Aerodynamics ~ Phase Lag
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Aerodynamics ~ Phase Lag
The following is submitted to provoke interest, boredom, or a scathing rebuttal.
[list=a][*]For a rotor that is freely articulated at the center of rotation, or a teetering rotor, the phase lag is 90-degrees.[*] For rotor that is totally rigid , except for feathering, the phase lag is 0-degrees.[/list=a]
The conditions are;
The rotation is CCW, when viewed from above.
Azimuth-0 is aft, azimuth-90 is to the right, etc.
The flight maneuver is a transition from hover to forward flight.
In the case of A/, the higher blade pitches will be found between azimuth-181 and azimuth-359, with the highest at azimuth-270. The 90-degree phase-lag will result in the disk being high between azimuth-271 and azimuth-89, with the highest at azimuth-360. This will cause the rotor disk to 'drag' the helicopter's nose down about its pitch axis.
In the case of B/, the higher blade pitches will be found between azimuth-271 and azimuth-89, with the highest at azimuth-360. The zero-degree phase-lag, in conjunction with the absolutely rigid coupling of the rotor to the fuselage will cause the rotor disk to 'pry' the helicopter's nose down about its pitch axis.
_____________
An addendum to the forgoing is that this extremely rigid rotor must have higher than normal strength and thus greater mass. The mass will invoke gyroscopic precession.
. However, the gyroscopic precession will be relatively small, and in the case of twin counter-rotating main rotors, such as the coaxial and intermeshing configurations, the opposing gyroscopic precessions should provide some stability.
__________
That's the story and I'm sticking with it; until a knowable person says that parts of it, or all of it, are BS.
[ 25 November 2001: Message edited by: Dave Jackson ]
[list=a][*]For a rotor that is freely articulated at the center of rotation, or a teetering rotor, the phase lag is 90-degrees.[*] For rotor that is totally rigid , except for feathering, the phase lag is 0-degrees.[/list=a]
The conditions are;
The rotation is CCW, when viewed from above.
Azimuth-0 is aft, azimuth-90 is to the right, etc.
The flight maneuver is a transition from hover to forward flight.
In the case of A/, the higher blade pitches will be found between azimuth-181 and azimuth-359, with the highest at azimuth-270. The 90-degree phase-lag will result in the disk being high between azimuth-271 and azimuth-89, with the highest at azimuth-360. This will cause the rotor disk to 'drag' the helicopter's nose down about its pitch axis.
In the case of B/, the higher blade pitches will be found between azimuth-271 and azimuth-89, with the highest at azimuth-360. The zero-degree phase-lag, in conjunction with the absolutely rigid coupling of the rotor to the fuselage will cause the rotor disk to 'pry' the helicopter's nose down about its pitch axis.
_____________
An addendum to the forgoing is that this extremely rigid rotor must have higher than normal strength and thus greater mass. The mass will invoke gyroscopic precession.
. However, the gyroscopic precession will be relatively small, and in the case of twin counter-rotating main rotors, such as the coaxial and intermeshing configurations, the opposing gyroscopic precessions should provide some stability.__________
That's the story and I'm sticking with it; until a knowable person says that parts of it, or all of it, are BS.
[ 25 November 2001: Message edited by: Dave Jackson ]
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Aargh not again! This one has been done to death again and again.
I for one am not going to get drawn into posting anything at all.
Oh bu@@er!
I for one am not going to get drawn into posting anything at all.
Oh bu@@er!
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To: Dave Jackson
The following is submitted to provoke interest, boredom, or a scathing rebuttal.
A. For a rotor that is freely articulated at the center of rotation, or a teetering rotor, the phase lag is 90-degrees.
Response:
There are no (to my knowledge) any helicopters that have articulated heads that flap at the centerline. The closest you can come to that configuration (to my knowledge) is a Sikorsky S-51 or a Boeing CH-47 or a CH-46. These helicopters have what is known as a spider and the blades are attached to the spider and flap about the hinge formed by the spider. This flapping takes place several inches from the center of rotation. These types of helicopters do have a 90-degree phase angle.
B. For rotor that is totally rigid, except for feathering, the phase lag is 0-degrees.
Response:
Possibly with the exception of the S-69 (?) which has a rigid head and blades there is no totally rigid system as all have some points where either the rotorhead deflects or, the blades bend. The precursor to the Cheyenne had a rigid head and it had a phase angle of 90-degrees. The Cheyenne was designed to have a similar phase angle but it didn’t work out that way due to both the aerodynamics of the blade and stiffness of the blade. The phase angle would vary due to speed, loading and air density. This variance was so severe on two occasions that the blades contacted the fuselage. On the first occasion the pilot was killed and on the second a wind tunnel was destroyed.
The conditions are;
The rotation is CCW, when viewed from above.
Azimuth-0 is aft; azimuth-90 is to the right, etc.
The flight maneuver is a transition from hover to forward flight.
In the case of A/, the higher blade pitches will be found between azimuth-181 and azimuth-359, with the highest at azimuth-270. The 90-degree phase-lag will result in the disk being high between azimuth-271 and azimuth-89, with the highest at azimuth-360. This will cause the rotor disk to 'drag' the helicopter's nose down about its pitch axis.
Response:
Your text should read the highest pitch is realized when the blade is at azimuth 270
And the highest point of flap is at azimuth 360 and the lowest point of flap (down) is at azimuth 180. This is true for a teetered rotor and an articulated or a flex rotor of some kind.
In the case of B/, the higher blade pitches will be found between azimuth-271 and azimuth-89, with the highest at azimuth-360. The zero-degree phase-lag, in conjunction with the absolutely rigid coupling of the rotor to the fuselage will cause the rotor disk to 'pry' the helicopter's nose down about its pitch axis.
