Robinson rotor head question.
 

Chiplight. Thanks for the input. Lets put it another way. As the machine moves foreward following foreward cyclic input one has to assume that the blade as it travels around the rear half of the disc is providing more push that the blade travelling round the front half of the disc as it not only changes the attitude of the disc but also pulls the machine foreward. In other words in order to move the machine foreward it must be doing more work during the period where the blades flap to equality. If this is the case what is it that stops the blade moving on its coning/flapping hinge.

Bugdevheli,

the blade at the rear is doing work to raise the blade a certain amount, while the other blade is allowing work to be done to it as the pitch is reduced. The net result is that the rotor tilts forward and both blades move together.
The differential in lift between the two blades cause a rotor tilt, but the total rotor thrust stays the same. One blade has LIFT+X and the other has LIFT-X, and if you add those quantities you get (2 X)LIFT. The direction of the thrust vector changes to be more in the forward direction.
For one blade to move up,while the other does not move down would require that the cyclic somehow only affected one blade, which of course is mechanically not possible.

bugdevheli,

The attitude of the disk will have fully changed before any significant forward pull takes place.

Like you, I questioned the activity on the coning/flapping hinges a while back. The calculations show that the centrifugal effect is vastly stronger than the out-of-plane force at these hinges.

Dave

Robinson rotor head question.
 

Chiplight. How about. You are in hover. Suddenly an enormous gust of wind blows up into your rotor, and the machine lurches backwards, forewards. whatever. the disc is now being pushed up on one side. No cyclic input. Do the same rules apply.

I've got a funny feeling Lu may return to the fray with a bang on this one! :O

Lu! Remember what the doctors said. Gently at first.

Three posts within a minute! Cor!

Robinson rotor head question.
 

Good evening Dave. Take your point, but how come if the centrifual forces on the 22 are sufficient to stop movement around the hinge, that on machines with much heavier blades and having flapping hinges, they flap

bugdevheli,
I presume a gust would affect both blades. It would have to be a very quick and narrow gust to act within a half revolution of the rotor and only on one side.
If it did do this, then one blade would have slightly more lift and yes, it might cause a very slight movement about the coning hinge, but not much. The same thing would happen on any rotor- one blade would move on it's flapping hinge and the others wouldn't. If it did happen, I would expect a brief vibration to be the result. As Dave said, the centrifugal forces are so large compared to the changes in lift we are talking about, that they dominate.
At any rate, this does not happen as a result of normal cyclic inputs.

bugdevheli, don\'t confuse flapping hinges with coning hinges.
The R22 blades flap about the teeter bolt, just as on the Rotorway or any two-bladed heli.
They do not need to flap about the outboard hinges, as these are for coning, despite the fact that they appear to look like they would act as flapping hinges.

Blade flapping
 

Hi


I agree with the responses, but .... as has been discussed before..., things are more complicated

For instance in the case discussed here, the static equilibrium is not Lift-X versus Lift+X, but flapping moment blade1 versus flapping moment blade2, that is Lift blade1*arm blade1 versus blade 2. Because of twisting and speed differences, the centre of lift is not the equal for the two blades, for the forward moving blades it tends to be more in the centre, for the backwards moving blade it is more to the edge, so the forward moving blade may produce more lift, while still maintaining static moment-equilibrium. And this is still a simplification ignoring the (dynamic) inertia effects which tend to be more important if the rotor disk is tilted with respect to the mast, because then inertia effects come into play, namely having the rotor turn not perpencular to the rotor mast (=flapping). Helicopter design (trade off between drag, tail wing design, centres of gravity, coning, delta3 and blowback, kind off take care that in cruise the rotor does run close to perpendicular to the mast, because important flapping can reduce rotor efficiency and increases higher order harmonics = vibrations.



Delta3

Robinson rotor head question.
 

If it is possible under extreme conditions for a blade to move on its coning hinge,just a little bit, whilst the opposite blade has remained in the same position relative to the hub, does this mean that for a finite period of time the rotor system could be said to be out of balance. Hence the slight shudder you describe might occur.

I think the answer is yes, bugdevheli. A slight shudder is what I guess might happen in that unusual event.

