New Rotorhub Invention

Why, in your opinion is that inset, self centreing, double cardan joint better than a conventional CV joint?

It is a neat idea, but it is far more complex than a simple 6 groove balljoint as used at the wheel end of any FWD car.

I'd hazard a guess that size for size and weight for weight its torque capacity is much lower. It probably has a better friction vs endload characteristic.

Given that the entire weight and drive torque of the machine is going through this joint the failure mode needs to be benign. Is it?

If you want a friction-less CV joint at the rotor head why not use a rubber bush? They can be made to have very low coning rates (as we call them) while still being able to transmit torque and thrust.







Cheers

Greg Locock

GregLocock,

Thanks for the comments and concerns. I must think each one through.

Here are some initial thoughts, which are also open for criticism or comment by you or others.

"Why, in your opinion is that inset, self centering, double Cardan joint better than a conventional CV joint?'

1. A large portion of the load on the rotor of a helicopter is vertical and I don't know how well the Rzeppa joint can handle axial loading.

2. The bearings can be elastomeric and is this will overcome the difficulty of lubricating a vertically mounted Rzeppa.

3. Custom Rzeppa joints may be very expensive to design and build.

Dave

Ah, a variant of the Thompson coupling:


The only problem I can see is that the intermediate shaft could gimble, producing the problem you are trying to overcome. The Thompson coupling has additional mechanism precisely to overcome that.

Other than that it's a neat idea, but maybe an elastomeric couple would be more reliable/fail-safe. I think the fact of having a "rigid" rotor head will improve the dynamics anyway, since drive shaft must already handle lateral loads.

Mart

Mart,

You are right. The basic operation of the Concentric Double Universal Joint and the Thompson Coupling are identical. As you say, they differ in their means of assuring that the 'central component' exactly bisects the angle.

The Thompson coupling could probably be used to advantage on a three or more blade teetering rotor. However, to take full advantage of the rotor, a hub spring should be used, also.

The 'central component' requires very little force to move, whereas the hub spring must exert very large forces to be effective. Simplistically put, the hub spring is divided into two components with equal force x length characteristics and the actuator arm (ring) of the 'central component' is located between the two. This way it's angle is always 1/2 that of the CVJ.
________

Correct me if I am wrong, but I don't believe that anyone has come up with an effective elastomeric CVJ yet.

Dave

"...the hub spring is divided into two components...and the actuator arm (ring) of the 'central component' is located between the two."

Oh yeah, hadn't picked up on that. Very neat.

"...I don't believe that anyone has come up with an effective elastomeric CVJ yet."

No, but that is because there is no percieved requirement. Not much call for stiff CV joints in auto-engineering!Trelleborg (among others - Silvertown, Maskew) offer elastomeric joints for off-road suspension links, so the durability is there.

Basically you would have to allow the rubber to shear in the joint, so it would probably end up looking like an elastomeric CVT (as Greg suggests). Unlike a CVT the design would be more spherical than cylindrical, so as to react rotor thrust - so outer shell would be in two halves. The only real advantage over your design is that there would be no bearings, requiring regular greasing. You may also find it to be more compact, since the axial thrust is spread out over a large area.

I could put you in touch with various elastomeric suppliers, if you wanted to take the concept further. They should at least give you material properties for the elastomer.

Mart

"Correct me if I am wrong, but I don't believe that anyone has come up with an effective elastomeric CVJ yet."

It depends how much articulation you need. 1 or 2 degrees is easy, Jurid couplings are a long proven strong component.

If you need more then the standard Jurid's are no good, as the torque is transmitted by nylon cords, which makes them rather rigid.

For some of our suspension joints we use what we call rubber ball joints. These are conventional rubber cotton reel bushes, with an articulation of up to 15 degrees, and very low coning and (optionally)torsional rates, but high translational rates. The failure mode is non catastrophic as you can make sure that the inner metal cannot pull out past the outer metal. Here's a cross section showing the metal parts only. The ends have to be swaged over after assembly, or a separate part could be used.

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Cheers

Greg Locock

Mart,

Re "effective elastomeric CVJ"

Sikorsky has a couple of patents similar to what you are describing. One is entitled Elastomeric high torque, constant velocity joint. If interested, it can be seen at and the patent number is 4,714,450. However, I don't believe that it will work.

Bell has a number of patents for a rotor hub that is functionally the same but is configured quite differently.


"I could put you in touch with various elastomeric suppliers, if you wanted to take the concept further. They should at least give you material properties for the elastomer."

