Flight Dynamics: The Swashplate and Phase-angle
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
The devil made me do it.
Nick,
As a promoter of Sikorsky and a critic of the competition, no one can fault your marketing ability.
Does the purchase of Schweizer by Sikorsky now put you and Lu on the same side of the net?
Dave J.
As a promoter of Sikorsky and a critic of the competition, no one can fault your marketing ability.
Does the purchase of Schweizer by Sikorsky now put you and Lu on the same side of the net?
Dave J.
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
Dave,
It is strange that you should take the slant that you do. Consider that this entire thread has been since Sikorsky bought Schweizer, and the points I have held regarding the Robinson. How do you yet think my observations are for sale?
It is strange that you should take the slant that you do. Consider that this entire thread has been since Sikorsky bought Schweizer, and the points I have held regarding the Robinson. How do you yet think my observations are for sale?
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Hi Nick,
I was sitting at the computer producing detail drawings, which is as exciting as watching grass grow. So, for a little diversion it seemed like it would be fun to take your pacemaker up a notch or two.
It was all in good fun. You took it that way, didn't you
Dave
I was sitting at the computer producing detail drawings, which is as exciting as watching grass grow. So, for a little diversion it seemed like it would be fun to take your pacemaker up a notch or two.
It was all in good fun. You took it that way, didn't you
Dave
Joined: Sep 2001
Posts: 301
Likes: 16
From: Australia.
Interesting thread, although as one of "the masses" at times it steps beyond my noodle's capacity.....
From a pilot's perspective, I think most like myself reading this are still trying to understand the hazards from untrimmed flight in teetering head systems.
I'm a big believer in simple "laymans terms", which I find is the most effective way to explain something to most pilots so that they can actually vaguely comprehend it, take it with them and then apply it practically. That's where the real benefit of the knowledge lies for a pilot eh?
My understanding with regard to flapping and the potential hazards of untrimmed flight goes like this:
If at anytime the tip path plane is not by and large perpendicular to the axis of rotation, for whatever reason, then the blades must/will flap, or are flapping, and forces are not in equilibrium.
In stabilised unaccelerated flight, (ignoring turbulence etc) the tip path plane is basically perpendicular to the mast (the axis of rotation) and the blades by and large are not flapping. This is achieved by the pilot finding the right cyclic position, thus providing the respective pitch angles and ultimate angles of attack required to eliminate any dissymetry of lift, and therefore the requirement for the blades to flap. This does not just have to be forward flight, but stabilised where all the forces are in equilibrium.
So if you are bombing along in stabilised flight and then put the boot in, the aircraft departs from stabilised flight. The swashplate (and the respective pitch angles) rotate to a new position in relation to the direction of flight, which is like making a cyclic input. The forces on the fuselage change (drag and stabilisers) and it moves the mast in relation to the rotor. All these forces get busy, equilibrium goes out the window, and this takes the tip path plane away from being perpendicular to the mast and bingo the blades start/increase their flapping.
My understanding is such that the hazard lies in making a RAPID change to a new flight condition, introducing a lot of unbalanced forces, without having time to correct the cyclic position that is required to stabilise flight in this condition.
I have done a lot of film/photo work (in Robinsons and Jetrangers) where you are constantly in and out of trimmed flight at all kinds of speeds and it wasn't unheard of to reach pedal limits. But I always made the inputs smoothly to allow me time for the corresponding cyclic input that was required, and for things to stabilise. And I always displaced the cyclic in the direction the rotor was flying.
You are flying the rotor with the sticks and the fuselage with the pedals (and air), and the hazard lies when one asks the other to do too much too quickly.
Have I got a vague handle on it, or am I way off target and need to go back to the books?
From a pilot's perspective, I think most like myself reading this are still trying to understand the hazards from untrimmed flight in teetering head systems.
I'm a big believer in simple "laymans terms", which I find is the most effective way to explain something to most pilots so that they can actually vaguely comprehend it, take it with them and then apply it practically. That's where the real benefit of the knowledge lies for a pilot eh?