Response:
Rigid rotor helicopters fly just like any other type of helicopter the only difference being the response to control input.
_____________
An addendum to the forgoing is that this extremely rigid rotor must have higher than normal strength and thus greater mass. The mass will invoke gyroscopic precession. . However, the gyroscopic precession will be relatively small, and in the case of twin counter-rotating main rotors, such as the coaxial and intermeshing configurations, the opposing gyroscopic precessions should provide some static stability.
Response:
The rotors on a CH-47 can be viewed as individual rotors, both of which respond to control input. In forward flight up to about 60 knots there is no gyroscopic precession. Above 60 knots there is an automatic cyclic input and at this time there is gyroscopic precession or if you are a non-believer aerodynamic precession. Only when lateral cyclic or any other input that would cause the discs to tilt there is gyroscopic precession or the other thing if you are a non-believer. The fact that the blades are turning in opposition there is no stabilizing effect due to gyroscopic forces acting in opposition. I believe the same is true for coax helicopters and those like the Kaman.
Over.
__________
The following is submitted to provoke interest, boredom, or a scathing rebuttal.
A. For a rotor that is freely articulated at the center of rotation, or a teetering rotor, the phase lag is 90-degrees.
Response:
There are no (to my knowledge) any helicopters that have articulated heads that flap at the centerline. The closest you can come to that configuration (to my knowledge) is a Sikorsky S-51 or a Boeing CH-47 or a CH-46. These helicopters have what is known as a spider and the blades are attached to the spider and flap about the hinge formed by the spider. This flapping takes place several inches from the center of rotation. These types of helicopters do have a 90-degree phase angle.
B. For rotor that is totally rigid, except for feathering, the phase lag is 0-degrees.
Response:
Possibly with the exception of the S-69 (?) which has a rigid head and blades there is no totally rigid system as all have some points where either the rotorhead deflects or, the blades bend. The precursor to the Cheyenne had a rigid head and it had a phase angle of 90-degrees. The Cheyenne was designed to have a similar phase angle but it didn’t work out that way due to both the aerodynamics of the blade and stiffness of the blade. The phase angle would vary due to speed, loading and air density. This variance was so severe on two occasions that the blades contacted the fuselage. On the first occasion the pilot was killed and on the second a wind tunnel was destroyed.
The conditions are;
The rotation is CCW, when viewed from above.
Azimuth-0 is aft; azimuth-90 is to the right, etc.
The flight maneuver is a transition from hover to forward flight.
In the case of A/, the higher blade pitches will be found between azimuth-181 and azimuth-359, with the highest at azimuth-270. The 90-degree phase-lag will result in the disk being high between azimuth-271 and azimuth-89, with the highest at azimuth-360. This will cause the rotor disk to 'drag' the helicopter's nose down about its pitch axis.
Response:
Your text should read the highest pitch is realized when the blade is at azimuth 270
And the highest point of flap is at azimuth 360 and the lowest point of flap (down) is at azimuth 180. This is true for a teetered rotor and an articulated or a flex rotor of some kind.
In the case of B/, the higher blade pitches will be found between azimuth-271 and azimuth-89, with the highest at azimuth-360. The zero-degree phase-lag, in conjunction with the absolutely rigid coupling of the rotor to the fuselage will cause the rotor disk to 'pry' the helicopter's nose down about its pitch axis.
Response:
Rigid rotor helicopters fly just like any other type of helicopter the only difference being the response to control input.
_____________
An addendum to the forgoing is that this extremely rigid rotor must have higher than normal strength and thus greater mass. The mass will invoke gyroscopic precession. . However, the gyroscopic precession will be relatively small, and in the case of twin counter-rotating main rotors, such as the coaxial and intermeshing configurations, the opposing gyroscopic precessions should provide some static stability.
Response:
The rotors on a CH-47 can be viewed as individual rotors, both of which respond to control input. In forward flight up to about 60 knots there is no gyroscopic precession. Above 60 knots there is an automatic cyclic input and at this time there is gyroscopic precession or if you are a non-believer aerodynamic precession. Only when lateral cyclic or any other input that would cause the discs to tilt there is gyroscopic precession or the other thing if you are a non-believer. The fact that the blades are turning in opposition there is no stabilizing effect due to gyroscopic forces acting in opposition. I believe the same is true for coax helicopters and those like the Kaman.
Over.
__________
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i dissagree dave, the blade has to fly around 90deg from where the pitch input was, in order to change the spining axes. weather the changing axes is a tetering hub which is easily done or the drive shaft which would be a bit heavier to move because the helicopter is atatched. the axis still has to be changed to offset lift.
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Dave, even though we are mostly in agreement on these topics, it still seems to me that there is something about rotational dynamics that you just don't see.
Take your fully rigid rotor, in a hover, and have someone walk underneath and push up on the tail (avoiding the tail rotor, of course.) What happens?
Now take a bicycle wheel, suspend it on a string attached to the axle, attach a weight on the other end of the axle to represent the helicopter, spin it in the horizontal plane and then push up on the weight as far from the axis of rotation as you can. What happens here is what happens to your rigid helicopter when you push up on the tail.
I'll try use words where a picture should be used. The highest pitch at azimuth zero means the blade creates the most force at azimuth zero. That force creates a moment about the center of mass. The entire helicopter already has some angular momentum, once again these angular momenta must be vector summed. Result is the helicopter wants to roll left. The force at azimuth zero continues, so the helicopter wants to continue to roll left.
