coriolis effect & static droop
Guest
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On the subject of Torque.
Often torque and power are mistakenly imagined as the same thing.
Think of a spanner, you apply a force to one end which is tranlated into a TORQUE, which is simply the magnitude of the force you apply multiplied by the distance of the applied force from the fulcrum (or the centre of rotation although it does not HAVE to rotate). In the olden days it was foot pounds, now newton metres but could be hundredweight cubits, it doesn't matter! It could even be two forces acting as a couple about a fulcrum.
The first thing to bear in mind is that you could measure the force applied at the tail, the rotor blade tips, the gearbox coupling; all would allow you to measure a torque which, ignoring losses, should be the same. You can therefore say that the engines exert a torque at the rotor tips AND the tail boom. Without an anti-torque system the torque couple would cause the fuselage and rotors to turn in opposition.
Second point of academic interest only. As 212man stated, one way power can be described in a rotating system is as a function of torque and Nr. As you increase rotational speed at a fixed power setting so the torque drops, which is why drive shafts always spin so fast, the faster the spin the lighter the shaft (up to a limit). But if you apply full power at low NR you can seriously over torque as we all know (and a collegue of mine can tell you all about that!)
Often torque and power are mistakenly imagined as the same thing.
Think of a spanner, you apply a force to one end which is tranlated into a TORQUE, which is simply the magnitude of the force you apply multiplied by the distance of the applied force from the fulcrum (or the centre of rotation although it does not HAVE to rotate). In the olden days it was foot pounds, now newton metres but could be hundredweight cubits, it doesn't matter! It could even be two forces acting as a couple about a fulcrum.
The first thing to bear in mind is that you could measure the force applied at the tail, the rotor blade tips, the gearbox coupling; all would allow you to measure a torque which, ignoring losses, should be the same. You can therefore say that the engines exert a torque at the rotor tips AND the tail boom. Without an anti-torque system the torque couple would cause the fuselage and rotors to turn in opposition.
Second point of academic interest only. As 212man stated, one way power can be described in a rotating system is as a function of torque and Nr. As you increase rotational speed at a fixed power setting so the torque drops, which is why drive shafts always spin so fast, the faster the spin the lighter the shaft (up to a limit). But if you apply full power at low NR you can seriously over torque as we all know (and a collegue of mine can tell you all about that!)
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To: Crab
What you seem to forget is that when the blade flaps up or down it is attached to the pitchlink and the pitchlink is fixed by its’ attachment to the swashplate. The blade however is free to feather in relation to the pitchlink. When the blade flaps up, the fixed pitchlink will cause the blade to feather and decrease pitch. The opposite is true for the downward flapping blade. It moves down on the fixed pitch link and pitch is added. This is pitch coupling. Here is how it is explained in the FAA Rotorcraft Flying handbook when they were addressing dissymmetry of lift. “If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize the lift across the rotor disc”. It works exactly the same on a tail rotor. The mechanism that allows this to happen on the tail rotor is a delta hinge. The delta effect also works on the main rotor but only when the pitchlink / pitch horn connect point is above or below the flapping hinge or the cone hinges and teeter hinges on the Robinson or, the teeter hinge on a Bell. If the points are coincident with each other when the blade flaps the blade will pivot about the flapping hinge and the pitchlink/pitch horn attach point and there will be no pitch coupling. This would be extremely rare for these points on all of the blades to be aligned in flight.
I don’t dispute what you said about the drag induced by pitch change and, that the drag is different on the advancing and retreating side. What I said previously is that the drag forces have minimal if no effect on the Coriolis forces that cause lead and lag.
The advancing blade has a higher drag than the retreating side but the advancing blade leads and the retreating blade lags which is in the opposite direction of the respective drag forces.
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The Cat
[This message has been edited by Lu Zuckerman (edited 18 January 2001).]
What you seem to forget is that when the blade flaps up or down it is attached to the pitchlink and the pitchlink is fixed by its’ attachment to the swashplate. The blade however is free to feather in relation to the pitchlink. When the blade flaps up, the fixed pitchlink will cause the blade to feather and decrease pitch. The opposite is true for the downward flapping blade. It moves down on the fixed pitch link and pitch is added. This is pitch coupling. Here is how it is explained in the FAA Rotorcraft Flying handbook when they were addressing dissymmetry of lift. “If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize the lift across the rotor disc”. It works exactly the same on a tail rotor. The mechanism that allows this to happen on the tail rotor is a delta hinge. The delta effect also works on the main rotor but only when the pitchlink / pitch horn connect point is above or below the flapping hinge or the cone hinges and teeter hinges on the Robinson or, the teeter hinge on a Bell. If the points are coincident with each other when the blade flaps the blade will pivot about the flapping hinge and the pitchlink/pitch horn attach point and there will be no pitch coupling. This would be extremely rare for these points on all of the blades to be aligned in flight.
I don’t dispute what you said about the drag induced by pitch change and, that the drag is different on the advancing and retreating side. What I said previously is that the drag forces have minimal if no effect on the Coriolis forces that cause lead and lag.
The advancing blade has a higher drag than the retreating side but the advancing blade leads and the retreating blade lags which is in the opposite direction of the respective drag forces.
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The Cat
[This message has been edited by Lu Zuckerman (edited 18 January 2001).]
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Lu,
The pitch-flap couple you describe is the Delta 3 hinge effect discussed on previous threads which, as you correctly stated, can only occur if the pitch change horn is not in line with the flapping hinge. Most examples of this effect are found on tail rotors where the flapping limits must be constrained to keep the blades clear of the tail structure and droop stops or flapping restrainers are undesireable alternatives because they add weight.
Back to the main rotor; the reason the blades rate of flapping reduces is due to a reduction in cyclic pitch as the POAs follow the control orbit and aerodynamic damping - as a blade flaps up, it's relative airflow changes direction and gradually returns the angle of attack towards it's starting value.
Tilt the swashplate forwards (towards the nose) the cyclic pitch measured at the 3 and 9 0'clock positions is the same - the minimun pitch is 12 0'clock and the maximum is 6 o'clock.
As a blade moves from 3 o'clock towards the nose it begin to see a reduction in pitch (POAs follow swashplate) and therefore AoA and starts to move (flap) downwards. The rate of pitch change is not uniform and the maximum rate (vertical movement of the POA for a given angular displacement) of pitch change is at the 12 o'clock position.
At this point the rate of flapping down is at it's maximum as well and although the cyclic pitch from 12 to 9 0'clock is increasing again, it is still less than the 3 and 9 0'clock settings and the blade continues to flap down until it reaches it's low point at 9 0'clock. The blade is slowed in it's rate of flapping in the manner described above but as the blade is flapping down the relative airflow is coming from a greater angle so the AoA is increasing.
The 90 degree disparity between swashplate tilt and blade low point is Phase Lag - the way the blades AoA is reduced or increased by flapping is Flapping to Equality and exactly the same phenomenon is the reason for Flapback (equalising the changes in lift caused by speed differences between advancing and retreating blades in forward flight) and Inflow Roll (equalising changes in lift between the front and rear of the disc caused by disc tilt and the resulting differences in relative airflow).
That hurt my brain so I'm off to work for a rest!
The pitch-flap couple you describe is the Delta 3 hinge effect discussed on previous threads which, as you correctly stated, can only occur if the pitch change horn is not in line with the flapping hinge. Most examples of this effect are found on tail rotors where the flapping limits must be constrained to keep the blades clear of the tail structure and droop stops or flapping restrainers are undesireable alternatives because they add weight.
