Cyclics, Semantics and Teetering Rotors ~ A question
You chaps just don't take the hint about using the term precession when talking rotor blade movement do you? Read Nicks' post again, if the professionals don't believe it happens why do you keep talking about it and worse, muddying the waters by making up terms like 'aerodynamic precession'?
DJ I know you want to build the worlds best and cheapest helicopter but I do think you should get out more!!
DJ I know you want to build the worlds best and cheapest helicopter but I do think you should get out more!!
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Does the application of forward cyclic on a hovering helicopter with a 'basic' teetering rotor cause an immediate tilt of the disk to the right?
Dave, picture this for a moment; a spinning solid disk in an ideal setting used to demonstrate precession will have a force at one particular point, applied to make the disk/wheel etc tilt. … now back to your rotor……
Still moving the cyclic forward in a steady state hover ……the difference here to the solid disk demo., is that in your helicopter rotor, the force/couple from the blades changing pitch acts in a relatively gradual manner!
The blade pitch angle, which determines the amount of cyclic force, takes 180 degrees before it gets to its maximum or minimum value. During that gradual change the force/couple has already started to tilt the rotor disk to an angle that precedes the desired disk tilt.
In this instance, the overall tilt down is to the front but a with a slight right hand component, which was instigated by the lighter but increasing force/couple prior to the blade reaching its max or min pitch angle.
Is that what you mean by “washed out coupling”?
The tilt down at the front becomes a tilt to the right by a few degrees. Therefore, my answer to your last question is “yes”.
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crab,
This thread is an attempt, by a few interested people, to acquire a better understanding of some complex activities. I don't think anyone is questioning the accuracy of Nick's remarks. They are contributory and appreciated.
The phrase 'Aerodynamic Precession' is used by Gareth Padfield &/or Gordon Leishman, two very respected helicopter dynamists.
Sprocket,
I think that the results from gyroscopic precession and from aerodynamic precession are identical, when one is considering a 'basic' teetering rotor.
" a spinning solid disk in an ideal setting used to demonstrate precession will have a force at one particular point, applied to make the disk/wheel etc tilt."
OK, but, remember that this is a 'solid disk" [structurally rigid disk] and therefor "a force at one particular point" will be transmitted to all points on the disk. The amount transmitted to the various points is a trigonometric function of the azimuths and the moment arm. The amount of this force that is transmitted to each radii-azimuth is identical to the force of a blade element at the same radii-azimuth.
When the consideration turns to a more complex rotor, Gyroscopic precession start to 'fly out the window'. In fact, IMHO, is better to use Aerodynamic precession in all circumstances; with the possible exception of when considering the hub portion of an articulated rotor with a large flapping hinge offset.
__________________
There is Rate cross-coupling, Acceleration cross-coupling and apparently a much less known coupling called Washed-out cross-coupling. I am guessing that Washed-out cross-coupling is associated with time and therefore the speed with which the cyclic stick is pushed forward etc. This seem to be inline with Nick's remarks and some information he provided in an earlier thread, where he mentioned that the flight controls on the Comanche could tailored to the specific flight situation.
If this is correct, then the relevance of Washed-out cross-coupling [Wee-wa] may or may not be important in a 'basic' teetering rotor
This thread is an attempt, by a few interested people, to acquire a better understanding of some complex activities. I don't think anyone is questioning the accuracy of Nick's remarks. They are contributory and appreciated.
The phrase 'Aerodynamic Precession' is used by Gareth Padfield &/or Gordon Leishman, two very respected helicopter dynamists.
Sprocket,
I think that the results from gyroscopic precession and from aerodynamic precession are identical, when one is considering a 'basic' teetering rotor.
" a spinning solid disk in an ideal setting used to demonstrate precession will have a force at one particular point, applied to make the disk/wheel etc tilt."
OK, but, remember that this is a 'solid disk" [structurally rigid disk] and therefor "a force at one particular point" will be transmitted to all points on the disk. The amount transmitted to the various points is a trigonometric function of the azimuths and the moment arm. The amount of this force that is transmitted to each radii-azimuth is identical to the force of a blade element at the same radii-azimuth.
When the consideration turns to a more complex rotor, Gyroscopic precession start to 'fly out the window'. In fact, IMHO, is better to use Aerodynamic precession in all circumstances; with the possible exception of when considering the hub portion of an articulated rotor with a large flapping hinge offset.
