R22 Corner
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Well done Lu for being so candid.
I do feel that in general your arguments have been too diverse and all encompassing, when it comes to R22 bashing. If you had just concentrated on one flaw (as perceived by you) at a time I think things may have been clearer. The threads have covered all manner of topics such as low g behaviour, low Nr behaviour, excessive sideslip problems and even the consequences of showing off at low level to your friends. To have pinned all this on Frank and his 18 degrees of offset was a little bold I feel, and smacked of an obsession.
Some of your statements simply don't show themselves in practice,as quite a few pilots tried to point out. The R22 is a very clever piece of kit and reasonable competence in handling it should yield dividends when converting to larger types. I appreciate the high loss rate, but this is more of a regulatory and training issue than a design flaw. You simply cannot expect a 50 hour PPL to fly a helicopter with the safety that an equivelant fixed wing pilot could fly a Cessna (and that's debatable in it's own right). Similarly, a very experienced pilot on larger types, such as Bells, with more benign handling traits may find problems in some predicaments.
Anyway, keep up the contributions, they certainly keep the forum alive!
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Another day in paradise
I do feel that in general your arguments have been too diverse and all encompassing, when it comes to R22 bashing. If you had just concentrated on one flaw (as perceived by you) at a time I think things may have been clearer. The threads have covered all manner of topics such as low g behaviour, low Nr behaviour, excessive sideslip problems and even the consequences of showing off at low level to your friends. To have pinned all this on Frank and his 18 degrees of offset was a little bold I feel, and smacked of an obsession.
Some of your statements simply don't show themselves in practice,as quite a few pilots tried to point out. The R22 is a very clever piece of kit and reasonable competence in handling it should yield dividends when converting to larger types. I appreciate the high loss rate, but this is more of a regulatory and training issue than a design flaw. You simply cannot expect a 50 hour PPL to fly a helicopter with the safety that an equivelant fixed wing pilot could fly a Cessna (and that's debatable in it's own right). Similarly, a very experienced pilot on larger types, such as Bells, with more benign handling traits may find problems in some predicaments.
Anyway, keep up the contributions, they certainly keep the forum alive!
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Another day in paradise
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To: Imlanphere
It is a well known fact that when a student graduates from a university, a technical school or say, a flight school he will have only that knowledge that was imparted to him by his instructors. If the student never expands on that knowledge he will only know what his instructors told him and nothing more.
Most helicopter flight schools start the student out in an R22. The instructors at that flight school may have been students at that same school only months before. Some of those instructors may have even attended a safety school at Robinson or at some field site where a Robinson Pilot taught the course. So, when one of the graduate students is asked a specific question he will provide an answer that fits the subject as he was taught. All Robinson helicopter pilots will respond in unison as if they were parroting the words as spoken by Frank Robinson.
First of all, the response that Frank Robinson made as quoted by you is totally false and this is why. Mr. Robinson mixed chickens and eggs so that you must really look at what he says in order to determine which came first.
The rotor head was designed from the very beginning as a teetering rotorhead with coning capability. Robinson knew from the beginning that the aerodynamics of a lightly loaded low inertia rotor would involve severe blade bending and blade flapping. In order to minimize those two negative actions they incorporated coning hinges.
Once coning hinges were installed they could never have a pitch horn that lead the blade by 90-degrees. To do so, would render the helicopter uncontrollable due to massive pitch coupling. By definition, the pitch horns could not pass the cone hinges. A 90-degree pitch horn would not force the pilot to compensate in the low speed regime. There would be no low speed or, for that matter high speed regime because when the pilot brought the helicopter to a hover, the pitch coupling would make it extremely difficult to maintain that hover and if he pushed forward cyclic the helicopter would most likely crash.
Robinsons’ theories, are fashioned to provide the neophyte pilot with information that the neophyte pilot believes to be true, and if that pilot never learns or accepts another alternate theory, then, when he is asked a question about a Robinson Helicopter, he will reply as if Robinson Helicopters signs his pay checks.
To: 212man
The reason that my posts are so diverse and I don’t address a particular point is because I have many interested individuals responding to the many points put forth in my report. Once I respond to a posting I get several more differing points of view that I in turn have to respond to. It is like playing a tennis match where I have 20-30 opponents at one time and they are all lobbing fuzzy yellow balls at me.
Regarding those statements that don’t show themselves in actual practice, I would refer you to those pilots that took up my challenge, and then reported back, that I was correct.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
It is a well known fact that when a student graduates from a university, a technical school or say, a flight school he will have only that knowledge that was imparted to him by his instructors. If the student never expands on that knowledge he will only know what his instructors told him and nothing more.
Most helicopter flight schools start the student out in an R22. The instructors at that flight school may have been students at that same school only months before. Some of those instructors may have even attended a safety school at Robinson or at some field site where a Robinson Pilot taught the course. So, when one of the graduate students is asked a specific question he will provide an answer that fits the subject as he was taught. All Robinson helicopter pilots will respond in unison as if they were parroting the words as spoken by Frank Robinson.