Another effect starts happening immediately. In the hover, lift and weight were aligned. When the helicopter rolled left, the lift vector is no longer in line with the center of mass. Because of stability considerations, I believe that most, if not all, helicopters have the center of lift above the center of mass. This means that with the helicopter rolled left, the lift vector creates a moment that, if the rotor weren't turning, would roll the helicopter left. Since the helicopter already has angular momentum, the result of the lift vector not acting through the center of mass is the vector sum of all the angular momenta. The helicopter continues to roll left because of the cyclic position, but also starts to pitch forward due to the lift vector's alignment.
It doesn't end here. In the long term, the helicopter does a roll to the left while the vertical axis actually precesses (first example we've discussed yet of true precession). The lift vector of the helicopter starts spinning around an intersecting but non-parallel axis.
Of course, in the longer term, the motion is not easy to discuss here as the "gyroscope" will topple.
My point is that when you make everything unearthly rigid, you have turned the helicopter into a gyroscope. Relative RPM, mass or anything else don't matter. I could make a piece of paper that rotates once every millenium act like a gyroscope.
Matthew.
Take your fully rigid rotor, in a hover, and have someone walk underneath and push up on the tail (avoiding the tail rotor, of course.) What happens?
Now take a bicycle wheel, suspend it on a string attached to the axle, attach a weight on the other end of the axle to represent the helicopter, spin it in the horizontal plane and then push up on the weight as far from the axis of rotation as you can. What happens here is what happens to your rigid helicopter when you push up on the tail.
I'll try use words where a picture should be used. The highest pitch at azimuth zero means the blade creates the most force at azimuth zero. That force creates a moment about the center of mass. The entire helicopter already has some angular momentum, once again these angular momenta must be vector summed. Result is the helicopter wants to roll left. The force at azimuth zero continues, so the helicopter wants to continue to roll left.
Another effect starts happening immediately. In the hover, lift and weight were aligned. When the helicopter rolled left, the lift vector is no longer in line with the center of mass. Because of stability considerations, I believe that most, if not all, helicopters have the center of lift above the center of mass. This means that with the helicopter rolled left, the lift vector creates a moment that, if the rotor weren't turning, would roll the helicopter left. Since the helicopter already has angular momentum, the result of the lift vector not acting through the center of mass is the vector sum of all the angular momenta. The helicopter continues to roll left because of the cyclic position, but also starts to pitch forward due to the lift vector's alignment.
It doesn't end here. In the long term, the helicopter does a roll to the left while the vertical axis actually precesses (first example we've discussed yet of true precession). The lift vector of the helicopter starts spinning around an intersecting but non-parallel axis.
Of course, in the longer term, the motion is not easy to discuss here as the "gyroscope" will topple.
My point is that when you make everything unearthly rigid, you have turned the helicopter into a gyroscope. Relative RPM, mass or anything else don't matter. I could make a piece of paper that rotates once every millenium act like a gyroscope.
Matthew.
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vorticey
These two examples (90-degrees & 0-degrees phase lag) were given because they may represent the two extremities that phase lag is capable of.
The 90-degrees example [A.] is indicative of gyrocopters and early helicopters; all of which had their out-of-plane rotation centered on the mast's axis.
The 0-degrees example [B.] is the direction that modern rotors, with flapping hinge offset and high rigidity etc., are heading. The Sikorsky ABC, with its fairly-rigid rotors, operated with a phase lag down to 20-degrees. Granted, it may be impossible to get all the way down to 0-degrees, because of gyroscopic precession and the inability to achieve infinite rotor rigidity.
Example [B.] has absolute (& impossible) rigidity between the blades and the fuselage. The only allowable motion between a blade and the hub is pitch change. The only allowable motion between the hub and the fuselage is RRPM.
In [A.] high pitch tilts the rotor disk. In [B.] high pitch must tilt the whole helicopter. In both cases the pitch link may be preceding the pitch bearing by 90-degrees for reasons of mechanical efficiency, but in the case of [B.] the inputs into the swashplate will have been rotated 90-degrees CCW also.
_______________
heedm
You're correct. My knowledge of rotational dynamics is far from complete, particularly when considering a situation where the speed differential between the two angular velocity is relatively small.
In your two examples, there is no significant aerodynamic activity. I agree that a 'pushing up' at tail will cause a roll to the left. I do not deny the presence of gyroscopic activity in a helicopter, only the importance of the role it plays.
In your two examples. it is assumed that the weight and the fuselage are 'welded' to the axle and the axis of primary rotation. If the non-rotating mass is negligible, then the precession will be 90-degrees. If the non-rotating mass is infinite, then the precession will be 0-degrees. As well, if the helicopter in your example is coaxial then, irrespective of the masses, the precession will be 0-degrees, I think. Does this paragraph sound correct to you?
The gist of your last five paragraphs is interesting. It takes the consideration of the rotor activity to a depth that had not been considered.
__________
This discussion appears to interface well with the Sikorsky ABC helicopter. I theorize that the rotors in the ABC gave the pilot a crisp response to cyclic inputs and an enhanced stability. Nick Lappos is in the enviable position of commenting on fact not theory.
There is a listing of revisions that would have been made if a second generation ABC was produced. Superficially, it looks like it could have been a candidate for the next generation of helicopters. Nick, if you happen to read this posting would you be willing to comment on the demise of the ABC? Was it due to politics or to a limiting feature of the advancing blade concept?
[ 25 November 2001: Message edited by: Dave Jackson ]
These two examples (90-degrees & 0-degrees phase lag) were given because they may represent the two extremities that phase lag is capable of.
