Robinson's rotor revisited

[Dave Jackson]

This posting is a hypothetical musing about the Robinson's rotor. There is no claim that the following is true or that it has caused, or could cause any misfortune.

The Robinson's use of coning/flapping hinges, which are in addition to its teetering hinge, appears to be unique. Therefore, a clearer understanding of the purpose and functioning of these coning/flapping hinges may be interesting and informative.

The following is an attempt to look at this unique rotor and to understand what might be percieved as an unfavorable characteristic.

_________________

I read somewhere that a high-speed camera was placed on a rotor mast and focused on one of the blades. The craft was then put through some demanding maneuvers. After a viewing the film, a comment that was made that the blade looked like a piece of spaghetti and that the film should not be shown to helicopter pilots because it may cause some to reassess their vocation.

The little story above was only inserted to emphasize that rotors also operate with harmonics that are greater than the commonly discussed one per revolution flapping and teetering. It therefore follows that there will instances when a segment of a rotor blade is flapping up while another segment of the same blade is flapping down.

Let's assume the following. The rotor has been placed in a situation where it is dynamically very active. One blade is flapping up at its root and down at 75% of its span. The other blade is flapping down at its root and up at 75% of its span. The two blades are flapping in opposite directions at their roots, and because the coning/flapping hinges allow these two moments to be transmitted to the teetering hub, it is reasonable to assume that this hub will subjected to a larger amount of rotational displacement. A degree of displacement that is greater than a hub with only a teetering hinge would experience.

The root of the first blade is flapping up and this is causing the delta3 to pull pitch out of the blade. But the segment at 75% of the blade, which is providing much larger aerodynamic forces than the root, is flapping down. What we now have is a blade that is flapping down and a delta3 which increasing this downward motion.

It would seem that the delta3 is not removing flap. It is adding to it.

From my limited perspective, it appears that the coning/flapping hinge is increasing the teetering in this scenario, to the possible detriment of stability. In other word the delta3, which is incorporated into light helicopters to increase the stability by lengthening period of oscillation, is now working against stability.

Is anyone in a position to dispute the above argument and/or comment on the reason for Robinson's inclusion of these additional hinges?

___________

Again, it must be repeated that there is no intent or any specific knowledge to imply that the above is true, or if true, that it is detrimental.

[Arm out the window]

Well, the gyroscopic forces......just joking, had you worried but, eh?

What you say sounds feasible, though.

[helimutt]

Robinson helicopters + delta 3 hinges=?

New to me!!!!!

Delta 3 hinges are more likely to be found in tail rotors, allowing closer location of tail rotor to tail boom, hence shorter and stronger linkages possible which in turn reduce the flapping amplitude, helping reduce the vibration.

[tgrendl]

Dave,

Always enjoy your posts.

When you said "let's assume" above followed by the blades spaghetti like description and it's flapping I think the word bending should have been used instead of flapping.

I believe we're concerned about the totallity of flapping force here not the individual blade pockets.

In the same way that the twist of a blade still delivers a lifting force even though the individual pockets may not be providing lift at a given moment.

The totallity of this force is then transmitted back to the fuselage through the flex plate, strap pack etc to affect the desired action.

I've tried listing and depicting all of the different forces and their pathways outbound (towards the tips) and back inbound. This has been really illuminating for me along with the really good side of the discussions here.

I have to run to work and make some money but think that the force pathways would make a great thread too. (as long as we left some of the labeling of types of forces absent)

Fly safe !

tom

[Lu Zuckerman]

To: Dave Jackson

The primary purpose of the cone hinges on the Robinson head is to reduce the bending moments on the blade. Your story about the camera on the rotorhead actually took place a long time ago and it differed from the story that you were told and it is most probable that both are true. The first time it was done was on an S-51 rotorhead that was on a whirl tower at Sikorsky and sure as hell the blade was moving up and down like a shaken piece of limp spaghetti. In this same test it was found that advancing blades lead and retreating blades lag which was contrary to the popular theory at that time. And like in your story it did have an effect on the pilots as many of them went back to fixed wing or quit flying altogether.

The movement you describe takes place but not in the manner in which you describe it. The point of movement does not remain stationary as it travels from the tip to the root and is referred to as a traveling wave. I do not believe it has any effect on pitch coupling as when it hits the blade root it is reacted and felt as a vertical beat. Many helicopters have pendular weights at the blade root to absorb this vertical beat and some have two pendular weights because the traveling wave moves at two different frequencies on those helicopters and are superimposed on each other. Some Bell systems incorporate these weights and some bell blades have weights imbedded in the blades at the main nodal point. On some Bell systems and on the Boeing CH-47 they have active vibration canceling systems that operate on the same principle as noise canceling headsets. Sikorsky systems use the Bifilar system mounted on the rotorhead and Aerospatial helicopters have a spring supported bob weight to dampen out the traveling wave this is also mounted on the rotorhead.

To: Helimutt

On flapping tail rotors you do not wish to dampen the flapping in any way as by flapping the tail rotor equalizes the lift across the disc. As the blade advances into the relative wind it will flap out or in depending on rotation and the opposite blade will flap in the opposite direction. When flapping the advancing blade will have pitch removed and the retreating blade will have pitch added. This equalizes the lift across the disc and reduces bending at the blade attachment to the drive shaft. All of this is done through the delta hinge, which in most cases is 60-degrees. The same type of coupling exists on the rotorhead.

[helimutt]

Lu:
Lu:
My understanding is that the degree of flapping must be reduced as much as possible (within reason of course) when referring to tail rotors.
I also believe that a Robinson helicopter does not have a delta 3 hinge on the rotor head. If it does, please send me a picture of it so I can ID it on the aircraft that I fly.(R22)

My earlier statement above was supposed to be in reference to the reduction of vibration in tail rotors.
Anyway, as I have only just found out this week that gyroscopic precession doesn’t really exist in the theory of rotorheads,etc then my knowledge of helo systems (albeit limited anyway) is shot away.

I wait in anticipation of enlightenment !!!!!
:confused:

[t'aint natural]

AAAAARRGGGHHH! NO! NOT ANOTHER ROBINSON ROTORHEAD THREAD WITH LU ZUCKERMAN IN IT!!! FLEE FOR YOUR LIVES!!!

[chips_with_everything]

Dave,

Referring to the original post... I don't think the coning or flapping hinges really add to the amount of flap.

The bendy blades essentially go where they like, as determined by aerodynamics and all the other freaky forces.

The hinges are for the most part passengers there for the ride and are relieving bending loads on the blades in the process.

I am deliberately being a touch coarse here, I'm sure someone with solid domain knowledge such as Nick Lappos could add a little more precision. I am confident that the general picture I am painting is accurate however.


To go on further... instability in the Robbie is pretty well understood. The rotor system is not that mysterious, and in many ways quite clever in my opinion. If you keep within its limitations it will work amazingly well, and reliably. It's robust and makes a very manoeuverable aircraft.

There is not much margin for abuse and error however.

Low inertia and low stability, which are there by design, just aren't forgiving of low RRPM, low g, large gusts or sudden and inappropriate cyclic movement.

If you can live within the flight manual limitations then the Robinson system is absolutely fine. The converse is also true.

[ 18 August 2001: Message edited by: chips_with_everything ]

[Dave Jackson]

helimutt;

The web page http://www.synchrolite.com/0941.html#delta3 may answer your concern regarding delta3. It shows the two different styles of pitch-flap delta3. In regard to teetering rotors, type B/ is used on the Robinson's main rotor, and type A/ is used on the Kaman intermeshing main rotors. These sketches are from 'Helicopter Theory' by Wayne Johnson and it is the only comprehensive source of information on delta3, that I have been able to find.

The web page http://www.synchrolite.com/B185.html#Layout_of_Robinson has a sketch of the Robinson's rotor hub layout. The drawing on the left is what you want and it shows that the pitch link is offset in respect to the teetering axis. The drawing on the right is of no interest.

Can't help much with tail rotors 'cause I don't believe in them. :eek: Long live the intermeshing helicopter! :) At the top of second web page is a link to a site with excellent pictures of main and tail rotors, which might be of interest.

________

tgrendl & Lu

Your points are going to take time, for me at least, to digest. Perhaps others with a better understanding of the subject can add to your remarks.

Lu;

Don't want to take the thread off topic (just joking) but your mention of a 60-degree delta3 on the tail rotor is interesting. A 45-degree delta3 pulls out all the pitch, a 26.5-degree pulls out half the pitch, and a 0-degree, of course, pulls out none of the pitch.

A 60-degree delta3 will give -1.73-degrees of pitch for each 1-degree of flap. Is it a case that the tail rotor wants to 'over-react' to flapping as a means of assuring that there will be an absolute minimum of flap?

[ 18 August 2001: Message edited by: Dave Jackson ]

[Lu Zuckerman]

To: helimutt

Please send me your email address. The pprune address does not work. I have some material to send you.

To: Dave Jackson

Hopefully you have received the info and pictures of tail rotors.

[Dave Jackson]

chips_with_everything

Please don't think that this is an attempt to discredit any helicopter. Every type of vehicle has its safety envelope and it shouldn't be exceeded. This thread is intended to be an interesting and informative search for technical understanding.

The following is an attempt at responding to two of your points

>Low inertia and low stability, which are there by design<

I believe that low inertia and low stability aren't intentionally designed in. They are an inherent and undesirable characteristic of small helicopters. Bell's stabilizer bar, Hiller's paddles, Robinson's delta3 and Ultrasports relatively high tip weights are methods that are used to increase the stability of light helicopters.

__________________

>I don't think the coning or flapping hinges really add to the amount of flap.<

I agree that they probably don't add to the amount of overall blade flap but I do believe that they will significantly add to the flap at the blade's root.

A component of this thread's argument entails the upward force at the root of one blade and the simultaneous downward force at the root of the other blade.

On a helicopter with a teetering rotor, one of the forces is attempting to lift the teetering axle while the other force is attempting to lower the teetering axle. Therefore, the forces tend to cancel each other.

On a helicopter with offset flapping hinges, the rotor hub is rigidly attached to the body of the helicopter. The moment developed by the above two forces is well restrained because this moment must roll or pitch the whole helicopter.

On the Robinson, there is a 4" long yoke, with hinges at both ends and one in the middle. This 3-4 pound yoke offers little resistance to the moment developed by the above two forces. The yoke's maximum vertical motion (rotation) will take place at the hinges at each end. The pitch links are in line with these hinges and they do not move. The implication is that the delta3 will impart a larger pitch change to the blades because of the coning/flapping hinges then it would without them.

One blade will pitch up and the other will pitch down. This aerodynamic spring is the intent of the Robinson designers and is very viable for the 1/rev oscillation.

What takes place due to large higher frequency oscillations is the subject of interest.


Hope this makes sense.

[ 19 August 2001: Message edited by: Dave Jackson ]

[chips_with_everything]

Yeah it sort of does make sense Dave, I think we are agreeing on the second point and disagreeing on the first.

In other words I feel the Robbie is intentionally lively. I guess that's something in Frank Robinson's mind, only he could answer. There are lighter helis with more inertia and stability out there, so I'll assert that the R22 characteristics are deliberately as they are.

BTW I miss living in Vancouver.

[helimutt]

All:
I still have trouble identifying the actual delta 3 'hinge' I mentioned earlier. What I see is delta 3 caused by positioning of the pitch links.
The web site posted above also mentions stuff about delta 3 and P/C so I'm lost.
I'm giving up! Too much information for me when I have other stuff to learn.
The thing flies so that's all that i'm interested in. If a student ever asks me how it works I'll say " by magic". :confused:

[tgrendl]

Helimut,

The talk above is not actually about a delta 3 "hinge".

The referance is to the idea that a mainrotors delta 3 can be changed by various mechanical design changes.

IE. stiffer blades, larger hinge offset etc will increase the responsiveness of the rotor system changing the delta 3.

I'm being drug around by the nose on some of this but it's interesting to plod through portions of my meager little brain that haven't been used in a while.

I think some of the fundamental assertions above are incorrect but it doesn't stop me from learning the good bits.

fly safe

[HeloTeacher]

I find it hard to accept the assertion above that the blade exerts a downward force. Remember, even on the retreating side the blades are lifting.

The way I envision it, the blade is exerting less upward force, not downward. This definitely changes the dynamics of what we are talking about.

BTW: does anyone know of a rough estimate of the centripetal force exerted outward on the blade root of the R22? It must be at least several thousand pounds, compared to the 685 it is lifting.

[Dave Jackson]

Lu

Agreed. Its doubtful that the normal harmonics of the rotor will, by themselves, represent any problem. I am thinking of an extremely unique occurrence. An occurrence that would result from a unusual combination of normal harmonics, vertical sharp-edged-gust, tip vortex, trailed vorticity and a sudden strong cyclic input.

A reply to your e-mail was sent.


tgrendl

>I .... think that the force pathways would make a great thread too. (as long as we left some of the labeling of types of forces absent)<

It would be interesting and informative to hear more about this.

chips-with-everything

You left Vancouver! You must have been here during the annual 10 months of rain.

I really enjoyed a year and a half in Australia. Never got up to the Gold Coast, unfortunately.
Learnt how to be brutally honest and never couch remarks in polite correctness :)


HelloTeacher

You're correct in saying one blade is exerting less upward force. My use of 'up & down' was merely to try to simplify the argument.

[helimutt]

heloteacher,
I may have misheard but I think the force exerted is in the region of 3 tons!! I also think this increases to more than twice that with a ten percent increase in rotor speed.
No doubt I'll be corrected in good time.
:eek:

[chips_with_everything]

Must be basic physics here.

F=ma

energy= (mass * velocity**2) /2

and all that.


Now who here is the physics teacher??


More interestingly if the forces double with a 10% speed increase, and those forces keep the disc a disc then it's no suprise that low rotor RPM sends the trajectory haywire. :eek:

[Vortex what...ouch!]

Be <very> carefull with the RRPM overspeeding then :)

[Vfrpilotpb]

Good afternoon Rotoheads,

Please dont shoot me down , but the other day whan I was doing a pre-flite on a R22, I aligned the tail rotor blades to emulate a total neutral stance or posistion when at rest, to my eye the blades were not truly parralell, I am sorry if I seem not to make sense but if you took the centre spindle as the centre and therfore at position 0 or zero the tips of both blades were a few degrees nearer to the upright tailplane in other words the blades were proscribing the profile of a Chinamans hat, can any of you throw any light on this, the Cfi said it was Ok and had just flown, so tentativley I went up and everything was OK but I am puzzelled by this! :confused:

[HeloTeacher]

centripetal force = mv^2/r if i remember so...

110% RRPM should increse the load by 21% compared to that at 100% RRPM.

to Vfrpilotpb:

the tailrotor has a pre-cone angle of a little over 1 degree. perhaps this is what you were seeing.

thanks for the replies

[JohnJ]

vfrpilotpb

I seem to recall from the Factory Course that the R22 tail rotor is pre-coned 1°11” to gearbox. Is this what you saw?

[heedm]

HeloTeacher, a centripetal force doesn't act outwards. By definition it acts inwards. A centrifugal force acts outwards, but many are told that it is not a real force. That is true, but it is still valid to use the terminology. The reasons why it is not a real force don't change how helicopters fly.

The centrifugal force is directly proportional to the square of the angular velocity. If you increase rotor speed by 10% then you increase the force by 21%.

If you assume uniform mass, then the force exerted from one blade at the hub is F=m x r x Nr^2 where m is the mass of one blade and r is the rotor radius. Of course, that is opposed by an equal force from the other rotor. Tip weights increase this force.


Dave, I think you may be correct that the blade flex may cause an opposite effect to what is desired from the delta 3, but when you consider that the blade doesn't stay in it's bent configuration, instead it oscillates, then sometimes delta 3 will give the desired effect, sometimes it won't.

The mean position of the blade flapping up would cause the delta 3 to remove pitch. The amount of pitch removed would oscillate with the oscillations of the blade root,perhaps to the extent that pitch is increased momentarily, but overall the pitch will be reduced for a blade flapping up.


It's a good question, though. It finally made me really look at the Robinson head (from online sources only). I think it's a good design for weight reduction. I imagine the compromise is that the aerodynamics become more complex. Perhaps new oscillations are found and instability is increased. I'd have to think about this much more before I'd be able to present a convincing argument.

Matthew.

[ 20 August 2001: Message edited by: heedm ]

[Dave Jackson]

heedm

The Robinson's maximum cyclic is +11-degrees in the longitudinal direction.
Its 18-degree delta3 will pull 3.58-degrees of pitch out of an 11-degree flap.

To play with hypothetical theory;

Assuming that pitch-to-flap and flap-to-pitch give identical results, then a full cyclic stick travel of 11-degrees of pitch will result in a (11 - 3.58) = 7.42-degree flap. Now if a violent vertical force at the root was to teeter the yoke to the opposite extreme, then the pitch would change to ( 7.42 + (2 * 3.58)) = 14.58-degrees.

This means that the pilot is expecting 7.42-degrees of pitch but experiences 14.58-degrees of pitch.

The above was done during a moment of boredom; :rolleyes: Only TV has this much violence; but the flexibility at the blade roots does present an area of interest.

[ 21 August 2001: Message edited by: Dave Jackson ]

[tgrendl]

Hi Dave,

I'm working quite a bit so don't have lots of time just now but two quick things;

The above seems at first glance to be the way you would figure tailrotor delta and I don't think that you can transfer that method to the mainrotor delta3. (I admit to not having time to run it all through)

And the bendy motion at the blade root.

I'd like to see a rotorhead drawing (cutaway, 3D) can you link me to a site?

I'm still under the assumption that the totallity of force transmitted is the key. Unless you can lift a robbie blade root and move the rotor out of plane at its' root the bendy motion remains just one part of the total lift and flap perfomance for the system. One part that adds to the totallity of the force transmitted from the blade to the fuselage.

Fly safe !

Tom

[Dave Jackson]

tgrendl

The following two web pages give some illustrative information on the Robinson hub.

Two sectional views; http://www.synchrolite.com/Robinson_Rotorhead.htm

Description and picture; http://www.cybercom.net/~copters/mech/mr_semi.html


Delta3 should work the same in the main rotor and the tail rotor. In the tail rotor, it is only converting flap to pitch. In the main rotor, it is doing both flap to pitch and pitch to flap.

I agree with your last paragraph. The tip path plane and the hub (root path) plane would have to move out of alignment in respect to each other. This amount of misalignment would have to be quite extreme, and it is only speculation that it could even happen.

[Lu Zuckerman]

To: Dave Jackson

The illustrations provided on the Robinson’s site are quite good but a bit misleading. On the head on picture where the cyclic is displaced both left and right the blades are not at their maximum relative pitch change. To get the maximum pitch change the blades should not be disposed over the longitudinal axis but instead should be rotated an additional 18-degrees past the longitudinal axis. This is the position of the blades when setting left and right pitch ranges. The other point is when you click on gyroscopic precession the illustration is that of a Bell swashplate and not that of a Robinson. I personally believe that if they were to use an illustration of a Robinson swashplate with the 18-degree offset it would cause a great deal of confusion as they could not use the verbal description regarding gyroscopic precession and apply it to the Robinson dynamics.

[Dave Jackson]

Hi Lu,

The page that you are referring to is from a web site by a Paul Cantrell. It's probably the best site on helicopters, which I have seen. This specific page is from the [Mechanical Components] section.

Originally this site was on the MIT server but had to be relocated a few months ago. The introductory page to this very interesting site is; http://www.cybercom.net/~copters/helicopter.html

_______________________

The following is my position regarding precession and phase angle. Your comments or critique will be appreciated.


The first point that must be dispelled is that there is no singular lifting force at 90-degrees azimuth, which results in an increased elevation at 180-degrees azimuth. The lifting force exists at all degrees from 1 to 179. This very point is part of the mathematical proof of gyroscopic precession. The location(s) of this force is also obvious when envisioning aerodynamic precession, since the blade does not snap to some steep pitch at 89-degrees and then snap back down at 91-degrees.

The only thing of interest about 90-degress is that;
1/ It is the location of the greatest lifting force, between 0 and 180,and
2/ it is the mid-location of all the degrees that exhibit a lifting force.


The second point is that it has been mathematically and experimentally proven that precession, both aerodynamic and gyroscopic, can have phase angles of less than 90- degrees.


The third point is that flapping hinge offset and delta3 cause the tip path plane to realign itself in less than 180-degrees, after a change in the control plane (swashplate).


The above shows that by the use of a flapping offset hinge or delta3 hinge it is possible to start the lifting at 18-degrees, have maximum lifting force at 99-degrees and alignment of the two planes at 180-degrees. Once the two planes are reoriented, in respect to each other, the remaining 18-degrees becomes irrelevant.