A hub for the SynchroLite is currently being designed. The building of it should start next month. Any information on elastomeric suppliers will definitely be of help.
_______________

GregLocock

The maximum angle is 8-degrees.

Your second idea is interesting, but for the helicopter, there must be absolutely no linear motion, or rotational motion about the vertical axis. The use of rubber suggests unwanted motion.


Dave J

"Your second idea is interesting, but for the helicopter, there must be absolutely no linear motion, or rotational motion about the vertical axis. The use of rubber suggests unwanted motion"

No such thing as 'absolutely no motion'in engineering, unless there are absolutely no forces.

Everything has a compliance.

Cheers

Greg Locock

"Sikorsky has a couple of patents similar to what you are describing... I don't believe that it will work"

True, any vertical flex will cause potentially dangerous control problems. Something similar eventually killed the Cheyenne, although it was cured. Actually, i really like your hub concept. Have you got any particular bearings in mind? Naturally these must take the vertical load, plus any vibration load.

"Any information on elastomeric suppliers will definitely be of help."

Trelleborg (VERY slow to respond):

dermot.fitzgerald@trelleborg.com


Silvertown (Seem to be pretty good):

Eddie.Deeming@silvertown.co.uk


Maskew:

darryl@allanmaskew.co.za

I'm sure greg can give you some supplier details too...

Mart

People I've dealt with over the years:

Boge

Avon

Peradin

Cooper

Lord

Freudenberg

(Trelleborg are good as well)

Once you come up with a compliance and max load definition for each axis (ie 6 numbers) then you can get serious.

Cheers

Greg Locock

Graviman.

" True, any vertical flex will cause potentially dangerous control problems."

In addition, when the Sikorsky Elastomeric CVJ teeters it will compress the elastomer at some locations, and this is a no-no.

" Have you got any particular bearings in mind?"

Victor [zeeoo] mentioned composite bearings, so I when hunting on the net and came up with Rexnord Tuflite They look ideal, but if anyone has a concern, please say so.


Greg and Mart,

Thanks for the lists of suppliers.

Dave

"Victor [zeeoo] mentioned composite bearings, so I when hunting on the net and came up with Rexnord Tuflite..."

For some reason I couldn't download pics. Even so they read pretty impressive, especially if they are maintenance free. I take it a small amount of stiction will not adversely affect the dynamics? This can be thought of as high damping at small displacements. Worst case is that the control link lead angles may alter slightly for small inputs, since flapping/teetering damping will raise resonant frequency (i.e. it becomes more rigid). Practical upshot might lead to very mild cross coupling - my guess is that there will be no real problem.

Can they handle the end thrusts that teetering would generate? During startup, lateral loads due to (say) gear/tyre resonance will cause end thrusts. Did you reject preloaded conical tapers on cost or reliability/maintenance grounds?

Mart

Mart

Graviman,

The composite Tuflite bearings were chosen for their space saving and stated good characteristics.

"I take it a small amount of stiction will not adversely affect the dynamics?"
The aerodynamic force and hub force are so large that a little stiction probably won't be a problem. The teetering joints on this hub are mechanically identical to those on the Robinson's hub. Robinson torques their three teetering and coning/flapping nut/bolts to 15 ft-lb.

Interestingly, this 'advanced?' hub has only one more hinge than Robinson's hub.

Dave

"Interestingly, this 'advanced?' hub has only one more hinge than Robinson's hub."

It does look like a good solid design. In fairness, i am really just curious as to the constraints when choosing a bearing for this application.

The Jaguar X-type is basically a Ford Mondeo, but (among other things) the MacPhearson strut top bush was replaced by a bearing because the engineers felt it would allow driver more steering feel. I didn't know if the same constraints applied.

Out of interest do you have any predictions about the basic dynamics? I am curious, since I wasn't sure whether lateral roll was collective or cyclically controlled. Will the synchrocopter improve lateral control in hover, over say an R22? Have you considered any gyro mechanisms in the control system, for ultimate in pilot feel? I am still impressed at tales of a fixed wing pilot (with 0 helicopter hours) jumping into the Lockheed CL475 and flying it home!

Mart

Graviman,


"In fairness, i am really just curious as to the constraints when choosing a bearing for this application."

For interest, a view of the Robinson R22 hub, from their maintenance manual, has been placed on this web page.


"I wasn't sure whether lateral roll was collective or cyclically controlled."

Roll is by lateral cyclic.


"Will the synchrocopter improve lateral control in hover, over say an R22?"

The SynchroLite will have stronger lateral control than the R22. This will be due to the CVJ rotor's hub-spring. This rotorhead will give a stronger moment than a teetering rotor. In addition, this type of head can be lower and this results in a shorter moment arm between the rotor and the craft's CG.

