Flapping, Lead Lag, Hooke's Effect
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Flapping, Lead Lag, Hooke's Effect
As a private helo pilot with about 600 hours of experience, I’ve spent a bunch of time on and off thinking about rotor systems and how they operate. While I understood most of the bits and pieces, I never quite had a sense of how it all fit together, and there were a couple of things I didn’t understand at all.
I bought the Wagtendonk book recently, which helped a lot, and began writing stuff down as a way of reinforcing my understanding. What follows are the notes I wrote for my own benefit, which I am posting in hope of corrections if I got things wrong, and of helping someone else if I got things right. Items that are asterisked are things that I have thought through but not seen clearly explained or documented in my reading.
So: I begin by assuming a three bladed, fully articulated rotor head mounted to a fixed mast. The mast is perfectly rigid, is attached to the ground, and is more than several rotor spans long. I chose this setup to eliminate distractions caused by movement of the mast, by ground effect, and by the tail rotor. I’m also assuming the rotor head never gets out of balance, so I don’t have to worry about ground resonance.
At rest, the blades droop, as they are flexible by design.
As an engine begins to turn the mast counter-clockwise, several things happen, each of which I describe individually, although they occur simultaneously:
- Centrifugal force ‘stiffens’ the blades, such that at rough equilibrium, which I will call flight RPM, the blades seem to extend straight out from the mast at a right angle. This assumes a zero pitch angle (no lift generated). *In reality, the blades still droop a small amount, as gravity continues to exert a down force, and there is no offsetting up force.
- *Conservation of Angular Momentum (Coriolis Effect) will try to slow the blade as droop dissipates, because the blade center of gravity is moving away from the mast. This effect is overcome by the acceleration force of the engine.
- The inertia of the blades will cause them to lag (by way of the lead lag hinge) as they begin to accelerate. This is because the blades are being accelerated at the root, and resisted by inertia at the center of mass of the blade.
- When the blades reach rough equilibrium at flight RPM, the lag will diminish, both because the blades are subject to less acceleration, and because centrifugal force will act to center the blades at a right angle to the mast. Profile drag ensures that some acceleration will still be required, thus keeping the blades slightly lagged relative to the right angle position. Wagtendonk refers to the this as ‘the mean lag position’.
As pitch is applied, several things happen, each of which I will again describe individually:
- Lift is generated by the blades. The blades begin to cone. Coning is a product of lift acting at a right angle to centrifugal force. The greater the lift, the greater the coning. Coning has nothing to do with the weight of the helicopter, save that when a helicopter is heavier, more lift will be required to get it off the ground, and hence the observed coning at takeoff will be greater.
- As the blades cone, the blade center of gravity moves closer to the mast. Coriolis effect may initially accelerate the blades slightly, such that they move forward (lead) on the lead lag hinge. Forward movement beyond the right angle position is resisted by centrifugal force.
- *Induced drag develops, which will decelerate the blades. This deceleration is offset by the acceleration force of the engine. This combination of acceleration at the root and induced drag at the center of lift will cause the blade to lag. I will call this combination of forces a ‘lag couple’.
- *At rough equilibrium, with the rotor disk exerting a fixed lift, the blades should lag more than at the mean lag position, as additional acceleration is required to offset the induced drag added to the profile drag.
- The tip path plane should remain parallel to the mast axis of rotation.
Now let’s blow a 30 knot wind across the rotor disk from a direction I arbitrarily call the front. This simulates a helicopter beginning to move forward in flight. For the moment, I will ignore Hooke’s Joint Effect i.e I will assume that even as the rotor disk tilts relative to the mast, the mast is still spinning the disk at a constant velocity (more on this below):
- As undisturbed air (air not acted upon by the rotor system) blows across the rotor system, induced drag will decrease, and lift will increase. Hence the rotor system will operate more efficiently. This phenomena is known as Translational Lift.
- As the air is only truly undisturbed at the front of the rotor system, the rotor system will operate more efficiently (generate relatively greater lift) at the front than at the rear. Due to phase lag, a counter-clockwise rotating rotor system will tilt to the right as observed from the rear. This phenomena is known as Transverse Flow Effect.
- The advancing blade will experience a higher velocity relative wind than the retreating blade. The advancing blade will therefore develop greater lift, and flap up. As the blade flaps up, the relative wind shifts slightly such that the angle of attack declines, and the increase in lift is reduced. The up flap is caused by increased lift, and is resisted by gravity and centrifugal force.