The 90-degrees example [A.] is indicative of gyrocopters and early helicopters; all of which had their out-of-plane rotation centered on the mast's axis.
The 0-degrees example [B.] is the direction that modern rotors, with flapping hinge offset and high rigidity etc., are heading. The Sikorsky ABC, with its fairly-rigid rotors, operated with a phase lag down to 20-degrees. Granted, it may be impossible to get all the way down to 0-degrees, because of gyroscopic precession and the inability to achieve infinite rotor rigidity.
Example [B.] has absolute (& impossible) rigidity between the blades and the fuselage. The only allowable motion between a blade and the hub is pitch change. The only allowable motion between the hub and the fuselage is RRPM.
In [A.] high pitch tilts the rotor disk. In [B.] high pitch must tilt the whole helicopter. In both cases the pitch link may be preceding the pitch bearing by 90-degrees for reasons of mechanical efficiency, but in the case of [B.] the inputs into the swashplate will have been rotated 90-degrees CCW also.
_______________
heedm
You're correct. My knowledge of rotational dynamics is far from complete, particularly when considering a situation where the speed differential between the two angular velocity is relatively small.
In your two examples, there is no significant aerodynamic activity. I agree that a 'pushing up' at tail will cause a roll to the left. I do not deny the presence of gyroscopic activity in a helicopter, only the importance of the role it plays.
In your two examples. it is assumed that the weight and the fuselage are 'welded' to the axle and the axis of primary rotation. If the non-rotating mass is negligible, then the precession will be 90-degrees. If the non-rotating mass is infinite, then the precession will be 0-degrees. As well, if the helicopter in your example is coaxial then, irrespective of the masses, the precession will be 0-degrees, I think. Does this paragraph sound correct to you?
The gist of your last five paragraphs is interesting. It takes the consideration of the rotor activity to a depth that had not been considered.
__________
This discussion appears to interface well with the Sikorsky ABC helicopter. I theorize that the rotors in the ABC gave the pilot a crisp response to cyclic inputs and an enhanced stability. Nick Lappos is in the enviable position of commenting on fact not theory.
There is a listing of revisions that would have been made if a second generation ABC was produced. Superficially, it looks like it could have been a candidate for the next generation of helicopters. Nick, if you happen to read this posting would you be willing to comment on the demise of the ABC? Was it due to politics or to a limiting feature of the advancing blade concept?
[ 25 November 2001: Message edited by: Dave Jackson ]
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Dave, I never try to pretend that the theory is more correct than fact. I do think that even the best helicopter aerodynamicist ultimately admits that the helicopter just "beats the air into submission". The goal of helicopter aerodynamics, in my opinion, is to try to understand what is already happening. Nick et al have the inside scope on what is happening, how it is happening is gradually becoming to be understood.
My goal in this whole process is to learn more about the aerodynamic processes and to help educate on the physics.
Rather than taking the extreme of an infinite mass, consider what happens with a finite but very large mass. You apply the upwards force to the tail, a moment is created, angular momenta are summed, and the helicopter rolls a very small amount to the left. Since the mass of the helicopter is now very large, the effects I discussed in the last five paragraphs dominate. True precession of mast axis takes place.
I keep talking about summing angular momenta, but I haven't specified where you can and where you cannot do this. Don't try it with offset hinges. Teetering and rigid heads should be fine. What is best is understanding why angular momenta are summed the way they are. Once you know that then start considering Lock's number and blade flapping frequencies, apply that to the rotational physics, and phase lag jumps out at you. I think.
Matthew.
My goal in this whole process is to learn more about the aerodynamic processes and to help educate on the physics.
Rather than taking the extreme of an infinite mass, consider what happens with a finite but very large mass. You apply the upwards force to the tail, a moment is created, angular momenta are summed, and the helicopter rolls a very small amount to the left. Since the mass of the helicopter is now very large, the effects I discussed in the last five paragraphs dominate. True precession of mast axis takes place.
I keep talking about summing angular momenta, but I haven't specified where you can and where you cannot do this. Don't try it with offset hinges. Teetering and rigid heads should be fine. What is best is understanding why angular momenta are summed the way they are. Once you know that then start considering Lock's number and blade flapping frequencies, apply that to the rotational physics, and phase lag jumps out at you. I think.
Matthew.
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Hi heedm:
Gyroscopic precession or aerodynamic precession or both?
From ~ Helicopter Flight Dynamics: The Theory and Application of Flying Qualities and Simulation Modeling ~ 1996 ~ by Gareth D. Padfield.
The phase angle "stems from the two components ...... one aerodynamic due to the distribution of airloads from the angular motion, the other from the gyroscopic flapping motion."
I am certainly not qualified to question Gareth Padfield, but what the heck, this is the Internet and anything goes. As mentioned before, since a rotor's mass and RRPM (excluding minor adjustments) are constants. it is possible to consider his reference to "gyroscopic flapping motion" from an aerodynamic perspective.
Nick has mentioned that the optimum phase angle for a specific helicopter will vary as the flight aerodynamics change. This is also reinforced by the Sikorsky ABC and its widely variable Gamma.
Virtually all the inputs are aerodynamic, even a non-rotor input such as parasitic drag is aerodynamic. Trim, stability and control are looked at primarily from an aerodynamic perspective.
To me, it appears that an aerodynamic perspective, where possible, is the preferred method.
=======
Having concluded the aerodynamic sales pitch; lets talk rotational physics.
=======
>"My point is that when you make everything unearthly rigid, you have turned the helicopter into a gyroscope. "<
Only the rotor is a gyroscope; in the horizontal plane
The whole helicopter is *starting* to become a gyroscope in the vertical-longitudinal plane, but, this will only last until the desired nose-down pitch is achieved. Then all precessions stop.