Delta 3, interesting points, but to a first approximation, flapping equalizes lift between the blades. any remaining difference, due to cyclic pitch change probably wouldn't have much impact in terms of pitching moments, etc, would it?

Higher order harmonics
 

The ones I studied are induced due to aerodynamic differences and vortices.

For the flapping versus coning (see Daves remarks) I always presume that the hub is straithtend out by the dominant centrifugal forces.
For instance in a R22/R44 the second order harmonic is important (5-10%) at high speed, high coning, and flapping. These disturbances could temporary dis-align the head if flapping or coning friction were significant and coning would have more friction than flapping). I assume that they are not significant in the case of R22/R44.
During important transients (turbulence, wind gust) I guess some minor temporary (0,5 to 1,5 rev) unbalances could occur. (not calculated yet)

Delta3

Chiplight

This is what I wanted to stress, the first order simplified condition is not lift equilibrium but flapping moment equilibrium, see previous 3D rotor plots I published.

The physical equation at the foundation of rotor calculations is flapping moment equilibrium (not static but with inertia, coriolis etc).

In certain flight profiles the rotor is nicely running perpendicular to the mast
(blow back equal to forward cyclic, delta3 or other compesations schemes offsetting lateral effects) In that case there is no flapping, but there is an important aerodynamic force difference, and the rule is then that moments are equal, not lifts.

An important corollary in this simplified condition is that rotor trust will be laterally inclined to the backward blade (to the left in an R44) because of coning, so the total lift is not perpendicular to the tip path.


Delta3

It's against doctors orders but......
 

To:bugdevheli

The coning hinges allow coning and they also allow the blades to flap to equality to alleviate gusting. The advancing blade is most generally effected and it can flap independently from the retreating blade without causing any vibration.

The NTSB has reported in several accident investigations that the blade system on the Robinson is dynamically unstable under certain conditions resulting in extreme flapping of the blade system. Since the blades are independent of the rotor head most of this flapping takes place about the coning hinges and some flapping will take place about the teeter hinge. This excessive flapping will manifest itself in either mast bumping where the entire rotor system is flapping about the teeter hinge or fracturing of the blade tusks due to excessive flapping about the coning hinge.

The coning angle is a balance between the lift generated to raise the helicopter and the (I said I wouldn’t refer to this in my posts but…) centrifugal force acting on the blade. If Nick is reading this change centrifugal to centripetal. In the gust alleviation mode (flapping to equality) the blade will deviate from the tip path plane countering the centrifugal loading on the blade. If this did not happen then you would get vibration.

In response to Dave Jackson’s comment about the centrifugal force being so much stronger than any cause to flap: If this were the case the blades would not cone.

Please note there is no reference to 18 or 72.


:E :E

Delta 3,

I'm trying to visualize the above situation. It seems to me that in this instance, the forward cyclic is exactly neutralizing disymmetry of lift,which is another way of saying aerodynamic force difference.
It seems as if you are saying that since the tip path plane happens to be parallel to the rotorhead spin plane and therefore no teetering, that dissymmetry of lift still exists somehow.

Can you help me out here?

Thanks, this is a good, thought provoking discussion!

Hi Lu

So, bugdevheli's mention of the word 'Robinson' got you off the couch. :D I didn't mean to imply that the blades would not cone when the rotor was under load. It's just that I used to suspect that an extreme perturbation might send a wave action back into the hub that would cause the two coning angles to be different, for a very short period. After reading the patent and doing the math, this possibility appears to be somewhere between very remote and impossible.

Dave

Robinson rotor head question.
 