- The retreating blade will experience a lower velocity relative wind than the advancing blade. The retreating blade will therefore develop less lift, and flap down. As the blade flaps down, the relative wind shifts slightly such that the angle of attack increases, and the decrease in lift is reduced. The down flap is caused by gravity and centrifugal force, and is resisted by the residual lift.
- If forward is thought of as the 12 o’clock position, the greatest increase in lift will be at the 3 o’clock position, and the greatest decrease at the 9 o’clock position. Due to phase lag, the rotor system will tilt to the rear. This phenomena is known as Blowback.
- *As the advancing blade rises, the center of gravity moves closer to the mast, and Coriolis effect may initially accelerate the blade slightly, such that it moves forward on the lead lag hinge.
- *As the retreating blade falls, the center of gravity moves farther from the mast, and Coriolis effect may initially decelerate the blade slightly, such that it moves backward on the lead lag hinge.
- *The easiest way for me to visualize the overall movement of the disk relative to the flapping and lead lag hinges is to think of the tip path plane as supporting a solid plate. Prior to the introduction of wind, the plate was centered directly over the mast, and the plane of rotation of the plate was parallel to the plane of rotation of the mast. Ignoring Transverse Flow Effect for the moment, Blowback causes the disk to tilt backward. In my mind, I think about simply tilting the plate backward, such that the plate is no longer centered over the mast, and the the plane of rotation of the plate is no longer parallel to the plane of rotation of the mast. It’s relatively easy to realize that for the tips to stay in contact with the plate, a blade at the 12 o’clock position would have to flap up, a blade at the 6 o’clock position would have to flap down, a blade at the 3 o’clock position would have to lag on the lead lag hinge, and a blade at the 9 o’clock position would have to lead on the lead lag hinge. Wagtendonk makes this same point by referencing Hooke’s Joint Effect, which I’m not quite sure is correct.
- *An intuitive understanding of why the 3 o’clock (advancing) blade lags is that it is developing greater lift, which leads to greater induced drag, which requires more acceleration at the root, which creates the ‘lag couple’ described above. The reverse is true of the 9 o’clock (retreating) blade.
- *Hooke’s Joint Effect. Hooke’s Joint is simply another name for a universal joint. The underlying principle here is that if two shafts are connected by a universal joint, and if those shafts are not parallel, then when the driving shaft spins at a constant velocity, the driven shaft will turn at a non-constant velocity. More specifically, the output shaft velocity relative to the input shaft velocity will be low-high/high-low/low-high/high-low over the period of a single rotation, with each shaft having achieved 90 degrees of rotation after each low-high or high-low episode. Thinking back to the plate supported by the tip plane path that I imagined above, the axis of rotation of the mast represents the driving shaft, and the axis of rotation of the plate represents the driven shaft. As rotational force (acceleration) is applied unequally to the rotor blades across each 90 degrees of rotation, there must be some resultant lead lag movement of the blades.
- *Wagtendonk says: “Although Hooke’s joint effect causes movement on the lead-lag hinge almost continuously during a blade’s rotation, most of the effect occurs as the blade passes the axis of disk tilt, that is, the middle of the advancing and retreating sides.” While I agree with the first half of the sentence, I do not agree with the second half. Again, Wagtendonk seems to claim that the extreme lagging of the advancing blade at the 3 o’clock position, and the extreme leading of the retreating blade at the 9 o’clock position, is because of Hooke’s Joint Effect. I believe that it is because of increases or decreases in the ‘lag couple’.
Now let’s imagine that the mast, although still attached to the ground, and still rotating at a constant velocity, is permitted to move.
- As the lift vector is perpendicular to the backwards tilted rotor disk, the mast should tilt backwards, and become parallel to the lift vector.
- *The mast should also tilt backward and become parallel to the lift vector because of the uneven centripetal force acting on the mast. The blades are spinning at a constant rate, but the center of gravity of a rear, lower blade is further from the mast than the center of gravity of a forward, higher blade. This dissymmetry of force should tug the mast backwards.
(Note in a real helicopter, when the rotor disk tilts forward, the mast tilts forward as well, both for the reasons described above, and because of the coupled movement created by the lift vector pulling the rotor disk forward, and profile drag pushing the fuselage rearward.)
Now let’s apply cyclic.
- The point of applying cyclic is that the pilot increases or decreases pitch at various points in the blade disk rotation such that the blade disk - and the resultant lift vector - tilt in the direction of movement desired. All of the resultant effects on the rotor system have already been described above.
