(another) question about C Beaty's rotor

Victor Duarte

troublemaker
Joined
Aug 25, 2004
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Hello Chuck,

as you know, i find a great interest in your "floating hub rotor" and it happens that a rotor could be built using that system.

If you don't mind (and if you ask no fees :D), i'd like to ask you 2 things :
1 - if you could measure it, what was the angle of "flapping" allowed by the bushings ?
2 - did you encouter some "resonnance" due to the bushings ? given that when the bushings get loaded, the blade pitch changes (delta 3 effect), this could decrease the pitch, then the rotor gets less loaded, the pitch increases etc etc, causing a possibly destructive resonnance.
It happenned to a friend whose helicopter entered a vertical resonnance du to the exitement of the gearbox suspensions.

thank you in advance Chuck, as usual.

Victor
 
Victor, “flap” is a term that can be confusing if not clearly defined.

There are 3 axes of concern in a rotorcraft:

(1) tip plane axis
(2) control axis (the swash plate axis when feathering cyclic control is present or the rotorhead axis when it is not).
(3) The powered shaft axis

A rotor always “flaps” with respect to the control axis while in translational flight.

It need not “flap” with respect to the shaft axis, in fact, it can even flap forward, depending upon horizontal CG location.

The attached drawings, from “Aerodynamics of the Helicopter” by Gessow & Myers illustrate this quite clearly.

In my specific case, the horizontal CG was positioned so as to eliminate flapping with respect to the rotorhead in normal cruise flight. The rubber bushings permitted ~5º of hub tilt with respect to the rotorhead.
 

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Chuck,
Thank you for your fast reply, as usual.
Yes i meant the tilting allowed by the rubber bushings. 5° sound stiff.

i would be grateful if you could tell me if you encoutered some resonnance problems due to the rubber bushings.
A friend of mine, while we were drawing some ideas, told me that, with one of his prototypes, entered a severe vertical resonnance that could have sent him in the rotor if his belt was not fastened. very violent, his soulders hurted but he could land and stop it. After some thinking, he realized that that was the rubber mounts that allowed that resonnance, the sawhplate was dissociated from the mast suspension, hence, when the mast+main transmission began to oscillate, the pitch would change due to delta 3 effect...
we actually wonder if you encountered this kind of resonnance or, at least, vibration. It may be a serious problem to overcome, maybe not .

Thank you for your reply chuck.

Victor
 
I believe delta 3 is a change in pitch as the rotor teeters, to command the disk to align with the spindle. What you described is an uncommanded input from uncoordinated pivoting points between the controll/mast. As far as the 5* enough or not goes, it depends on your designed top speed.
 
Victor, your friend probably had coupling between collective pitch and the compression of the rubber bushings under load.

I was aware of that and had a collective stop that in essence locked collective movement to the hub at normal flying pitch. That way, coupling between collective and compression of the rubber bushings was eliminated.

It would be better to connect the hub to the rotorhead by a spherical roller bearing such as a Shafer DAL26-46A and use the rubber bushings to supply driving torque and flap stiffness.
 
Hello Phil, Chuck.

Chuck, excuse me if i don't explain things very well, yes it is collective/compression coupling, not d3 link.

Thank you for the information, it seems that this problem is going to be a serious issue.

i had a look at the SHAFER bearings and i don't know if they can worl properly under continuous axial loads.
adding a roller bearing will seriously complicate the simplicity of your design and it may loose interest.

another solution would be to link the pith horns to the triling edge of the blades but the instability would be worse..

there's still some food for thought, i suppose.

thank you for your help Chuck.

Victor

PS : how can i contact you in private ? does your "send a message via email" option work on the forum?
 
Victor, the Doblhoff tip jet helicopter used a spherical roller bearing for connection of hub to mast in a completely free floating arrangement.

With tip jet rotor drive, there was obviously no need for transmitting torque.

What I visualize is something like the attached sketch with the rubber bushings designed for radial stiffness but soft axially.
 

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Hi Chuck,
oh yes tip jets are wonderfull in many aspects...
yes torque is a problem, your arrangement sounds good, we also plan to use pseudo-laminated bushings to have axial softness but radial stiffness.
I have read the Shafer catalog and had a look at the refecence you gave me and it sounds like it could do the job.

BMW have a kind of donut that is designed to transmit torque with softness..we may also investigate that way.

Thank you Chuck

is there a way to contact you in PM ?
 
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Sounds like BMW may be using a Lovejoy coupling.
 

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that's it.
i actually wonder if this system could be adapted.
Or maybe we could shape the fixed hub and the floating hub, then mold an elastomer, to reproduce the same effect+replace the bushings.

Chuck, you really know a lot of useful products and manufacturers, have you got an idea where i could find a laminated bushing that could be used as thrust bearing, like the ones that are used in the exec hub ?
i could'nt find a manufacturer.

Thank you
 
The earlier Mosquito used a flat Torrington needle bearing thrust washer.

The later models appear to have a ball bearing stack.
 
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