Back to the main rotor; the reason the blades rate of flapping reduces is due to a reduction in cyclic pitch as the POAs follow the control orbit and aerodynamic damping - as a blade flaps up, it's relative airflow changes direction and gradually returns the angle of attack towards it's starting value.
Tilt the swashplate forwards (towards the nose) the cyclic pitch measured at the 3 and 9 0'clock positions is the same - the minimun pitch is 12 0'clock and the maximum is 6 o'clock.
As a blade moves from 3 o'clock towards the nose it begin to see a reduction in pitch (POAs follow swashplate) and therefore AoA and starts to move (flap) downwards. The rate of pitch change is not uniform and the maximum rate (vertical movement of the POA for a given angular displacement) of pitch change is at the 12 o'clock position.
At this point the rate of flapping down is at it's maximum as well and although the cyclic pitch from 12 to 9 0'clock is increasing again, it is still less than the 3 and 9 0'clock settings and the blade continues to flap down until it reaches it's low point at 9 0'clock. The blade is slowed in it's rate of flapping in the manner described above but as the blade is flapping down the relative airflow is coming from a greater angle so the AoA is increasing.
The 90 degree disparity between swashplate tilt and blade low point is Phase Lag - the way the blades AoA is reduced or increased by flapping is Flapping to Equality and exactly the same phenomenon is the reason for Flapback (equalising the changes in lift caused by speed differences between advancing and retreating blades in forward flight) and Inflow Roll (equalising changes in lift between the front and rear of the disc caused by disc tilt and the resulting differences in relative airflow).
That hurt my brain so I'm off to work for a rest!
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To: Crab
First of all we have to get on the same piece of paper. In this discussion I will not address the Lynx or the Robinson because of their respective offsets (Lynx 15-degrees and Robinson 18-degrees). I will not address swashplate position; instead I will address cyclic stick displacement, as this is uniform on all helicopters where swashplate displacement is not.
First lets address the Bell. If the blades are displaced in line with the lateral axis and the cyclic stick is moved forward the blade at the 3:00 position will be at the minimum pitch angle. The blade at the 9:00 position will be at the highest pitch angle. Gyroscopic precession will tilt the disc down over the nose and up over the tail. (If you don’t agree with gyroscopic precession you can use your own terminology). If the blades were disposed over the longitudinal axis and the cyclic stick was moved forward or backward the blades would not change pitch. If the blades were disposed over the longitudinal axis and the stick was moved to the right the blade at 12:00 would be at its’ highest pitch and the blade at 6:00 would be at its’ lowest pitch. The opposite would be true if the stick were moved left.
Now for clarity’s sake I will use a four-blade rotor system turning counterclockwise in this illustration. With the blades disposed over the lateral and longitudinal axes of the helicopter and the cyclic was moved forward the blade at 3:00 would be at its lowest pitch and the blade at 9:00 will be at its’ highest pitch. The blades at 6:00 and 12:00 will reflect the collective pitch setting. If the cyclic were moved to the right, the blade at 12:00 would be at its’ highest pitch and the blade at 6:00 would be at its’ lowest pitch. The blades at 3:00 and 9:00 would reflect the collective pitch setting.
All of this is allowed by the lead angle of the pitch horn and the tilting of the swashplate in respect to the direction in which the cyclic stick is displaced.
With all of this in mind, you are free to go back to the “drawing board” and rethink your post.
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The Cat
First of all we have to get on the same piece of paper. In this discussion I will not address the Lynx or the Robinson because of their respective offsets (Lynx 15-degrees and Robinson 18-degrees). I will not address swashplate position; instead I will address cyclic stick displacement, as this is uniform on all helicopters where swashplate displacement is not.
First lets address the Bell. If the blades are displaced in line with the lateral axis and the cyclic stick is moved forward the blade at the 3:00 position will be at the minimum pitch angle. The blade at the 9:00 position will be at the highest pitch angle. Gyroscopic precession will tilt the disc down over the nose and up over the tail. (If you don’t agree with gyroscopic precession you can use your own terminology). If the blades were disposed over the longitudinal axis and the cyclic stick was moved forward or backward the blades would not change pitch. If the blades were disposed over the longitudinal axis and the stick was moved to the right the blade at 12:00 would be at its’ highest pitch and the blade at 6:00 would be at its’ lowest pitch. The opposite would be true if the stick were moved left.
Now for clarity’s sake I will use a four-blade rotor system turning counterclockwise in this illustration. With the blades disposed over the lateral and longitudinal axes of the helicopter and the cyclic was moved forward the blade at 3:00 would be at its lowest pitch and the blade at 9:00 will be at its’ highest pitch. The blades at 6:00 and 12:00 will reflect the collective pitch setting. If the cyclic were moved to the right, the blade at 12:00 would be at its’ highest pitch and the blade at 6:00 would be at its’ lowest pitch. The blades at 3:00 and 9:00 would reflect the collective pitch setting.
All of this is allowed by the lead angle of the pitch horn and the tilting of the swashplate in respect to the direction in which the cyclic stick is displaced.
With all of this in mind, you are free to go back to the “drawing board” and rethink your post.
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The Cat
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Lu writes (to Crab):
"With all of this in mind, you are free to go back to the “drawing board” and rethink your post."
Very condescending to distinguished (and modest IMO) Crab ... it's fairly clear that he just accidentally shifted his points of reference accidentally (twice)… but still has a better understanding than U Lu..
..AND excuse me for suggesting but I think Lu YOU might benefit from a bit of a rethink.
Since you do not seem to understand the nature of helicopter rotors. Hiding behind the term Gyroscopic Precession (GP) is becoming boring...
Not understanding Flapping to Equality is pretty odd (not that it actually occurs much in practice)...
Try this for an unusual (but simpler) approach:
(For S&L cruise, teetering head)
Terms;
Disc Attitude (angle of Tip Path Plane Axis* from vertical) I'll call X
X=invTan(DRAG/WEIGHT)
(….because DRAG=Tsin(X) and WEIGHT=Tcos(X), where T=TotalRotorThrust)
So the Plane of Rotation for a given cruise is predetermined!...*
Now all you have to do is arrange for that to continue being the case...
For this to continue the 'Lift' generated by a blade through the Cycle must remain the same through one half rotation as the other half (any of the infinite possible halves will do).
IF it does not THEN the consequent response of the blades to Flap to Equality* (Lift actually still remaining the same* all the way around the disc) will RESULT in a change of Attitude* ... This change will be opposed by the PILOT, who varies the asymmetry in Pitch (Cyclic Pitch) such that the attitude does not need to change*.
So really it IS actually the pilot, using the feedback of Attitude, who eliminates dissymmetry of lift*, by running his Swash Plate at whatever attitude is required.
It would make NO material difference if Delta3 Pitch Coupling existed or not, the pitch the blades ran would still have to be the same* with it or without it it's just that the pilot would have to hold his stick in a different place to achieve this… in the same way he would if the passengers walked around (assuming no drag change from fuselage attitude change) …
Pilots generally do not mind moving the stick. They often actually enjoy it … which I suppose is why 'Auto' Pilots (airline pilots) love to learn to fly helicopters … if they didn't already…
Good morning.
* = Things Lu won't understand….