A couple of other things:
Ground Effect: Lift is a function of low pressure on top of the blade, and high pressure underneath the blade. When hovering close to the ground, downwash creates an artificially high pressure beneath the blade, thus increasing the rotor systems efficiency.
Recirculation: If hovering in ground effect close to a solid structure like a hangar, the downwash attempting to escape horizontally from beneath the helicopter may encounter the solid structure and be forced upward. As the downwash rises, it can get drawn back into the rotor system, creating a circular airflow with an ever increasing velocity. The downward velocity of this airflow into the rotor system reduces the angle of attack, thus decreasing lift, and can force the helicopter into a hard landing.
Thoughts welcomed.
I bought the Wagtendonk book recently, which helped a lot, and began writing stuff down as a way of reinforcing my understanding. What follows are the notes I wrote for my own benefit, which I am posting in hope of corrections if I got things wrong, and of helping someone else if I got things right. Items that are asterisked are things that I have thought through but not seen clearly explained or documented in my reading.
So: I begin by assuming a three bladed, fully articulated rotor head mounted to a fixed mast. The mast is perfectly rigid, is attached to the ground, and is more than several rotor spans long. I chose this setup to eliminate distractions caused by movement of the mast, by ground effect, and by the tail rotor. I’m also assuming the rotor head never gets out of balance, so I don’t have to worry about ground resonance.
At rest, the blades droop, as they are flexible by design.
As an engine begins to turn the mast counter-clockwise, several things happen, each of which I describe individually, although they occur simultaneously:
- Centrifugal force ‘stiffens’ the blades, such that at rough equilibrium, which I will call flight RPM, the blades seem to extend straight out from the mast at a right angle. This assumes a zero pitch angle (no lift generated). *In reality, the blades still droop a small amount, as gravity continues to exert a down force, and there is no offsetting up force.
- *Conservation of Angular Momentum (Coriolis Effect) will try to slow the blade as droop dissipates, because the blade center of gravity is moving away from the mast. This effect is overcome by the acceleration force of the engine.
- The inertia of the blades will cause them to lag (by way of the lead lag hinge) as they begin to accelerate. This is because the blades are being accelerated at the root, and resisted by inertia at the center of mass of the blade.
- When the blades reach rough equilibrium at flight RPM, the lag will diminish, both because the blades are subject to less acceleration, and because centrifugal force will act to center the blades at a right angle to the mast. Profile drag ensures that some acceleration will still be required, thus keeping the blades slightly lagged relative to the right angle position. Wagtendonk refers to the this as ‘the mean lag position’.
As pitch is applied, several things happen, each of which I will again describe individually:
- Lift is generated by the blades. The blades begin to cone. Coning is a product of lift acting at a right angle to centrifugal force. The greater the lift, the greater the coning. Coning has nothing to do with the weight of the helicopter, save that when a helicopter is heavier, more lift will be required to get it off the ground, and hence the observed coning at takeoff will be greater.
- As the blades cone, the blade center of gravity moves closer to the mast. Coriolis effect may initially accelerate the blades slightly, such that they move forward (lead) on the lead lag hinge. Forward movement beyond the right angle position is resisted by centrifugal force.
- *Induced drag develops, which will decelerate the blades. This deceleration is offset by the acceleration force of the engine. This combination of acceleration at the root and induced drag at the center of lift will cause the blade to lag. I will call this combination of forces a ‘lag couple’.
- *At rough equilibrium, with the rotor disk exerting a fixed lift, the blades should lag more than at the mean lag position, as additional acceleration is required to offset the induced drag added to the profile drag.
- The tip path plane should remain parallel to the mast axis of rotation.
Now let’s blow a 30 knot wind across the rotor disk from a direction I arbitrarily call the front. This simulates a helicopter beginning to move forward in flight. For the moment, I will ignore Hooke’s Joint Effect i.e I will assume that even as the rotor disk tilts relative to the mast, the mast is still spinning the disk at a constant velocity (more on this below):
- As undisturbed air (air not acted upon by the rotor system) blows across the rotor system, induced drag will decrease, and lift will increase. Hence the rotor system will operate more efficiently. This phenomena is known as Translational Lift.
- As the air is only truly undisturbed at the front of the rotor system, the rotor system will operate more efficiently (generate relatively greater lift) at the front than at the rear. Due to phase lag, a counter-clockwise rotating rotor system will tilt to the right as observed from the rear. This phenomena is known as Transverse Flow Effect.