First of all, the response that Frank Robinson made as quoted by you is totally false and this is why. Mr. Robinson mixed chickens and eggs so that you must really look at what he says in order to determine which came first.
The rotor head was designed from the very beginning as a teetering rotorhead with coning capability. Robinson knew from the beginning that the aerodynamics of a lightly loaded low inertia rotor would involve severe blade bending and blade flapping. In order to minimize those two negative actions they incorporated coning hinges.
Once coning hinges were installed they could never have a pitch horn that lead the blade by 90-degrees. To do so, would render the helicopter uncontrollable due to massive pitch coupling. By definition, the pitch horns could not pass the cone hinges. A 90-degree pitch horn would not force the pilot to compensate in the low speed regime. There would be no low speed or, for that matter high speed regime because when the pilot brought the helicopter to a hover, the pitch coupling would make it extremely difficult to maintain that hover and if he pushed forward cyclic the helicopter would most likely crash.
Robinsons’ theories, are fashioned to provide the neophyte pilot with information that the neophyte pilot believes to be true, and if that pilot never learns or accepts another alternate theory, then, when he is asked a question about a Robinson Helicopter, he will reply as if Robinson Helicopters signs his pay checks.
To: 212man
The reason that my posts are so diverse and I don’t address a particular point is because I have many interested individuals responding to the many points put forth in my report. Once I respond to a posting I get several more differing points of view that I in turn have to respond to. It is like playing a tennis match where I have 20-30 opponents at one time and they are all lobbing fuzzy yellow balls at me.
Regarding those statements that don’t show themselves in actual practice, I would refer you to those pilots that took up my challenge, and then reported back, that I was correct.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
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Lu, the response I spoke of in the above post was from Ray Prouty, not from a recent training school graduate. I completely agree that asking fellow pilots in a training school will more than likely yield similar results, but you have assumed too much for this was not the case.
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To: Imlanphere
If Ray Prouty said that about the design of the Robinson rotorhead either he had not fully examined the problem or, if he did, he had his head up-and-locked.
A ninety degree pitch horn could never have been used on the R22/R44 rotorheads unless they got rid of the coning hinges.
If you would refer to my response to Frank Robinson I stated that I would buy the positioning of the cone hinges that resulted in the eighteen degree offset as a means of reducing certain pitch coupling. However, the lead in to this thread is that there is no 18-degree offset of the two parts of the swashplate.. The rigging is done with the pitch links over the lowest/highest point of the stationary swashplate which offsets the blades by 18-degrees. This makes Frank Robinsons' argument go away and as far as I am concerned, Ray Proutys' input is moot.
You mentioned having to compensate at higher speeds, discounting transverse flow effect, what compensation are you refering to.
Before you answer that I am including a response by The NR Fairy regarding my statements about the impreciseness of the Robinson cyclic control. This was in response to my claim about having to compensate for the so called 18 degree offset.
"On this occasion, I'd have to agree with Lu that the Robbie's handling is far from precise, but only when I'm trying to beat the little b*****d into submission" !!
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
If Ray Prouty said that about the design of the Robinson rotorhead either he had not fully examined the problem or, if he did, he had his head up-and-locked.
A ninety degree pitch horn could never have been used on the R22/R44 rotorheads unless they got rid of the coning hinges.
If you would refer to my response to Frank Robinson I stated that I would buy the positioning of the cone hinges that resulted in the eighteen degree offset as a means of reducing certain pitch coupling. However, the lead in to this thread is that there is no 18-degree offset of the two parts of the swashplate.. The rigging is done with the pitch links over the lowest/highest point of the stationary swashplate which offsets the blades by 18-degrees. This makes Frank Robinsons' argument go away and as far as I am concerned, Ray Proutys' input is moot.
You mentioned having to compensate at higher speeds, discounting transverse flow effect, what compensation are you refering to.
Before you answer that I am including a response by The NR Fairy regarding my statements about the impreciseness of the Robinson cyclic control. This was in response to my claim about having to compensate for the so called 18 degree offset.
"On this occasion, I'd have to agree with Lu that the Robbie's handling is far from precise, but only when I'm trying to beat the little b*****d into submission" !!
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
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Lu,
You CANNOT continue to preach that gyroscopic precession is the cause of rotor phase lag; it is incorrect and misleading. There are many good academic texts on the subject. Please see my post on page 9 of the R22 certification thread.
Grey Area
You CANNOT continue to preach that gyroscopic precession is the cause of rotor phase lag; it is incorrect and misleading. There are many good academic texts on the subject. Please see my post on page 9 of the R22 certification thread.
Grey Area
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this is an excerpt from a letter written by Ray Prouty:
"...If you are on the ground and apply maximum cyclic pitch to the rotor when it is over the nose, the tip path plane will tilt up 90 degrees later on the left side (and down on the right) following the laws that apply to a gyroscope.
If you are in the air, the same thing happens as you call for an acceleration of a right roll by applying maximum pitch over the nose. (If the rotor has offset flapping hinges, the angle is not 90 degrees, but something less--maybe 85 degrees). However, once you achieve some rate of right roll, the situation changes and the tip path plane will not be responding at 90 degrees, but at some smaller angle depending on the design of the blades. This angle can be as low as 60 degrees depending on the design of the blades and results in rate crosscoupling.