__________________
There is Rate cross-coupling, Acceleration cross-coupling and apparently a much less known coupling called Washed-out cross-coupling. I am guessing that Washed-out cross-coupling is associated with time and therefore the speed with which the cyclic stick is pushed forward etc. This seem to be inline with Nick's remarks and some information he provided in an earlier thread, where he mentioned that the flight controls on the Comanche could tailored to the specific flight situation.
If this is correct, then the relevance of Washed-out cross-coupling [Wee-wa] may or may not be important in a 'basic' teetering rotor
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The phrase aerodynamic precession is most apt, and is intended to draw us away from any idea that gyroscopes are involved. In truth, the varable rotor phase angle (varies with time, with speed, with density altitude, with rate of input, with rotor blade inertia, with hinge offset, with blade chord,......) is a complex motion that tells us how the blade's flapping oscillation compares with the 1 per revolution frequency of the aircraft.
Gareth Padfield (a friend) and Gordon Leishman (an acquaintance) are very respected experts in our field, they would have no problem discussing how rotors behave without once using gyroscopes in the explanation.
Gareth Padfield (a friend) and Gordon Leishman (an acquaintance) are very respected experts in our field, they would have no problem discussing how rotors behave without once using gyroscopes in the explanation.
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Didn't we all agree long ago that the gyroscope thing was just a demonstration tool and an easy way to dumb down a complex topic?
In truth, gyroscopic precession is a crazy thing that tops and the like do when they lean from the vertical. The explanation for the GP goes something like the explanation for what a helicopter's rotors do.
Okay, Regis, FINAL ANSWER.....(from a physicist and helicopter guy) not only does gyroscopic precession have nothing to do with helicopters, the rotors don't even PRECESS!!!! (unless you stir the cyclic)
Matthew.
In truth, gyroscopic precession is a crazy thing that tops and the like do when they lean from the vertical. The explanation for the GP goes something like the explanation for what a helicopter's rotors do.
Okay, Regis, FINAL ANSWER.....(from a physicist and helicopter guy) not only does gyroscopic precession have nothing to do with helicopters, the rotors don't even PRECESS!!!! (unless you stir the cyclic)
Matthew.
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Who is on first?
QUOTE: The phrase 'Aerodynamic Precession' is used by Gareth Padfield &/or Gordon Leishman, two very respected helicopter dynamists.
When Messrs. Padfield and Leishman were attending physics and aerodynamics courses at university level what theory was taught to them? Was it gyroscopic precession or was it aerodynamic precession? Did the term gyroscopic precession evolve into aerodynamic precession by way expansion of engineering knowledge where computer analyses and complex testing proved the gyro theory to be incorrect? Or, was the new aerodynamic precession “invented” in order for these two gentlemen and other writers of engineering texts to sell more textbooks. Granted, I don’t have the engineering knowledge to fully understand all of what is in these textbooks but I have read several where the author(s) changed their theories over a series of textbooks.
In a case like this if the student were exposed to the first textbook and not all of them he /she would enter the world of engineering with a limited knowledge of the subject.
If in fact aerodynamic precession is the case there are a lot of pilots and mechanics in this world that were never exposed to this theory and they fully believe in gyroscopic precession. This does not make them stupid or place them in a time warp.
I have the training texts from different helicopter firms and although this material is dated these firms are still teaching that the rotor has two characteristics of a gyro. Rigidity in space and precession. The present FAA Rotorcraft Flying Handbook slightly downplays the importance of gyroscopic theory but they do not offer a replacement theory. Who is right and who is wrong. It depends on how and WHEN you were taught while in university or flight school and by extension mechanics school. Although some of you might object, both theories work.
Here is a question to ponder. On the Cheyenne helicopter control inputs to the rotor pitch horns are input from a “gyroscope rotor” suspended above the main rotor. When the main rotor responds to the input is it being displaced by gyroscopic precession or, aerodynamic precession?
QUOTE: Okay, Regis, FINAL ANSWER.....(from a physicist and helicopter guy) not only does gyroscopic precession have nothing to do with helicopters, the rotors don't even PRECESS!!!! (unless you stir the cyclic)
You can’t have it both ways. If rotors do not precess then both theories (gyroscopic precession and aerodynamic precession) go out the window.