Lu: "Here is how it is explained in the FAA Rotorcraft Flying handbook when they were addressing dissymmetry of lift. “If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize the lift across the rotor disc”. " - trying to explain how Pitch coupling is responsible for eliminating Diss-o-Lift - VERY ODD ... now Stability ...that's another story…
"With all of this in mind, you are free to go back to the “drawing board” and rethink your post."
Very condescending to distinguished (and modest IMO) Crab ... it's fairly clear that he just accidentally shifted his points of reference accidentally (twice)… but still has a better understanding than U Lu..
..AND excuse me for suggesting but I think Lu YOU might benefit from a bit of a rethink.
Since you do not seem to understand the nature of helicopter rotors. Hiding behind the term Gyroscopic Precession (GP) is becoming boring...
Not understanding Flapping to Equality is pretty odd (not that it actually occurs much in practice)...
Try this for an unusual (but simpler) approach:
(For S&L cruise, teetering head)
Terms;
Disc Attitude (angle of Tip Path Plane Axis* from vertical) I'll call X
X=invTan(DRAG/WEIGHT)
(….because DRAG=Tsin(X) and WEIGHT=Tcos(X), where T=TotalRotorThrust)
So the Plane of Rotation for a given cruise is predetermined!...*
Now all you have to do is arrange for that to continue being the case...
For this to continue the 'Lift' generated by a blade through the Cycle must remain the same through one half rotation as the other half (any of the infinite possible halves will do).
IF it does not THEN the consequent response of the blades to Flap to Equality* (Lift actually still remaining the same* all the way around the disc) will RESULT in a change of Attitude* ... This change will be opposed by the PILOT, who varies the asymmetry in Pitch (Cyclic Pitch) such that the attitude does not need to change*.
So really it IS actually the pilot, using the feedback of Attitude, who eliminates dissymmetry of lift*, by running his Swash Plate at whatever attitude is required.
It would make NO material difference if Delta3 Pitch Coupling existed or not, the pitch the blades ran would still have to be the same* with it or without it it's just that the pilot would have to hold his stick in a different place to achieve this… in the same way he would if the passengers walked around (assuming no drag change from fuselage attitude change) …
Pilots generally do not mind moving the stick. They often actually enjoy it … which I suppose is why 'Auto' Pilots (airline pilots) love to learn to fly helicopters … if they didn't already…
Good morning.
* = Things Lu won't understand….
Lu: "Here is how it is explained in the FAA Rotorcraft Flying handbook when they were addressing dissymmetry of lift. “If this condition were allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize the lift across the rotor disc”. " - trying to explain how Pitch coupling is responsible for eliminating Diss-o-Lift - VERY ODD ... now Stability ...that's another story…
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Actually to be fair I think this forum is really helping Lu learn about helicopters - and he's starting to make some sense, it's just amazing that he keeps his crediblity (retrospectively) throughout his learning process ... hats off to all
How?
How?
Guest
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Lu,
I agree with all you say regarding the blade pitch response to cyclic position - I should have stated that I was using as simple an example as possible where the pitch change arms would be positioned 90 degrees ahead of the blade pitching axis on a Bell type 2 bladed rotor. In my example, to get the blade low at 12 'o'clock when you move the cyclic forward, the swash plate is tilted to the 3 o'clock as the blade will reach it's low position 90 degrees after the minimum pitch position (I say due to flapping - you say due to precession). We are both on the same piece of paper and if you go back and re-read my post with that in mind you will see we agree.
I was trying to clarify Flapping to Equality as used by Brit Mil and many others as it is not the same as the pitch-flap coupling you described which we regard as the Delta 3 hinge effect. Your previous post stated that the blade was free to feather in relation to the pitch-link which is not the case - it would be impossible to control the aircraft if the blades could increase or decrease pitch except as controlled by the pitch operating arms which are fixed to the swashplate.
You might not like the idea of aerodynamic forces and prefer precession but if it's good enough for the Empire Test Pilot's School (ETPS) who's notes I have read, then it's good enough for me!
I agree with all you say regarding the blade pitch response to cyclic position - I should have stated that I was using as simple an example as possible where the pitch change arms would be positioned 90 degrees ahead of the blade pitching axis on a Bell type 2 bladed rotor. In my example, to get the blade low at 12 'o'clock when you move the cyclic forward, the swash plate is tilted to the 3 o'clock as the blade will reach it's low position 90 degrees after the minimum pitch position (I say due to flapping - you say due to precession). We are both on the same piece of paper and if you go back and re-read my post with that in mind you will see we agree.
I was trying to clarify Flapping to Equality as used by Brit Mil and many others as it is not the same as the pitch-flap coupling you described which we regard as the Delta 3 hinge effect. Your previous post stated that the blade was free to feather in relation to the pitch-link which is not the case - it would be impossible to control the aircraft if the blades could increase or decrease pitch except as controlled by the pitch operating arms which are fixed to the swashplate.
You might not like the idea of aerodynamic forces and prefer precession but if it's good enough for the Empire Test Pilot's School (ETPS) who's notes I have read, then it's good enough for me!
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To: Crab
Your illustration of the Bell swashplate Vs blade tilt is wrong. When you push the cyclic forward on most Bell helicopters the swashplate will tilt down at the 12:00 position with the blades disposed over the 3:00 and 9:00 positions. With the blade in this position the pitch horns are at the 12:00 and 6:00 positions. That means that the blades have the maximum pitch change relative to their collective pitch angles. The advancing blade will have less pitch and the retreating blade has more pitch and some “unknown force” will cause the blade disc to tilt down over the nose.
In stating that the blade is free to feather a better choice of words could have been is that the blades can be made to feather. This feathering can be accomplished by moving the pitchlink in relation to the blade or, by moving the blade in relation to the pitch link. This is what happens when the blade flaps up or down in relation to the parallel paths of the disc and the swashplate resulting in pitch coupling.
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The Cat
Your illustration of the Bell swashplate Vs blade tilt is wrong. When you push the cyclic forward on most Bell helicopters the swashplate will tilt down at the 12:00 position with the blades disposed over the 3:00 and 9:00 positions. With the blade in this position the pitch horns are at the 12:00 and 6:00 positions. That means that the blades have the maximum pitch change relative to their collective pitch angles. The advancing blade will have less pitch and the retreating blade has more pitch and some “unknown force” will cause the blade disc to tilt down over the nose.
In stating that the blade is free to feather a better choice of words could have been is that the blades can be made to feather. This feathering can be accomplished by moving the pitchlink in relation to the blade or, by moving the blade in relation to the pitch link. This is what happens when the blade flaps up or down in relation to the parallel paths of the disc and the swashplate resulting in pitch coupling.
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The Cat
Is there a parrot around here, or is it an echo from before Christmas? Guest
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sps, the lose definition of torque is: a force which tends to produce rotation or torsion. That definition doesn't include anything about a "reaction". if you wrap your hand aroud a shaft and rotate it (don't try this at home) you are exerting the force (torque) directly to the shaft. maybe a more accurate name for the tail rotor would be anti-torque reaction system. maybe not.
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To: Joe Pilot et al.
The reason I am starting to make sense is summed up in the story of the young man that thought his father was so stupid, and when the boy reached 21 years of age, he was amazed at how smart his father got in six years.