Dave. I think your first line of thought ie the possibility of two different angles of blade to hub might still be a possibility. Experiments with a scrap machine showed that the slighest imbalance such as centre of mass variation on the blades despite the rotor system balancing across the hub resulted in the machine shaking like a wet dog,whilst the rotor system appeared to be stable. I concluded that if the rotor blade angles could alter unequally relative to the hub then a self destruct situation may be possible. I always liken the design of this hub to be somewhat like a potters wheel. Get the clay some where near the middle and you can throw a nice pot. Get it too far off centre and your scraping the clay off the wall. Bug

Chiplight


I have no means to publish computer simulation plots from where I am now, but let me put it in words

- coning provokes blowback (well approximated by a simplified first order relation: steady state blowback angle is proportional to forward speed and coning angle)

-coning provokes phase shifts, but let me IGNORE these here for the sake of discussion. This topic was (heavily) discussed with with Lu (Hello LU, I am really glad to see your back, but allow me to ask to stay on the question of this threat - at least for a moment)

So if the cyclic forward angle equals the coning angle then the rotor runs nicely perpendicular. That indeed happens in a lot of speed versus W&B conditions. I could give you examples for the R44

But even in this nice situation the blade a lift difference exits as explained before: the forward blade having a smaller arm around the hub (tip produces less lift because of twist, most lift is in the middle) is/needs to producing more lift than the retreating blade having a larger arm around the hub (because lift is closer to the tip). To visualise this further the forward cyclic necessary to compensate blowback makes the forward blade dive down, in some cases there is negative lift at the tip. In the retreating blade there is an area of turbulence and even negative lift close to the hub.

If you follow sofar, the larger lift on the upwards tilted/coned forward going blade at the 90° to the right position in case of R44 (tilt=coning angle, left and right wrt pilot sitting in his seat, so forward looking) creates a larger force component to the left (because of tilt) than the retreating blade compensates for (with a smaller right force component), so the sum of forces, the total rotor lift force, tilts to the left (remember in this situation blades are equally coned wrt mast axis, no flapping)

This effect clearly shows in the full scale 3D flight simulations I'am working on, and again surprised me first (thought at first there we go again, the model or the implementation goes wrong, sleeps less nights, checking .... etc..), until I made the above observations.

bugdevheli

Rotor inbalances with or without R44 type hub will shake the whole system. The unbalance spoken of (at least as I understand it) is where the rotor hub so to speak does not take on the average angle of the coned blades (this is exactly the formula used in the R44 simulator btw) As said centrifugal forces are great. But what is the horizontal force at the coning axis position. As a first static approximation you should take the lift of the blade and I showed above that these lifts are most of the time different, so even statically giving a reason for unbalance. The typical figures I would come up with : centrifugal order of 100.000N, lift difference or of max 500N, that will produce a very very small angle.

Now dynamically there are also and even more reasons for unbalances, but again I think they are small, unless some extreme higher order (2nd, 3th) components are acting (both aerodynamical higher orders and blade inertia higher orders), but this also would provoke large aeroelastic forces on the blade, shaking the whole system and not only the hub so that they are mostly, avoided/reduced in design and blade pairing.

Delta3

coning hinge and lead lag
 

Bugdevheli,
About a year ago I think, you posted a photo of a R-22 blade that was curved in the lead/lag plane. I thought maybe the reason for the bent blade was because the R-22 head lacks a lead/lag hinge. Without a lead/lag hinge the blade must bend in lead/lag when it moves about the coning (flapping) hinge.

My understanding is that any rotor that uses a flapping hinge must also incorporate a lead/lag hinge to relieve the strain caused by the imbalance in angular momentum. A teetering head doesn't need a lead/lag hinge because the two blades work together. And most teetering heads are very strong in lead/lag with wide chord and drag links. The Robinson has no drag links.

So its not clear to me how the Robinson head can survive strong turbulence without any lead lag hinges. (I assume the blades flap independently in a sharp edged gust)
Is this related to your question?


P.S. This may have come up before and I have tried to search but it is difficult to search on R-22 rotor head as you get about 400 hits.

bugdevheli,

You press a very interesting point, and one that might get Lu salivating. :D

To get a better understanding your concern, is the following a re-expression of your thoughts?
____________

Theoretically, any conceivable out-of-plane force at one of the coning hinges must create an infinitesimal change in the angle of the two coning hinges. Since both coning hinges already have a coning angle, one hinge will experience a slight increase in its angle and the other hinge will experience a slight decrease in its angle.

This means that the CG of one blade must move outward (very very slightly) and the CG of the other blade must move inward (very very slightly). This raises the possibility of a self-exciting vibration, perhaps due to a long flexible mast, with little or nothing to dampen it.
____________

Could this be somewhat similar to Air Resonance, but unique to the tri-hinged hub because it involves the coning hinges instead involving the lead/lag hinges, as is normally the case?