Be gentle Nick’
When Messrs. Padfield and Leishman were attending physics and aerodynamics courses at university level what theory was taught to them? Was it gyroscopic precession or was it aerodynamic precession? Did the term gyroscopic precession evolve into aerodynamic precession by way expansion of engineering knowledge where computer analyses and complex testing proved the gyro theory to be incorrect? Or, was the new aerodynamic precession “invented” in order for these two gentlemen and other writers of engineering texts to sell more textbooks. Granted, I don’t have the engineering knowledge to fully understand all of what is in these textbooks but I have read several where the author(s) changed their theories over a series of textbooks.
In a case like this if the student were exposed to the first textbook and not all of them he /she would enter the world of engineering with a limited knowledge of the subject.
If in fact aerodynamic precession is the case there are a lot of pilots and mechanics in this world that were never exposed to this theory and they fully believe in gyroscopic precession. This does not make them stupid or place them in a time warp.
I have the training texts from different helicopter firms and although this material is dated these firms are still teaching that the rotor has two characteristics of a gyro. Rigidity in space and precession. The present FAA Rotorcraft Flying Handbook slightly downplays the importance of gyroscopic theory but they do not offer a replacement theory. Who is right and who is wrong. It depends on how and WHEN you were taught while in university or flight school and by extension mechanics school. Although some of you might object, both theories work.
Here is a question to ponder. On the Cheyenne helicopter control inputs to the rotor pitch horns are input from a “gyroscope rotor” suspended above the main rotor. When the main rotor responds to the input is it being displaced by gyroscopic precession or, aerodynamic precession?
QUOTE: Okay, Regis, FINAL ANSWER.....(from a physicist and helicopter guy) not only does gyroscopic precession have nothing to do with helicopters, the rotors don't even PRECESS!!!! (unless you stir the cyclic)
You can’t have it both ways. If rotors do not precess then both theories (gyroscopic precession and aerodynamic precession) go out the window.
Be gentle Nick’
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Lu,
"Did the term gyroscopic precession evolve into aerodynamic precession by way expansion of engineering knowledge where computer analyses and complex testing proved the gyro theory to be incorrect? " ~
Time and knowledge progress (well ~ sometimes
). Would you not agree that back in the beginning, its doubtful that Leonardo da Vinci considered gyroscopic precession when contemplating his air-screw.
If this thread does move on into; phase angle, delta-3, and possibly flapping hinge offset, You will have to put gyroscopic precession totally aside, or there will be absolutely no way of grasping the interrelationship of the above three activities.
______________________________
Meanwhile, back to some of heedm's earlier posts, to get reacquainted with ~ [/]"the rotors don't even PRECESS!!!!"[/i]
heedm,
I tried to look up a previous point you had made, but the eyes started going. I think you previously commented on the precession of the azimuth of greatest tip. In addition, you mentioned that the azimuth of greatest tip advanced extremely slowly, until the force of the primary rotation had significantly decreased visa vie the perturbing force, i.e a wobble. Is this somewhat similar to what you're referring to by "stir the cyclic"?
Edited to add note to heedm
"Did the term gyroscopic precession evolve into aerodynamic precession by way expansion of engineering knowledge where computer analyses and complex testing proved the gyro theory to be incorrect? " ~
Time and knowledge progress (well ~ sometimes
). Would you not agree that back in the beginning, its doubtful that Leonardo da Vinci considered gyroscopic precession when contemplating his air-screw.If this thread does move on into; phase angle, delta-3, and possibly flapping hinge offset, You will have to put gyroscopic precession totally aside, or there will be absolutely no way of grasping the interrelationship of the above three activities.
______________________________
Meanwhile, back to some of heedm's earlier posts, to get reacquainted with ~ [/]"the rotors don't even PRECESS!!!!"[/i]
heedm,
I tried to look up a previous point you had made, but the eyes started going. I think you previously commented on the precession of the azimuth of greatest tip. In addition, you mentioned that the azimuth of greatest tip advanced extremely slowly, until the force of the primary rotation had significantly decreased visa vie the perturbing force, i.e a wobble. Is this somewhat similar to what you're referring to by "stir the cyclic"?
Edited to add note to heedm
Last edited by Dave_Jackson; 26th Jul 2003 at 07:12.
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Lu, your going to love this.