It must be understood that the major reason for so much antagonism from UK and OZ pilots and their not agreeing with my input on these threads is due to the fact that your training regarding flight theory is totally different from how it is taught in the USA. With that said, I find it difficult to understand why so many UK and OZ pilots quote books and theories postulated by people like Ray Prouty and other American text book writers. How do you reconcile the differences in aerodynamic and physical laws as taught in the US and the UK?
Regarding my quoting the FAA Handbook, the quoted statement should have been preceded with; …The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter while the relative wind speed acting on the retreating blade is reduced by the helicopters forward speed. Therefore, as a result of the relative wind speed, the advancing blade side of the rotor produces more lift than the retreating blade side. This situation is described as disymmetry of lift. Then comes the paragraph I quoted above. The automatic feathering they alluded to is pitch coupling.
If you have an argument, take it up with the FAA because I don’t agree with their explanation. I believe disymmetry of lift, if it exists, results in blow back. If you look at the rotor system that has disymmetry of lift across the rotor disc it appears that it is basically the same as that of retreating blade stall but a much milder version. That’s what I think. The FAA states that the helicopter would roll left under the stated conditions. That is the first stage of retreating blade stall. If the left roll were not countered the disc would blowback, and you might ask, what would cause the disc to blow back. It will blow back for the same reason the disc blows back with retreating blade stall. The culprit (are you ready for this?) is gyroscopic precession.
One of the reasons I disagree with the FAA is that they address individual subjects in an isolated manner much like the way helicopter aerodynamics is taught. They allude to the fact that the blades are automatically feathered (pitch coupling) but they neglect to discuss one very important point. That point is that in order to get the helicopter to fly through the relative wind that will create disymmetry of lift you have to move the cyclic forward which will decrease the pitch on the advancing blade and increase the pitch on the retreating blade thus tilting the disc and at the same time result in eliminating disymmetry of lift. There are other perturbing forces out there that will cause a helicopter to roll right or left one of, which is transverse, flow effect.
Let the race begin.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 January 2001).]
[This message has been edited by Lu Zuckerman (edited 21 January 2001).]
The reason I am starting to make sense is summed up in the story of the young man that thought his father was so stupid, and when the boy reached 21 years of age, he was amazed at how smart his father got in six years.
It must be understood that the major reason for so much antagonism from UK and OZ pilots and their not agreeing with my input on these threads is due to the fact that your training regarding flight theory is totally different from how it is taught in the USA. With that said, I find it difficult to understand why so many UK and OZ pilots quote books and theories postulated by people like Ray Prouty and other American text book writers. How do you reconcile the differences in aerodynamic and physical laws as taught in the US and the UK?
Regarding my quoting the FAA Handbook, the quoted statement should have been preceded with; …The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter while the relative wind speed acting on the retreating blade is reduced by the helicopters forward speed. Therefore, as a result of the relative wind speed, the advancing blade side of the rotor produces more lift than the retreating blade side. This situation is described as disymmetry of lift. Then comes the paragraph I quoted above. The automatic feathering they alluded to is pitch coupling.
If you have an argument, take it up with the FAA because I don’t agree with their explanation. I believe disymmetry of lift, if it exists, results in blow back. If you look at the rotor system that has disymmetry of lift across the rotor disc it appears that it is basically the same as that of retreating blade stall but a much milder version. That’s what I think. The FAA states that the helicopter would roll left under the stated conditions. That is the first stage of retreating blade stall. If the left roll were not countered the disc would blowback, and you might ask, what would cause the disc to blow back. It will blow back for the same reason the disc blows back with retreating blade stall. The culprit (are you ready for this?) is gyroscopic precession.
One of the reasons I disagree with the FAA is that they address individual subjects in an isolated manner much like the way helicopter aerodynamics is taught. They allude to the fact that the blades are automatically feathered (pitch coupling) but they neglect to discuss one very important point. That point is that in order to get the helicopter to fly through the relative wind that will create disymmetry of lift you have to move the cyclic forward which will decrease the pitch on the advancing blade and increase the pitch on the retreating blade thus tilting the disc and at the same time result in eliminating disymmetry of lift. There are other perturbing forces out there that will cause a helicopter to roll right or left one of, which is transverse, flow effect.
Let the race begin.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 January 2001).]
[This message has been edited by Lu Zuckerman (edited 21 January 2001).]
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I have read it. What of it? It could be saying many things that might have some bearing on the thread on flapback/forward, and then again it could have no relevance at all with regard to my actual question.
I know why the disc does not flap forward as much as it flaps back below 80% of BRC speed.
(Well, I think I do!) Part of the reason
that the flapping thread was started was to see what others have to say on the matter, and it was done with an open mind.
Gyroscopic precession has a part to play in the process but I do not beleive it to be a panacea for all P of F anomilies that initially seem to be unanswerable without reaching for it.
As I said to you before, US training may well
rely far too heavily on Gyroscopic Precession whilst UK training may not use it enough. I have a feeling that the truth lies somewhere in the middle. You didn't choose to reply to me on that point, your choice of course.
In any event, if you have something to add to what has been said on the thread then I'd be really pleased to hear it and have the opportunity to discuss it with you, OK?
SPS, watching the water go down the plughole the opposite way whilst my head still goes the same way as it did in the N. hemi after a few beers...?
I know why the disc does not flap forward as much as it flaps back below 80% of BRC speed.
(Well, I think I do!) Part of the reason
that the flapping thread was started was to see what others have to say on the matter, and it was done with an open mind.
Gyroscopic precession has a part to play in the process but I do not beleive it to be a panacea for all P of F anomilies that initially seem to be unanswerable without reaching for it.
As I said to you before, US training may well
rely far too heavily on Gyroscopic Precession whilst UK training may not use it enough. I have a feeling that the truth lies somewhere in the middle. You didn't choose to reply to me on that point, your choice of course.
In any event, if you have something to add to what has been said on the thread then I'd be really pleased to hear it and have the opportunity to discuss it with you, OK?
SPS, watching the water go down the plughole the opposite way whilst my head still goes the same way as it did in the N. hemi after a few beers...?
Guest
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Lu,
In an attempt at clarification I muddied the waters - my simple 2 bladed model had double correction for phase lag, both swash plate tilt to the right to produce blade low at the front and a 90 degree advance angle on the pitch operating arms. One or other of these corrections would have been alright and the aim was to agree that the blade reaches it's high or low position 90 degrees after the maximum or minimum cyclic pitch position.
Most real helicopters use a combination of jack positioning and advance angle of the POAs to achieve the obviously desireable state where forward cyclic makes the blades flap down at the front and up at the back, thereby angling the Total Reaction forwards and accelerating the helicopter.
If the FAA thinks that differential airspeeds on the advancing and retreating blades makes the aircraft roll left then good luck to anyone flying a helicopter certified by them. The true result as we all know is flapback (blowback).
The roll towards the advancing blade (to the right on a counter-clockwise rotor)is a result of disc tilt causing a reduction in the inflow angle at the front of the disc. The increase in AoA gives more lift and as the effect starts at the 3 o'clock position, has it's maximum at 12 o'clock and reduces by the 9 o'clock the disc flaps to the right (90 degree phase lag again). We call it Inflow Roll and you call it Transverse Flow.
In an attempt at clarification I muddied the waters - my simple 2 bladed model had double correction for phase lag, both swash plate tilt to the right to produce blade low at the front and a 90 degree advance angle on the pitch operating arms. One or other of these corrections would have been alright and the aim was to agree that the blade reaches it's high or low position 90 degrees after the maximum or minimum cyclic pitch position.