My understanding with regard to flapping and the potential hazards of untrimmed flight goes like this:
If at anytime the tip path plane is not by and large perpendicular to the axis of rotation, for whatever reason, then the blades must/will flap, or are flapping, and forces are not in equilibrium.
In stabilised unaccelerated flight, (ignoring turbulence etc) the tip path plane is basically perpendicular to the mast (the axis of rotation) and the blades by and large are not flapping. This is achieved by the pilot finding the right cyclic position, thus providing the respective pitch angles and ultimate angles of attack required to eliminate any dissymetry of lift, and therefore the requirement for the blades to flap. This does not just have to be forward flight, but stabilised where all the forces are in equilibrium.
So if you are bombing along in stabilised flight and then put the boot in, the aircraft departs from stabilised flight. The swashplate (and the respective pitch angles) rotate to a new position in relation to the direction of flight, which is like making a cyclic input. The forces on the fuselage change (drag and stabilisers) and it moves the mast in relation to the rotor. All these forces get busy, equilibrium goes out the window, and this takes the tip path plane away from being perpendicular to the mast and bingo the blades start/increase their flapping.
My understanding is such that the hazard lies in making a RAPID change to a new flight condition, introducing a lot of unbalanced forces, without having time to correct the cyclic position that is required to stabilise flight in this condition.
I have done a lot of film/photo work (in Robinsons and Jetrangers) where you are constantly in and out of trimmed flight at all kinds of speeds and it wasn't unheard of to reach pedal limits. But I always made the inputs smoothly to allow me time for the corresponding cyclic input that was required, and for things to stabilise. And I always displaced the cyclic in the direction the rotor was flying.
You are flying the rotor with the sticks and the fuselage with the pedals (and air), and the hazard lies when one asks the other to do too much too quickly.
Have I got a vague handle on it, or am I way off target and need to go back to the books?
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
IMHO
Lu is on one side of the 'argument' and Frank Robinson is on the other. As I understand it, Lu has a background in aircraft reliability and Frank Robinson is the principal of the Robinson Helicopter Company. For them to have divergent positions on the subject of the Robinson rotor does not seem to be totally illogical.
Lu appears to have an intuitive feeling that the R-22 rotor may not be perfect, and he has expressed it on numerous occasions. Frank Robinson has placed one post on PPRuNe where he presents an explaination about "a highly technical subject".
Between these two extremes are the participants of this forum. I would humbly suggest that none of the regular participants are fully conversant with the difficult subject delta3 & phase-lag. Even Nick has said that it took two or three revisions, during the development of the S-76, to get the phase-lag correct. This means that with all the 'scientific' mathematics on delta3 & phase-lag it still took additional work in the form of 'art' to obtain the optimal phase-angle. It should also be noted that the S-76' s tail rotor only has collective control, whereas the R-22 main rotor has collective and cyclic control to consider.
How can any PPRuNe participant take the side of Lu or of Frank Robinson?
Either, neither or both may be partially or totally, correct or incorrect.
_____________________
The coyote has given a very clear description. He concludes with; " You are flying the rotor with the sticks and the fuselage with the pedals (and air), and the hazard lies when one asks the other to do too much too quickly."
In support of Frank Robinson;
It is said that large helicopters have greater stability than small one. Since the R-22 is at the light end of certified helicopters, the use of a basic teetering rotor would have resulted in minimal stability. Frank Robinson probably implemented delta3 as a type of stability augmentation system. The objective being similar to that of the early Bell gyrobar and the Hiller paddle.
In support of Lu; (for those who say Lu does not technically back his arguments)
Robinson's patent 4,131,391 has a interesting segment.
The patent says; "Referring to FIG. 6, a condition is illustrated which the present invention avoids. If it is assumed that line 70 represents the true axis of rotation, destabilizing moments tend to create a condition in which the center of the rotor 71 is displaced, off the true axis of rotation. One destabilizing moment is due to the offset of the rotor thrust vector from the teeter hinge 61 and is equal to T .times. Z. The other destabilizing moment is due to the centrifugal force created by moving the center of gravity of the rotor off the center of rotation by a distance Z.