- The advancing blade will experience a higher velocity relative wind than the retreating blade. The advancing blade will therefore develop greater lift, and flap up. As the blade flaps up, the relative wind shifts slightly such that the angle of attack declines, and the increase in lift is reduced. The up flap is caused by increased lift, and is resisted by gravity and centrifugal force.
- The retreating blade will experience a lower velocity relative wind than the advancing blade. The retreating blade will therefore develop less lift, and flap down. As the blade flaps down, the relative wind shifts slightly such that the angle of attack increases, and the decrease in lift is reduced. The down flap is caused by gravity and centrifugal force, and is resisted by the residual lift.
- If forward is thought of as the 12 o’clock position, the greatest increase in lift will be at the 3 o’clock position, and the greatest decrease at the 9 o’clock position. Due to phase lag, the rotor system will tilt to the rear. This phenomena is known as Blowback.
- *As the advancing blade rises, the center of gravity moves closer to the mast, and Coriolis effect may initially accelerate the blade slightly, such that it moves forward on the lead lag hinge.
- *As the retreating blade falls, the center of gravity moves farther from the mast, and Coriolis effect may initially decelerate the blade slightly, such that it moves backward on the lead lag hinge.
- *The easiest way for me to visualize the overall movement of the disk relative to the flapping and lead lag hinges is to think of the tip path plane as supporting a solid plate. Prior to the introduction of wind, the plate was centered directly over the mast, and the plane of rotation of the plate was parallel to the plane of rotation of the mast. Ignoring Transverse Flow Effect for the moment, Blowback causes the disk to tilt backward. In my mind, I think about simply tilting the plate backward, such that the plate is no longer centered over the mast, and the the plane of rotation of the plate is no longer parallel to the plane of rotation of the mast. It’s relatively easy to realize that for the tips to stay in contact with the plate, a blade at the 12 o’clock position would have to flap up, a blade at the 6 o’clock position would have to flap down, a blade at the 3 o’clock position would have to lag on the lead lag hinge, and a blade at the 9 o’clock position would have to lead on the lead lag hinge. Wagtendonk makes this same point by referencing Hooke’s Joint Effect, which I’m not quite sure is correct.
- *An intuitive understanding of why the 3 o’clock (advancing) blade lags is that it is developing greater lift, which leads to greater induced drag, which requires more acceleration at the root, which creates the ‘lag couple’ described above. The reverse is true of the 9 o’clock (retreating) blade.
- *Hooke’s Joint Effect. Hooke’s Joint is simply another name for a universal joint. The underlying principle here is that if two shafts are connected by a universal joint, and if those shafts are not parallel, then when the driving shaft spins at a constant velocity, the driven shaft will turn at a non-constant velocity. More specifically, the output shaft velocity relative to the input shaft velocity will be low-high/high-low/low-high/high-low over the period of a single rotation, with each shaft having achieved 90 degrees of rotation after each low-high or high-low episode. Thinking back to the plate supported by the tip plane path that I imagined above, the axis of rotation of the mast represents the driving shaft, and the axis of rotation of the plate represents the driven shaft. As rotational force (acceleration) is applied unequally to the rotor blades across each 90 degrees of rotation, there must be some resultant lead lag movement of the blades.
- *Wagtendonk says: “Although Hooke’s joint effect causes movement on the lead-lag hinge almost continuously during a blade’s rotation, most of the effect occurs as the blade passes the axis of disk tilt, that is, the middle of the advancing and retreating sides.” While I agree with the first half of the sentence, I do not agree with the second half. Again, Wagtendonk seems to claim that the extreme lagging of the advancing blade at the 3 o’clock position, and the extreme leading of the retreating blade at the 9 o’clock position, is because of Hooke’s Joint Effect. I believe that it is because of increases or decreases in the ‘lag couple’.
Now let’s imagine that the mast, although still attached to the ground, and still rotating at a constant velocity, is permitted to move.
- As the lift vector is perpendicular to the backwards tilted rotor disk, the mast should tilt backwards, and become parallel to the lift vector.
- *The mast should also tilt backward and become parallel to the lift vector because of the uneven centripetal force acting on the mast. The blades are spinning at a constant rate, but the center of gravity of a rear, lower blade is further from the mast than the center of gravity of a forward, higher blade. This dissymmetry of force should tug the mast backwards.
(Note in a real helicopter, when the rotor disk tilts forward, the mast tilts forward as well, both for the reasons described above, and because of the coupled movement created by the lift vector pulling the rotor disk forward, and profile drag pushing the fuselage rearward.)
Now let’s apply cyclic.