Some helicopter designers arrange the cyclic control system such that a stick movement directly to the right produces maximum pitch over the nose and rely on the pilot to take care of the rate cross-coupling. Others, such as Robinson, rotate the controls to minimize this crosscoupling while accepting some acceleration crosscoupling.
There are some other sources of crosscoupling, so one value of control phasing will never be optimum....."
I typed this out because it was unfair of me to use Ray's name in the above post along with other information which was not given by him.
[This message has been edited by lmlanphere (edited 14 December 2000).]
"...If you are on the ground and apply maximum cyclic pitch to the rotor when it is over the nose, the tip path plane will tilt up 90 degrees later on the left side (and down on the right) following the laws that apply to a gyroscope.
If you are in the air, the same thing happens as you call for an acceleration of a right roll by applying maximum pitch over the nose. (If the rotor has offset flapping hinges, the angle is not 90 degrees, but something less--maybe 85 degrees). However, once you achieve some rate of right roll, the situation changes and the tip path plane will not be responding at 90 degrees, but at some smaller angle depending on the design of the blades. This angle can be as low as 60 degrees depending on the design of the blades and results in rate crosscoupling.
Some helicopter designers arrange the cyclic control system such that a stick movement directly to the right produces maximum pitch over the nose and rely on the pilot to take care of the rate cross-coupling. Others, such as Robinson, rotate the controls to minimize this crosscoupling while accepting some acceleration crosscoupling.
There are some other sources of crosscoupling, so one value of control phasing will never be optimum....."
I typed this out because it was unfair of me to use Ray's name in the above post along with other information which was not given by him.
[This message has been edited by lmlanphere (edited 14 December 2000).]
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To: Imlanphere
Ray Prouty said: "Some helicopter designers arrange the cyclic control system such that a stick movement directly to the right produces maximum pitch over the nose and rely on the pilot to take care of the rate cross-coupling. Others, such as Robinson, rotate the controls to minimize this crosscoupling while accepting some acceleration crosscoupling". My question is, how do the pilots counter this cross coupling. Which way do they move the cyclic?
The control movement and the resulting swashplate movement is exactly the same on a Robinson as it is on a Bell. Using the illustration above, when the pilot moves the cyclic to the right the Bell blade will have reached its' maximum pitch over the nose and its' minimum pitch over the tail and as a result the P R E C E S S I O N of 90 degrees will cause the blade to rise over the left side and drop over the right side and the helicopter is flying to the right.
The control movement for the Robinson is exactly the same. However, when the blades are over the nose or the tail they must rotate an additional 18 degrees to achieve maximum and minimum pitch change. When the have rotated this additional 18 degrees they will be at the maxim/minimum pitch change points and, the P R E C E S S I O N OF 90 degrees will cause the blade to rise 18-degrees past the lateral axis, and the helicopter will fly to the right, with a slight forward component. So, Mr. Prouty is wrong in this case. However, I do agree with his remarks about the laws that apply to a gyroscope. Grey Area are you reading this?
With regards to what Mr. Prouty said about articulated rotorheads or rotor heads with real or theoretical offset hinges he was talking about the positioning of the swashplate relative to the direction of flight. Most helicopters of this type follow what Sikorsky has been doing for years. The movement of the controls are the same as described above, however the movement of the swashplate is different. If the pilot moves his cyclic to the right, the swash plate will tilt up somewhere between the 10 and 11 o clock position and the swashplate will tip down somewhere between the4 and 5 o clock position. This offset is 45-degrees. The pitch horn lead is 45-degrees making a total phase angle of 90-degrees. With the right cyclic input as described above the disc will tilt up over the 9 o clock position and will tip down over the 3 o clock position. This movement is the result of PR E C E S S I O N which was caused by a force imbalance across the disc which was induced by the movement of the cyclic stick and the resultant pitch change.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
[This message has been edited by Lu Zuckerman (edited 15 December 2000).]
Ray Prouty said: "Some helicopter designers arrange the cyclic control system such that a stick movement directly to the right produces maximum pitch over the nose and rely on the pilot to take care of the rate cross-coupling. Others, such as Robinson, rotate the controls to minimize this crosscoupling while accepting some acceleration crosscoupling". My question is, how do the pilots counter this cross coupling. Which way do they move the cyclic?
The control movement and the resulting swashplate movement is exactly the same on a Robinson as it is on a Bell. Using the illustration above, when the pilot moves the cyclic to the right the Bell blade will have reached its' maximum pitch over the nose and its' minimum pitch over the tail and as a result the P R E C E S S I O N of 90 degrees will cause the blade to rise over the left side and drop over the right side and the helicopter is flying to the right.