Most real helicopters use a combination of jack positioning and advance angle of the POAs to achieve the obviously desireable state where forward cyclic makes the blades flap down at the front and up at the back, thereby angling the Total Reaction forwards and accelerating the helicopter.
If the FAA thinks that differential airspeeds on the advancing and retreating blades makes the aircraft roll left then good luck to anyone flying a helicopter certified by them. The true result as we all know is flapback (blowback).
The roll towards the advancing blade (to the right on a counter-clockwise rotor)is a result of disc tilt causing a reduction in the inflow angle at the front of the disc. The increase in AoA gives more lift and as the effect starts at the 3 o'clock position, has it's maximum at 12 o'clock and reduces by the 9 o'clock the disc flaps to the right (90 degree phase lag again). We call it Inflow Roll and you call it Transverse Flow.
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Lu WROTE: "The reason I am starting to make sense is summed up in the story of the young man that thought his father was so stupid, and when the boy reached 21 years of age, he was amazed at how smart his father got in six years."
- NONSENSE you are just learning, without anyone realising how little you knew...
- The Pilots job in setting a Cyclic dissymetry to counter aerodynamic dissymetries to ensure equal lift around the disc ... you have clearly just understood!
Shame you are taking so long to understand what you mean by Gyroscopic Precession:
GP: More pitch on ONE side will cause the blades of that side to flap (a process) up, the process of flapping up causes the Angle of Attack to be reduced. Result: the 'lift' stays the same but only because the blade is now RISING - this RISING (or Flapping Up) will finnish at the END of the half where there was extra pitch, ie. APPROXIMATELY 90degrees after point of MAX Pitch.
It is very convienient not to have to explain this, so the term 'Gyroscopic Precession' is used since it BLINDLY predicts approximately the same behaviour.
You must try to understand this - otherwise you miss the true 'elegance' of the helicopter.
Lu:"How do you reconcile the differences in aerodynamic and physical laws as taught in the US and the UK?" - Simplification for Americans... of course!
On reading Lu more carefully it is apparent that his use of English is often what causes the misunderstandings.
Allow me to illustrate Lu's ENGLISH being 90Deg 'out of phase':
Lu: "When you push the cyclic forward on most Bell helicopters the swashplate will tilt down at the 12:00 position with the blades disposed over the 3:00 and 9:00 positions. With the blade in this position the pitch horns are at the 12:00 and 6:00 positions. That means that the blades have the maximum pitch change relative to their collective pitch angles. The advancing blade will have less pitch and the retreating blade has more pitch and some "unknown force" will cause the blade disc"
In this paragraph the phrase: "blades have the maximum pitch change" implies that the rate of change of pitch is a max - whereas I'm sure he means the pitch (ammount, quantity or value) is a Maximum (or minimum). Pitch Change implies a rate, the max rate of which obviously occurs at 12&6 o'clock.
I'm sure CRAB is just getting HIS TERMINOLOGY 90deg out from our understanding in the same way, since he does at least appear to UNDERSTAND the process:
A blade at 3 has it's Pitch Link at 12 (approx), therefore to run min blade pitch at 3 the swash plate must be 'tilted down' at 12. The Max Rate of Pitch Change occurrs at 12(blade at 12, pitch link at 9) requiring upward Force on swash plate at 9 hence righthand stick force(not movement, just force) requirement. Right CRAB?
Lu: I guess the simplest way to get accross the concept you call GP is to think of the pitch variations flying the blades to a new plane of rotation - does that help?
(and/or likewise ...preventing changes in plane of rotation)
Lu you are clearly still 'screwed up' over things like 'Blow Back' - to us this is more violent version of 'Flap Back' - do you use that term? Retreating blade stall and Diss of Lift - similar ? - well yes ... but only in-as-much as that IS how a rotor system works!
Lu:"The culprit (are you ready for this?) is gyroscopic precession." - no! Again: A Falling blade (flappING down) will be low at the END of the half in which it is FALLING. .. getting it yet?
CRAB (and other mil-bible die hards)... just a side question : Do the military still teach (wrongly) that recirculation at cliffs results in attitude change TOWARDS the cliff? (worth a (see)new topic(s)... I think)
[This message has been edited by JoePilot (edited 21 January 2001).]
- NONSENSE you are just learning, without anyone realising how little you knew...
- The Pilots job in setting a Cyclic dissymetry to counter aerodynamic dissymetries to ensure equal lift around the disc ... you have clearly just understood!
Shame you are taking so long to understand what you mean by Gyroscopic Precession:
GP: More pitch on ONE side will cause the blades of that side to flap (a process) up, the process of flapping up causes the Angle of Attack to be reduced. Result: the 'lift' stays the same but only because the blade is now RISING - this RISING (or Flapping Up) will finnish at the END of the half where there was extra pitch, ie. APPROXIMATELY 90degrees after point of MAX Pitch.
It is very convienient not to have to explain this, so the term 'Gyroscopic Precession' is used since it BLINDLY predicts approximately the same behaviour.
You must try to understand this - otherwise you miss the true 'elegance' of the helicopter.
Lu:"How do you reconcile the differences in aerodynamic and physical laws as taught in the US and the UK?" - Simplification for Americans... of course!
On reading Lu more carefully it is apparent that his use of English is often what causes the misunderstandings.
Allow me to illustrate Lu's ENGLISH being 90Deg 'out of phase':
Lu: "When you push the cyclic forward on most Bell helicopters the swashplate will tilt down at the 12:00 position with the blades disposed over the 3:00 and 9:00 positions. With the blade in this position the pitch horns are at the 12:00 and 6:00 positions. That means that the blades have the maximum pitch change relative to their collective pitch angles. The advancing blade will have less pitch and the retreating blade has more pitch and some "unknown force" will cause the blade disc"
In this paragraph the phrase: "blades have the maximum pitch change" implies that the rate of change of pitch is a max - whereas I'm sure he means the pitch (ammount, quantity or value) is a Maximum (or minimum). Pitch Change implies a rate, the max rate of which obviously occurs at 12&6 o'clock.
I'm sure CRAB is just getting HIS TERMINOLOGY 90deg out from our understanding in the same way, since he does at least appear to UNDERSTAND the process:
A blade at 3 has it's Pitch Link at 12 (approx), therefore to run min blade pitch at 3 the swash plate must be 'tilted down' at 12. The Max Rate of Pitch Change occurrs at 12(blade at 12, pitch link at 9) requiring upward Force on swash plate at 9 hence righthand stick force(not movement, just force) requirement. Right CRAB?
Lu: I guess the simplest way to get accross the concept you call GP is to think of the pitch variations flying the blades to a new plane of rotation - does that help?
(and/or likewise ...preventing changes in plane of rotation)
Lu you are clearly still 'screwed up' over things like 'Blow Back' - to us this is more violent version of 'Flap Back' - do you use that term? Retreating blade stall and Diss of Lift - similar ? - well yes ... but only in-as-much as that IS how a rotor system works!
Lu:"The culprit (are you ready for this?) is gyroscopic precession." - no! Again: A Falling blade (flappING down) will be low at the END of the half in which it is FALLING. .. getting it yet?
CRAB (and other mil-bible die hards)... just a side question : Do the military still teach (wrongly) that recirculation at cliffs results in attitude change TOWARDS the cliff? (worth a (see)new topic(s)... I think)
[This message has been edited by JoePilot (edited 21 January 2001).]