- The point of applying cyclic is that the pilot increases or decreases pitch at various points in the blade disk rotation such that the blade disk - and the resultant lift vector - tilt in the direction of movement desired. All of the resultant effects on the rotor system have already been described above.
A couple of other things:
Ground Effect: Lift is a function of low pressure on top of the blade, and high pressure underneath the blade. When hovering close to the ground, downwash creates an artificially high pressure beneath the blade, thus increasing the rotor systems efficiency.
Recirculation: If hovering in ground effect close to a solid structure like a hangar, the downwash attempting to escape horizontally from beneath the helicopter may encounter the solid structure and be forced upward. As the downwash rises, it can get drawn back into the rotor system, creating a circular airflow with an ever increasing velocity. The downward velocity of this airflow into the rotor system reduces the angle of attack, thus decreasing lift, and can force the helicopter into a hard landing.
Thoughts welcomed.
Ground Effect: Lift is a function of low pressure on top of the blade, and high pressure underneath the blade. When hovering close to the ground, downwash creates an artificially high pressure beneath the blade, thus increasing the rotor systems efficiency.
Recirculation: If hovering in ground effect close to a solid structure like a hangar, the downwash attempting to escape horizontally from beneath the helicopter may encounter the solid structure and be forced upward. As the downwash rises, it can get drawn back into the rotor system, creating a circular airflow with an ever increasing velocity. The downward velocity of this airflow into the rotor system reduces the angle of attack, thus decreasing lift, and can force the helicopter into a hard landing.
Thoughts welcomed.
Recirculation: If hovering in ground effect close to a solid structure like a hangar, the downwash attempting to escape horizontally from beneath the helicopter may encounter the solid structure and be forced upward. As the downwash rises, it can get drawn back into the rotor system, creating a circular airflow with an ever increasing velocity. The downward velocity of this airflow into the rotor system reduces the angle of attack, thus decreasing lift, and can force the helicopter into a hard landing.
Thoughts welcomed.
But you are not quite right on ground effect, it is not a pressure increase below the disc, rather it is a change of direction of the downwash, which reduces the induced drag, giving the disc system apparently more power, or the ability to hover with less collective pitch.
And recirculation, which I have never experienced or seen in 45 years of choppering, should have a different result from what is commonly accepted. People say that because the lift is reduced on the side next to the obstacle, be it a hangar or a cliff, the aircraft should be drawn into that obstacle.
But the loss of lift happens at one point, and with phase lag (glad you didn't talk of precession) the result should happen 90 degrees or so later. This would make the aircraft drift PARALLEL to the obstacle.
Riddle me that one.
Experienced recirculation in Northern Ireland landing in a wriggly tin fort in S Armagh - as the rotor got down towards the top of the walls we basically fell out of the sky with the lever coming up rapidly - than goodness for the Wessex undercarriage!
LGV9 - as for the rest, you are rather over thinking things unless you plan to be a test pilot or helicopter designer.
However when you add your 30 kt wind, don't forget that flapback (blowback) and inflow roll (transverse flow) occur before the onset of translational lift and both have to be countered with cyclic input so that you can continue to accelerate to translational lift.
To help visualise the hookes joint effect, make a shallow paper cone and draw 4 equally spaced blades/lines from the centre to the circumference. Look straight down at it - to see all the blades exactly equally spaced - and then , without moving your viewpoint, tilt the cone and see how the blades appear.
LGV9 - as for the rest, you are rather over thinking things unless you plan to be a test pilot or helicopter designer.
However when you add your 30 kt wind, don't forget that flapback (blowback) and inflow roll (transverse flow) occur before the onset of translational lift and both have to be countered with cyclic input so that you can continue to accelerate to translational lift.
To help visualise the hookes joint effect, make a shallow paper cone and draw 4 equally spaced blades/lines from the centre to the circumference. Look straight down at it - to see all the blades exactly equally spaced - and then , without moving your viewpoint, tilt the cone and see how the blades appear.
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Lift according the Newtonian explanation.
Air is moved downwards, so there has to be an equal and opposite force.
Air is accelerated downwards: F = m x a
Acceleration is key.
Induced flow requires a greater vertical flow through the rotor therefore requiring an increased pitch angle when out of ground effect.
Flying close to the ground reduces the induced flow, therefore increasing the effective angle of attack, without changing pitch angle (collective) and without requiring additional power.
More explanation and illustration:
Helicopter Aviation
Ground effect and lift in general has not so much to do with pressure differences.
More on lift:
Air is moved downwards, so there has to be an equal and opposite force.
