Rotorcraft Dynamics - Hub Spring for Teetering Rotor

[Dave_Jackson]

Incorporating a hub spring (http://www.unicopter.com/B329.html#Hub_Spring) into a conventional 2-blade teetering rotor will result in more cyclical control authority and less chance of tail-boom incursion. Unfortunately, this hub spring will generate a 2P vibration when the tip-path-plane is tilted in respect to the mast.

It appears that the following concept will provide the above two advantages while minimizing the 2P vibration.

The basic idea consists of locating a 'trim-tube' between the teetering head and the mast. Hub springs are installed between the teetering head and the trim-tube, and 2 linear actuators are installed between this rotating trim-tube and a stationary support.


For the very interested or for constructive criticisms; Rotor - Hub - 2-blade - CVJ+HS (http://www.unicopter.com/0837.html)

[NickLappos]

the chief disadvantage of the hub spring is that it raises significantly the structural loads on the mast, defeating the only advantage of the teetering rotor - the fact that it delivers no moment to the mast/transmission allows extremey light structures, thus raising payload.

[Dave_Jackson]

You're probably right. It may be an idea without a viable application. :uhoh:

[Graviman]

Hmmm, complexity for no gain - bad idea. Stick to 3-rotor with CVJ+HS for effective hinge offset with no vibration, Dave.

This is the one instance where an interleaver scores highly. Both rotors may be teetering, while maintaining very positve roll authority. Pitch TPP angle would just help rotor to provide efficient forward thrust at high speed. It all falls down, of course, since the span of the aircraft for a given disk loading wipes out any weight advantage of not transferring rotor hub manouvreing torque on a single large rotor...

Nick, what was the construction of the Comanche hub? After seeing either you, or Bill Fell, doing unbelievable roll rates on Discovery Channel, i did some googling. I gather the construction was composite torque tubes, with hingless bearings? Is that using elastomeric feathering bushes then?

Mart

[NickLappos]

Dave,
The place where hub springs pay off is as safety bumpers near the edge of flapping limits, where the springs save the mast by creating a strong moment that repels the head (or forces the aircraft to pitch or roll away from the head.)
The teetering head on the tilt rotors use a hub spring design as a last resort, when the head begins to gimbol toward the mast.

Mart,
The Comanche used a very stiff blade root end (about 12% hinge offset) and a metal double-plate head. The blade had a stiff torque tube so that feathering could be accomplished by spar deformation without bearings. The blade had a damper on the torque tube that tied the spar to the tube, and absorbed inplane oscillations. Very snappy aircraft, you could roll from 60 degrees left bank to 60 degrees right bank and stop in 1 second, and hit 100 degrees per second roll rate while doing so!

[Dave_Jackson]

Mart,

IMO, the Interleaving configuration can never become viable until helicopter rotors have the ability to provide active and discrete pitch angles, at multiple zonal locations, about the combined disk area.

Active Blade Twist for the radial coordinate and Higher Harmonic Control for the azimuthal coordinate are a start; which can be undertaken today.

The Interleaving configuration should very likely become the optimum configuration for medium and heavy lift VTOL transport. Just consider that, the Interleaving configuration offers the advantage of a large disk area for hover, while also offering the advantages of short rotor-fuselage-rotor moment arms and fan-jets for high-speed cruise.


Nick,

Yes. Consideration was given to coupling the movement of the trim-tube to the movement of the cyclic stick. This would result in the hub springs only having an effect when the tip path plane was changed by an external perturbation.

It would have the advantage of reacting sooner than a fixed bumper. However, it's getting a little complex for a simple teetering rotor.

[Graviman]

"...Interleaving configuration offers the advantage of a large disk area for hover..."

But my arguement is that is doesn't. Although not as bad as say the V22, the machine will never be as compact as say coaxial for the same disk loading. It's just that if you consider only weight and not span, use of teetering will allow a very high roll rate for improved payload/MAUW over the coaxial. The size definately limits it's applications to large airfields only.

----

"...The (Comanche) blade had a stiff torque tube so that feathering could be accomplished by spar deformation without bearings."

Hard for me to visualise how this would work, unless you mean a compliant bush?

"The blade had a damper on the torque tube that tied the spar to the tube, and absorbed inplane oscillations."

