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[Graviman]

Slowrotor,

2P will happen everytime the cyclic is moved off-centre. Models get away with it because they carry no useful payload (and bending moments don't scale linearly) - so structure can withstand the vibration. The batteries don't scale up too well either.


Dave,

Interesting concept. My first thought would be why even allow hub compliance, if the gyro is there to remove 2P vibration. The only advantage i can think of is the convenient hangerage of a 2-blader. Probably once the gyro mass was taken into account, an additional blade or two works out lighter. Then again the hub compliance does allow a smaller gyro, at the expense of some zero-zero control loss (bladestrike? ).

Agree about designing in a system for dynamic stab aug though. A gyro could force helo to follow cyclic, giving linear displacement roll/pitch rate. The gyro could thus be embedded within the airframe (this is effectively how electromechanical SAS works).

Mart

[slowrotor]

Dave,
The 2/rev vibration may occur with pitch change also as you suggest. But should we care. If the pitch change or roll is done slowly, the vibration would be minimal.
The advantage of restricting the flapping with a rigid head would be worth the effort in spite of the additional issues. For a coaxial or intermesh the rigid head would be best I think, to avoid blade contact.
My point is:
The models work very well with rigid hubs. I dont see any vibration. The vibration would be hard to see on a model, I admit, but I certainly see other types of vibrations in my model. A model cannot be compared with a full size in some respects of course, but the models show the way for simpler design.

[Dave_Jackson]

Heliport,

Thanks.
It's looking like the 'bandwidth' usage ain't going to be too large.


Slowrotor,

I think that two, or more, recreational coaxial helicopters have rigid (no teetering or flapping) hubs with 2 blades each, as you suggest. However, I really question this approach. In addition, one of these coaxials has no ability to autorotate.

The following is a quick 'cut and past' from a UniCopter web page;
__________________________
Hiller Rigid Rotors: Hiller X-2-235, Hiller XH-44

"... the extremely rigid rotor of the Hiller lead to "Vibration problems curtailed these tests for fear of damaging the wind tunnel" When you worry about the aircraft wiping out the tunnel, you have problems! ~ Nick Lappos on RAR

Dual rotors with two blades each will result in a lateral vibration when longitudinal cyclic is applied. See: DESIGN: SynchroLite ~ Rotor - Disk - Lift Distribution re: Vibration ~ DBJ
___________________

This submitted idea may not work and it is not intended for any of my projects. It just seems like an idea where one device might satisfy 3 or 4 requirements and be fun for a number of people to work through.


Mart,

Would you elaborate on 'hub compliance' and on 'zero-zero control loss'. Thanks.


Dave

[slowrotor]

Dave,
The Hiller had super rigid blades. I am talking about flexible blades with a rigid hub.
The rigid hub works fine with models. We might find it would work with very small and slow helicopters.
If you know of any two blade single rotor with rigid hub, please let me know.
Thanks
slowrotor

[Graviman]

Dave,

'hub compliance' - 1/stiffness of hub spring. If spring is stiff, does it need to be there?

'zero-zero control loss' - reduced g rotor unloading and reduced foward speed tailplane surface ineffectiveness (just quicker to type).

Perhaps not relevent to this thread, i have been reconsidering reason for Lockheed blade forward sweep. Since flapback is natural to any rotorhead, the forward sweep was really there to compensate the tendancy for a hingeless to roll left (US rotation) in forward flight.


Slowrotor,

Hiller X-2-235 blades were super rigid for their day, but in the absence of data outside of that PR shot i would conservatively estimate offset to 15%. If pushed i might be convinced of 30%, but you really have to be careful how much engineering data you read into a PR photo...

Mart

[Dave_Jackson]

Mart,

Thanks for clarifications.

If spring is stiff, does it need to be there?

The intention is that the physical movement of a teetering rotorhub with a hub spring, or the shifting of the tie bar on a teetering hub with offset hinges provide the force to 'reset' the orientation of the gyro-assemblage; twice per rotor revolution.


Dave

[Graviman]

Dave, OK so a 21st century Hiller paddle system (with CVJ hub). In this forum i learned that the Hiller system is flawed, because the delay of a teetering rotor means pilot has to think a second or so ahead - pilot is flying the rotor rather than the machine which swings underneath.

An alternative objective might be to give the zero-zero control authority of hingeless rotors to a 2 bladed design. You could achieve this by having a gyro cyclically moved to minimise hub moment vibration (ie moving in opposition to blade cyclic 90' out of phase to blades). So the gyro could simply be put inside the fuselage as a motor driven vibration adsorbing/cancelling device.

IMHO the whole setup is too complex unless the benefit is improved pilot machine interface. Lockheed got closest to ideal for a mechanical only solution, using a hingeless rotor. The achilles heel was the blade flap feedback via the pitchlink, needed to stop roll towards retreating side (blades were forward swept). This pitch feedback does not suit modern aerofoils, while its removal would result in some control asymmetry. Maybe a small amount of hub compliance would allow flapback to trim gyro directly (without risk of control divergence).