The control movement for the Robinson is exactly the same. However, when the blades are over the nose or the tail they must rotate an additional 18 degrees to achieve maximum and minimum pitch change. When the have rotated this additional 18 degrees they will be at the maxim/minimum pitch change points and, the P R E C E S S I O N OF 90 degrees will cause the blade to rise 18-degrees past the lateral axis, and the helicopter will fly to the right, with a slight forward component. So, Mr. Prouty is wrong in this case. However, I do agree with his remarks about the laws that apply to a gyroscope. Grey Area are you reading this?
With regards to what Mr. Prouty said about articulated rotorheads or rotor heads with real or theoretical offset hinges he was talking about the positioning of the swashplate relative to the direction of flight. Most helicopters of this type follow what Sikorsky has been doing for years. The movement of the controls are the same as described above, however the movement of the swashplate is different. If the pilot moves his cyclic to the right, the swash plate will tilt up somewhere between the 10 and 11 o clock position and the swashplate will tip down somewhere between the4 and 5 o clock position. This offset is 45-degrees. The pitch horn lead is 45-degrees making a total phase angle of 90-degrees. With the right cyclic input as described above the disc will tilt up over the 9 o clock position and will tip down over the 3 o clock position. This movement is the result of PR E C E S S I O N which was caused by a force imbalance across the disc which was induced by the movement of the cyclic stick and the resultant pitch change.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
[This message has been edited by Lu Zuckerman (edited 15 December 2000).]
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Lu,
I am reading this and from personal experience know you are in error. For example the Lynx is rigged to accommodate a phase lag of 72 degs and is not too far off, a Squirrel at 78 degs and again does not suffer from too much cross couple. These cross couples will alter with density altitude (I have practical experience of this fact). In fact, with a cyclic pitch input aft it is possible to demonstrate on a cold, high pressure day, to demonstrate a pitch up and roll right whereas at high density altitude a pitch up and roll left. Please explain?
[This message has been edited by Grey Area (edited 15 December 2000).]
I am reading this and from personal experience know you are in error. For example the Lynx is rigged to accommodate a phase lag of 72 degs and is not too far off, a Squirrel at 78 degs and again does not suffer from too much cross couple. These cross couples will alter with density altitude (I have practical experience of this fact). In fact, with a cyclic pitch input aft it is possible to demonstrate on a cold, high pressure day, to demonstrate a pitch up and roll right whereas at high density altitude a pitch up and roll left. Please explain?
[This message has been edited by Grey Area (edited 15 December 2000).]
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To: Imlanphere
You stated above that a helicopter can be designed so that when the cyclic is moved to the left or, right the pilot can take care of the cross couple and fly forward (or at least that is what I think you said.
FAA Advisory Circular 27-1 which governs Certification Of Normal Catagory Rotorcraft states that the direction of flight must be in the same sense of cyclic stick displacement. They allow a 1-2 degree deviation for control coupling but not 90 degrees as you had indicated.
[This message has been edited by Lu Zuckerman (edited 20 December 2000).]
You stated above that a helicopter can be designed so that when the cyclic is moved to the left or, right the pilot can take care of the cross couple and fly forward (or at least that is what I think you said.
FAA Advisory Circular 27-1 which governs Certification Of Normal Catagory Rotorcraft states that the direction of flight must be in the same sense of cyclic stick displacement. They allow a 1-2 degree deviation for control coupling but not 90 degrees as you had indicated.
[This message has been edited by Lu Zuckerman (edited 20 December 2000).]
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Grey Area,
I'm afraid that I am going to have to disagree with you there about your statement about the Lynx. Firstly, the Phase Lag is 75 deg, secondly, with the AFCS OUT, it suffers massively due to this so called 'PITCH/ ROLL' couple - as in the R22 (note – this is not the ‘Acceleration Cross couple as explained by Ray Prouty/ not even he could explain this one!!). The only difference is that due to the nature of the Head - the rate of roll produced by an Aft/ Fwd movement of the cyclic is incredible. After a considerable amount of research and Flight testing into this subject, our conclusions are the same as for Lu Zuckerman. The only question that still remains un-answered is ‘Why has it been designed this way?’
Lu – regardless of the analogy of ‘GYROSCOPIC PRESSESSION’ that Ray Prouty uses to explain the flapping of the disc, it is WRONG. If indeed the disc did act as a GYRO – then where is the secondary/ tertiary etc pressession? What about the OTHER property of a gyro – RIGIDITY IN SPACE? If the disc were a gyro, then it would be uncontrollable!! There is a far easier/ factually correct explanation, which holds true for ALL helicopter Principles of Flight. It involves the application of Pitch – due to the swash plate, and its effect on RATE OF FLAPPING!!
I'm afraid that I am going to have to disagree with you there about your statement about the Lynx. Firstly, the Phase Lag is 75 deg, secondly, with the AFCS OUT, it suffers massively due to this so called 'PITCH/ ROLL' couple - as in the R22 (note – this is not the ‘Acceleration Cross couple as explained by Ray Prouty/ not even he could explain this one!!). The only difference is that due to the nature of the Head - the rate of roll produced by an Aft/ Fwd movement of the cyclic is incredible. After a considerable amount of research and Flight testing into this subject, our conclusions are the same as for Lu Zuckerman. The only question that still remains un-answered is ‘Why has it been designed this way?’