Padfield's book and a referenced report say "... washed-out coupling, which can occur in helicopters with feedback control systems". Delta-3 must be considered a mechanical 'feedback control systems', since it pull out pitch in a relationship to the amount of flap.
This raises the circular question; was delta-3 incorporated to minimize washed-out coupling, or did the incorporation of delta-3 cause washed-out coupling?
The following two quotations are from separate posts by Chuck Beaty, a gentleman with considerable technical knowledge about rotorcraft. They provide additional information.
1/
" Delta-3 coupling binds the rotor more tightly to the mast position, perhaps a good thing for tail rotors but a bad thing for main rotors."
2/
"Subject: Wee-wa
Bramwell in "Helicopter Aerodynamics" after tossing about some fancy math, derives some fairly simple expressions for cross coupling.
The rotor tilt in the commanded direction is proportional to: 16 * (tilt rate)/(angular velocity) * (1/y)
y is the mass constant of the rotorblade (Lock #), the ratio of aerodynamic force to mass. High inertia blades have lower Lock # than the other way around. Angular velocity is that of the rotor, everything being in radians/sec.
The tilt in the crosswise direction is: (tilt rate)/(angular velocity)
What this says is that cross coupling might not be noticed in a Bell or Hiller but might be a problem with a Robinson.
Irrelevant sentence snipped.
I've met Frank Robinson and remember having a discussion with him about his use of delta-3 coupling back when he was still working on the R-22 prototype. He's a very competent engineer and wrote a number of papers about tail rotors while he was a project engineer at Bell-Textron but I suspect tail rotors gave him his fixation about delta-3 coupling where suppression of cyclic flapping is considered desirable.
Mr. Robinson is not only president and chief engineer of Robinson Helicopter but is also chief promoter and head salesman. When forging dies for blade grips and pitch arms are bought and paid for, it's sometimes expedient to make theory fit practice.
With a model rotor at least, if one wants to see some really nasty "wee-wa," skew the teeter hinge and observe the nutating behavior. It will go crazy with a 45 degree skew angle. Model rotors don't exhibit any detectable cross coupling because the ratio of rotor rpm to tilt rate is so high.'
Confusion runs wild. It's all very interesting.
Lu,
Why not give Chuck Beaty a call. Hell, why not give Frank Robinson a call.
This raises the circular question; was delta-3 incorporated to minimize washed-out coupling, or did the incorporation of delta-3 cause washed-out coupling?
The following two quotations are from separate posts by Chuck Beaty, a gentleman with considerable technical knowledge about rotorcraft. They provide additional information.
1/
" Delta-3 coupling binds the rotor more tightly to the mast position, perhaps a good thing for tail rotors but a bad thing for main rotors."
2/
"Subject: Wee-wa
Bramwell in "Helicopter Aerodynamics" after tossing about some fancy math, derives some fairly simple expressions for cross coupling.
The rotor tilt in the commanded direction is proportional to: 16 * (tilt rate)/(angular velocity) * (1/y)
y is the mass constant of the rotorblade (Lock #), the ratio of aerodynamic force to mass. High inertia blades have lower Lock # than the other way around. Angular velocity is that of the rotor, everything being in radians/sec.
The tilt in the crosswise direction is: (tilt rate)/(angular velocity)
What this says is that cross coupling might not be noticed in a Bell or Hiller but might be a problem with a Robinson.
Irrelevant sentence snipped.
I've met Frank Robinson and remember having a discussion with him about his use of delta-3 coupling back when he was still working on the R-22 prototype. He's a very competent engineer and wrote a number of papers about tail rotors while he was a project engineer at Bell-Textron but I suspect tail rotors gave him his fixation about delta-3 coupling where suppression of cyclic flapping is considered desirable.
Mr. Robinson is not only president and chief engineer of Robinson Helicopter but is also chief promoter and head salesman. When forging dies for blade grips and pitch arms are bought and paid for, it's sometimes expedient to make theory fit practice.
With a model rotor at least, if one wants to see some really nasty "wee-wa," skew the teeter hinge and observe the nutating behavior. It will go crazy with a 45 degree skew angle. Model rotors don't exhibit any detectable cross coupling because the ratio of rotor rpm to tilt rate is so high.'
Confusion runs wild. It's all very interesting.