Again, difficult to visualise without pictures, since most of my web search revealed only descriptive tech paper abstracts. When looking at hub close-ups it seemed to me that hydraulics were used to slide blade along tube, using coriolis force to correct blade lead/lag - effectively damping the in plane vibrations.

100'/sec is an amazing roll rate for a heli - however it works!

Mart

[Edit: To clarify my confusion about Comanche hub design]

[Dave_Jackson]

Mart,

To consider 'disk loading' you must first define 'disk area'. You are considering the 'disk area' of the coaxial to be the sum of the areas of the two disks. Some rotorcraft gurus use your definition for the coaxial, whereas other rotorcraft gurus define it as being the area of a single disk. Here is information on this subject. (http://www.unicopter.com/0949.html)

I take the latter for two reasons.

1/
The latter is more logical when all twin-rotor configurations are being considered.

The effective disk area for twin-rotor craft should be defined as the sum of the areas of the two rotors; with any overlapping area divided by 2. This means that for craft with equal diameter rotors the following approximations apply;
~ A side-by-side helicopter = 2*1000 = 2000 sq-ft.
~ A tandem helicopter = 2*1000 - 200 = 1800 sq-ft.
~ An interleaving helicopter = 2*1000 - 500 = 1500 sq-ft.
~ An intermeshing helicopter = 2*1000 - 700 = 1300 sq-ft.
~ A coaxial helicopter = 2*1000 - 1000 = 1000 sq-ft.

2/
The use of the latter can also be justified when moving away from 'Momentum Theory' and looking at the rotor performances using 'Blade Element Theory'.

By comparing;
~ a single rotor with 2 blades,
~ a pair of coaxial rotors with 2 blades each (total of 4 blades),
~ a single rotor with 4 blades,
it can be shown that the performance of coaxial rotors with a small gap is closer to the 4-blade single than the 2-blade single.

Dave

[Graviman]

Dave, i'm just saying that if you consider the landing box required to be the restriction the coaxial fits better into the box than the interleaver. You need smaller rotors as you move them apart, hence the increase in disk loading. I'm thinking about the operator requirements for a practical cargo/pax transporting machine, rather than one that spends all day rolling in the hover...

Mart

[NickLappos]

Dave,
Disk loading refers to the single disk, as it is that diameter which acts on the air, and determines the induced velocity (the size of the pipe thru the air is pumped).


Mart,
The Black Hawk and S-76 cross beam tail rotor was the inspiration for the Comanche main rotor. The spar twists or bends as needed, without bearings. The torque tube is the way that the twisting for feathering is spread across a more length of spar so that the rotation is less per inch of spar, and thus the deformation stresses are much lower and more allowable.

[Graviman]

Thanks Nick. A brief google search revealed:

Blackhawk:
http://www.b-domke.de/AviationImages/Blackhawk/3416.html

S76:
http://www.kiwiaircraftimages.com/pages/s76a6.html

and a very neat S76 RC model :O :
http://www.v-eastonline.com/SikorskyS76.htm

So basically the compliance in the spar root behaves as if it was an elastomeric bearing. Very simple, and very reliable i imagine. Although these are all power augmented, i would imagine that a non power augmented heli would suffer high control forces.

What looked to me like hydraulic ram fittings in the hub must have been a red herring, or part of the pitch control mech.

Mart

[Dave_Jackson]

Mart,

The Coaxials were/are popular with the Russian navy because of their smaller "landing box". But what has this got to do with the subject of rotor aerodynamics; or with a hub spring for that matter? You keep moving the goalpost.


Nick,

'Disk loading' is an averaging of the loading over the surface of the disk. I agree, that the simplistic Momentum Theory, and its components of 'disk area' and 'disk loading' etc, are fine when comparing apples to apples, such as single-rotor configuration to single-rotor configuration.

However, Momentum Theory's simple 'gap equation' for the coaxial configuration and 'gap and stagger equation' for the tandem configuration are inadequate in defining effective disk area, disk loading and power loading etc for their respective comfigurations.

Interestingly, Stepniewski was required to use 'an extended' coaxial and tandem Momentum Theory to compare; the cold-jet-driven rotor configuration, the single rotor configuration, and the ABC Intermeshing configuration, in his final publication.


I strongly feel that today's rotorcraft would be much more advanced if the past 60 years of rotorcraft research had been spent on twin-rotor configurations and not on some quaint concept called a tail rotor.