Lu – regardless of the analogy of ‘GYROSCOPIC PRESSESSION’ that Ray Prouty uses to explain the flapping of the disc, it is WRONG. If indeed the disc did act as a GYRO – then where is the secondary/ tertiary etc pressession? What about the OTHER property of a gyro – RIGIDITY IN SPACE? If the disc were a gyro, then it would be uncontrollable!! There is a far easier/ factually correct explanation, which holds true for ALL helicopter Principles of Flight. It involves the application of Pitch – due to the swash plate, and its effect on RATE OF FLAPPING!!
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Lu, I can't find what you were refering to above about a 90 degree compensation (I'll assume a misunderstanding). All I've done is repeat what I've learned on the subject recently (which unfortunately does not include FAR's on allowable cyclic/rotor displacement discrepancies).
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To: Baque Flip
Regarding the second paragraph of your post about gyroscopic characteristics. The rotor disc like the rotor on a gyroscope have two things in common. One, is rigidity in space. In other words, it will stay in the position you put it. The other is precession, as it will move to where you command it. In order to make the gyro/rotor disc move from its' rigid in plane position you have to input an external force. In the lab or in training school the instructor will demonstrate these characteristics using a multi gimbal gyroscope. With the rotor spinning, the instructor will perturb the rigid rotor by applying a force to the outer gimbal ring. The results of that force input causes the rotor to tilt in the direction of the force but 90-degrees later in the direction of rotation.
In the case of the rotor system it is either a direct force on the swashplate in the case of a Robinson or by a hydraulic servo such as those used on larger helicopters. The precession is still the same, 90-degrees and the precession will continue until one of two things happens, 1) the force stops or, 2) the control system or the rotor system comes up hard on the stops. In the case of 2 above the pilot becomes aware of the problem immediately as there is a very large vibratory force applied to the airframe from the rotorhead.
What you stated in the last sentence of your post discounts precession and states that the rotor moves because the swashplate position changes the pitch on the blades and the disc moves as a result. The change in pitch due to swashplate movement is the external perturbing force. The change in pitch creates a force differential across the disc, which results in precession.
Regarding the massive pitch couple on the Lynx when the AFCS is turned off I would direct you to read my post on the Piaseki Compound Helicopter where I made a comment about the Cheyenne helicopter.
The Cheyenne because of the design and the stiffness of the blades had a constantly changing phase angle (90 + or,- 5-10 degrees of precession). This little problem killed one pilot and destroyed a very large and very important wind tunnel in California. It took two years to figure out how to solve the problem. What they did was to measure and monitor cyclic stick position and when the blades went where they wanted instead of where the pilot commanded them the computer would sense this and command the servo system to instantly compensate. Say, the pilot pushed the stick forward and the disc tilted to the left, the computer would alter the servo input as if the pilot moved the cyclic to the right. The system worked very well but it was so complex and contained many single point catastrophic failure modes that the Army cancelled the program. I would assume that the AFCS system does the same on the Lynx as the flight computer did on the Cheyenne. Its’ too bad they can’t put one on a Robinson R22/44.
This principle of external force causing a gyro to precess is used in electro mechanical and air driven gyro instruments on the instrument panel of your ride. Only in this case it is the movement of the aircraft around the instrument that provides the perturbing force and the movement of the rotor is connected to the indicating needle or the ball on the gyro horizon.
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The Cat
[This message has been edited by Lu Zuckerman (edited 20 December 2000).]
[This message has been edited by Lu Zuckerman (edited 20 December 2000).]
Regarding the second paragraph of your post about gyroscopic characteristics. The rotor disc like the rotor on a gyroscope have two things in common. One, is rigidity in space. In other words, it will stay in the position you put it. The other is precession, as it will move to where you command it. In order to make the gyro/rotor disc move from its' rigid in plane position you have to input an external force. In the lab or in training school the instructor will demonstrate these characteristics using a multi gimbal gyroscope. With the rotor spinning, the instructor will perturb the rigid rotor by applying a force to the outer gimbal ring. The results of that force input causes the rotor to tilt in the direction of the force but 90-degrees later in the direction of rotation.
In the case of the rotor system it is either a direct force on the swashplate in the case of a Robinson or by a hydraulic servo such as those used on larger helicopters. The precession is still the same, 90-degrees and the precession will continue until one of two things happens, 1) the force stops or, 2) the control system or the rotor system comes up hard on the stops. In the case of 2 above the pilot becomes aware of the problem immediately as there is a very large vibratory force applied to the airframe from the rotorhead.
What you stated in the last sentence of your post discounts precession and states that the rotor moves because the swashplate position changes the pitch on the blades and the disc moves as a result. The change in pitch due to swashplate movement is the external perturbing force. The change in pitch creates a force differential across the disc, which results in precession.
Regarding the massive pitch couple on the Lynx when the AFCS is turned off I would direct you to read my post on the Piaseki Compound Helicopter where I made a comment about the Cheyenne helicopter.