Lu,
Why not give Chuck Beaty a call. Hell, why not give Frank Robinson a call.
Last edited by Dave_Jackson; 26th Jul 2003 at 10:16.
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A precession is a motion that spinning masses do where the axis of the spin traces a cone. Stick a flashlight pointing upwards on a moving turntable that is mounted horizontally. If the flashlight makes a dot on the ceiling then there is no precession. If you then pick up the turntable and keep tilting it so that the flashlight traces a circle on the ceiling, then you have precession.
Helicopter rotors, in the simple case, do not exhibit any such behaviour. In the full analysis, even a simple spinning top has at least two frequencies/amplitudes of precession. So I imagine a helicopter's rotors would show many of these.
Gyroscopic precession occurs when a force acts continuously on a spinning object at an angle to the spinning axis. Like when a spinning top starts to lean over, gravity pulls down on the object causing the top to precess. This can be shown quite easily in the classroom. It can then be broken down in the classroom to show that it's caused by a force acting on a rotating object apparently being realized 90 degrees later in the rotation.
Because it's easy to show in the classroom, it's also an easy way of convincing prospective helicopter pilots that a control system that appears to be rigged 90 degrees out of sync will actually work.
If you agree with me to this point, we'll all have to agree that gyroscopic precession is a big lie told to make life easy.
--------
I haven't read Padfield's or Leishman's books yet, so I still don't know what aerodynamic precession really is. If it's an attempt to explain why the controls are approximately 90 degrees out of sync, then I'd question if they even used the word precession accurately.
However, if they are referring to some higher order response of the rotor system, then they may in fact be using the term accurately.
---------
Dave, "stirring the cyclic" is a term for overcontrolling. Lots of students do it. They get task saturated trying to hover, end up moving the cyclic all over the place which means more effort required in controlling yaw & power. Also bad when operating close to power limits since it results in your total thrust vector leaning from the vertical and time averaging to a shorter but vertical vector.
Matthew.
Helicopter rotors, in the simple case, do not exhibit any such behaviour. In the full analysis, even a simple spinning top has at least two frequencies/amplitudes of precession. So I imagine a helicopter's rotors would show many of these.
Gyroscopic precession occurs when a force acts continuously on a spinning object at an angle to the spinning axis. Like when a spinning top starts to lean over, gravity pulls down on the object causing the top to precess. This can be shown quite easily in the classroom. It can then be broken down in the classroom to show that it's caused by a force acting on a rotating object apparently being realized 90 degrees later in the rotation.
Because it's easy to show in the classroom, it's also an easy way of convincing prospective helicopter pilots that a control system that appears to be rigged 90 degrees out of sync will actually work.
If you agree with me to this point, we'll all have to agree that gyroscopic precession is a big lie told to make life easy.
--------
I haven't read Padfield's or Leishman's books yet, so I still don't know what aerodynamic precession really is. If it's an attempt to explain why the controls are approximately 90 degrees out of sync, then I'd question if they even used the word precession accurately.
However, if they are referring to some higher order response of the rotor system, then they may in fact be using the term accurately.
---------
Dave, "stirring the cyclic" is a term for overcontrolling. Lots of students do it. They get task saturated trying to hover, end up moving the cyclic all over the place which means more effort required in controlling yaw & power. Also bad when operating close to power limits since it results in your total thrust vector leaning from the vertical and time averaging to a shorter but vertical vector.
Matthew.
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Love it or leave it.
To: Dave Jackson
You say I’ll love this referring to the text that followed. Hell I not only do not love it I don’t understand it. I understand diagrams not words. Regarding my calling Frank Robinson I did just that about seven years ago. I told him that I could show him how to greatly simplify his rigging procedure, which could be performed in about 30 minutes with one man, and you didn’t have to level the helicopter. You could even rig the R-22/44 on a pitching and rolling tuna boat. All it took was two very simple tools that could be manufactured in their own shops with the tools being offered as special tools and sold to the operators. A few days later Franks’ son called me and stated that Robinson helicopters was not in the business of making special tools. As a result the Robinson helicopters are a bear to rig and the rigging instructions are very vague and misleading for the mechanic.