>" You apply the upwards force to the tail, a moment is created, angular momenta are summed, and the helicopter rolls a very small amount to the left. "<
The very small roll to the left can be also explained aerodynamically and the change in pitch is exactly what we commanded. In a very rigid coaxial or intermeshing helicopter, the tendency for a small roll to the left will be offset by a tendency for an equal roll to the right. These combined tendencies should slightly decrease the helicopters roll and pitch rate thereby, adding to its stability. I hope.
Gyroscopic precession or aerodynamic precession or both?
From ~ Helicopter Flight Dynamics: The Theory and Application of Flying Qualities and Simulation Modeling ~ 1996 ~ by Gareth D. Padfield.
The phase angle "stems from the two components ...... one aerodynamic due to the distribution of airloads from the angular motion, the other from the gyroscopic flapping motion."
I am certainly not qualified to question Gareth Padfield, but what the heck, this is the Internet and anything goes. As mentioned before, since a rotor's mass and RRPM (excluding minor adjustments) are constants. it is possible to consider his reference to "gyroscopic flapping motion" from an aerodynamic perspective.
Nick has mentioned that the optimum phase angle for a specific helicopter will vary as the flight aerodynamics change. This is also reinforced by the Sikorsky ABC and its widely variable Gamma.
Virtually all the inputs are aerodynamic, even a non-rotor input such as parasitic drag is aerodynamic. Trim, stability and control are looked at primarily from an aerodynamic perspective.
To me, it appears that an aerodynamic perspective, where possible, is the preferred method.
=======
Having concluded the aerodynamic sales pitch; lets talk rotational physics.
=======
>"My point is that when you make everything unearthly rigid, you have turned the helicopter into a gyroscope. "<
Only the rotor is a gyroscope; in the horizontal plane
The whole helicopter is *starting* to become a gyroscope in the vertical-longitudinal plane, but, this will only last until the desired nose-down pitch is achieved. Then all precessions stop.
>" You apply the upwards force to the tail, a moment is created, angular momenta are summed, and the helicopter rolls a very small amount to the left. "<
The very small roll to the left can be also explained aerodynamically and the change in pitch is exactly what we commanded. In a very rigid coaxial or intermeshing helicopter, the tendency for a small roll to the left will be offset by a tendency for an equal roll to the right. These combined tendencies should slightly decrease the helicopters roll and pitch rate thereby, adding to its stability. I hope.
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To: Dave Jackson
>"My point is that when you make everything unearthly rigid, you have turned the helicopter into a gyroscope. "<
Only the rotor is a gyroscope; in the horizontal plane
The whole helicopter is *starting* to become a gyroscope in the vertical-longitudinal plane, but, this will only last until the desired nose-down pitch is achieved. Then all precessions stop.
Response:
Why do you continually limit yourself to rigid rotors having gyroscopic tendencies and not consider that other rotors exhibit the same characteristics. The only difference is the level of interlock between the blades and the rotorhead, which effects the response to control input.
>" You apply the upwards force to the tail, a moment is created, angular momenta are summed, and the helicopter rolls a very small amount to the left. "<
Response:
In the Ditching thread, I indicated that when a helicopter is on a pitching and rolling deck the perturbing force on the spinning rotor in the form of ships motion will result in the rotor system moving in response to that input. This response was in the form of gyroscopic precession. Nick Lappos who said it never happens crucified me. Now a theory is being postulated that if on a rigid rotor helicopter the fuselage were moved upward by the tail boom the rotor system would precess. I totally agree but I also believe the same thing would happen on any other helicopter because I experienced this phenomenon on the back end of an icebreaker. Both types of heads were represented. One was a Bell and the other was a Sikorsky.
“The very small roll to the left can be also explained aerodynamically and the change in pitch is exactly what we commanded.
Response:
How can you explain this phenomenon by introducing aerodynamics as the partial cause of the rotor movement? There was no introduction of pitch change and the motivating (perturbing) force was someone pushing up on the tail boom.
[ 25 November 2001: Message edited by: Lu Zuckerman ]
>"My point is that when you make everything unearthly rigid, you have turned the helicopter into a gyroscope. "<
Only the rotor is a gyroscope; in the horizontal plane
The whole helicopter is *starting* to become a gyroscope in the vertical-longitudinal plane, but, this will only last until the desired nose-down pitch is achieved. Then all precessions stop.
Response:
Why do you continually limit yourself to rigid rotors having gyroscopic tendencies and not consider that other rotors exhibit the same characteristics. The only difference is the level of interlock between the blades and the rotorhead, which effects the response to control input.
>" You apply the upwards force to the tail, a moment is created, angular momenta are summed, and the helicopter rolls a very small amount to the left. "<
Response:
In the Ditching thread, I indicated that when a helicopter is on a pitching and rolling deck the perturbing force on the spinning rotor in the form of ships motion will result in the rotor system moving in response to that input. This response was in the form of gyroscopic precession. Nick Lappos who said it never happens crucified me. Now a theory is being postulated that if on a rigid rotor helicopter the fuselage were moved upward by the tail boom the rotor system would precess. I totally agree but I also believe the same thing would happen on any other helicopter because I experienced this phenomenon on the back end of an icebreaker. Both types of heads were represented. One was a Bell and the other was a Sikorsky.
“The very small roll to the left can be also explained aerodynamically and the change in pitch is exactly what we commanded.
Response:
How can you explain this phenomenon by introducing aerodynamics as the partial cause of the rotor movement? There was no introduction of pitch change and the motivating (perturbing) force was someone pushing up on the tail boom.