The Cheyenne because of the design and the stiffness of the blades had a constantly changing phase angle (90 + or,- 5-10 degrees of precession). This little problem killed one pilot and destroyed a very large and very important wind tunnel in California. It took two years to figure out how to solve the problem. What they did was to measure and monitor cyclic stick position and when the blades went where they wanted instead of where the pilot commanded them the computer would sense this and command the servo system to instantly compensate. Say, the pilot pushed the stick forward and the disc tilted to the left, the computer would alter the servo input as if the pilot moved the cyclic to the right. The system worked very well but it was so complex and contained many single point catastrophic failure modes that the Army cancelled the program. I would assume that the AFCS system does the same on the Lynx as the flight computer did on the Cheyenne. Its’ too bad they can’t put one on a Robinson R22/44.
This principle of external force causing a gyro to precess is used in electro mechanical and air driven gyro instruments on the instrument panel of your ride. Only in this case it is the movement of the aircraft around the instrument that provides the perturbing force and the movement of the rotor is connected to the indicating needle or the ball on the gyro horizon.
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The Cat
[This message has been edited by Lu Zuckerman (edited 20 December 2000).]
[This message has been edited by Lu Zuckerman (edited 20 December 2000).]
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Robinson: Tech Q's
Just A quickie..........
why have the robinson r22 and r44 got such long masts , ie the distance between the swashplate and the cabin. many different reasons sugested just wondering which one is right. whats the difference between the 206 setup and the r22/44 setup??
why have the robinson r22 and r44 got such long masts , ie the distance between the swashplate and the cabin. many different reasons sugested just wondering which one is right. whats the difference between the 206 setup and the r22/44 setup??
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The precession theory for the rotor does not hold any water. Yes maybe as a ‘basic’ analogy/ comparison, but that is as far as it goes. To make the disc change attitude, you don’t just apply a force 90deg prior to its desired deflection. Consider the swash plate as a ‘swash plate’ and the blades as individual wings. All that the swash plate is doing is changing the AoA/ pitch of the ‘wings’ therefore causing them to increase/ decrease lift. Because they are attached to the rotor head, the blades flap up/ down respectively. This journey does not simply start 90 deg prior, but a full 180 deg prior.
Consider this. In order to get the blade to fly low over the front of the aircraft and high over the rear – you apply fwd cyclic. This will, in blades that turn counter clockwise, tilt the swash plate to the right (starboard)….As the blade leaves the 6 o’clock position (tail) pitch is slowly, but progressively being taken off – assume that it starts with neutral pitch. The blade starts to lose AoA = lift…therefore starts to flap down. As the journey of the blade continues to the 3 o’clock pos (starboard), the MAX amount of pitch is removed = MAX RATE of flap down. Towards the 12 o’clock pos (nose), note that the blade is still continuing to flap down, but at a ‘sustained rate’ because the amount of pitch is returning to neutral. Note – this blade will continue to flap down until positive pitch is applied. Funnily enough, this is what happens once the blade passes through the 12 o’clock. The swash plate now starts to apply pitch onto the blades, therefore the blades start to climb.
This is easily demonstrated in the hangar. Apply fwd cyclic and slowly rotate the disc and follow the journey of a blade from the tail to the nose. You will find that as the blade travels, the amount of pitch subjected to the blades is directly proportional to the swash plate. You don’t even need a swash plate – ANY rotor system.
Now, with a GENERAL helicopter, you would assume that with FWD cyclic applied, the blade over the 12 o’clock pos would have neutral pitch (indeed it shold not be effected) – and where you apply the phase lag offset – the max amount of pitch removed. As soon as the blade moves from the 12, the pitch 'should' start to increase again….Well, in the Lynx it is different. As the blade is moved through the 12, it is clear that it is still being subjected to pitch. It is not until 15deg past the 12, that the pitch becomes neutral. Therefore, the longitudinal axis does not run as we would expect - 12-6 o’clock. So AFT cyclic not only produces a nose up component, but also a roll to the right.. This can also be seen when applying collective pitch in fwd flight. One would expect the nose to ‘just’ flap back when raising the collective (try using precession to explain that one) and vice versa for lowering the collective. It doesn’t – you also get a roll, therefore confirming the theory. But you already know this.
As for the AFCS being advanced, believe me it is not. Maybe 25 yrs ago….It simply compares your control inputs with its vertical gyros and applies direct control inputs to the hydraulic servos.
[This message has been edited by Baque Flip (edited 20 December 2000).]
Consider this. In order to get the blade to fly low over the front of the aircraft and high over the rear – you apply fwd cyclic. This will, in blades that turn counter clockwise, tilt the swash plate to the right (starboard)….As the blade leaves the 6 o’clock position (tail) pitch is slowly, but progressively being taken off – assume that it starts with neutral pitch. The blade starts to lose AoA = lift…therefore starts to flap down. As the journey of the blade continues to the 3 o’clock pos (starboard), the MAX amount of pitch is removed = MAX RATE of flap down. Towards the 12 o’clock pos (nose), note that the blade is still continuing to flap down, but at a ‘sustained rate’ because the amount of pitch is returning to neutral. Note – this blade will continue to flap down until positive pitch is applied. Funnily enough, this is what happens once the blade passes through the 12 o’clock. The swash plate now starts to apply pitch onto the blades, therefore the blades start to climb.