The author of the text you quoted said if I understand that there is no pitch coupling on a high speed rotor system simply because the blades can’t react fast enough. If that is the case, how can a Robinson rotor system respond so quickly with a 72-degree lead angle. If this same reasoning was applied to the tail rotor which rotates considerably faster than the main rotor and there were no pitch flap coupling the tail rotor would eventually fatigue and it and possibly the tail rotor gearbox would would break away from the tail boom.
When collective is pulled and cyclic input is made the pitch link connect point on the horn is above the flapping axis and any flapping up will subtract pitch and when the blade flaps down pitch will be added just like on a tail rotor. Now maybe the blades can’t react due to their rotational speed but the pitch flap coupling is still there.
You say I’ll love this referring to the text that followed. Hell I not only do not love it I don’t understand it. I understand diagrams not words. Regarding my calling Frank Robinson I did just that about seven years ago. I told him that I could show him how to greatly simplify his rigging procedure, which could be performed in about 30 minutes with one man, and you didn’t have to level the helicopter. You could even rig the R-22/44 on a pitching and rolling tuna boat. All it took was two very simple tools that could be manufactured in their own shops with the tools being offered as special tools and sold to the operators. A few days later Franks’ son called me and stated that Robinson helicopters was not in the business of making special tools. As a result the Robinson helicopters are a bear to rig and the rigging instructions are very vague and misleading for the mechanic.
The author of the text you quoted said if I understand that there is no pitch coupling on a high speed rotor system simply because the blades can’t react fast enough. If that is the case, how can a Robinson rotor system respond so quickly with a 72-degree lead angle. If this same reasoning was applied to the tail rotor which rotates considerably faster than the main rotor and there were no pitch flap coupling the tail rotor would eventually fatigue and it and possibly the tail rotor gearbox would would break away from the tail boom.
When collective is pulled and cyclic input is made the pitch link connect point on the horn is above the flapping axis and any flapping up will subtract pitch and when the blade flaps down pitch will be added just like on a tail rotor. Now maybe the blades can’t react due to their rotational speed but the pitch flap coupling is still there.
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heedm,
Got it, Thanks.
PS. Don't tell Lu, but Frank Robinson used the 'G' word when he said; "an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor."
Lu,
Help! Forget the math. Just reread Chuck Beaty's conclusions. You will find that he is somewhat in agreement with you.
I believe that most tail rotors have a delta-3 of 45-degrees. This means that any flapping is instantly pulled out. No 90-degrees. No discernible delay. No sh1t.
Got it, Thanks.
PS. Don't tell Lu, but Frank Robinson used the 'G' word when he said; "an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor."
Lu,
Help! Forget the math. Just reread Chuck Beaty's conclusions. You will find that he is somewhat in agreement with you.
I believe that most tail rotors have a delta-3 of 45-degrees. This means that any flapping is instantly pulled out. No 90-degrees. No discernible delay. No sh1t.
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Delta Hinge.
To: Dave Jackson
Edited to remove the words no ****.
On two blade tail rotors the delta hinge is offset by 60 degrees. You say that any flapping is instantly pulled out. Please explain. When the tail rotor flaps one blade will decrease in pitch and the opposite blade will increase in pitch thus equalizing the lift across the tail rotor disc. On multi blade tail rotors that are free to flap the blades act like those on the main rotor. When the blade flaps in either direction the pitch flap coupling will tend to return the blade to the radial position. This is a form of relieving stress on the blades and that’s why they are free to flap. However with flapping you get lead an lag and this is catered to by the design of the blade (S-58 , S-61) or by the incorporation of a lead lag hinge and a damper (CH-37).
I believe that most tail rotors have a delta-3 of 45-degrees. This means that any flapping is instantly pulled out. No 90-degrees. No discernible delay. .
On two blade tail rotors the delta hinge is offset by 60 degrees. You say that any flapping is instantly pulled out. Please explain. When the tail rotor flaps one blade will decrease in pitch and the opposite blade will increase in pitch thus equalizing the lift across the tail rotor disc. On multi blade tail rotors that are free to flap the blades act like those on the main rotor. When the blade flaps in either direction the pitch flap coupling will tend to return the blade to the radial position. This is a form of relieving stress on the blades and that’s why they are free to flap. However with flapping you get lead an lag and this is catered to by the design of the blade (S-58 , S-61) or by the incorporation of a lead lag hinge and a damper (CH-37).
