapping)

It does not mean both blades move up at the same time or down at the same time (like a bird would)

Typically the advanving blade FLAPS up because it sees more air and the retreating blade FLAPS down because less air , this applies to both teetering and multi-blade rotors.

The reason I say that is because there is a well known and well respected CFI who thinks it is a bird-like flapping ... he demonstrated it on a parked gyro (stationary rotor) by standing at the tail vigorously whipped the blades so both tips moved up and down simultaneously like flapping bird wings would .

He was demonstrating how the rear blade can flex down enough to hit the tail. He said this is rotor FLAP.

Even though the CFI misunderstands the term FLAP there is a pinch of truth to his demonstration but it is because the teetering rotor has hit its maximum downward design limits (hits the stops) and the blade itself will continue to flex down enough at the tips to hit the rudder ... however the opposite blade is not doing the same at that moment as was suggested in his demonstration..
 

Vance

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Good point Martin W.

I feel that flap is a poorly chosen work for what is happening.

I feel worse still is using the term flap to describe a divergent blade event as in; “he flapped his blades and hit the tail.”

In my opinion a rotorcraft won’t fly forward without rotor blade flap to deal with dissymmetry of lift.

In my opinion this flapping is the key to cyclic control.

There is a good explanation of rotor flap and gyroplane rotor aerodynamics in the Rotorcraft Flying Handbook under “Aerodynamics of the Gyroplane” chapter 16.

It is available for a free down load here:

http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/faa-h-8083-21.pdf

The Helicopter part is obsolete but still has some useful information.
 

Tyger

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Interestingly, the use of "flap" to mean make a motion like a bird's wing only dates from about the 1770s. Before that, its meaning was much like the word "slap", i.e. to hit.
 

C. Beaty

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The front wheels on a front wheel drive automobile flap in the same sense as the rotor on your gyro flaps. Jack up the front end, start the front wheels spinning and with a steering angle applied, the valve stem moves nearer and farther away from the viewer when viewed along the drive shaft axis. The wheel is flapping in the same way as a rotor flaps.
The "flapping" wheel is the equivalent of a floating hub rotor.
The floating hub rotor requires a constant velocity “U” joint as does the outer “U” joint of an automobile.
 
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The floating hub rotor requires a constant velocity “U” joint as does the outer “U” joint of an automobile.
I know the trandictional U joints are not constant velocity; but how does this apply to a gyroplane rotor as it is not driving anything/driven by the shaft?
 

C. Beaty

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You’re right, 168, a gyro doesn't require a CVJ because the inertia of the rotorhead is so low.
 

XXavier

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You’re right, 168, a gyro doesn't require a CVJ because the inertia of the rotorhead is so low.
A CVJ is necessary only for a helicopter, where the rotation of the engine has to be correctly transmitted to the rotor, but a gyro rotor turns freely. The Doblhoff helicopter/gyro worked without one, because in helicopter mode, the rotor was driven by tip jets.

I'm not sure, but perhaps the usual gyro see-saw rotor head could be considered as a sort of two-blade floating hub, with indirect cyclic command and the blades hinged in a universal joint...
 
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The see-saw type rotor head cannot be a type of floating hub. The pitches of the blades on a float hub actually change relative to the hub where as the pitches of the blades on a see-saw rotor head do not change. They flap instead. I always think pitch change blades relative to the hub is more superior. It might also has much less chance of tail strike.
 

XXavier

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The see-saw type rotor head cannot be a type of floating hub. The pitches of the blades on a float hub actually change relative to the hub where as the pitches of the blades on a see-saw rotor head do not change. They flap instead. I always think pitch change blades relative to the hub is more superior. It might also has much less chance of tail strike.


Look at this sketch of a floating hub. Take away the CV joint and the transmission, and keep the spherical bearing, with a lateral and longitudinal command in tilt. Add two non-flapping, fixed-pitch blades fixed as in the sketch, and there you have a two-blade floating hub. Now, replace the spherical bearing with a cardan joint, and we are left with our usual semi-rigid rotor head...

Captura de pantalla 2021-01-19 a las 7.48.36.png


In our semi-rigid see-saw rotors, (and in any articulated rotor) the blades do not flap. It's an illusion, as Chuck Beaty has often pointed out in this forum. The articulation works as a universal joint, and forces the blades to change pitch periodically.

This animation may help to see that fact... https://en.wikipedia.org/wiki/Universal_joint#/media/File:Universal_joint.gif

It's clear for everyone who has observed a 'flapping' rotor that the tip-path surface is perfectly flat, and not a warped surface due to flapping blades... Flapping 'exists' only as an illusion perceived by the observer placed at the rotor axis. Dissymmetry of lift is compensated by the periodic change of pitch of the blades, and that feathering is –apart from rotation– the only real movement of the blades, since they don't flap at all... Yes, the blades oscillate around flapping hinge, but the flapping oscillation disappears 'absorbed' by the rotation. That oscillation has the same frequency as the rotation of the blades, and the addition of those two periodic movements of equal frequency results in pure rotation with a tilt of the tip-path plane; that's the 'blown-back' rotor...


This illustration is due to Jean Claude:


Captura de pantalla 2021-01-17 a las 12.31.37.png
 
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Tyger

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what do the numbers on the vertical axes represent in those graphs?
 
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Tyger

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I thought the azimuth was represented on the horizontal axis, in degrees. I was asking about the +2 to -2 numbers.
 
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