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Joe,
I was trying to show Lu that although we disagree on the Precession issue, he was arguing correctly about blade low/high being 90 degrees after the min/max pitch position.
I wanted to highlight that you either have to tilt the swashplate 90 degrees early or mount the pitch operating arms 90 degrees ahead of the blade axis. Unfortunately I applied a both to my second post because Lu wanted to refer to cyclic displacement rather than swashplate tilt and rather confused the very thing I was trying to clarify.
I can see you have been a student of the "In your face" school of communication - did you study with RW-1?
I was trying to show Lu that although we disagree on the Precession issue, he was arguing correctly about blade low/high being 90 degrees after the min/max pitch position.
I wanted to highlight that you either have to tilt the swashplate 90 degrees early or mount the pitch operating arms 90 degrees ahead of the blade axis. Unfortunately I applied a both to my second post because Lu wanted to refer to cyclic displacement rather than swashplate tilt and rather confused the very thing I was trying to clarify.
I can see you have been a student of the "In your face" school of communication - did you study with RW-1?
Guest
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To: Joe Pilot et Al.
First of all when I wrote that bit about the father and son I was visualizing you as the son. In other words sooner or later you would come to realize that the father was correct. As long as you stay in the UK or OZ you will always be right and I will be perceived as an idiot. However if you ever have to come the USA for training say at Sikorsky and they start talking about theory of flight they will have to put you in restraints because based on how you respond to me on this forum you will most likely become violent.
Now I’m going to tell you a story about the life cycle of a helicopter blade or should I better use the term rotational cycle as that would be proper English. What I am about to say is applicable to every counterclockwise rotor system with the exception of a Robinson and a Westland Lynx. It applies also to clockwise rotation blade systems but the clock positions of 9:00 and 3:00 are switched, otherwise everything is the same. All numbers denoting pitch settings are applicable to this description and do not apply to a specific helicopter. Another point, when I use the term maximum pitch change relative to collective I am not addressing rate of change as that is constant as it reflects the angular deflection of the swashplate. There are several other disclaimers one of, which is cyclic pitch change to counter tail rotor propeller effect, is not considered. Another is that the collective pitch once set will not change to compensate for translational lift. We will use a four-blade rotor for simplification.
1) The rotor system is at rest. We measure the basic collective pitch setting at the root of the blade and it is 6-degrees with the cyclic in neutral.
2) The collective range is set at 18-degrees.
3) The rotor is started and the helicopter is brought to a hover. If we could measure the collective pitch setting in a hover it is 18-degrees on all blades as they rotate.
4) The pilot pushes forward cyclic. If we could stop the rotor with the blades disposed over the longitudinal and lateral axes the pitch readings would be 3:00= 12-degrees, 12:00= 18-degrees, 9:00= 24-degrees and 6:00= 18-degrees.
5) At these respective positions 3:00 and 9:00 the blades have the maximum variation relative to the basic collective setting of 18-degrees and rate of change has nothing to do with it.
6) When the blades move from the respective positions of 3:00 and 9:00 the pitch will start to change until the blades are disposed over the longitudinal center line of the helicopter and at that time they will be at the full collective position of 18-degrees. They have increased and decreased pitch respectively.
Now try to visualize this. If what you say is true about the advancing blade flapping up and the retreating blade flapping down due to changing aerodynamic loads then for gods sake please explain how or what forces are involved to tip the disc down over the nose. You say the advancing blade flaps up and the retreating blade flaps down. How much is the flap up and flap down and why won’t the delta three effect cancel this movement. Nobody ever tells you how much, he or she just says it happens and the rest is left up to your imagination.
Another point to ponder, if the advancing blade is increasing pitch between 3:00 and 12:00 and the retreating blade is decreasing pitch between 9:00 and 6:00 how can the aerodynamic forces cause the disc to tilt down over the nose and up over the tail? The way you explain it the disc would rise over the nose and down over the tail if it were pure aerodynamics. Yet, when the pilot pushes forward cyclic the disc tips down over the nose. The next time you fly your helicopter, watch the tip path when you move the cyclic in any direction. If when you address flapping up and down in forward flight and the rotor disc tips back this phenomenon also results from an increased lift on one side of the disc causing the disc to blow back(FAA definition) the blowing back is caused by the gyroscopic precession effect.
In the United States where they respect the laws of physics and the laws as they apply to gyroscopic phenomenon they would look at it in this way. When the pilot first pushed forward cyclic he changed the pitch relationship between the advancing side and the retreating side. This creates a differential of lift due to the pitch differences and this equates to a force that is applied to a spinning rotor. The lift differential is just like the perturbing force on a gyroscope and the application of the greater force is on the left side of the disc. And like all good gyros the response will be 90-degrees later in the direction of rotation. Once the forces are stabilized (at the time the cyclic stopped moving) the perturbing force is removed and since the rotor exhibits the gyroscopic characteristic it will remain rigid in that position unless the pilot moves his cyclic or some aerodynamic generated force perturbs the rotor. In this case read Transverse flow effect or a lift imbalance cause during retreating blade stall or the lift imbalance resulting in blowback as described above.
I have said this many times and it still goes un- noticed. Do not look at the spinning rotor as individual blades. The disc moves as a unit and is a composite of the blades made up of that disc. The only time blades act individually and fly out of track the delta 3 pitch coupling will return the wayward blade.
One last thing, when you all went through pilot training 101 you must have been taught how the instruments on your panel work. How did they address gyroscopic precession as it applied to the specific gyro operated instruments? What you rely on for your compasses, your attitude indicator and your turn indicator and some not mentioned all work using gyros and they all work under exactly the same rules that govern your rotorheads.
I think if you cut back on the trifle and the spotted dick your system might clear up so that you can accept an idea that is foreign to you.
Here is another thought and that is the AH 56 Cheyenne was not controlled directly by the pilot when he moved his cyclic. When he did move the cyclic it actuated the servo and the output of the servo was coupled to a spring. The spring was in turn linked to the swashplate which was in turn linked to a very powerful gyroscope mounted on top of the rotor head. When the pilot made the perturbing force via his servo the gyro responded to its’ maximum deflection 90-degrees later and it was linked to the pitch horns on the blade and they responded 90-degrees later at least they did most of the time. Sometimes they would respond early or late and on two occasions they responded with such fury that the blades impacted the fuselage. On one ship it killed the pilot and on the other it destroyed a wind tunnel at Ames Labs in San Francisco, California.
Guess who designed the blades? Ray Prouty.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
First of all when I wrote that bit about the father and son I was visualizing you as the son. In other words sooner or later you would come to realize that the father was correct. As long as you stay in the UK or OZ you will always be right and I will be perceived as an idiot. However if you ever have to come the USA for training say at Sikorsky and they start talking about theory of flight they will have to put you in restraints because based on how you respond to me on this forum you will most likely become violent.
Now I’m going to tell you a story about the life cycle of a helicopter blade or should I better use the term rotational cycle as that would be proper English. What I am about to say is applicable to every counterclockwise rotor system with the exception of a Robinson and a Westland Lynx. It applies also to clockwise rotation blade systems but the clock positions of 9:00 and 3:00 are switched, otherwise everything is the same. All numbers denoting pitch settings are applicable to this description and do not apply to a specific helicopter. Another point, when I use the term maximum pitch change relative to collective I am not addressing rate of change as that is constant as it reflects the angular deflection of the swashplate. There are several other disclaimers one of, which is cyclic pitch change to counter tail rotor propeller effect, is not considered. Another is that the collective pitch once set will not change to compensate for translational lift. We will use a four-blade rotor for simplification.