Dynamic lateral stability may be of concern. However, the gyroscopic precession of the two counter-rotating rotors may reduce this concern. The bottom line is that the SynchroLite will have less cross-coupling than a single rotor helicopter has. Of course, the UniCopter will have even less cross-coupling than the SynchroLite.

Dave

"For interest, a view of the Robinson R22 hub..."

Simple, effective, neat...

"Roll is by lateral cyclic."

Makes sense if the hubs are close together. Will there be any collective differential? I don't know if any advantage would exist, admitedly. It may increase the roll damping - reducing pilot workload.

"Dynamic lateral stability may be of concern. However, the gyroscopic precession of the two counter-rotating rotors may reduce this concern."

This was sort of my query. No doubt that the hingless rotor will offer exceptional pilot control. I was really thinking from the point of view of the effective damping factor in pitch/roll response. I gather (from Prouty) that this is the main problem in hover control. The pilot effectively ends up inputing the damping control to the hover "resonance" - this is why it is so hard to stay on the spot.

The Lockheed system allowed the pilot to hover hands off. This was accomplished by the gyro controlling rate of pitch/roll directly. The pilot thus had direct control of attitude, while the helicopter control system kept the dynamics under check (ie well damped, in practice).

It just seems that if you are going to the trouble of designing such a neat hub mech, it may seriously be worth considering such a control system. From drawings I am always amazed at how simple the system actually is, despite the very clever dynamics. I know that a low cg, relative to sideslip centre of pressure, will give positive dihedral. Depending on your market, the idea of the stable helicopter may have significant appeal...

Have you thought out the control mech yet? I imaging that, unlike the CL475, the best position for the gyro is actually very near the control stick - for direct input/feedback.

Just a thought.

Mart

Graviman,

" Will there be any collective differential? "

Answer(s) here.

" pitch/roll response"

The opposite turning rotors should mitigate this.


" The Lockheed system allowed the pilot to hover hands off."

For a short period of time the US Military used the Kaman Huskie (intermeshing) to start the training of helicopter pilots. They then stopped using them because they were too easy to fly.


" Have you thought out the control mech yet?"

The intent is to use the control system that was developed a few years ago for the SynchroLite. All the design work has been done.
Dave

Graviman,

A little story for the heck of it.

The SynchroLite was ready for construction of the prototype a few years ago. Just before starting, I took a reality check and decided that there were two strong reasons for not going ahead with the build.

Firstly, was that the craft's construction would be very expensive, if the intent was to keep it under 254 lbs (US Ultralight category ~ no licensing required). This would totally remove it from the 'homebuilt' market.

Secondly, it did not have sufficient 'ease of piloting' to attract those people who had more disposable money but who had no interest in building it, maintaining it, or getting a full helicopters pilot license. The UniCopter, with its absolutely rigid rotors, was started to overcome this limitation. Then the UniCopter project evolved into an attempt to overcome other impediments of current rotorcraft, such as slow forward velocities etc.

It now looks like this Concentric Double Universal Joint and Hub Spring should overcome the second of the above concerns and thereby make the SynchroLite a more viable craft.

Is there a smiley for 'fingers crossed?

Dave

All interesting stuff - i enjoy perusing your site.

I think you are misunderstanding my query about the control system though. I realise that the unicopter will be a very flyable machine, due to the rigid counter-rotating rotors. I understand that the controls will be very intuitive, with no cross coupling effects.

I really am just thinking about single channel response to either pitch or roll. If a chopper is in stable hover and is given a brief forward cyclic movement, the ideal is that the machine will settle with a forward speed until the same is applied rearwards. What I suspect happens is that the chopper continues forwards, until the centroid "pendulums" the chopper back to hover again. Likely it will swing a couple of times before settling back into a hover.

This means that the pilot, in addition to controlling the attitude of the chopper, is also controlling the pendulum swing. The practical upshot is that an inexperienced chopper pilot (or me, if i ever got the chance ) will end up struggling to match the pendulum frequency. This is what I mean by control damping, which I suspect to be an inverse function of disk loading (since rotor movement will damp motion). A gyro in the control system would effectively null this pendulum effect, or at least push the frequency down so low that it wasn't noticeable.

This is why I suggest a gyro in the control system. It's purpose wouldn't be to correct the basic characteristics of a very flyable machine, but to enhance them. The machine would become as easy to fly as the car that got the pilot to the airfield. If I was a "well-to-do" looking for a plaything, that would be high on my list.

Mart