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Lu,
I should have removed the tail rotor paragraph from Beaty's post, 'cause I know very little about them, and do not want to. The tail rotor is about to become a historical relic.
Should have probably removed his math paragraph, as well. 
My limited understanding is that collective pitch change is wanted in a tail rotor but teetering or flapping is an undesirable. They use delta-3 to minimize the flapping. 45-degrees will eliminate any pitch change caused by flapping. 60-degrees will put in an opposing pitch, which will drive the blade back to the "home position'. You are probably correct, and my guestimate of 45-degrees was wrong.
While discussing delta-3, there are two different ways of implementing it.
See the two sketches at the top of this web page.
[A/ By flap hinge geometry] has the lead/lag component in it, which you refer to.
[B/ By control system geometry] is the arraignment that Robinson uses in its main rotors.
____________________
Meanwhile, back on track.
The initial post on this thread was an attempt to see if lateral cyclic is required, when the stick is being advanced on hovering craft, which has a basic teetering rotor. No practical answer has yet been give, but Prouty seems to imply that lateral cyclic is not required at this point in time.
If this is the case, then what heck is "Wee-wa' and where is this number one problem that Frank Robinson apparently sees?
I should have removed the tail rotor paragraph from Beaty's post, 'cause I know very little about them, and do not want to. The tail rotor is about to become a historical relic.
Should have probably removed his math paragraph, as well. My limited understanding is that collective pitch change is wanted in a tail rotor but teetering or flapping is an undesirable. They use delta-3 to minimize the flapping. 45-degrees will eliminate any pitch change caused by flapping. 60-degrees will put in an opposing pitch, which will drive the blade back to the "home position'. You are probably correct, and my guestimate of 45-degrees was wrong.
While discussing delta-3, there are two different ways of implementing it.
See the two sketches at the top of this web page.
[A/ By flap hinge geometry] has the lead/lag component in it, which you refer to.
[B/ By control system geometry] is the arraignment that Robinson uses in its main rotors.
____________________
Meanwhile, back on track.
The initial post on this thread was an attempt to see if lateral cyclic is required, when the stick is being advanced on hovering craft, which has a basic teetering rotor. No practical answer has yet been give, but Prouty seems to imply that lateral cyclic is not required at this point in time.
If this is the case, then what heck is "Wee-wa' and where is this number one problem that Frank Robinson apparently sees?
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One from column A and one from column B
To: Dave Jackson
QUOTE: While discussing delta-3, there are two different ways of implementing it.
See the two sketches at the top of this web page.
[A/ By flap hinge geometry] has the lead/lag component in it, which you refer to.
[B/ By control system geometry] is the arraignment that Robinson uses in its main rotors.
In B above the only time there is no pitch flap coupling is when there is no collective input. At this time the pitch link / pitch horn interface is in alignment with the cone hinge. When pitch is added the pitch horn / pitch link interface will rise above the cone hinge and in this condition when the blades flap about the cone hinge you will get pitch flap coupling. The same is true when the blades flap about the teeter hinge this will also result in pitch flap coupling. So, I ask the question what is the control system geometry espoused by Frank Robinson?
QUOTE: While discussing delta-3, there are two different ways of implementing it.
See the two sketches at the top of this web page.
[A/ By flap hinge geometry] has the lead/lag component in it, which you refer to.
[B/ By control system geometry] is the arraignment that Robinson uses in its main rotors.
In B above the only time there is no pitch flap coupling is when there is no collective input. At this time the pitch link / pitch horn interface is in alignment with the cone hinge. When pitch is added the pitch horn / pitch link interface will rise above the cone hinge and in this condition when the blades flap about the cone hinge you will get pitch flap coupling. The same is true when the blades flap about the teeter hinge this will also result in pitch flap coupling. So, I ask the question what is the control system geometry espoused by Frank Robinson?
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PS. Don't tell Lu, but Frank Robinson used the 'G' word when he said; "an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor."
Dave, reference your initial post, the downwards flapping of the forward blade and upwards flapping of the aft blade will result in angle of attack changes that cause the disk to roll right (ccw system). Unfortunately, I think it is inaccurate to state that "...if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward" because that indicates this effect (don't know if it's weewa) has an end result of a laterally rolled disk. It seems clear this effect puts in an input, but there are many other effects going on.
I think the best way to deal with this is to flight test a simple system.
Matthew.