1) The rotor system is at rest. We measure the basic collective pitch setting at the root of the blade and it is 6-degrees with the cyclic in neutral.
2) The collective range is set at 18-degrees.
3) The rotor is started and the helicopter is brought to a hover. If we could measure the collective pitch setting in a hover it is 18-degrees on all blades as they rotate.
4) The pilot pushes forward cyclic. If we could stop the rotor with the blades disposed over the longitudinal and lateral axes the pitch readings would be 3:00= 12-degrees, 12:00= 18-degrees, 9:00= 24-degrees and 6:00= 18-degrees.
5) At these respective positions 3:00 and 9:00 the blades have the maximum variation relative to the basic collective setting of 18-degrees and rate of change has nothing to do with it.
6) When the blades move from the respective positions of 3:00 and 9:00 the pitch will start to change until the blades are disposed over the longitudinal center line of the helicopter and at that time they will be at the full collective position of 18-degrees. They have increased and decreased pitch respectively.
Now try to visualize this. If what you say is true about the advancing blade flapping up and the retreating blade flapping down due to changing aerodynamic loads then for gods sake please explain how or what forces are involved to tip the disc down over the nose. You say the advancing blade flaps up and the retreating blade flaps down. How much is the flap up and flap down and why won’t the delta three effect cancel this movement. Nobody ever tells you how much, he or she just says it happens and the rest is left up to your imagination.
Another point to ponder, if the advancing blade is increasing pitch between 3:00 and 12:00 and the retreating blade is decreasing pitch between 9:00 and 6:00 how can the aerodynamic forces cause the disc to tilt down over the nose and up over the tail? The way you explain it the disc would rise over the nose and down over the tail if it were pure aerodynamics. Yet, when the pilot pushes forward cyclic the disc tips down over the nose. The next time you fly your helicopter, watch the tip path when you move the cyclic in any direction. If when you address flapping up and down in forward flight and the rotor disc tips back this phenomenon also results from an increased lift on one side of the disc causing the disc to blow back(FAA definition) the blowing back is caused by the gyroscopic precession effect.
In the United States where they respect the laws of physics and the laws as they apply to gyroscopic phenomenon they would look at it in this way. When the pilot first pushed forward cyclic he changed the pitch relationship between the advancing side and the retreating side. This creates a differential of lift due to the pitch differences and this equates to a force that is applied to a spinning rotor. The lift differential is just like the perturbing force on a gyroscope and the application of the greater force is on the left side of the disc. And like all good gyros the response will be 90-degrees later in the direction of rotation. Once the forces are stabilized (at the time the cyclic stopped moving) the perturbing force is removed and since the rotor exhibits the gyroscopic characteristic it will remain rigid in that position unless the pilot moves his cyclic or some aerodynamic generated force perturbs the rotor. In this case read Transverse flow effect or a lift imbalance cause during retreating blade stall or the lift imbalance resulting in blowback as described above.
I have said this many times and it still goes un- noticed. Do not look at the spinning rotor as individual blades. The disc moves as a unit and is a composite of the blades made up of that disc. The only time blades act individually and fly out of track the delta 3 pitch coupling will return the wayward blade.
One last thing, when you all went through pilot training 101 you must have been taught how the instruments on your panel work. How did they address gyroscopic precession as it applied to the specific gyro operated instruments? What you rely on for your compasses, your attitude indicator and your turn indicator and some not mentioned all work using gyros and they all work under exactly the same rules that govern your rotorheads.
I think if you cut back on the trifle and the spotted dick your system might clear up so that you can accept an idea that is foreign to you.
Here is another thought and that is the AH 56 Cheyenne was not controlled directly by the pilot when he moved his cyclic. When he did move the cyclic it actuated the servo and the output of the servo was coupled to a spring. The spring was in turn linked to the swashplate which was in turn linked to a very powerful gyroscope mounted on top of the rotor head. When the pilot made the perturbing force via his servo the gyro responded to its’ maximum deflection 90-degrees later and it was linked to the pitch horns on the blade and they responded 90-degrees later at least they did most of the time. Sometimes they would respond early or late and on two occasions they responded with such fury that the blades impacted the fuselage. On one ship it killed the pilot and on the other it destroyed a wind tunnel at Ames Labs in San Francisco, California.
Guess who designed the blades? Ray Prouty.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
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OK Lu let's start again.
1. Assume that the pitch operating arms are mounted to change the pitch of the blade exactly matching the tilt of the swashplate (no advance angle).
2. Assume that the swashplate tilts in the same direction as the cyclic is moved.
3. Use the same 18 degree collective pitch setting as you did.
4. Push the cyclic forward to set the same pitch settings as you had = 18 degrees at 3:00 and 9:00, 24 degrees at 6:00 and 18 degrees at 12:00.
Happy so far?
5. Start with a blade at 3:00 - as it moves towards 12:00 it sees a change in cyclic pitch that reduces tha AoA so the blade loses lift and begins to flap down.
6. At 12:00 it sees the minimum cyclic pitch setting and so has the least lift - it cannot start to flap up as it has 6 degrees less pitch than when it started to flap down. Therefore it continues to flap down past the 12:00 BUT at a reducing rate because the cyclic pitch is increasing back towards the neutral 18 degree setting at 9:00. THE BLADE IS AT IT'S LOWEST POINT AT 9:00 DUE TO PHASE LAG.
7. For the remaining 180 degrees the opposite happens and this is why maximum/minimum cyclic pitch must be applied 90 degrees before the desired blade high/low point. This is achieved by tilting the swashplate and/or mounting the pitch operating arms ahead of the blade feathering axis. THIS IS NOT A MYTH AND ALL HELICOPTERS (except oddballs like the one you described) USE VARIATIONS OF THIS IDEA.
8. The phase lag is only truly 90 degrees on a teetering head rotor system and the figure reduces as the flapping hinge is moved outboard (hinge offset).
The rate of cyclic pitch change is not constant although the rate of angular displacement is :
Draw a clock face with a horizontal line underneath it so 6:00 is touching the line. Draw vertical lines down from 1:00, 2:00, 3:00 and 4:00 to the horizontal line. From 12:00 to 1:00 is 30 degrees and so is 1:00 to 2:00 and 2:00 to 3:00 and 3:00 to 4:00. This is constant angular displacement.
Now look at the distances between each of the vertical lines where they cross the horizontal line - they are not equal. The distance between 12:00 and 1:00 is greater than 1:00 to 2:00 which is greater than 2:00 to 3:00. The pitch change arm moves up and down at different amounts depending on which 30 degree segment of the circle it is in and if plotted would produce a perfect sine wave.
On a swashplate system (known as a control orbit) the maximum rate of pitch change occurs during the 30 degrees arc either side of the maximum/minimum pitch setting which is surely proof enough that the blade cannot start to flap up again at 12:00 in the above example.
The only time the precession causes the 90 degree phase change is in a theoretical model of a rotor system in a vacuum where aerodynamic forces are completely discounted. This model is used to illustrate what subsequently happens when aerodynamic forces are introduced - ie the real world and not theoretical physics. The 2 effects have the same end state but only the aerodynamic effects are generally considered since we don't fly helicopters in space!