The condition illustrated in FIG. 6 cannot occur, however, since by the present invention, the stabilizing centrifugal moment is always greater than the destabilizing moments for all conditions. Structurally this is achieved by separate coning hinges 63, 64, for each blade, the coning hinges being offset a sufficient distance such that the stabilizing centrifugal moment (2 .times. CF .times. y) is always greater than the destabilizing moments where y is the displacement between the pitch change axes of the opposed rotor blades."
Will not his stabilizing centrifugal moment be half of what is stated I.e (2 .times. CF .times. half of y)? Might a sudden out-of-plane excitation, which had the same harmonic as the flexible blade, cause the condition illustrated in FIG. 6 to occur? Would not his second destabilizing concern then attempt to cause lead-lag between the blades, since the coning hinges will be acting as flapping hinges and the blades will want to independently adjust their speeds to compensate for their changes in radius? Could delta3 then exasperate the problem?
I am certainly not suggesting that there IS a problem here. However, if the coning hinges are to do all that he wants them to and definitely not do what is shown in FIG 6, IMHO, the two coning hinges should have been linked together. Linked together so that they can cone in unison and cannot flap independently. This concern has been brought up previously. The last time was the posting and picture in Lu's thread [R-44 Rotorhead. Sorry guys I just couldn't help myself.]
______________
No one, including Lu, has given cause to believe that the R-22 rotor is flawed, but conversely, can anyone say that this light rotor is without fault?
Dave J.
Edited for clarification.
Lu appears to have an intuitive feeling that the R-22 rotor may not be perfect, and he has expressed it on numerous occasions. Frank Robinson has placed one post on PPRuNe where he presents an explaination about "a highly technical subject".
Between these two extremes are the participants of this forum. I would humbly suggest that none of the regular participants are fully conversant with the difficult subject delta3 & phase-lag. Even Nick has said that it took two or three revisions, during the development of the S-76, to get the phase-lag correct. This means that with all the 'scientific' mathematics on delta3 & phase-lag it still took additional work in the form of 'art' to obtain the optimal phase-angle. It should also be noted that the S-76' s tail rotor only has collective control, whereas the R-22 main rotor has collective and cyclic control to consider.
How can any PPRuNe participant take the side of Lu or of Frank Robinson?
Either, neither or both may be partially or totally, correct or incorrect.
_____________________
The coyote has given a very clear description. He concludes with; " You are flying the rotor with the sticks and the fuselage with the pedals (and air), and the hazard lies when one asks the other to do too much too quickly."
In support of Frank Robinson;
It is said that large helicopters have greater stability than small one. Since the R-22 is at the light end of certified helicopters, the use of a basic teetering rotor would have resulted in minimal stability. Frank Robinson probably implemented delta3 as a type of stability augmentation system. The objective being similar to that of the early Bell gyrobar and the Hiller paddle.
In support of Lu; (for those who say Lu does not technically back his arguments)
Robinson's patent 4,131,391 has a interesting segment.
The patent says; "Referring to FIG. 6, a condition is illustrated which the present invention avoids. If it is assumed that line 70 represents the true axis of rotation, destabilizing moments tend to create a condition in which the center of the rotor 71 is displaced, off the true axis of rotation. One destabilizing moment is due to the offset of the rotor thrust vector from the teeter hinge 61 and is equal to T .times. Z. The other destabilizing moment is due to the centrifugal force created by moving the center of gravity of the rotor off the center of rotation by a distance Z.
The condition illustrated in FIG. 6 cannot occur, however, since by the present invention, the stabilizing centrifugal moment is always greater than the destabilizing moments for all conditions. Structurally this is achieved by separate coning hinges 63, 64, for each blade, the coning hinges being offset a sufficient distance such that the stabilizing centrifugal moment (2 .times. CF .times. y) is always greater than the destabilizing moments where y is the displacement between the pitch change axes of the opposed rotor blades."
Will not his stabilizing centrifugal moment be half of what is stated I.e (2 .times. CF .times. half of y)? Might a sudden out-of-plane excitation, which had the same harmonic as the flexible blade, cause the condition illustrated in FIG. 6 to occur? Would not his second destabilizing concern then attempt to cause lead-lag between the blades, since the coning hinges will be acting as flapping hinges and the blades will want to independently adjust their speeds to compensate for their changes in radius? Could delta3 then exasperate the problem?