I can see why SAS systems have developed the way they have...

Mart

[forget]

The public disclosure of a new idea stops others from patenting it. However, a patent application must be applied for within a year of the public disclosure.

Dave, I'd say you are wrong here. Only the first part is correct. The public disclosure of a new idea stops others from patenting it.

Second part - once an idea is publically disclosed but without an issued Patent or application, you've no chance of stopping anyone, worldwide, from using the idea.

[Dave_Jackson]

forget,

The primary objective is to stop the ideas from being patented. This way all can freely use these basic ideas. However, an individual still has the ability to patent related or subordinate ideas, which he/she develops.


The second part of your posting could be a concern for 'open group inventions'.

The following is from a patent lawyer. "In Canada and the United States, you may still apply for a patent if such public use, sale, disclosure, etc. occurred no more than a year ago, and if a patent application can be prepared and filed within a year of the first such public use, disclosure, sale, etc."

The fact that he uses the word 'etc.' may make his statement questionable.

Any additional information that you can offer on this subject will be appreciated.

Thanks,
Dave
____________

Mart,

Will get back to your posting.

D

[Dave_Jackson]

Objective: 2-blade rotor with improved control and stability.

OK forget the gyro ~ for a moment.
Another idea added to the stack.

The following regards a 2-blade coaxial configuration that incorporates any means and amount of 'rotor rigidity' (IE. not a simple teetering rotor).

For a given amount of cyclic input, it appears that the amplitude of the vibration will differ when the direction of the cyclic input is the same as one of the blade crossing azimuths from when the direction of the cyclic input is midway between two of the blade crossing azimuths.

It therefore appears that the vibration may be lessened if the blade-crossing azimuths can be actively varied so that they always have a constant relationship with the ever-changing cyclical input azimuth.

This coaxial transmission should be able to change the crossing azimuths.

Just a wild idea.

Dave

[forget]

Any additional information that you can offer on this subject will be appreciated.

I'm right - and I'm wrong, depending on which side of the Atlantic you're sitting. The US has unilaterally gone sideways on this, despite Patents being very much an International thing.

From US Patent Office web

http://www.uspto.gov/web/offices/pac/provapp.htm

Provisional Application for Patent Filing Date Requirements

The provisional application must be made in the name(s) of all of the inventor(s). It can be filed up to 12 months following the date of first sale, offer for sale, public use, or publication of the invention. (These pre-filing disclosures, although protected in the United States, may preclude patenting in foreign countries.)

Such as the rest of the world.

[slowrotor]

Dave,
The Yamaha RMAX helo has two-blade hingeless hub.
see page 6 at this site: http://controls.ae.gatech.edu/papers/schrage_erf_99.pdf

I don't think the hiller paddle control would be needed with a manned helo.
The Mosquito ultralight apparently does not need a hiller system.
slowrotor

[Graviman]

Originally Posted by Dave Jackson

OK forget the gyro ~ for a moment.

Still worth considering, Dave. Flapback provides static stability, but a gyro is perfect for dynamic (ie transient input) stability.

One concept i have often wondered about is designing a 4-blade hingeless rotor with hiller bars. Basically two 2-blade designs at 90 degrees. Again complexity of this concept tells me that Lockheed got it right with final system (perfected after cancellation ).

Originally Posted by Dave Jackson

It therefore appears that the vibration may be lessened if the blade-crossing azimuths can be actively varied so that they always have a constant relationship with the ever-changing cyclical input azimuth.

My concern here is that you are introducing complexity for an otherwise simple to solve problem (extra blades). Ignoring the control complexity problems, the real headache is that every time you move the cyclic around the box you will perform an unrequested turn! The solution would simply be to provide additional drive to a smaller yaw control rotor, fitted aft of the main rotor.

Vibration is best cured from source, which to my mind means designing a main rotor assy which does not respond to frequencies to which it might be exposed. Continuously trimmable RPM rotors are feasible, but may increase the problem of rotor resonances (active or internal damping clearly helps - more complexity). Combine this with possible problems associated with high speed and it looks more daunting.

Mart

[Dave_Jackson]

forget,

Thanks for the information related to foreign patentability.
_______________________

slowrotor,

IMIgnorantO, the RMAX may have good stability. However, like the Mosquito it may not have good controllability.

IMO, the stiffer the RMAX blades are, the greater the 2/rev vibration is. Perhaps in models the rotor speed is so fast that the high frequency is not considered detrimental.
_______________________

Mart,

I'm not giving up on the gyro, it's just that the mind got temporarily side tracked.

Your last two paragraphs are very valid.
_______________________

The thinking behind a small light gyro with an exceptionally fast RPM, is that it might be justifiable if it (a single item) can satisfy more than one or two rotorcraft shortcomings.
Hopefully, stability and controllability are two of them.



Dave

[Graviman]

Originally Posted by DJ

IMO, the stiffer the RMAX blades are, the greater the 2/rev vibration is. Perhaps in models the rotor speed is so fast that the high frequency is not considered detrimental.