What is the answer? Suspend the craft at its CG and then start shaking the rotor head to obtain it's frequency at various azimuths? Then see if there is any correlation between this and the range of the rotor of rpm?

Dave :confused: :confused:

Robinson rotor head question.
 

DJ You have it in one. Given the length of the main rotor shaft that is unsupported between the bearings, and given the flexible nature of the shaft itself (not very thick wall you know) I would think its a possibility. Its interesting to observe how much the exposed part of the shaft above the top bearing will flex when you pull down hard on a blade. If it were permissable to demonstrate this on an airworthy machine I think people would not risk mast bumping.
Slowrotor. Yes I have a pair of blades that have a gentle curve in them. when I posted the article it was suggested I got my eyes tested.

Lead-Lag on the Robinson head
 

To: slowrotor

The Robinson head like those on the Bell machines is underslung to minimize if not eliminate any tendency for the blades to lead and lag. With forward cyclic and with the blades equally coned there is no leading and lagging. However in a maneuvering situation or during gust alleviation the blades will flap about the cone hinge and then there is leading and lagging. Since there is no vertical hinge to permit the leading and lagging the forces are absorbed in spanwise blade bending. These loads are transmitted to the head and from there to the mast, which results in a cyclic twisting of the mast. All of the lead lag loads are transmitted via the cone hinges with resultant elongation of the cone hinge, which is in the form of an ellipse. All of these loads are generated at twice the rotor speed during maneuvering.

:E :E

Hub resonance
 

Dave, bugdevheli

Your reasoning would point to the existance of a mast bending resonance (whole system including hud and blades), with a resonance frequency equal to the main rotor rpm (or first multiples ,say 2 and/or 3). Only in this case small excitated disturbances, such as lift inbalances) can provoque large oscillations.

It is my understanding that the mast is on rubber bearings, making the likely resonance frequency pretty low, so very unlikely in mho. Take for instance also small inbalances in the geometry of the mast it self, these would also be amplified. So this system can not realistically be resonant at these frequencies.

I'll put it on my simulation todo list...

Delta3

Lu,
Glad your back, its been a little slow here without you.
You said the blades absorb the lead and lag loads in a manuever or gust. Are the blades designed for this (in your opinion)?

The R-22 blades are attached at the root with a small hub. And several photos posted on this forum show cracked and separated blades at the root. Seems like a highly stressed blade attachment without the lead and lag hinge. Other teetering rotors (Bell, Hiller) are much larger with drag link struts.

I suspect Mr. Robinson used coning hinges to allow for a lighter rotor system. A good idea as long as both blades cone together. But if one flaps alone for a fraction of a second then it would be stressed in lead/lag ,I think. The R-22 rotor just looks weak in the lead and lag plane to me.

If someone wanted to build a rotor similar to theR22/44 but eliminate the potential problem under discussion, why not simply link the coning hinges so that they cone in unison.

Alternative, just take the weight of the above modification plus the weight saving from the removal of the coning hinges and put this weight into stronger composite blades and simple hub.
Then call it a Bell 47. :D

Dave

__________________

Edited to add the following thought.

1/ Teetering rotors are more difficult to control than rotors with offset flapping hinges.
2/ Lighter helicopters are more difficult to control than heavier helicopters.
3/ A new US helicopter does not have to comply with the new FAA regulations if it maintains commonality with a previously certified helicopter. (This statement is based on very limited knowledge.)

Perhaps number 2 + 3 is one of the reasons why Frank Robinson is rumored to be stopping the production of the R22. He can use his approved rotorhead in the R44 and have the advantage of a more controllable craft.

Delta 3, thanks for trying to explain.

I'm still "collating", as Ash said in the movie Aliens.

rhm, thanks for your reply.

maybe frank robinson knows the right answer... ;)

regards,
Franz

Funny enough you should ask - was tolled the other day why the tailrotor is pre-coned on the R22 ..