I am certainly not suggesting that there IS a problem here. However, if the coning hinges are to do all that he wants them to and definitely not do what is shown in FIG 6, IMHO, the two coning hinges should have been linked together. Linked together so that they can cone in unison and cannot flap independently. This concern has been brought up previously. The last time was the posting and picture in Lu's thread [R-44 Rotorhead. Sorry guys I just couldn't help myself.]
______________
No one, including Lu, has given cause to believe that the R-22 rotor is flawed, but conversely, can anyone say that this light rotor is without fault?
Dave J.
Edited for clarification.
Last edited by Dave_Jackson; 11th December 2004 at 04:35.
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
Dave,
I really think this thread has wandered all over the map. You now present one enormous tome that seems to be a detailed mechanical speculation about the possible problems (none with any outward symptoms except Lu's ranting).
I read your post twice and was struck to ask this question:
What does the rotor do that you don't like? If you can't say it in one coherent sentence, then it probably isn't a problem. Or as you say, "I am certainly not suggesting that there IS a problem here"
If you were speculating so wildly about your own helicopter, we'd all be amused. I am sure owners operators and makers of the Robinson expect more than what you have offered (or that Lu regularly offers) as foundation for your (non) concerns.
I really think this thread has wandered all over the map. You now present one enormous tome that seems to be a detailed mechanical speculation about the possible problems (none with any outward symptoms except Lu's ranting).
I read your post twice and was struck to ask this question:
What does the rotor do that you don't like? If you can't say it in one coherent sentence, then it probably isn't a problem. Or as you say, "I am certainly not suggesting that there IS a problem here"
If you were speculating so wildly about your own helicopter, we'd all be amused. I am sure owners operators and makers of the Robinson expect more than what you have offered (or that Lu regularly offers) as foundation for your (non) concerns.
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
Nick,
You say; "I really think this thread has wandered all over the map."
The lead post in this thread was addressed to Frank Robinson and it is a questioning of his rotorhead. Is it off topic to discuss the mechanical fundamentals of this rotorhead?
You say; "You now present one enormous tome that seems to be a detailed mechanical speculation about the possible problems."
Lu is criticized for not presenting technical support. I am criticized for presenting technical support?
Please don't assume that I am faulting Robinson's rotorhead, OR approving of it. From my perspective, this head only serves as the foundation for interesting and technically educational discussions.
As in the past, I would respectfully ask " in one coherent sentence"
that you stay on thread and attack the argument, not the messenger.
Thanks,
Dave J.
You say; "I really think this thread has wandered all over the map."
The lead post in this thread was addressed to Frank Robinson and it is a questioning of his rotorhead. Is it off topic to discuss the mechanical fundamentals of this rotorhead?
You say; "You now present one enormous tome that seems to be a detailed mechanical speculation about the possible problems."
Lu is criticized for not presenting technical support. I am criticized for presenting technical support?
Please don't assume that I am faulting Robinson's rotorhead, OR approving of it. From my perspective, this head only serves as the foundation for interesting and technically educational discussions.
As in the past, I would respectfully ask " in one coherent sentence"
that you stay on thread and attack the argument, not the messenger.
Thanks,
Dave J.
Last edited by Dave_Jackson; 11th December 2004 at 19:54.
Joined: Feb 2003
Posts: 777
Likes: 0
From: Harwich
This thread has gone all over the place. Here it goes further.
1. Experienced people have killed themselves in machines since there were machines. So have inexperienced people. Most of the time you can put the cause of the crash down to a list of three or four factors. Bad luck takes care of the rest.
2. The Wrights first flew in 1903. Lanchester and Prandtl had independently worked out the vortex theory of lift by 1913. In other words, people could fly a decade before anyone knew how it was possible.
3. The first practical helicopter flew in the early forties. Nick Lappos became an IP about 30 years later. Later still they discovered some more things about teetering rotors they hadn't known before. That's another example of practice leading theory, but more so.
4. There is a gap - a positive abyss - between the steady states and the 'transients'. Engineers have been modelling steady states for decades and decades. Modelling transients accurately is Nobel-winning stuff. Especially when it's rotating.
