Cyclics, Semantics and Teetering Rotors ~ A question
Iconoclast
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No such thing in practice.
To: Dave Jackson
In a steady state no wind condition you cannot hover without having input some degree of cyclic pitch. So it is purely theoretical to say different. When the pilot lifts off to hover he makes adjustments in cyclic to compensate for tail rotor translation and for CG location. In this case he is not starting with a level disc when he adds forward cyclic. As the helicopter moves forward the tail rotor translation is not as strong so the pilot can adjust his cyclic and just about then he is into translational lift and is into Induced flow and Transverse Flow Effect and the pilot is making all sort of cyclic adjustments. So, if Wee Wa exists it is most likely mixed up in all of the other things and is most likely a theory that can only be proved on paper or in closely controlled testing.
IMHO as a non pilot.
In a steady state no wind condition you cannot hover without having input some degree of cyclic pitch. So it is purely theoretical to say different. When the pilot lifts off to hover he makes adjustments in cyclic to compensate for tail rotor translation and for CG location. In this case he is not starting with a level disc when he adds forward cyclic. As the helicopter moves forward the tail rotor translation is not as strong so the pilot can adjust his cyclic and just about then he is into translational lift and is into Induced flow and Transverse Flow Effect and the pilot is making all sort of cyclic adjustments. So, if Wee Wa exists it is most likely mixed up in all of the other things and is most likely a theory that can only be proved on paper or in closely controlled testing.
IMHO as a non pilot.
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Lu,
You said; "So, if Wee Wa exists it is most likely mixed up in all of the other things and is most likely a theory that can only be proved on paper or in closely controlled testing."
I agree that a number of things are taking place. But, if 'Wee-wa' does not have a noticeable effect, why did Frank Robinson want to address it when he designed his rotorhead and later when he explained his rotorhead on this forum?
It would be nice to know what 'Wee-wa' is an acronym for, but it appears that he has defines it as ".. the rotor disk will have some lateral tilt while the rotor disk is tilting forward ..". Perhaps it stands for 'small washed-out coupling'. The following is from the NASA Annual Report for 1993 "It is speculated that the washed-out coupling configurations were a higher frequency and lower amplitude coupling than the control and rate coupling."
He also says; " .. the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive."
I can accept this if there is significant forward velocity, but I question it when he specifically says " In a steady no-wind hover".
It may or may not be detectable in actual hovering flight, but 17º is not insignificant. I suspect (from a purely theoretical perspective) that if a Bell 47 pilot was to snap the cyclic forward he would only see a forward tilt of the rotor disk. If a Robinson pilot was to snap the cyclic forward he would see a forward tilt of the rotor disk, plus an amount of left tilt.
Disclaimer:
In no way does this speak to the pros or cons of using delta-3 in a main rotor. It is only an attempt to better understand rotor characteristics.
You said; "So, if Wee Wa exists it is most likely mixed up in all of the other things and is most likely a theory that can only be proved on paper or in closely controlled testing."
I agree that a number of things are taking place. But, if 'Wee-wa' does not have a noticeable effect, why did Frank Robinson want to address it when he designed his rotorhead and later when he explained his rotorhead on this forum?
It would be nice to know what 'Wee-wa' is an acronym for, but it appears that he has defines it as ".. the rotor disk will have some lateral tilt while the rotor disk is tilting forward ..". Perhaps it stands for 'small washed-out coupling'. The following is from the NASA Annual Report for 1993 "It is speculated that the washed-out coupling configurations were a higher frequency and lower amplitude coupling than the control and rate coupling."
He also says; " .. the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive."
I can accept this if there is significant forward velocity, but I question it when he specifically says " In a steady no-wind hover".
It may or may not be detectable in actual hovering flight, but 17º is not insignificant. I suspect (from a purely theoretical perspective) that if a Bell 47 pilot was to snap the cyclic forward he would only see a forward tilt of the rotor disk. If a Robinson pilot was to snap the cyclic forward he would see a forward tilt of the rotor disk, plus an amount of left tilt.
Disclaimer:
In no way does this speak to the pros or cons of using delta-3 in a main rotor. It is only an attempt to better understand rotor characteristics.
Iconoclast
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Wee What?
To: Dave Jackson
I'm surprised our friends from OZ never mentioned it. I went on the internet to find something about Wee Wa and came up with Wee Waa which is located in NSW Australia.
Strange!!!!
I'm surprised our friends from OZ never mentioned it. I went on the internet to find something about Wee Wa and came up with Wee Waa which is located in NSW Australia.
Strange!!!!
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Cause and Effect;
Nick, on the related thread thinks that I mixed Cause and Effect. 
I think that Frank Robinson may have mixed Cause and Effect.
[list=1][*]Frank Robinson's description of 'Wee-wa' appears to be the same as 'washed-out coupling effect'.[*]I agree with Nick's remark that "Delta three ..... acts like a stability augmentation system."[*]The AHS report says that 'washed-out coupling effect' can occur in helicopters with stability augmentation systems'[/list=1] This creates an enigma.
How can Frank Robinson use a 'stability augmentation system' (delta-3) to overcome the 'washed-out coupling effect' (Wee-wa) when it is the 'stability augmentation system' that caused the 'washed-out coupling effect'?
I think that Frank Robinson may have mixed Cause and Effect.
[list=1][*]Frank Robinson's description of 'Wee-wa' appears to be the same as 'washed-out coupling effect'.[*]I agree with Nick's remark that "Delta three ..... acts like a stability augmentation system."[*]The AHS report says that 'washed-out coupling effect' can occur in helicopters with stability augmentation systems'[/list=1] This creates an enigma.How can Frank Robinson use a 'stability augmentation system' (delta-3) to overcome the 'washed-out coupling effect' (Wee-wa) when it is the 'stability augmentation system' that caused the 'washed-out coupling effect'?