This is easily demonstrated in the hangar. Apply fwd cyclic and slowly rotate the disc and follow the journey of a blade from the tail to the nose. You will find that as the blade travels, the amount of pitch subjected to the blades is directly proportional to the swash plate. You don’t even need a swash plate – ANY rotor system.
Now, with a GENERAL helicopter, you would assume that with FWD cyclic applied, the blade over the 12 o’clock pos would have neutral pitch (indeed it shold not be effected) – and where you apply the phase lag offset – the max amount of pitch removed. As soon as the blade moves from the 12, the pitch 'should' start to increase again….Well, in the Lynx it is different. As the blade is moved through the 12, it is clear that it is still being subjected to pitch. It is not until 15deg past the 12, that the pitch becomes neutral. Therefore, the longitudinal axis does not run as we would expect - 12-6 o’clock. So AFT cyclic not only produces a nose up component, but also a roll to the right.. This can also be seen when applying collective pitch in fwd flight. One would expect the nose to ‘just’ flap back when raising the collective (try using precession to explain that one) and vice versa for lowering the collective. It doesn’t – you also get a roll, therefore confirming the theory. But you already know this.
As for the AFCS being advanced, believe me it is not. Maybe 25 yrs ago….It simply compares your control inputs with its vertical gyros and applies direct control inputs to the hydraulic servos.
[This message has been edited by Baque Flip (edited 20 December 2000).]
Guest
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Lu,
Baque Flip has amplified what I sent you on e-mail regarding our thinking on precession versus aerodynamic forces causing phase lag.
Grey Area - you stated earlier that the pitch roll couple on the Lynx could be demonstrated to occur in different directions depending the Density Altituded. This might have been true with metal blades but the CMRBs only go one way - pitch up=roll right. As phase lag reduces with increase of DA you cannot achieve a reversal in this. Equally the negative DA required to increase the phase lag by the appropriate amount would be impossible to find even over the dead sea in the middle of winter.
[This message has been edited by [email protected] (edited 20 December 2000).]
Baque Flip has amplified what I sent you on e-mail regarding our thinking on precession versus aerodynamic forces causing phase lag.
Grey Area - you stated earlier that the pitch roll couple on the Lynx could be demonstrated to occur in different directions depending the Density Altituded. This might have been true with metal blades but the CMRBs only go one way - pitch up=roll right. As phase lag reduces with increase of DA you cannot achieve a reversal in this. Equally the negative DA required to increase the phase lag by the appropriate amount would be impossible to find even over the dead sea in the middle of winter.
[This message has been edited by [email protected] (edited 20 December 2000).]
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Baque flip and Crab etc
I have been trying to argue the flapping to equality point with Lu for a while (see the big R22 thread). You are going to have fun convincing him.
I am in the process of putting together a missive on the subject of phase lag. Struggling with simplifying offset (displaced) flapping hinges at the mo. Watch this space. For the lynx the short answer is increased AUM, increased NR and new blades since the head was designed.
Crab. True about CRMB. I also agree that phase lag reduces with altitude, so the right roll component should decrease as you climb (Point of max tilt moves clockwise). In a CMRB cab the whole dynamics have changed I agree, but the roll rate should be reducing with altitude.
[This message has been edited by Grey Area (edited 21 December 2000).]
I have been trying to argue the flapping to equality point with Lu for a while (see the big R22 thread). You are going to have fun convincing him.
I am in the process of putting together a missive on the subject of phase lag. Struggling with simplifying offset (displaced) flapping hinges at the mo. Watch this space. For the lynx the short answer is increased AUM, increased NR and new blades since the head was designed.
Crab. True about CRMB. I also agree that phase lag reduces with altitude, so the right roll component should decrease as you climb (Point of max tilt moves clockwise). In a CMRB cab the whole dynamics have changed I agree, but the roll rate should be reducing with altitude.
[This message has been edited by Grey Area (edited 21 December 2000).]
Guest
Posts: n/a
To: Baque Flip, Grey Area and everybody else that I have tried to force my beliefs on.
Our arguments are like trying to convince the other guy that your religion is better than his. You believe in God and he believes in Allah. You believe in Jesus Christ as being the Messiah and your Jewish neighbor believes that the Messiah is yet to come and when he does come, most likely he will find that it is Jesus Christ. When you get to heaven or your Muslim friend gets to Paradise and you meet up there you find that one god is judging you. What you see suits your beliefs and what the Muslim sees suits his belief. You both believe in the same thing but you see it in different ways.
What you learn about helicopters in the UK training programs suits you very well and you have a complete understanding of the subject. The student and pilot in the USA training programs gets a different understanding and that suits them very well, and they have a complete understanding of the subject. As long as you stay in the UK and the American stays in the States every thing is OK. Switch places and your whole concept about how a helicopters flies will immediately come under challenge.
I think Grey Area said it, it is a difference in language. I have often wanted Danny to incorporate a Lexicon or, thesaurus so that us Yanks understand what you Brits and especially the Wizards of OZ are talking about.