Please read this carefully Lu and then go and read Prouty/Padfield/Gessow and Myers. Then come back and tell me if anything they say is different to the above (it's not).
Or.. look at a helicopter rotor head and examine carefully where the swash plate tilts when you move the cyclic and also how far angularly displaced ahead of the blade are the pitch operating arms.
Sorry to go on but this does need sorting.
If you still disagree then I will e-mail you extracts from appropriate publications to show I am not making this up or blindly quoting Brit-Mil dogma.
1. Assume that the pitch operating arms are mounted to change the pitch of the blade exactly matching the tilt of the swashplate (no advance angle).
2. Assume that the swashplate tilts in the same direction as the cyclic is moved.
3. Use the same 18 degree collective pitch setting as you did.
4. Push the cyclic forward to set the same pitch settings as you had = 18 degrees at 3:00 and 9:00, 24 degrees at 6:00 and 18 degrees at 12:00.
Happy so far?
5. Start with a blade at 3:00 - as it moves towards 12:00 it sees a change in cyclic pitch that reduces tha AoA so the blade loses lift and begins to flap down.
6. At 12:00 it sees the minimum cyclic pitch setting and so has the least lift - it cannot start to flap up as it has 6 degrees less pitch than when it started to flap down. Therefore it continues to flap down past the 12:00 BUT at a reducing rate because the cyclic pitch is increasing back towards the neutral 18 degree setting at 9:00. THE BLADE IS AT IT'S LOWEST POINT AT 9:00 DUE TO PHASE LAG.
7. For the remaining 180 degrees the opposite happens and this is why maximum/minimum cyclic pitch must be applied 90 degrees before the desired blade high/low point. This is achieved by tilting the swashplate and/or mounting the pitch operating arms ahead of the blade feathering axis. THIS IS NOT A MYTH AND ALL HELICOPTERS (except oddballs like the one you described) USE VARIATIONS OF THIS IDEA.
8. The phase lag is only truly 90 degrees on a teetering head rotor system and the figure reduces as the flapping hinge is moved outboard (hinge offset).
The rate of cyclic pitch change is not constant although the rate of angular displacement is :
Draw a clock face with a horizontal line underneath it so 6:00 is touching the line. Draw vertical lines down from 1:00, 2:00, 3:00 and 4:00 to the horizontal line. From 12:00 to 1:00 is 30 degrees and so is 1:00 to 2:00 and 2:00 to 3:00 and 3:00 to 4:00. This is constant angular displacement.
Now look at the distances between each of the vertical lines where they cross the horizontal line - they are not equal. The distance between 12:00 and 1:00 is greater than 1:00 to 2:00 which is greater than 2:00 to 3:00. The pitch change arm moves up and down at different amounts depending on which 30 degree segment of the circle it is in and if plotted would produce a perfect sine wave.
On a swashplate system (known as a control orbit) the maximum rate of pitch change occurs during the 30 degrees arc either side of the maximum/minimum pitch setting which is surely proof enough that the blade cannot start to flap up again at 12:00 in the above example.
The only time the precession causes the 90 degree phase change is in a theoretical model of a rotor system in a vacuum where aerodynamic forces are completely discounted. This model is used to illustrate what subsequently happens when aerodynamic forces are introduced - ie the real world and not theoretical physics. The 2 effects have the same end state but only the aerodynamic effects are generally considered since we don't fly helicopters in space!
Please read this carefully Lu and then go and read Prouty/Padfield/Gessow and Myers. Then come back and tell me if anything they say is different to the above (it's not).
Or.. look at a helicopter rotor head and examine carefully where the swash plate tilts when you move the cyclic and also how far angularly displaced ahead of the blade are the pitch operating arms.
Sorry to go on but this does need sorting.
If you still disagree then I will e-mail you extracts from appropriate publications to show I am not making this up or blindly quoting Brit-Mil dogma.
Guest
Posts: n/a
How is disymmetrty of lift compensated for?
I know most text books I've read describe the advancing blade flapping up due to the increase in airspeed. The flapping up then reduces angle of attack because the now upward direction of the blade changes the relative wind. And the retreating blade basically does the opposite. While I argee that by itself this theory of flapping to equality is valid I don't see it being valid in the question of dysimmetry of lift with a helicopter in forward flight.
First of all the book says that the advancing blade flaps up and the retreating blade flaps down. If we simply look at a helicopter in forward flight we instantly see that this cannot be true. If the disc is tilted forward, and, as someone else stated earlier, the tip path plane remains at a constant angle to maintain thrust and lift the same, then how can the advancing blade be moving up? With the disc tilted forward the advancing blade is moving DOWNWARD and the retreating blade is moving UPWARD. This is the opposite of what the book says. Forward cyclic is what takes angle of attack away from the advancing blade and gives more to the retreating.
If you go back to the days of the gyro planes you will see where the theory of flapping to equality compensating for dysymmetry of lift came from. Juan de la cierva's gyros had no cyclic or any type of attitude control in their rotor systems, they also did not have any flapping hinges. So he came up with the idea of putting a flapping hinge on each blade theorizing that the blades would flap to equality thereby reducing an unwanted rolling moment (disymmetry of lift). It seems to work, because there was no cyclic in these designs. I think he did however design the flapping hinge at an angle to produce delta 3 pitch coupling. Think about the disc orientation on a gyro though. In forward flight it is tilted back and you can see that the advancing blade is flapping up and the retreating blade is descending, just like the book says. Maybe whoever wrote the book just figured this must apply to helicopters too. And everyone else who wrote a book just followed suit.
[This message has been edited by helisphere (edited 22 January 2001).]
I know most text books I've read describe the advancing blade flapping up due to the increase in airspeed. The flapping up then reduces angle of attack because the now upward direction of the blade changes the relative wind. And the retreating blade basically does the opposite. While I argee that by itself this theory of flapping to equality is valid I don't see it being valid in the question of dysimmetry of lift with a helicopter in forward flight.
First of all the book says that the advancing blade flaps up and the retreating blade flaps down. If we simply look at a helicopter in forward flight we instantly see that this cannot be true. If the disc is tilted forward, and, as someone else stated earlier, the tip path plane remains at a constant angle to maintain thrust and lift the same, then how can the advancing blade be moving up? With the disc tilted forward the advancing blade is moving DOWNWARD and the retreating blade is moving UPWARD. This is the opposite of what the book says. Forward cyclic is what takes angle of attack away from the advancing blade and gives more to the retreating.
If you go back to the days of the gyro planes you will see where the theory of flapping to equality compensating for dysymmetry of lift came from. Juan de la cierva's gyros had no cyclic or any type of attitude control in their rotor systems, they also did not have any flapping hinges. So he came up with the idea of putting a flapping hinge on each blade theorizing that the blades would flap to equality thereby reducing an unwanted rolling moment (disymmetry of lift). It seems to work, because there was no cyclic in these designs. I think he did however design the flapping hinge at an angle to produce delta 3 pitch coupling. Think about the disc orientation on a gyro though. In forward flight it is tilted back and you can see that the advancing blade is flapping up and the retreating blade is descending, just like the book says. Maybe whoever wrote the book just figured this must apply to helicopters too. And everyone else who wrote a book just followed suit.
[This message has been edited by helisphere (edited 22 January 2001).]