[NickLappos]

Mart,
On the Comanche blade root area were elsatomeric and fluidic dampers, which absorbed the lag oscillations of the blade. The Black hawk and S-76 tail rotors have them, but they are buried inside the cuff and not visible from the outside. Comanche was similar to the EC-135, the brown cylinders on the lower and upper side shown here:

http://212.158.133.3/hwa/antony_morgan/ec135(01)/zakutwo_ec135(01)_06.jpg

Dave,
There are some effects that help the power efficiency of a coax, but they are generally cancelled by others that affect power in the wrong direction (swirl effects, extra drag, reduced figure of merit from too much blade area). For the simplistic analysis of disk loading as a hover power determinant, I think things are roughly equal, regardless if there are two disks.

Actually, Igor Sikorsky found that helicopter development was actually set back for decades by those who insisted on the seemingly simpler symmetrical solutions of coaxials, whose mechanical arrangement drove the designs to dizzyingly complexity. Sort of like the old saw, "If its ugly, but it works, its not ugly."

[Dave_Jackson]

Nick,

I totally agree with the first paragraph of your last post. There is little difference in the power requirements between two closely spaced 2-blade coaxial rotors, and a 4-blade single rotor; if all the blades are identical in span, chord, rpm etc. etc.

However, there is a large difference in the power requirements between two widely spaced 2-blade rotors and two closely spaced 2-blade rotors. In other words, as the two separated disks approach each other (vertically, horizontally, or diagonally in the case of the Tandem) their slipstreams experience an increasing overlap and the power requirement increases.

http://www.unicopter.com/Configurations.gif

This is the supporting [Blade Element Theory] data for the above sketch. (http://www.unicopter.com/B360.html#Required_Power_Comparison)

The power requirements for the Interleaving, the Intermeshing and the Tandem will be somewhere between these two extremes.

Dave

[Graviman]

Dave,

"...what has this got to do with ... hub spring ...? You keep moving the goalpost."

You were proposing a 2-blade teetering design with hub compliance. Nick commented that the only real advatage of teetering is it's low mass. In response i commented that you could apply teetering to an interleaver, allowing low mass with fast roll rate. I also commented that such a machine would be ungainly, unless you pushed up the diskloading which then undoes the mass advantage.

The practical upshot is that for fast roll rate a "rigid" rotor is the best solution. This is why i have being trying to understand the Comanche head...

"Stepniewski was required to use 'an extended' coaxial and tandem Momentum Theory ... in his final publication."

Where can i get hold of this? I've read "Rotary-Wing Aerodynamics", but no mention was made of this publication. It sounds pretty good.

----

Nick,

"Comanche was similar to the EC-135, the brown cylinders on the lower and upper side shown here:"

Ah, this is starting to make a lot more sense to me now. Having the hydromount lead/lag damper buried definately had me puzzled. I'll have another look at that Comanche footage, to put it all into place.

Out of interest has anyone tried the concept of actively moving mass inboard/outboard along the spar? The control system would be:
Blade lead error ---> move mass outboard : Coriolis applies lag force.
Blade lag error -----> move mass inboard : Coriolis applies lead force.

Conceptually at least, fairly simple...

Mart

[Dave_Jackson]

Mart,

A 23-page excerpt, in Polish, can be purchased from NASA. There also appears to be a book, published in Polish. Information on these two publications can be found at this web page (http://www.unicopter.com/1093.html) .

[Graviman]

Thanks Dave - or is it D. Jakziowski? My Polish isn't so good, although i will soon be involved in some durability/fatigue testing in Czech Rep which is just next door...

Looking at your page, i'm hoping you can now easilly answer the "thoughts":

"Direction of rotation differs from all other intermeshing helicopters."

Inboard advance offers aero sideslip/yaw stability and gyro roll/yaw stability, outboard advance offers better aero efficiency...

"Static Stability might be affected by anhedral, resulting from the slope of the advancing blades. ???:"

Rotation direction will adversely affect stability, but a nice gyro stability aumented control system will make it flyable...

Remember the jury is still out on the Intermesher Vs Coaxial debate...

Mart

[Dave_Jackson]

Mart,

I apologize for the curtness of the preceding post, but I'm not in a position to say anything more.