[ 25 November 2001: Message edited by: Lu Zuckerman ]
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heedm
An apology;
>"The very small roll to the left can be also explained aerodynamically....." <
Please disregard the above portion of that sentence.
I forgot that there are no aerodynamics involved in your two examples. The pitch referred to in that sentence is helicopter pitch not blade pitch.
______
While on the soapbox;
>"Another effect starts happening immediately. In the hover, lift and weight were aligned. When the helicopter rolled left, the lift vector is no longer in line with the center of mass............. This means that with the helicopter rolled left, the lift vector creates a moment that, if the rotor weren't turning, would roll the helicopter left. Since the helicopter already has angular momentum, the result of the lift vector not acting through the center of mass is the vector sum of all the angular momenta. The helicopter continues to roll left because of the cyclic position, but also starts to pitch forward due to the lift vector's alignment."<
Both the longitudinal and the lateral component of the lift vector will not be in line with the mass while an upward vertical motion is taking place on the tail. When this motion stops both lift vectors will realign with the mast
The sketches on web page http://www.synchrolite.com/0906.html and http://www.synchrolite.com/0907.html show examples of the thrust vector for rotors with absolute rigidity. The first is hover and the second is an intermeshing configuration with lateral cyclic.
[ 26 November 2001: Message edited by: Dave Jackson ]
An apology;
>"The very small roll to the left can be also explained aerodynamically....." <
Please disregard the above portion of that sentence.
I forgot that there are no aerodynamics involved in your two examples. The pitch referred to in that sentence is helicopter pitch not blade pitch.______
While on the soapbox;
>"Another effect starts happening immediately. In the hover, lift and weight were aligned. When the helicopter rolled left, the lift vector is no longer in line with the center of mass............. This means that with the helicopter rolled left, the lift vector creates a moment that, if the rotor weren't turning, would roll the helicopter left. Since the helicopter already has angular momentum, the result of the lift vector not acting through the center of mass is the vector sum of all the angular momenta. The helicopter continues to roll left because of the cyclic position, but also starts to pitch forward due to the lift vector's alignment."<
Both the longitudinal and the lateral component of the lift vector will not be in line with the mass while an upward vertical motion is taking place on the tail. When this motion stops both lift vectors will realign with the mast
The sketches on web page http://www.synchrolite.com/0906.html and http://www.synchrolite.com/0907.html show examples of the thrust vector for rotors with absolute rigidity. The first is hover and the second is an intermeshing configuration with lateral cyclic.
[ 26 November 2001: Message edited by: Dave Jackson ]
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To: Dave Jackson
“Both the longitudinal and the lateral component of the lift vector will not be in line with the mass while an upward vertical motion is taking place on the tail. When this motion stops both lift vectors will realign with the mast.”
Response:
There is no lift vector. The helicopter is on the ground at low pitch and therefore is not generating lift when the brave individual got under the tail cone and lifted the helicopter. The response to the movement is gyroscopic in nature and does not involve aerodynamics.
There is also a POSSIBILITY that the rotor will maintain itself in the commanded position due to gyroscopic rigidity. It would depend on the stiffness of the system or, if there was any pitch/flap coupling which would aerodynamically realign the rotor to the original position.
“Both the longitudinal and the lateral component of the lift vector will not be in line with the mass while an upward vertical motion is taking place on the tail. When this motion stops both lift vectors will realign with the mast.”
Response:
There is no lift vector. The helicopter is on the ground at low pitch and therefore is not generating lift when the brave individual got under the tail cone and lifted the helicopter. The response to the movement is gyroscopic in nature and does not involve aerodynamics.
There is also a POSSIBILITY that the rotor will maintain itself in the commanded position due to gyroscopic rigidity. It would depend on the stiffness of the system or, if there was any pitch/flap coupling which would aerodynamically realign the rotor to the original position.
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Dave said, "In a very rigid coaxial or intermeshing helicopter, the tendency for a small roll to the left will be offset by a tendency for an equal roll to the right."
The intermeshing configuration would create a system that has zero angular momentum. If you lift the tail, you're creating a moment to pitch the nose down, add that to the existing angular momentum (0) and the result is that the nose pitches down. A different way of coming to the same result, but I find this one easier to accept.
"Virtually all the inputs are aerodynamic...."
Do those high school physics experiments with spinning things. Sitting on a bar stool with a bicycle wheel, etc. All the energy and inputs into those experiments come from human muscle power. Following your "aerodynamic...is the preferred method" argument, then those aren't demonstrations of gyroscopic effects, rather demonstrations of physiological effects.
I have no problem dropping the word gyroscope (or any related forms). I would really like the word precession dropped (it's wrong to use it).
I think a purely aerodynamic explanation of why rotors behave the way they do works if you also talk about some basic physics.
For example, you can track a stationary rotor blade up and down sinusoidally using a wind tunnel and sinusoidal pitch changes. Problem with this is, as the blade rises to it's maximum flap, you must use blade pitch to actually decelerate the flapping motion. You stabilize with the blade pitch and flapping position to be 180 degrees out of phase (ie minimum pitch at maximum flap).
You then realize that the blade doesn't have a restoring force to return it to it's median position. You set up a spring or pendulum as a restoring force, run the experiement, and find that if you're not too aggressive with the amount of pitch change, you get approximately a 90 degree phase angle.
Since the turning blade has a restoring force, this seems to support an aerodynamics only explanation. Problem is the restoring force on the helicopter is centrifugal force (aarrrgghh...I hate that term). In order for this to work, you need to use some rotational dynamics.
