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:

http://www.cvcoupling.com/thompson_coupling/index_...

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.

XXXXXXXXXXXXXXXXXXXXXXXXXX
X                        X
X      XXXXXXXXXXXXX     X
       XXXXXXXXXXXXX
Xxxxxxxxxxxxxxxxxxxxxxxxxx


xxxxxxxxxxxxxxxxxxxxxxxxxx
       XXXXXXXXXXXXX
X      XXXXXXXXXXXXX     X
X                        X
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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 http://www.uspto.gov/patft/index.html 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 http://www.synchrolite.com/1350.html 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 http://aerospace.rexnord.com/CatalogPDF/tuflite.pd.... 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.
http://www.synchrolite.com/1385.html.


"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.  http://www.synchrolite.com/B257.html


" 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. :)http://www.synchrolite.com/ControlFlight.html

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

Hi Graviman,

You said:

"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."

The pendulum effect doesn't work like it might seem due to aerodynamic effects on the rotor system with any sort of lateral airflow across it. There is really no such thing as an automatically "stable" hover with conventional flying controls; any displacement from the hover becomes divergent due to something called flapback. The rotor system (disc) progressively tilts away from the direction of the initial travel. The "pendulum" effect actually causes an "overswing" of the aircraft's fuselage, resulting in a larger travel in the opposite direction and the situation gets quickly worse. Present day helicopters often have an autopilot, using gyros as a trimmed attitude reference, to help the pilot out.

Graviman,

OK. I understand what your objective is.

A couple of related quotes;
"The ubiquitous nature of cross-coupling constitutes one of the chief reasons why piloting this type of aircraft [helicopter] requires such high skill levels developed through long training programmes" ~ Gareth D. Padfield - 1996

"In more distracting circumstances, however, he [the pilot] would benefit if the helicopter control system would make the aircraft quickly do what the pilot wants it to do, and nothing more." ~ R.W. Prouty


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

A book called 'Whirlybirds ~ Americas first 50 years' talks about hands-off hover for the Cheyenne.  In a separate chapter, it talked about hands-off for the Hiller with paddles.  There was a picture of the Hiller hovering in ground effect and the only things in the pilot's seat were some bags of sand.

This is the stuff that I've pulled together on the subject.
http://www.synchrolite.com/B306.html
http://www.synchrolite.com/B326.html
http://www.synchrolite.com/B318.html

Dave

"The pendulum effect doesn't work like it might seem due to aerodynamic effects..."

Thanks PEW. That is a much better explanation of what i was getting at.

"Present day helicopters often have an autopilot, using gyros as a trimmed attitude reference, to help the pilot out."

What really impressed my about the original Lockheed system (as fitted to CL475) was that it was entirely mechanical. Electrohydraulics are fine, but not really in the budget of a recreational craft.

"...Hiller hovering in ground effect and the only things in the pilot's seat were some bags of sand."

Wow! I have never been sure how well the Hiller system worked in aerodynamic regimes other than hover. I imagine it's weakness was that the trim forces vary with helicopter velocity (in all 3 axes).

"This is the stuff that I've pulled together on the subject..."

A usual, very well researched. How do you use the equations of motion? In automotive the dynamics are checked out in a package called ADAMS. By having an explicit (in time) simulation of the modal behavior, it is possible to see (graphically) the vehicle dynamics at a very early stage. This has been helpful in the truck project i am on, in spotting potential ride/handling quirks.

Usually Nastran and ADAMS are used in combination to pick up dynamic/structural interaction. This is similar to aeroflexing used in fixed wing design. I imagine the licensing costs of these packages is prohibitive, but if you have a friendly university with research budget...

Mart

Been thinking a little about a gyro stabilised control system. Although it doesn't matter where the gyro goes, for packaging convenience lets say it is fitted to the base of the cyclic. This would require a very high rpm gyro, so may not be the best solution.

Basically the gyro is connected to swash plate through the normal controls. In this way, especially with a rigid rotor, the helicopter manouvres to keep level with the gyro (linear direction changes being ignored). The cyclic then connects to the gyro with a lead of 90 degrees. The input force into the gyro then processes it, resulting in helicopter roll/pitch rate being proportional to input force. For pilot feel, the cyclic connection to the gyro has inbuilt springs, thus giving the helicopter a linear roll/pitch rate for given cyclic movement.