Air is accelerated downwards: F = m x a
Acceleration is key.
Induced flow requires a greater vertical flow through the rotor therefore requiring an increased pitch angle when out of ground effect.
Flying close to the ground reduces the induced flow, therefore increasing the effective angle of attack, without changing pitch angle (collective) and without requiring additional power.
More explanation and illustration:
Helicopter Aviation
Ground effect and lift in general has not so much to do with pressure differences.
More on lift:
Two clearly intelligent gentlemen but they both have weaknesses in their 'theories' that they gloss over in order to try and prove their points.
They left me with more questions than answers and the idea that some people are too clever for their own good and don't know when to say 'we don't really know why this happens'.
They left me with more questions than answers and the idea that some people are too clever for their own good and don't know when to say 'we don't really know why this happens'.
OOOOHhhh you'll go straight to hell for that one
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? er 212 are you sayin that the PoF question that MightyGem* asked the Man from Boscome was THAT question?
If so then hardly surprising he was unable to answer it, there being no correct answer to that question AFAIK .
MG I was just genuinely interested to know what part of PoF the MfB does not know?
*(who notably acheived a great result with a tail rotor issue once)
If so then hardly surprising he was unable to answer it, there being no correct answer to that question AFAIK .
MG I was just genuinely interested to know what part of PoF the MfB does not know?
*(who notably acheived a great result with a tail rotor issue once)
MG I was just genuinely interested to know what part of PoF the MfB does not know?
On a list to Boscombe, I asked the TP about it and that was his answer.
Similar to its behaviour in a turning quickstop to the right where you had to use left cyclic to stop it rolling in any further and even more left cyclic to get it to roll out again.
All down to the odd pitch-roll coupling effects most have seen on the Lynx - and possibly down to a combination of the high AoA that the retreating side of the disc can operate at (thanks to Berp blades) and the high effective hinge offset that gives such excellent roll and pitch response.
Noticed a similar thing - but the other way - on an Airbus helo during an airtest.
All down to the odd pitch-roll coupling effects most have seen on the Lynx - and possibly down to a combination of the high AoA that the retreating side of the disc can operate at (thanks to Berp blades) and the high effective hinge offset that gives such excellent roll and pitch response.
Noticed a similar thing - but the other way - on an Airbus helo during an airtest.
the orientation of the swashplate
All down to the odd pitch-roll coupling effects most have seen on the Lynx - and possibly down to a combination of the high AoA that the retreating side of the disc can operate at (thanks to Berp blades) and the high effective hinge offset that gives such excellent roll and pitch response.
Dunno - I only ever demoed inflow roll and flapback with the ASE out and it always went towards the advancing side as normal.
Trouble with the Lynx was you also had the effects of the interlinks to consider.
Trouble with the Lynx was you also had the effects of the interlinks to consider.
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interesting
so how does it transmit control inputs to vary pitch cyclically? (not swashplate?)
is the control axis aligned with the controlled axis or is it offset? (and by how many degrees/)
so how does it transmit control inputs to vary pitch cyclically? (not swashplate?)
is the control axis aligned with the controlled axis or is it offset? (and by how many degrees/)
It has a spider.
As for the other bits, try asking Westlands - it's not the depth of geekery one needs to know in order to fly it.
As for the other bits, try asking Westlands - it's not the depth of geekery one needs to know in order to fly it.
MightyGem - looking through some old documents, I found a reply from Ray Prouty to an email a colleague had sent about cross-coupling, explaining that the control response from the semi-rigid rotor meant that phase lag was not 90 degrees but more like 80 degrees. He then mentioned that in steady pitch or roll rate (rather than accelerative), that figure might be as low as 60 degrees and doesn't depend on the hub geometry but the Lock number (ratio of aerodynamic parameters of the blade and its flapping inertia).
I don't know whether that would account for the steady state drift into the turn to the right but the Lynx dynamics were rather complex and anything is possible.
I don't know whether that would account for the steady state drift into the turn to the right but the Lynx dynamics were rather complex and anything is possible.
Dunno - I only ever demoed inflow roll and flapback with the ASE out and it always went towards the advancing side as normal.
I found a reply from Ray Prouty to an email a colleague had sent about cross-coupling, explaining that the control response from the semi-rigid rotor meant that phase lag was not 90 degrees but more like 80 degrees. He then mentioned that in steady pitch or roll rate (rather than accelerative), that figure might be as low as 60 degrees and doesn't depend on the hub geometry but the Lock number (ratio of aerodynamic parameters of the blade and its flapping inertia).