[/thread creep]
1. Experienced people have killed themselves in machines since there were machines. So have inexperienced people. Most of the time you can put the cause of the crash down to a list of three or four factors. Bad luck takes care of the rest.
2. The Wrights first flew in 1903. Lanchester and Prandtl had independently worked out the vortex theory of lift by 1913. In other words, people could fly a decade before anyone knew how it was possible.
3. The first practical helicopter flew in the early forties. Nick Lappos became an IP about 30 years later. Later still they discovered some more things about teetering rotors they hadn't known before. That's another example of practice leading theory, but more so.
4. There is a gap - a positive abyss - between the steady states and the 'transients'. Engineers have been modelling steady states for decades and decades. Modelling transients accurately is Nobel-winning stuff. Especially when it's rotating.
[/thread creep]
Joined: Oct 2004
Posts: 184
Likes: 0
From: south of France
About the Wright Brothers and the helicopter :
" As all the novices, we began with the helicopter but we saw that there was no future and we abandoned it. The helicopter makes in big effort, only what the balloon makes without labour and is not better endowed than the balloon for the fast horizontal flight. If its engine stops, he has to fall with a mortal violence because he cannot float as the balloon or glide as the aeroplane. The helicopter is easier to conceive that the plane but once realized, it is not worth. "
WILBUR WRIGHT
The job of an engineer is often to say no.
" As all the novices, we began with the helicopter but we saw that there was no future and we abandoned it. The helicopter makes in big effort, only what the balloon makes without labour and is not better endowed than the balloon for the fast horizontal flight. If its engine stops, he has to fall with a mortal violence because he cannot float as the balloon or glide as the aeroplane. The helicopter is easier to conceive that the plane but once realized, it is not worth. "
WILBUR WRIGHT
The job of an engineer is often to say no.
Joined: Jul 2001
Posts: 68
Likes: 0
From: CT, USA
R22 down on LA rooftop
http://abclocal.go.com/kabc/news/121...elicopter.html
Helicopter Makes Emergency Landing on Roof
LOS ANGELES — A helicopter pilot tried to land on the roof of a downtown high-rise due to an "in-flight emergency" today, but the craft missed the heliport and landed hard on the roof, a fire official said.
The two men aboard the two-seat Robinson R-22 were uninjured and there was no fire, but the chopper was seriously damaged, said Brian Humphrey of the Los Angeles Fire Department.
Fire crews were called to the 54-story Wells Fargo building at 333 S. Grand Ave. about 10:15 a.m., said Humphrey.
"There was some type of in-flight emergency," said Humphrey. "The pilot attempted to land on the rooftop heliport, but he missed the heliport and the helicopter impacted the rooftop.""The pilot stated that he suspected something was wrong with the design of the rotorhead after seeing some posts on a "pilots' rumour network" on the internet." "I was flying along and everything was fine, then I suddenly had this thought- what if the coning hinges don't really work the way Robinson says they do and the rotorhead decides to go divergent on me right at this moment?" "I'd better make a precautionary landing and ask Dave Jackson if he thinks its safe to continue."
The hard landing left the tail section of the craft twisted and the fuselage damaged.
There was no obvious damage to the roof, said Humphrey.
Fire crews were sent to the 723-foot building -- the seventh tallest in Los Angeles -- after getting a call from security personnel there, he said.
The hard landing will be investigated by the National Transportation Safety Board.
Humphrey said the helicopter is 15 years old.
(Copyright 2004 by The CNS. All Rights Reserved.) (with additional comments thrown in by Chiplight)
Helicopter Makes Emergency Landing on Roof
LOS ANGELES — A helicopter pilot tried to land on the roof of a downtown high-rise due to an "in-flight emergency" today, but the craft missed the heliport and landed hard on the roof, a fire official said.
The two men aboard the two-seat Robinson R-22 were uninjured and there was no fire, but the chopper was seriously damaged, said Brian Humphrey of the Los Angeles Fire Department.
Fire crews were called to the 54-story Wells Fargo building at 333 S. Grand Ave. about 10:15 a.m., said Humphrey.