However, while looking to see if the English version of the book had been published I did stumble across Open Airscrew VTOL Concepts (232 page 'pdf' file) (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930008694_1993008694.pdf) by Stepniewski and Tarczynski in 1992. It does not appear to include twin-rotor craft but it does look like an interesting evaluation of a large number of other rotorcraft configurations. I've downloaded the file and plan to read it in the near future.

Another excellent overview of the coaxial configuration is
A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550_1997024330.pdf).

Dave

______________________________
Edited to respond to your last post.

"Direction of rotation differs from all other intermeshing helicopters."

Inboard advance offers aero and gyro roll/yaw stability, outboard advance offers better aero efficiency...Flettner is the only person, to my knowledge, who ever tested the 'outside forward' rotation. I believe that your answer is correct, and in addition, there is a pitch-torque coupling to be considered.

These disadvantages will be reduced due to the smaller 'V' angle allowed by 'absolutly' rigid rotors.


"Static Stability might be affected by anhedral, resulting from the slope of the advancing blades. ???:"

Rotation direction will adversely affect stability, but a nice gyro stability aumented control system will make it and flyable... Agreed, however I would like to look for a 'natural' solution, such as the smaller 'V' angle, before having to consider the weight, complexity and reliability of an add-on.


Remember the jury is still out on the Intermesher Vs Coaxial debate...Not for this kid.
I've sat on the jury (and on the fence) long enough to know where the grass is greener.

This thread and the above two links are a microcosm of your court case. It is a case of too few having the motivation to consider the Intermeshing configuration.

Flettner had it right. Kellett sensed the possibilities. Kaman took it in the wrong direction. And Stepniewski saw the greener grass, one year before he was placed six feet under it.


Edited to change 'Interleaving' to 'Intermeshing' ~ after Graviman's following post.

[Graviman]

Thanks for the PDF links Dave. I will read them with interest. I admire your sticking to your guns about intermeshers.

For reference purposes, i remain unconvinced about the efficiency of a practically sized interleaver in hover (not to mention drivetrain complexity/reliability). Intermeshers (feathered retreating) make more sense to me, and i suspect Nick (in the light of the X2) will stick to ABC coaxials. Flight aerodynamics and cost will decide. :ok:

Hover power requirement really is the limiting factor in helicopter design, which was highlighted very well in the post (pilot let speed bleed off - potentially fatal in a fixed wing):

http://www.pprune.org/forums/showthread.php?s=&threadid=142923

A machine which has an even steeper -ve power curve from hover to foward flight (ie transition) will be more likely to suffer this kind of mishap. The effective foward flight "wing span" of an inteleaver will be higher than it's intermesher or coaxial counterpart, leading to false pilot perception of hover capability in high altitude approaches.


Regarding Lockheed style gyro stability (i still feel privileged that Lu took the time to explain the AH56 technicalities to me), i will only comment that reading the following thread made me think about helicopter flight in IFR. They are difficult, without experience, to hover in VFR. Having flown gliders in cloud, i hate to think what an R22 would be like (deadly i suspect):

http://www.pprune.org/forums/showthread.php?s=&threadid=193920

Note kissmysquirrel's comment "Good job he wasnt in an R22 then!!"...

My thought on reading this was that a Lockheed gyro system (ideally with "rigid" head) would significantly reduce the likelyhood of accident. The stick could also have a mass on it's base (or somewhere in control linkage), so that hands-off the helicopter would by default right itself into a stable hover. This mass could be trimmed, allowing inherent speed stability. I remain convinced that this system would be both simple enough for a light heli, and would significantly reduce accident rates.

There is of course nothing that says you can't fit large horizontal and vertical stabilisers. I suspect that this will go a long way to help your passive stability machine. Even with these the machine will never demonstrate the sort off hands-off controlability, particularly in hover, that i suggest is required to really open up the light heli market.

The addition of a weak servo on the collective for autorotation entry, would be the final piece to low accident rates. Since auto throttle is common, this is also easy to implement as part of the same system.

Sorry if this has dragged your original thread too far off topic - i genuinely do enjoy the future rotocraft debate...

Mart

[Edit: in response to the edit in Dave's previous post! :ok: ]

[Dave_Jackson]

Another report by Stepniewski;

A Comparative Study of Soviet vs. Western Helicopters (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830010377_1983010377.pdf)

[Graviman]

Thanks Dave! A brief skim shows this to be an interesting report.

Mart