With a restoring force and the flapping motion of the blade, the blade looks like a pendulum, swinging about the flapping hinge. If the flapping frequency is the same as the rotor speed, then the phase angle should be 90 degrees. Flapping frequency depends on mass distribution and apparently many aerodynamic things (density, chord, lift curve slope...from Kyrilian's ref to the Lock number).
This may not be the aerodynamic precession that you refer to, but every time I think I understand an aerodynamics only explanation, I find something that requires rotational dynamics.
I would tend to go along with Gareth D. Padfield (except for using the word, "gyroscopic"). Rotors do generate aerodynamic forces, but unlike fixed wing aerodynamics, some sort of spinning physics is critical.
Matthew.
The intermeshing configuration would create a system that has zero angular momentum. If you lift the tail, you're creating a moment to pitch the nose down, add that to the existing angular momentum (0) and the result is that the nose pitches down. A different way of coming to the same result, but I find this one easier to accept.
"Virtually all the inputs are aerodynamic...."
Do those high school physics experiments with spinning things. Sitting on a bar stool with a bicycle wheel, etc. All the energy and inputs into those experiments come from human muscle power. Following your "aerodynamic...is the preferred method" argument, then those aren't demonstrations of gyroscopic effects, rather demonstrations of physiological effects.
I have no problem dropping the word gyroscope (or any related forms). I would really like the word precession dropped (it's wrong to use it).
I think a purely aerodynamic explanation of why rotors behave the way they do works if you also talk about some basic physics.
For example, you can track a stationary rotor blade up and down sinusoidally using a wind tunnel and sinusoidal pitch changes. Problem with this is, as the blade rises to it's maximum flap, you must use blade pitch to actually decelerate the flapping motion. You stabilize with the blade pitch and flapping position to be 180 degrees out of phase (ie minimum pitch at maximum flap).
You then realize that the blade doesn't have a restoring force to return it to it's median position. You set up a spring or pendulum as a restoring force, run the experiement, and find that if you're not too aggressive with the amount of pitch change, you get approximately a 90 degree phase angle.
Since the turning blade has a restoring force, this seems to support an aerodynamics only explanation. Problem is the restoring force on the helicopter is centrifugal force (aarrrgghh...I hate that term). In order for this to work, you need to use some rotational dynamics.
With a restoring force and the flapping motion of the blade, the blade looks like a pendulum, swinging about the flapping hinge. If the flapping frequency is the same as the rotor speed, then the phase angle should be 90 degrees. Flapping frequency depends on mass distribution and apparently many aerodynamic things (density, chord, lift curve slope...from Kyrilian's ref to the Lock number).
This may not be the aerodynamic precession that you refer to, but every time I think I understand an aerodynamics only explanation, I find something that requires rotational dynamics.
I would tend to go along with Gareth D. Padfield (except for using the word, "gyroscopic"). Rotors do generate aerodynamic forces, but unlike fixed wing aerodynamics, some sort of spinning physics is critical.
Matthew.
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Lu, the brave individual was going under a hovering helicopter. Sorry if I wasn't clear on that.
I don't think there would be any pitch/flap coupling as I thought that this unholy rigid head didn't flap.
Dave said, “Both the longitudinal and the lateral component of the lift vector will not be in line with the mass while an upward vertical motion is taking place on the tail. When this motion stops both lift vectors will realign with the mast.”
There is not a longitudinal and a lateral component to the lift, there is only the one vector. Decomposing that vector into two orthogonal vectors is a tool that often makes the physics easier to deal with. In this case, decomposing it means you have to add two more spins to the already spinning and rolling system. I'd just stick to one.
Lu is right that the response in this example is purely gyroscopic. If it were in a vacuum the same left roll would occur after our brave individual pushes up on the tail. Of course, in the vacuum there would be little left of our brave individual, not even considering the helicopter that's bound to fall on him.
Matthew.
I don't think there would be any pitch/flap coupling as I thought that this unholy rigid head didn't flap.
Dave said, “Both the longitudinal and the lateral component of the lift vector will not be in line with the mass while an upward vertical motion is taking place on the tail. When this motion stops both lift vectors will realign with the mast.”
There is not a longitudinal and a lateral component to the lift, there is only the one vector. Decomposing that vector into two orthogonal vectors is a tool that often makes the physics easier to deal with. In this case, decomposing it means you have to add two more spins to the already spinning and rolling system. I'd just stick to one.
Lu is right that the response in this example is purely gyroscopic. If it were in a vacuum the same left roll would occur after our brave individual pushes up on the tail. Of course, in the vacuum there would be little left of our brave individual, not even considering the helicopter that's bound to fall on him.
Matthew.
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heedm
Again, we take different routes, but arrive at the same town at the same time.
The aerodynamic route isn't all that 'bumpy'. The centrifugal, or is it centripetal, force helps in returning the flap and aerodynamic force helps with damping, with any luck.
Picture of unholy rigid head
[ 26 November 2001: Message edited by: Dave Jackson ]
and again
[ 26 November 2001: Message edited by: Dave Jackson ]
Again, we take different routes, but arrive at the same town at the same time.
The aerodynamic route isn't all that 'bumpy'. The centrifugal, or is it centripetal, force helps in returning the flap and aerodynamic force helps with damping, with any luck.
Picture of unholy rigid head
[ 26 November 2001: Message edited by: Dave Jackson ]
and again
[ 26 November 2001: Message edited by: Dave Jackson ]
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heedm
No one else is probably reading this thread, so we can work the subject to death.
Thanks for the education on the cone effect caused by precession in gyroscopic precession. I guess that this true gyroscopic precession will be quite minimal in a helicopter since the rotor will have oriented itself to the commanded position within half a revolution.