Iconoclast
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Mixing of Metafours.
To: Dave Jackson
THe Stability Augmentation System or on some helicopters the Stability Control Augmentation System (SCAS) monitors fuselage displacement both from rate and magnitude and does not respond to the movement of the blades relative to gusting. In this case it is an auto pilot that will maintain the last commanded position of the fuselage. The Delta 3 will cause the blade to reposition itself to the in track condition with this movement being the result of pitch flap coupling (Delta 3).
The SCAS also monitors rate and magnitude of cyclic input and through the built in control laws it will limit the amount of pilot valve input to the servo in order to protect the airframe from overstress. This occurs so fast that in some cases the blades have not responded to the cyclic input. Apache has this system.
IMHO
THe Stability Augmentation System or on some helicopters the Stability Control Augmentation System (SCAS) monitors fuselage displacement both from rate and magnitude and does not respond to the movement of the blades relative to gusting. In this case it is an auto pilot that will maintain the last commanded position of the fuselage. The Delta 3 will cause the blade to reposition itself to the in track condition with this movement being the result of pitch flap coupling (Delta 3).
The SCAS also monitors rate and magnitude of cyclic input and through the built in control laws it will limit the amount of pilot valve input to the servo in order to protect the airframe from overstress. This occurs so fast that in some cases the blades have not responded to the cyclic input. Apache has this system.
IMHO
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Dave,
I hope I'm not droning on about something you completely understand. I got the feeling you didn't see one important part of the coupling we're discussing.
You mentioned, "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." Consider the angle of attack of the blade as it cycles around. The craft is stationary in zero wind, no cyclic inputs, so the angle of attack is pretty close to constant.
You suddenly apply forward cyclic, causing blade pitch and thus angle of attack to decrease at the 3 o'clock, and increase at the 9 o'clock. At 12 and 6, the blade pitch doesn't change from the steady state, but now that the blade is flapping down at 12 and up at 6 the relative airflow changes. It comes more from below the blade as the blade moves down (12 o'clock) which cause angle of attack to increase even though blade pitch is the same. Of course angle of attack increased means greater lift, which means the disk will tilt to the right.
I think Frank Robinson probably did mean hover when he wrote "In a steady no-wind hover..."
I hope I'm not droning on about something you completely understand. I got the feeling you didn't see one important part of the coupling we're discussing.
You mentioned, "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." Consider the angle of attack of the blade as it cycles around. The craft is stationary in zero wind, no cyclic inputs, so the angle of attack is pretty close to constant.
You suddenly apply forward cyclic, causing blade pitch and thus angle of attack to decrease at the 3 o'clock, and increase at the 9 o'clock. At 12 and 6, the blade pitch doesn't change from the steady state, but now that the blade is flapping down at 12 and up at 6 the relative airflow changes. It comes more from below the blade as the blade moves down (12 o'clock) which cause angle of attack to increase even though blade pitch is the same. Of course angle of attack increased means greater lift, which means the disk will tilt to the right.
I think Frank Robinson probably did mean hover when he wrote "In a steady no-wind hover..."
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heedm,
You're raising a good point and I certainly don't understand it all.
Nick Lappos may be the only person on the forum who can give a definitive answer, but here is my take on it.
On one side, we must assume that Frank Robinson knows what he is talking about and is therefore correct.
On the other side, I think what you are describing may be acceleration (control) cross-coupling. In addition, at hover I question if there even is an acceleration cross-coupling. The reason for this thinking is that blade is consistently adding pitch between 3:00 and 12:00 and this could be considered aerodynamic precession. This aerodynamic precession would be functionally identical to gyroscopic precession (please excuses the use of the word 'precession'
) on this basic teetering rotor.
Your argument may be more appropriate for forward flight, were there is 'transverse flow effect' and the effect of the coning angle. To have an increased angle of attack at the front during hover and have it be higher on the left than the right, something will have to be pulling down on the front of the disk ~ I think.
The final reason for questioning Frank Robinson's remark was mentioned in my last post where "'Wee-wa' appears to be the same as 'washed-out coupling effect'.", and 'washed-out coupling' is a different type of cross-coupling from 'acceleration cross-coupling' and 'rate cross-coupling'
Then again, you may very well be correct.
You're raising a good point and I certainly don't understand it all.
Nick Lappos may be the only person on the forum who can give a definitive answer, but here is my take on it.
On one side, we must assume that Frank Robinson knows what he is talking about and is therefore correct.
On the other side, I think what you are describing may be acceleration (control) cross-coupling. In addition, at hover I question if there even is an acceleration cross-coupling. The reason for this thinking is that blade is consistently adding pitch between 3:00 and 12:00 and this could be considered aerodynamic precession. This aerodynamic precession would be functionally identical to gyroscopic precession (please excuses the use of the word 'precession'
) on this basic teetering rotor. Your argument may be more appropriate for forward flight, were there is 'transverse flow effect' and the effect of the coning angle. To have an increased angle of attack at the front during hover and have it be higher on the left than the right, something will have to be pulling down on the front of the disk ~ I think.
The final reason for questioning Frank Robinson's remark was mentioned in my last post where "'Wee-wa' appears to be the same as 'washed-out coupling effect'.", and 'washed-out coupling' is a different type of cross-coupling from 'acceleration cross-coupling' and 'rate cross-coupling'
Then again, you may very well be correct.
'.