Last edited by heedm; 27th Jul 2003 at 07:09.
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Lu,
I agree with what you are saying, and more. Actually, a variation of the collective will change the coning angle and thereby slightly affect the pitch-flap coupling in both A/ and in B/. You are also correct in that Robinson's flapping/coning hinges could affect the pitch-flap coupling.
But, this thread is not specifically about the Robinson. In addition, bring in Robinson's flapping/coning hinges will make a complex subject even more complex. Lets just discuss pitch-roll cross-coupling, and include Frank Robinson's related 'Wee-wa'.
In fact, it looks like Mr. Robinson may have created the word, and if so, he is the only person who can define it.
Heedm,
I do not fully understand your comments, but like you, I am beginning to disagree with the quoted statement by Frank Robinson.
_____________
To try to keep this thread on topic, I am going to start a separate thread on delta-3 and phase-lag. It will be the definitive answer to both subjects 
and it should answer Lu's running question about 'what happened to the remaining 17-degrees of teetering'.
I agree with what you are saying, and more. Actually, a variation of the collective will change the coning angle and thereby slightly affect the pitch-flap coupling in both A/ and in B/. You are also correct in that Robinson's flapping/coning hinges could affect the pitch-flap coupling.
But, this thread is not specifically about the Robinson. In addition, bring in Robinson's flapping/coning hinges will make a complex subject even more complex. Lets just discuss pitch-roll cross-coupling, and include Frank Robinson's related 'Wee-wa'.
In fact, it looks like Mr. Robinson may have created the word, and if so, he is the only person who can define it.
Heedm,
I do not fully understand your comments, but like you, I am beginning to disagree with the quoted statement by Frank Robinson.
_____________
To try to keep this thread on topic, I am going to start a separate thread on delta-3 and phase-lag. It will be the definitive answer to both subjects
and it should answer Lu's running question about 'what happened to the remaining 17-degrees of teetering'.
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Does the term weasleword come to mind?
To: Dave Jackson
That's strange. I seem to remember a participant on this forum once saying that it was a bunch of crap.
I do not fully understand your comments, but like you, I am beginning to disagree with the quoted statement by Frank Robinson.
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Thought through this a little more and came up with something that I think is worth presenting here.
When you apply forward cyclic, you reduce pitch on the right and increase it on the left. This means that while tilting the disk, you generate a dissymetry of lift. However, since the forward blade experiences increased alpha and thus generates more lift, the disk tilts to the right.
So you decrease lift on the right but at the same time tilt the disk to the right. Wouldn't that just be crazy if those two effects cancelled each other out with the result that no lateral cyclic results in no roll during abrupt manoeuvring?
What I was trying to say before is not quite that the quote was a load of crap, just that it wasn't complete. It didn't consider other transient effects on the disk, and it didn't consider whether the (weewa?) effect was desirable or not.
Matthew.
When you apply forward cyclic, you reduce pitch on the right and increase it on the left. This means that while tilting the disk, you generate a dissymetry of lift. However, since the forward blade experiences increased alpha and thus generates more lift, the disk tilts to the right.
So you decrease lift on the right but at the same time tilt the disk to the right. Wouldn't that just be crazy if those two effects cancelled each other out with the result that no lateral cyclic results in no roll during abrupt manoeuvring?
What I was trying to say before is not quite that the quote was a load of crap, just that it wasn't complete. It didn't consider other transient effects on the disk, and it didn't consider whether the (weewa?) effect was desirable or not.
Matthew.
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heedm,
You said; "However, since the forward blade experiences increased alpha and thus generates more lift, the disk tilts to the right." This is probably true when the craft has forward velocity. When the craft is stationary, I believe that the blade only flies to position and the coning angle will not result in a change in alpha between the front and the back
Frank Robinson stated; "In a steady no-wind hover, when forward cyclic pitch is applied ...". His intent was not to write a legal document, so perhaps he did not really mean hover.
You said; "However, since the forward blade experiences increased alpha and thus generates more lift, the disk tilts to the right." This is probably true when the craft has forward velocity. When the craft is stationary, I believe that the blade only flies to position and the coning angle will not result in a change in alpha between the front and the back
Frank Robinson stated; "In a steady no-wind hover, when forward cyclic pitch is applied ...". His intent was not to write a legal document, so perhaps he did not really mean hover.