The model can be strengthened against the resonance and fatigue failure which would occur if it had to carry a useful payload. Besides the pilot has no direct feedback of the vibration.

Originally Posted by DJ

The thinking behind a small light gyro with an exceptionally fast RPM, is that it might be justifiable if it (a single item) can satisfy more than one or two rotorcraft shortcomings.
Hopefully, stability and controllability are two of them.

Agreed. With a rigid rotor the gyro can force a pitch/roll velocity response from the cyclic. With feedback of flapback you can maintain the speed/sideslip displacement response too. Sounds flyable to me.

----

At times i feel helidynamics is like quicksand - you think it all makes sense then explaining it to someone causes you to reconsider. Thinking about whether the response of a teetering is acceleration response to cyclic, until fuselage drag and flapback make it velocity (maybe) then displacement response, has made me reconsider Robinson "wee-wa". If the rotor was changing attitude, i can see how the relative upflow in one half and downflow in the other would cause an azimuthal reduction in lead angle from 90 degrees.

I think this goes under the theory vs experience folder...

Mart

[Dave_Jackson]

This is my thinking on the subject; subject to objections or improvements by others.


Objective:

To create a constant moment at the top of the mast when the cyclic stick of a 2-bladed rotor is moved off of center.


Physical Layout:

The rotor is a teetering rotor with a hub spring.

The gyro-assemblage consists of two counter-rotating gyros. Their rotational inertia is obtained from high rotational speeds and not from large masses. The two gyros are independently gimbaled on the mast, just below the rotor-hub. The two gyros are linked together so that they tip in unison. Their rotation is provided by electric motors. Perhaps the mass of the gyro is the rotor of its axial flux motor.

Description of Operation:

When the rotor disk is tilted down, the aerodynamic force that applies this tilting is partially resisted when the blades are in the rotor disk's two quadrants of tilt. Half of this resistance comes from the hub spring and therefore a moment is created at the top of the mast. The other half of this resistance comes from the act of tilting the gyro-assemblage in the same direction.

When the blades have rotated 90-deg and are in the two other quadrants, a cam-follower device at the bottom of the rotor-hub applies a force to the gyro-assemblage. This force reorients the gyro-assemblage to its normal position. The opposing force of this reorientation on the cam followers applies a moment at the top of the mast.

Theoretically, The moment on the hub spring from the teetered blades and the moment from the normalizing of the gyro-assemblage create an equal and constant moment at the top of the mast.

[Graviman]

Dave, gyros will need to be actuated out of phase or forces cancel. Otherwise the principle is sound. My question remains why would you want to add this additional complexity to the design of a helo? Additional blades are mechanically much simpler, at the cost of increased hangar space.

The reason i ask is that helos are driven by cost, which reflects on us mortals as $/hour. Introducing additional mechanical componentry increases both the initial outlay and the maintenance cost (inspections even if parts have infinite life). I'm not against the use of gyros, far from it since i believe that significant handling improvements are possible, but preference is to use them sparingly and for good reason.

The only real reason i champion the unpopular view of a mechanical SAS over electromechanical systems like the Honeywell SPZ 7600 is that for a light heli a mechanical system will be less likely to be overlooked. Combine this with the fact that failure is likely to be a progressive reduction in performance by wear and it strikes me as a good engineering soln. It does not allow all the advanced features that a full FBW system would, and naturally i accept that FBW will be the long term winner.

Mart

[slowrotor]

Dave,
I agree with Mart about keeping design simple. Two blades is simple, partly the reason for the large sales of Robinson compared with Enstrom and S-300 with the complication and cost of three blades.

Mart,
The Yamaha RMAX is more than a model( at 128lbs). It has also carried a videocam, no reports of vibration on the camera. More than 1200 have been sold mostly for crop dusting by non-pilot farmers in Japan. It has automatic hover. Pretty impressive.

[Dave_Jackson]

Yes, the 2-blade w/ gyro will have pros and cons and 3-blade rotors have their own pros and cons. God knows which could be the best. Its just that the desire to remove the 2P vibration from a 2-blade rotor, which has some amount of rigidity, has bugged me for years.

Mart,

gyros will need to be actuated out of phase or forces cancel.

Correct me if this is wrong but I think that having 2 gyros counter-rotating about the same axis, but having their own independent gimbals should work.

The rotating members would be linked by 3 tie-rods, with spherical bearings at both ends of each rod. This way the two gyro disks always tip in the same direction, by the same amount.

Because they are counter-rotating, the opposing gyroscopic moments should be canceling each other in the tie-rods and not in their common axle (mast).

Hopefully, this will will result in a gyro-assemblage with only a resistance to a tipping of it's plane. It is this resistance that provides the foundation of the moment when the two blades are not providing it.

If you find that this is flawed, please say so.


Dave

[slowrotor]

Dave and Mart,
I just tested my two blade hingeless model and found there is a vibration with cyclic input. It seems to be about the same magnitude as the other normal vibrations.
So Dave, I hope your invention works.