The tailrotor has to produce a certain amount of thrust to one side all the time so the helicopter doesn't start spinning around itself ..
To limit the stress of the bending moment on the tailrotors blades hinges/center the natural coning would make when it produces thrust, it is pre-coned to the side where it would cone anyway to minimize the stress ..


- madman

Dave
 

As many times suggested in similar topics perhaps only Frank knows the answer.....

The others perhaps speculate...

My speculation

- coning seams good in order to have tailboom clearance, it allows greater pitch angles of the MR. In order to have the same angles the MR hub would have to be higher. Making it higher creates other problems/compromises

- the fact that the R44 (not R22 as far as I know and as we discussed some time ago, cfr your pictures) has a positive coning delta3, induces me to speculate that the 'designer wanted that', that is at high loads the MR cones even more, what surprised me. I am speculating that this is for the above reason, he really want it coned upwards (difference could be that the heavier R44 blades would cone less that the lighter R22 blades)

- making the thing rigid would mechaninally speaking
- create too big stresses at the foot (you could make it semi-ridig, but I let the creativity on that route over to you...)

- make it very unpractible on the ground (could use your spring)

As I read this back one possible design alternative comes to mind
- have a UH-1 style of configuration, with an angle in the boom, but this would be heavier


My simulator is now in 'Full 3D validation mode'. As soon as I'am comfortable I will release it (target date august-september). Before that I want to check the yaw and low-G induced tail boom strikes (my original goal)


Delta3

Madman
 

As the rotor is more rigid, indeed a preconing in the direction where it would be on average if coning were free will reduce stresses at the foot

But, since tail rotor has no cyclic and still takes a lot of different aerodynamic disturbances, the aerodynamic design is more subtile than it would appear (see previous threats and answer of Nick several months ago).


Delta3

Robinson rotor head question.
 

Is it not the case that lead and lag on any rotor system would occur regardless of whether blades were coned or running flat, in any condition other than a no wind hover.

Three cheers for the axis.
 

To: bugdevheli

There are two axes of rotation on the rotor system. One is the driving axis and the other is the driven axis. The driving axis is coincident with the centerline of the mast. The driven axis is an imaginary line that is coincident with and perpendicular to the center of the rotor disc. As long as the two axes are coincident with each other such as when there is no cyclic input and the cyclic is in the neutral position there is no leading and lagging On the Robinson head with forward cyclic and equal coning of the blades the two axes will separate but since the head is underslung leading and lagging will be at an absolute minimum. However in maneuvering situation the blades will flap about the cone hinge resulting in leading and lagging because the blades have deviated from the coned position.

:E :E

delta3,

Robinson's coning hinges serve the same general function as the precone angle on other teetering rotors. They reduce the stresses in the blades.

If you are referring to the comment in my previous post, the idea is not to 'freeze' the coning hinges. The idea is to allow the blades to still cone about their hinges BUT insure that both hinges have the same amount of cone at all times. This should theoretically eliminate the concern that was raised by bugdevheli in this thread.


Lu,

You said "Three cheers for the axis."

Is this the old axis of evil, Iraq, Iran and North Korea?
Or, is this the newer axis of evil, Cuba, Libya and Syria? :D

Robinson rotor head question.
 

Mr Zuckerman. Sorry to spit hairs but. Two blades travelling foreward subject to different loads either side how do they remain directly opposite each other. Cyclic input or no cyclic input.

Hokay, I've been following this debate for quite awhile. There seems to be two (or three, but I'll focus on two) somewhat seperate issues. Let's see if I have a handle on them.

1. Symmetrical blade coning: If the blades cone excessively, the CG of the blades will "rise above" the optimally designed position when referred to the underslung rotor hub. This will lead to increased lead/lag forces as the CG/driven axis wil be "above" the driving axis. The contention is that the coning hinges on the Robinson allow this to happen to a greater degree than will happen on a "rigid-in-plane" system common to other 2-bladed systems.

But does this happen? I would think that Frank Robinson's team would have designed the underslung hub to be aligned with the normally coned axis of the blades. Perhaps, though, this is why the Robbie is so twitchy about vibration during startup and shut down - when the blades are not coned, the driven axis is "below" the drive axis. (Could this also be an issue in low-"G"?)