The term 'aerodynamic precession' may be an valid one, if the basic definition of the word 'precession' is used and not the mechanical connotation. I.e. the control input precedes the aerodynamic output.
Real nit-picking. Chow for now.
No one else is probably reading this thread, so we can work the subject to death.
Thanks for the education on the cone effect caused by precession in gyroscopic precession. I guess that this true gyroscopic precession will be quite minimal in a helicopter since the rotor will have oriented itself to the commanded position within half a revolution.
The term 'aerodynamic precession' may be an valid one, if the basic definition of the word 'precession' is used and not the mechanical connotation. I.e. the control input precedes the aerodynamic output.
Real nit-picking.
Chow for now.
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Dave, you're right, this does appear to be an almost private conversation.
Physics text books will say don't use centrifugal force because it's not a real force. They're totally right. There is no mechanism for the force, no work done, no energy expended, it is only felt by someone in a non-inertial reference frame.
That's all nice to talk about, but go to the fair grounds and ride the sizzler, then tell me there's no such thing as centrifugal force.
If we sit on the rotor blade in flight (unholy rigid?) and forget that the blade that we're sitting on is spinning, then we feel it swinging like a pendulum with a restoring force acting away from the hub. The only appropriate term for that force is centrifugal force.
If we looked at the same thing on the ground, then we can't feel or see a centrifugal force. We see a moving blade that keeps getting pulled to the rotor hub and having it's direction changed. That force that pulls it to the hub is the centripetal force. The centripetal force doesn't work as a restoring force.
___________________
I guess we have different dictionaries. The basic definition of precession in the one I use is, "the regular motion of a spinning body such as a spinning top or a planet, in which the axis of rotation sweeps out a cone".
Nit-picking is okay. Truth is in the details. Drive for show, putt for dough.
Matthew.
[ 27 November 2001: Message edited by: heedm ]
Physics text books will say don't use centrifugal force because it's not a real force. They're totally right. There is no mechanism for the force, no work done, no energy expended, it is only felt by someone in a non-inertial reference frame.
That's all nice to talk about, but go to the fair grounds and ride the sizzler, then tell me there's no such thing as centrifugal force.
If we sit on the rotor blade in flight (unholy rigid?) and forget that the blade that we're sitting on is spinning, then we feel it swinging like a pendulum with a restoring force acting away from the hub. The only appropriate term for that force is centrifugal force.
If we looked at the same thing on the ground, then we can't feel or see a centrifugal force. We see a moving blade that keeps getting pulled to the rotor hub and having it's direction changed. That force that pulls it to the hub is the centripetal force. The centripetal force doesn't work as a restoring force.
___________________
I guess we have different dictionaries. The basic definition of precession in the one I use is, "the regular motion of a spinning body such as a spinning top or a planet, in which the axis of rotation sweeps out a cone".
Nit-picking is okay. Truth is in the details. Drive for show, putt for dough.
Matthew.
[ 27 November 2001: Message edited by: heedm ]
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heedm
Thank for the explanation of centrifugal & centripetal.
May the force be with you.
>"I guess we have different dictionaries." <
The primary (non Mechanical) definition in my 1774AD dictionary is "the act of preceding".
A change to the control plane or a gust must 'precede' the change in the rotor plane, so the expression 'Aerodynamic Precession' could be considered valid.
And, you have to admit it has a nice sound to it.
___________
Is it correct to assume that 'gyroscopic precession' will always have a phase offset of 90-degrees, if the true precession that you have referred to is not considered?
[ 27 November 2001: Message edited by: Dave Jackson ]
Thank for the explanation of centrifugal & centripetal.
May the force be with you.
>"I guess we have different dictionaries." <
The primary (non Mechanical) definition in my 1774AD dictionary is "the act of preceding".
A change to the control plane or a gust must 'precede' the change in the rotor plane, so the expression 'Aerodynamic Precession' could be considered valid.
And, you have to admit it has a nice sound to it.
___________
Is it correct to assume that 'gyroscopic precession' will always have a phase offset of 90-degrees, if the true precession that you have referred to is not considered?
[ 27 November 2001: Message edited by: Dave Jackson ]
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"Is it correct to assume that 'gyroscopic precession' will always have a phase offset of 90-degrees, if the true precession that you have referred to is not considered?"
Yes, for small perturbations.
If the applied force is too big, relative to the mass and rpm, then the result is much more dynamic than merely a 90 degree apparent lag. If you set up the experiment of a bicylcle wheel spinning horizontally, hanging on a string, a small rap on the wheel will demonstrate the 90 degree phase angle. Hit it too hard and you start noticing some gyrating movements of the spin axis.
Those gyrations are predictable and are a result of conservation of angular momentum, but to describe them without pictures and without vector calculus would be challenging.
I needed only to check online dictionaries to find a number of definitions of precession that mentioned going before or forward. In that light, I guess the use of the word precession is not wrong.
Matthew.
Yes, for small perturbations.
If the applied force is too big, relative to the mass and rpm, then the result is much more dynamic than merely a 90 degree apparent lag. If you set up the experiment of a bicylcle wheel spinning horizontally, hanging on a string, a small rap on the wheel will demonstrate the 90 degree phase angle. Hit it too hard and you start noticing some gyrating movements of the spin axis.
Those gyrations are predictable and are a result of conservation of angular momentum, but to describe them without pictures and without vector calculus would be challenging.
I needed only to check online dictionaries to find a number of definitions of precession that mentioned going before or forward. In that light, I guess the use of the word precession is not wrong.
Matthew.