I think we should bring this aspect of these threads to a halt. No more talking about precession or phase lag.
I just received this from a pilot in the UK. I asume he was trained in the military.
It explains everything.
One aspect of rotor control that is not taught in the British Military is that of the rotor system acting as a gyroscope. Our belief is that the phase lag of 90 degrees exists between input of pitch change and desired blade position only because of aerodynamic forces. The blade starts to flap as the pitch operating arms follow the circular control orbit and the rate of change of pitch is at it's maximum 90 degrees later - for the next 90 degrees the blade continues to flap but at a reducing rate until at a point 180 degrees from where the pitch angle started to change - the blade reaches it's high or low point before completing the other 180 degrees doing exactly the opposite.
Whilst a theoretical solid rotor disc might exhibit some tendency to precession, the greatly reduced mass of a real rotor and the fact that the blades are free to flap mean that gyroscopic theory is not wholly applicable - are aerdynamic loads on a blade inertial or external and what produces the torque required to induce precession? - the pitch change happens gradually and is not the single point force that most textbooks illustrate when depicting precession in a gyro.
Grey Area appears to be ex-British Military, probably Navy and has questioned your use of the term "flapping to equality", we use it to describe the reaction of the blades to overcome dissymmetry of lift such as flapback or inflow roll, whereas you seemed to use it to describe the coning angle achieved once the lifting and centrifugal forces were in balance in the rotor system.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 December 2000).]
Our arguments are like trying to convince the other guy that your religion is better than his. You believe in God and he believes in Allah. You believe in Jesus Christ as being the Messiah and your Jewish neighbor believes that the Messiah is yet to come and when he does come, most likely he will find that it is Jesus Christ. When you get to heaven or your Muslim friend gets to Paradise and you meet up there you find that one god is judging you. What you see suits your beliefs and what the Muslim sees suits his belief. You both believe in the same thing but you see it in different ways.
What you learn about helicopters in the UK training programs suits you very well and you have a complete understanding of the subject. The student and pilot in the USA training programs gets a different understanding and that suits them very well, and they have a complete understanding of the subject. As long as you stay in the UK and the American stays in the States every thing is OK. Switch places and your whole concept about how a helicopters flies will immediately come under challenge.
I think Grey Area said it, it is a difference in language. I have often wanted Danny to incorporate a Lexicon or, thesaurus so that us Yanks understand what you Brits and especially the Wizards of OZ are talking about.
I think we should bring this aspect of these threads to a halt. No more talking about precession or phase lag.
I just received this from a pilot in the UK. I asume he was trained in the military.
It explains everything.
One aspect of rotor control that is not taught in the British Military is that of the rotor system acting as a gyroscope. Our belief is that the phase lag of 90 degrees exists between input of pitch change and desired blade position only because of aerodynamic forces. The blade starts to flap as the pitch operating arms follow the circular control orbit and the rate of change of pitch is at it's maximum 90 degrees later - for the next 90 degrees the blade continues to flap but at a reducing rate until at a point 180 degrees from where the pitch angle started to change - the blade reaches it's high or low point before completing the other 180 degrees doing exactly the opposite.
Whilst a theoretical solid rotor disc might exhibit some tendency to precession, the greatly reduced mass of a real rotor and the fact that the blades are free to flap mean that gyroscopic theory is not wholly applicable - are aerdynamic loads on a blade inertial or external and what produces the torque required to induce precession? - the pitch change happens gradually and is not the single point force that most textbooks illustrate when depicting precession in a gyro.
Grey Area appears to be ex-British Military, probably Navy and has questioned your use of the term "flapping to equality", we use it to describe the reaction of the blades to overcome dissymmetry of lift such as flapback or inflow roll, whereas you seemed to use it to describe the coning angle achieved once the lifting and centrifugal forces were in balance in the rotor system.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 December 2000).]
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To: whatsarunway
I would assume that if the Robinson swashplate were lower down on the mast like a Bell 206 the pitch links would have to be very stiff in order to counter the centrifugal loads that would cause them to bow outward. Because of this tendency to bend or, bow outward the attaching adjustable hardware at each end of the pitch link would have to be strengthened as would the swashplate and blade attach points and this would add weight. If you add weight to the pitch links the stiffness must increase again because of the higher centrifugal loading.
The mast length on the Robbie like that on the 206 has to be long because of the low rotor blade/rotorhead interlock. It supplies a longer moment arm.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 December 2000).]
I would assume that if the Robinson swashplate were lower down on the mast like a Bell 206 the pitch links would have to be very stiff in order to counter the centrifugal loads that would cause them to bow outward. Because of this tendency to bend or, bow outward the attaching adjustable hardware at each end of the pitch link would have to be strengthened as would the swashplate and blade attach points and this would add weight. If you add weight to the pitch links the stiffness must increase again because of the higher centrifugal loading.
The mast length on the Robbie like that on the 206 has to be long because of the low rotor blade/rotorhead interlock. It supplies a longer moment arm.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 December 2000).]