R22 Corner
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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.
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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.
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.
Last edited by Chiplight; 13th Apr 2005 at 21:59.
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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
..... as it not only changes the attitude of the disc but also pulls the machine foreward.
... what is it that stops the blade moving on its coning/flapping hinge.
Dave
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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.
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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
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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.
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.
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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
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
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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.
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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?
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?
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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
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
Last edited by delta3; 13th Apr 2005 at 22:30.
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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.
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.
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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.
(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.
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!
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Hi Lu
So, bugdevheli's mention of the word 'Robinson' got you off the couch.
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
So, bugdevheli's mention of the word 'Robinson' got you off the couch.
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.
Dave
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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
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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
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
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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.
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.
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bugdevheli,
You press a very interesting point, and one that might get Lu salivating.
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
You press a very interesting point, and one that might get Lu salivating.
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
Last edited by Dave_Jackson; 14th Apr 2005 at 19:39.
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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.
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.