In the Lockheed system the gyro was part of the rotor assy, since this avoided the need for seperate drive. some sort of geared system may be better suited. The Hiller system was also force controlled, but the force was generated through the aero trim tabs - this may have given awkward dynamics in air velocity vector fields other than hover.

One major advantage of the system is that it tunes out ANY helicopter handling quirks. The rate of roll/pitch is directly proportional to cyclic position. This will result in a machine with very easy to learn characteristics. There may even be CAA FAA incentives for such an easy to fly machine - especially if light. Maybe an rpm controlled rotor, to avoid auto-rotation stalling, would also improved safety factors...

Any thoughts?

Mart

Graviman,

A few thoughts.


Lu Zuckerman on PPRuNe is very knowledgeable about the Lockheed gyro system. You may wish to search past threads on PPRuNe on this subject, or start a new one.


"There may even be CAA FAA incentives for such an easy to fly machine"

The reason for trying to get the SynchroLite under 254 lbs is to escape all mandatory FAA piloting restrictions.  At the risk of being boring ~ In my opinion, the intermeshing configuration is the first step toward ease of piloting.


"Maybe an rpm controlled rotor, to avoid auto-rotation stalling, would also improved safety factors..."

The world's first production helicopter, the Flettner FL-282, had a rotor governor. Here is some information on rotor governors, which might be of interest. http://www.synchrolite.com/Governor.html

Dave


 

"Lu Zuckerman on PPRuNe is very knowledgeable about the Lockheed gyro system."

Thanks for this - interesting read. From Lu's comments i gather that the (3 bladed) CL475 was a success, and that the system only really struggled when scaled up to meet the increasing weight demands of any program.

"The reason for trying to get the SynchroLite under 254 lbs is to escape all mandatory FAA piloting restrictions."

Neat. Not sure how that works with the CAA (uk). Hopefully not too different, but I'm not sure if there is a microlight rotorcraft class...

"...the intermeshing configuration is the first step toward ease of piloting."

Agreed, but it might still be worth getting a friendly university to simulate the dynamics. This is old hat in the auto world, then again the volumes are different. You would be suprised how many handling bugs you an pick up from an early stage. I realise rigids will not have the flapping problem, but I imagine that there will still be some "pendulum" tendancy. Even though the craft has lateral symmetry, there will still be characteristics that need to be learned.

I'm happy to put this in another thread if you feel there is threadjacking going on. Perhaps I should revive:
http://eng-tips.com/viewthread.cfm?qid=97039

I really like your hub design, and just wonder if it is worth goint a bit further. My thought processes find it hard to constrain themselves!

"Here is some information on rotor governors, which might be of interest."

Interesting, but I wasn't sure how these helped with low RRPM stalling. I presume the collective minimum is governed by RRPM and variometer (or vertical flow velocity), to not allow blades to stall. A hard landing is better than a crash...

Mart

"A little story for the heck of it...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."

I take it that your emphasis has now shifted back from Unicopter to Synchrolite? Why not design Synchrolite II? It could be built using tubular steel, to keep cost and weight down. Once up and running, the project would also then provide a testbed to try out concepts eventually destined for the Unicopter. At that stage you could look at composites. I would suggest the gyro control system as something that might be worth prototyping...

"Is there a smiley for 'fingers crossed?"

Not sure, but my fingers is crossed anyhow.

Mart

Graviman,

The SynchroLite and the Unicopter have a lot in common. They both are intermeshing and they both will have 3-blade rotors.  One of the differences is that the UniCopter is to have the 'Absolutely' Rigid Rotors [http://www.synchrolite.com/0815.html], whereas the SynchroLite is to now have the Concentric Double Universal Joint partially rigid rotors [http://www.synchrolite.com/1377.html].

"I take it that your emphasis has now shifted back from Unicopter to SynchroLite?"

Yes. Both are being developed.  The 'flip-flop' comes about when the current project hits a small snag or a neat idea comes about for the other one.


Your comments regarding the building and testing of the new concepts makes good sense.


" I really like your hub design, and just wonder if it is worth going a bit further. My thought processes find it hard to constrain themselves!"

Any or all of your wild or wonderful thoughts will be appreciated.


" rotor governors" & "low RRPM stalling"

A rotor governor controls collective blade pitch and thereby controls the inertia being removed from the rotor disk.

If you're interested in a very different rotor governor idea, look at this one. http://www.synchrolite.com/0575.html


Dave

"The SynchroLite and the Unicopter have a lot in common..."

Hence the shift back to Synchrolite. I take it that an ARR has more potential aeroflexure problems to overcome/investigate? I like the idea of a rigid mast, and thrust bearings - sensible approach. I never liked the idea of rotating shafts taking bending moments - to much fatigue risk at (accidental) high loads...

"...'flip-flop' comes about when the current project hits a small snag or a neat idea comes about for the other one."

Probably the best way. So the emphasis is always:
Synchrolite - affordable microlight (intermeshing) rotorcraft;
Unicopter - High speed, high performance (intermeshing) rotorcraft.

"Your comments regarding the building and testing ... wild or wonderful thoughts will be appreciated."

Well i may not be aerospace (yet), but I'm used to juggling prototype requirements during a program. I'm the ideas man on my current project (you'd be suprised how technically clever an off-road truck can be), although i usually state durability, dynamics & design. I get involved in pretty much everything!

"A rotor governor controls collective blade pitch"

Yes, I definately appreciate the concept of cone/pitch coupling. This makes sense in removing uncommanded responses to lift/sink.

I didn't know if there was a design to limit maximum collective to just below blade stall. It could be done with either a combination of vertical velocity and RRPM, or a blade aoa (rel airflow) servo in rotor assy. Might stop some of those nasty falling from the sky type of incidents, by overiding a panicky pilot (me for example )...

"If you're interested in a very different rotor governor idea..."

Hmmm, i can see the point. Don't know if a machine with no direct vertical control is wise though. OK fixed wing pilots get used to having to manage the glide slope, but it would mean a change of flight procedure in rotorcraft engine failure. Might push workload that little bit too high...

I have often wondered if a simple one-way clutch between engine and flywheel would provide spare inertia to flare the blades. The collective could have a stiff spring, normally retracted with engine running, that would give a pilot a the feel that he was pulling the rotor into a potentially dangerous state. Rotor would detent to autorotate position, and pilot would have to "fight" collective to flare. Gives a safe landing, but also communicates the flight characteristics to the pilot.

Actually I think that the feel of controls is sometimes overlooked in aircraft. The dynamics guru at Peugeot used to reckon that you had a general idea of what your car was doing from the inner ear. The sensation of skidding (particularly at the back) came through the driver's backside. The most direct way to communicate the level of grip, though, came from the torque in the steering wheel. Only the mark I, fully gimballing, eyeball would override that input...

Mart

Graviman,

"I didn't know if there was a design to limit maximum collective to just below blade stall. It could be done with either a combination of vertical velocity and RRPM, or a blade aoa (rel airflow) servo in rotor assy. Might stop some of those nasty falling from the sky type of incidents, by overiding a panicky pilot (me for example "

At the bottom of an autorotative landing you may want to pull full collective to use up the rotor's inertia.  

With a conventional rotor and loss of power, the pilot must take action to lower the collective.  With a rotor governor, the pilot must take no action to lower the rotor. The 'theory' is that of letting the craft put itself into the authoritative state.
_________________

" Don't know if a machine with no direct vertical control is wise though.'

The collective pitch (lift) is determined from the engine's torque.  Increasing the engine's torque immediately increases the lift. Then the engine slowly picks up the rotor's RPM.

Dave

"Increasing the engine's torque immediately increases the lift."

Works fine as long as engine is producing torque. I was thinking along the lines of chopper puts itself into autorotative state (ie no height velocity curve, using same governing mech), but still allows pilot to pull RRPM off to stall aircraft into ground. By having a (normally retracted) spring there, pilot feels when he is asking rotor to stall. Avoids pilot trying to stretch the glide, when he/she/it realises that chopper won't make the chosen landing spot...

Mart