"There was some type of in-flight emergency," said Humphrey. "The pilot attempted to land on the rooftop heliport, but he missed the heliport and the helicopter impacted the rooftop.""The pilot stated that he suspected something was wrong with the design of the rotorhead after seeing some posts on a "pilots' rumour network" on the internet." "I was flying along and everything was fine, then I suddenly had this thought- what if the coning hinges don't really work the way Robinson says they do and the rotorhead decides to go divergent on me right at this moment?" "I'd better make a precautionary landing and ask Dave Jackson if he thinks its safe to continue."
The hard landing left the tail section of the craft twisted and the fuselage damaged.
There was no obvious damage to the roof, said Humphrey.
Fire crews were sent to the 723-foot building -- the seventh tallest in Los Angeles -- after getting a call from security personnel there, he said.
The hard landing will be investigated by the National Transportation Safety Board.
Humphrey said the helicopter is 15 years old.
(Copyright 2004 by The CNS. All Rights Reserved.) (with additional comments thrown in by Chiplight)
Last edited by Chiplight; 12th December 2004 at 17:29.
Joined: Jul 2001
Posts: 1,084
Likes: 1
From: the cockpit
Lu is the reason my helmet is on fire.
Did anyone else notice that Lu has deleted all his posts from the 2001 RBS, Phase Lag and LZ thread? hmmmmm.....
When we go back and read the threads Heliport posted, you will notice that the one thing that constantly emerges is that Lu cannot grasp first principles in aerodynamics OR in physics. This problem subsequently distorts all his theories because they are based on flawed start points. Unfortunately, this lack of first prinicple understanding is mated to an evangelical belief that he cannot be mistaken, thus it is an impossibility for him to acknowledge, understand, or even learn from any of the posters here. Even the basics.
But enjoy trying anyway......
Did anyone else notice that Lu has deleted all his posts from the 2001 RBS, Phase Lag and LZ thread? hmmmmm.....
When we go back and read the threads Heliport posted, you will notice that the one thing that constantly emerges is that Lu cannot grasp first principles in aerodynamics OR in physics. This problem subsequently distorts all his theories because they are based on flawed start points. Unfortunately, this lack of first prinicple understanding is mated to an evangelical belief that he cannot be mistaken, thus it is an impossibility for him to acknowledge, understand, or even learn from any of the posters here. Even the basics.
But enjoy trying anyway......
Joined: Apr 2003
Posts: 1,635
Likes: 1
From: Vancouver, BC, Canada
The two additional hinges in the Robinson rotorhead
Nick,
As previously mentioned, I do not see a problem, just an opportunity to acquire a better understanding of rotorheads.
I believe that Frank Robinson wanted to utilize the advantages of active coning hinges, which rotate in unison, while at the same time not risking any disadvantages that might come about from flapping hinges, which rotate independently. To achieve these two objectives, the essence of the patent is that "the stabilizing centrifugal moment is always greater than the destabilizing moments for all conditions."
In the continuing hope of having a discussion, I started to rephrase, what had previously been said, but it was becoming Tome II. To make a long story short, I talked to myself, made a few small assumptions and then did the math. Thanks for the reply.
Chiplight,
Funny post but, just in case some pilots take it the wrong way, the above calculations show the stabilizing moment to be far stronger than the two mentioned destabilizing moments.
Dave J.
As previously mentioned, I do not see a problem, just an opportunity to acquire a better understanding of rotorheads.
I believe that Frank Robinson wanted to utilize the advantages of active coning hinges, which rotate in unison, while at the same time not risking any disadvantages that might come about from flapping hinges, which rotate independently. To achieve these two objectives, the essence of the patent is that "the stabilizing centrifugal moment is always greater than the destabilizing moments for all conditions."
In the continuing hope of having a discussion, I started to rephrase, what had previously been said, but it was becoming Tome II. To make a long story short, I talked to myself, made a few small assumptions and then did the math. Thanks for the reply.
Chiplight,
Funny post but, just in case some pilots take it the wrong way, the above calculations show the stabilizing moment to be far stronger than the two mentioned destabilizing moments.
Dave J.