However, I would think that the Robbie would be no more prone to "over-coning" than any 2-bladed system. Perhaps with the hinge allowing the whole blade to cone up rather than just cone by flexing (as happens in other 2-bladed systems), the over-coned CG for the disk would be slightly higher than in a system like the 206.

2. Asymmetrical blade coning: Aerodynamic forces cause a coning hinge to activate instead of teetering the rotor. While theoretically possible, I see the enormous coupling force across the coning hinges versus the very small force that inhibits teetering. Even in a very steep turn, there might be 2.5 tons of force felt across the almost infinitesimal couple at the teetering hinge versus 10 tons across the much larger couple of the coning hinges. So would the Robbie actually be more susceptable to asymmetric coning than any 2-bladed system?

Now about that phase angle...

Blade relationship. Opposites do not attract.
 

To: bugdevheli

I don’t know if I fully understand your question. On most helicopters with an even number of blades with the blades at rest the centers of pitch rotation are not directly opposite each other. The blade root attachment is offset forward in direction of rotation so that the center of pitch rotation does not intersect the center of the mast. This offset is usually about a ¼” possibly more on larger helicopters. Uneven blade rotor heads are similarly constructed. This is done to minimize spanwise bending of the blades thus reducing structural stresses in the blades.

In forward flight with no maneuvering the blades will be coned and in the pure radial position due to (if I may use the term) centrifugal force. With maneuvering the blades will flap about the cone hinges and due to the different forces (lift) acting on them they will flap to equality (to use a British phrase) and the degree of flap may not be the same for each blade. Under ideal (design) conditions the disc is equally loaded on both the advancing side and retreating side however the helicopter is not operating under ideal conditions resulting in the pilot making corrections with the cyclic stick.

If this does not fully answer your question please restate it.


:E :E

bugdevheli
There is symmetry of lift (actually it is symmetry of moment) between the advancing and the retreating blades. The lift/drag ratio is quite constant across the angles of attack seen by the blade elements. Therefor there is symmetry of drag.

Corollas is, by far, the largest contributor to lead/lag. When a 2-blade teetering disk is tilted, during 'mean' flight conditions, the two blades will accelerate and decelerate together, twice per revolution. The remaining causes of lead/lag on a teetering rotor are relatively insignificant and the long flexible mast absorbs them.

On a 'theoretical' 4-blade teetering rotor, lead/lag would be a big problem because one pair of blades would be accelerating as the other pair was decelerating.

If interested, more information is available on the web page;
OTHER: Flight Dynamics - General - Lead/Lag


Flingwing207
:ok:

Dave

Robinson rotor head question.
 

Lu Zuckerman. Upon reading your earlier post again, you did in fact verify my suggestion that there must be some degree of lead and lag.


On the Robinson head with forward cyclic and equal coning of the blades the two axes will separate but since the head is underslung leading and lagging will be at an absolute minimum.


I was making the point that some degree of lead and lag is present in this hub. Thanks Bug.

R22 Servicing Costs
 

Can anyone quote the appoximate servicing costs for the R22.

50Hr ? 100Hr ? 200 ? Annual ? etc.:ok:

No, there is not necessarally any lead lag force at all in normal flight conditions.
Undersling puts the teeter hinge above the blade root, at the imaginary point where a straight line could be drawn through the cg of each blade and through the teeter joint. Now, when the rotor tilts about the teeter hinge, which is another way of saying flapping, the cg will not move off the mast, it will stay at the hinge point.
Without undersling, when the coned rotor tilts or flaps, the cg of the rotor, being above the hinge, will move away from the mast or towards it- and vibration will result as the cg of the rotor resists being moved away from it's center by the rotor head which is now spinning in a different plane.
With more than two blades, there is no hinge that can float in a way that keeps the cg of the rotor on the mast. Each blade rises and falls seperately and the resulting cg shift creates a coriolis force and the need for a lead lag hinge.

Contact Hugh @ PDG Helicopters, Kintore, Aberdeen.

I know that they do fixed service costs for pretty much most types.

Not suggesting you go there, but at least it will give you a ball park figure!:ok: