Lead or lag ?
HeedM,
Coriolis effect is an inertial force described by the 19th-century French engineer-mathematician Gustave-Gaspard Coriolis in 1835. Coriolis showed that, if the ordinary Newtonian laws of motion of bodies are to be used in a rotating frame of reference, an inertial force--acting to the right of the direction of body motion for counterclockwise rotation of the reference frame or to the left for clockwise rotation--must be included in the equations of motion.
The effect of the Coriolis force is an apparent deflection of the path of an object that moves within a rotating coordinate system. The object does not actually deviate from its path, but it appears to do so because of the motion of the coordinate system.
I don't think the above (which was gleaned from an internet search) can be confused with conservation of angular momentum.
The blades C of G is trying to move towards the rotor hub as the blade flaps up - the only impact of coriolis would be the apparent movement to the right as it tried to do so (counterclockwise rotation viewed from above).
If you assume (rightly or wrongly) that relative to the rotor system this produces a leading tendency of the blade as it flaps up then that is why you need to modify the effects of conservation of angular momentum with this term.
Does the blade really try to move forwards on it's hinge due to coriolis or is it just apparent movement relative to the rotor hub?
Newtons Laws governing conservation of angular momentum would argue that the blade will speed up as the C of G moves inwards but would not explain why.
Maybe you are completely correct and Coriolis effect is the driving force behind the C of A M - I dunno I'm not a graduate just a pilot so if you do know please explain.
Coriolis effect is an inertial force described by the 19th-century French engineer-mathematician Gustave-Gaspard Coriolis in 1835. Coriolis showed that, if the ordinary Newtonian laws of motion of bodies are to be used in a rotating frame of reference, an inertial force--acting to the right of the direction of body motion for counterclockwise rotation of the reference frame or to the left for clockwise rotation--must be included in the equations of motion.
The effect of the Coriolis force is an apparent deflection of the path of an object that moves within a rotating coordinate system. The object does not actually deviate from its path, but it appears to do so because of the motion of the coordinate system.
I don't think the above (which was gleaned from an internet search) can be confused with conservation of angular momentum.
The blades C of G is trying to move towards the rotor hub as the blade flaps up - the only impact of coriolis would be the apparent movement to the right as it tried to do so (counterclockwise rotation viewed from above).
If you assume (rightly or wrongly) that relative to the rotor system this produces a leading tendency of the blade as it flaps up then that is why you need to modify the effects of conservation of angular momentum with this term.
Does the blade really try to move forwards on it's hinge due to coriolis or is it just apparent movement relative to the rotor hub?
Newtons Laws governing conservation of angular momentum would argue that the blade will speed up as the C of G moves inwards but would not explain why.
Maybe you are completely correct and Coriolis effect is the driving force behind the C of A M - I dunno I'm not a graduate just a pilot so if you do know please explain.
Iconoclast
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Who's on first?
To: imabel and Crab
"lead and lag, (hunting); the tendency for a rotor blade to hunt for the centre of pressure of the blade, forward on the advancing blade, (lead), and rearward on the retreating blade, (lag). simple aerodynamics".
The blade that flaps up is the retreating blade and the blade that flaps down is the advancing blade. Please note that the two statements above (quoted from your post) are diametrically opposed to each other. In the first quote (Coriolis effect) you state that the blade that flaps up accelerates and the blade that flaps down decelerates and in the second (lead and lag) you state that the advancing blade leads and the retreating blade lags which is what I have stated in my posts. Lead and lag results from the aerodynamic effect that causes the flapping but the actual movement of the blade about the lead-lag hinge is a purely mechanical effect that has nothing to do with aerodynamics.
Regarding my ignorance I have attended 18 helicopter factory schools and I have taught POF and I never heard of Hooke’s joint effect until I worked on the A-310 and had many English colleagues on the program. In the USA what you call a Hooke’s joint is called a universal joint.
coriolis effect; the acceleration of the blade that flaps up and deceleration of the blade that flaps down. simple aerodynamics.
The blade that flaps up is the retreating blade and the blade that flaps down is the advancing blade. Please note that the two statements above (quoted from your post) are diametrically opposed to each other. In the first quote (Coriolis effect) you state that the blade that flaps up accelerates and the blade that flaps down decelerates and in the second (lead and lag) you state that the advancing blade leads and the retreating blade lags which is what I have stated in my posts. Lead and lag results from the aerodynamic effect that causes the flapping but the actual movement of the blade about the lead-lag hinge is a purely mechanical effect that has nothing to do with aerodynamics.
Regarding my ignorance I have attended 18 helicopter factory schools and I have taught POF and I never heard of Hooke’s joint effect until I worked on the A-310 and had many English colleagues on the program. In the USA what you call a Hooke’s joint is called a universal joint.
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heedm,
In an earlier posting, you mentioned " ... look down the mast axis at the tip path plane. If they aren't perpendicular then the tip path plane will be an oval.".
Your statement applies to both a teetering rotor and an articulated rotor; but, it raises an interesting consideration. When looking down the tip path axis; the tip path plane of a teetering rotor will be a circle, whereas the tip path plane of an articulated rotor will be an oval.
This leads one to think that the Hooke's Joint Effect may not be truly representative of articulated rotors.
_________________
imabell
You may be oversimplifying things.
"coriolis effect; ..... deceleration of the blade that flaps down. simple aerodynamics."
True; but if the blade flaps down below the position of no coning angle (normal to the mast) the mass of the blade will start moving inward towards the mast, and this will result in both blades accelerating. Incidentally, when a teetering rotor tips, the undersling will result in mass of both blades immediately moving inward, and "wanting to' accelerate.
"hookes joint effect; ... a multi bladed fully articulated rotor ..."
A Universal Joint, a Hooke's Joint and a Cardan Joint are just different names for the same device. Actually, when referring to a helicopter rotor, the teetering and the flapping hinges might be more accurately refereed to as Knuckle Joints, not Hooke's Joints. This is because the teetering, the flapping and the knuckle joint have only one hinge, whereas the Hooke's joint has two hinges. Perhaps the original use of the phrase 'Hooke's Joint' came from the Bell 47, which had two hinges in its teetering rotorhead.
I believe that when viewing the activity of a tipped teetering disk, Cyclical Coriolis achieves the same results as the, so-called, Hooke's Joint. The algorithms of the two are different, but they appear to arrive at the same answer.
_____________
RotorRooter,
Ah, forget it.
It's no fun talking to oneself.
In an earlier posting, you mentioned " ... look down the mast axis at the tip path plane. If they aren't perpendicular then the tip path plane will be an oval.".
Your statement applies to both a teetering rotor and an articulated rotor; but, it raises an interesting consideration. When looking down the tip path axis; the tip path plane of a teetering rotor will be a circle, whereas the tip path plane of an articulated rotor will be an oval.
This leads one to think that the Hooke's Joint Effect may not be truly representative of articulated rotors.
_________________
imabell
You may be oversimplifying things.
"coriolis effect; ..... deceleration of the blade that flaps down. simple aerodynamics."
True; but if the blade flaps down below the position of no coning angle (normal to the mast) the mass of the blade will start moving inward towards the mast, and this will result in both blades accelerating. Incidentally, when a teetering rotor tips, the undersling will result in mass of both blades immediately moving inward, and "wanting to' accelerate.
"hookes joint effect; ... a multi bladed fully articulated rotor ..."
A Universal Joint, a Hooke's Joint and a Cardan Joint are just different names for the same device. Actually, when referring to a helicopter rotor, the teetering and the flapping hinges might be more accurately refereed to as Knuckle Joints, not Hooke's Joints. This is because the teetering, the flapping and the knuckle joint have only one hinge, whereas the Hooke's joint has two hinges. Perhaps the original use of the phrase 'Hooke's Joint' came from the Bell 47, which had two hinges in its teetering rotorhead.
I believe that when viewing the activity of a tipped teetering disk, Cyclical Coriolis achieves the same results as the, so-called, Hooke's Joint. The algorithms of the two are different, but they appear to arrive at the same answer.
_____________
RotorRooter,
Ah, forget it.
It's no fun talking to oneself.
Iconoclast
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Of all of the Hookes,Carden and universal joints I had to come into this joint.
Maybe this will help clarify this matter of what kind of joint we are discussing. On a fully articulated rotor there are two axes. The driving axis and the driven axis. Under ideal conditions in a hover these two axes are coincident with each other. When the pilot places a cyclic input (in any direction) the driven axis will deviate from the driving axis as a result of flapping and the flapping results in leading and lagging. Does the Hookes’ joint effect or the universal joint effect or the Carden joint effect cause this? The answer is no.
The characteristic of a Hookes’ joint is that it has the capability to drive off of the primary drive axis and that the resultant is that the driven shaft is not rotating at exactly the same speed as the driving shaft. However, the speed differential will be the same around the shaft rotation. The speed differential is dependent on the angular difference between the driving and driven shafts and this will continue until the Hookes’ joint locks up. However on the articulated rotorhead the speed differential is not constant as it varies around the rotational plane.
The lead and lag is the resultant of conservation of angular momentum resulting from the variation in the mass of the blade shifting relative to the driving axis during the flapping up and down.
But then again I never studied Newtonian Physics.
The characteristic of a Hookes’ joint is that it has the capability to drive off of the primary drive axis and that the resultant is that the driven shaft is not rotating at exactly the same speed as the driving shaft. However, the speed differential will be the same around the shaft rotation. The speed differential is dependent on the angular difference between the driving and driven shafts and this will continue until the Hookes’ joint locks up. However on the articulated rotorhead the speed differential is not constant as it varies around the rotational plane.
The lead and lag is the resultant of conservation of angular momentum resulting from the variation in the mass of the blade shifting relative to the driving axis during the flapping up and down.
But then again I never studied Newtonian Physics.
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Could someone explain why the effect of the earths rotation and the movement of air which is affected by that rotation (corriolis) is used to explain the lead/lag of a rotor blade during one cycle?
Main Rotor Blades are affected real or apparent by, Cconing, Lift, Drag, Density, Rotational speed, Surface area, Flapping, Pitch changes, Angles of attack, Profile (cross sectional design) and Bending. Tail Rotor Blades have addionally a Delta hinge.
My understanding is;-
When the rotors are turning at right angles to the rotor shaft and in still air, the forces on a blade are constant through one complete cycle.
Now introduce a forward cyclic command. This will cause the advancing blade to reduce pitch, reduce the A/A, reduce lift and therefore reduce drag. Does the blade velocity increase and therefore 'leads' due to the reduction in drag? Or because the blade has flapped down the radius has increased (coning angle decreased) that the blade slows down causing the blade to 'lag'?
Main Rotor Blades are affected real or apparent by, Cconing, Lift, Drag, Density, Rotational speed, Surface area, Flapping, Pitch changes, Angles of attack, Profile (cross sectional design) and Bending. Tail Rotor Blades have addionally a Delta hinge.
My understanding is;-
When the rotors are turning at right angles to the rotor shaft and in still air, the forces on a blade are constant through one complete cycle.
Now introduce a forward cyclic command. This will cause the advancing blade to reduce pitch, reduce the A/A, reduce lift and therefore reduce drag. Does the blade velocity increase and therefore 'leads' due to the reduction in drag? Or because the blade has flapped down the radius has increased (coning angle decreased) that the blade slows down causing the blade to 'lag'?
[email protected]
Yes you're right and I'm wrong, just never seen it that way before, thank you for correcting me and expanding my mind even more!
I wonder what will happen to my head the day it states that I can no longer fill it up like thi?
Lu
You stated that the advancing blade flaps down, but this is contrary to all the aerodynamics texts we use in ground school, and teach...
You may be right, but I simply don't see it. I thought the advancing blade had to flap UP to reduce the AOA, thus creating less lift, to balance out the Dissymmetry of lift?
May be right may be wrong, but please explain
Thanx for listening
Yes you're right and I'm wrong, just never seen it that way before, thank you for correcting me and expanding my mind even more!
I wonder what will happen to my head the day it states that I can no longer fill it up like thi?
Lu
You stated that the advancing blade flaps down, but this is contrary to all the aerodynamics texts we use in ground school, and teach...
You may be right, but I simply don't see it. I thought the advancing blade had to flap UP to reduce the AOA, thus creating less lift, to balance out the Dissymmetry of lift?
May be right may be wrong, but please explain
Thanx for listening
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To flap or not to flap or is it to lead or to lag.
To: Winnie
The upward flapping of the advancing blade you are addressing is also accompanied by downward flapping of the retreating blade. This occurs at about 20-25-knots during translational lift and is referred to as flap back or blow back depending on your religious persuasion. The technical term is Transverse Flow Effect.
If Frank Robinson is your religious leader then it is called "We-Wa".
To answer another post the movement of the blade is mechanical and has nothing to do with profile drag. The advancing blade leads where drag is at its' greatest.
The upward flapping of the advancing blade you are addressing is also accompanied by downward flapping of the retreating blade. This occurs at about 20-25-knots during translational lift and is referred to as flap back or blow back depending on your religious persuasion. The technical term is Transverse Flow Effect.
If Frank Robinson is your religious leader then it is called "We-Wa".
To answer another post the movement of the blade is mechanical and has nothing to do with profile drag. The advancing blade leads where drag is at its' greatest.
Last edited by Lu Zuckerman; 9th Apr 2003 at 09:55.
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Crab,
If you're sitting in the rotating reference frame, you observe a deflection of an object that is moved you call it Coriolis effect. Becuase you're sitting in that reference frame, you don't see that the object is rotating before it's moved, you don't see that the object is being pulled to a point that is beneath you (ie center of earth or the rotor hub) so you don't think about angular momentum.
If you're outside that reference frame in an inertial reference frame (non-rotating, non-accelerating...laws of inertia apply) then you see that the object has angular momentum prior to it moving. You also see the object has a constraint that causes it to deflect in an arc when pushed in a line. You realize that pushing the object imparts more rotation on it and thus to determine the net effect, you must conserve angular momentum and do a vector sum of the individual angular momenta.
Matthew.
If you're sitting in the rotating reference frame, you observe a deflection of an object that is moved you call it Coriolis effect. Becuase you're sitting in that reference frame, you don't see that the object is rotating before it's moved, you don't see that the object is being pulled to a point that is beneath you (ie center of earth or the rotor hub) so you don't think about angular momentum.
If you're outside that reference frame in an inertial reference frame (non-rotating, non-accelerating...laws of inertia apply) then you see that the object has angular momentum prior to it moving. You also see the object has a constraint that causes it to deflect in an arc when pushed in a line. You realize that pushing the object imparts more rotation on it and thus to determine the net effect, you must conserve angular momentum and do a vector sum of the individual angular momenta.
Matthew.
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Head Turner
coriolis happends when something moves closer to the spining axis its spinning around, eg wind heading south or north from the equater (closer to the poles) advances, ice scaters arms advance when pulled in and so do blades, they all work the same.
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Yeah but.....
To: vorticey
Are you saying that Coriolis only works when the skater moves his / her arms closer to their respective bodies causing the rotation of the skater to speed up. If that is the case what force is involved when the skater moves their arms outward causing the speed of rotation to slow down.
In most American POF texts and even in the FAA helicopter handbook they mention Coriolis force and then immediately go into conservation of angular momentum as the reason for the leading and lagging.
Here is another point to ponder. In the FAA Helicopter handbook they state that when a rotor blade flaps upward the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place. Conversely, when the blade flaps downward the center of mass moves further from the axis of rotation and blade deceleration takes place. As the blade moves past the longitudinal centerline of the helicopter it starts to climb. And, when the blade passes over the tail it starts to flap downward. The climbing blade is the retreating blade and the blade flapping downward is the advancing blade. Contrary to what is stated in the FAA handbook and many other POF texts the diving blade leads and the retreating blade lags. Try and reconcile that with the textbooks.
coriolis happends when something moves closer to the spining axis its spinning around, eg wind heading south or north from the equater (closer to the poles) advances, ice scaters arms advance when pulled in and so do blades, they all work the same.
In most American POF texts and even in the FAA helicopter handbook they mention Coriolis force and then immediately go into conservation of angular momentum as the reason for the leading and lagging.
Here is another point to ponder. In the FAA Helicopter handbook they state that when a rotor blade flaps upward the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place. Conversely, when the blade flaps downward the center of mass moves further from the axis of rotation and blade deceleration takes place. As the blade moves past the longitudinal centerline of the helicopter it starts to climb. And, when the blade passes over the tail it starts to flap downward. The climbing blade is the retreating blade and the blade flapping downward is the advancing blade. Contrary to what is stated in the FAA handbook and many other POF texts the diving blade leads and the retreating blade lags. Try and reconcile that with the textbooks.
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lu
i seem to remember a previese thred not so different to this one.
no lu, coriolis works both ways. when clowds move toward the equater they 'slow down'. skaters slow down when arms go out.
i think of it as more of a distance thing. skaters hands dont speed up when pulled in, they just travel the same distance in the same amount of time. if the hands are pulled in half way, the hands will need to travel twice aroud the body to cover the same distance in the same time. sure the body rpm increcese by 2. (angular momentum maybe?)
naturally in forward flight the advancing blade is flapping down and retreating flapping up to get the corect disc attitude. if you let it flap back you stop!
i would imagine the blade at 0' lagging and 180' leading. does this cause a movement of the mast toward the leading/lagging side?
no lu, coriolis works both ways. when clowds move toward the equater they 'slow down'. skaters slow down when arms go out.
i think of it as more of a distance thing. skaters hands dont speed up when pulled in, they just travel the same distance in the same amount of time. if the hands are pulled in half way, the hands will need to travel twice aroud the body to cover the same distance in the same time. sure the body rpm increcese by 2. (angular momentum maybe?)
naturally in forward flight the advancing blade is flapping down and retreating flapping up to get the corect disc attitude. if you let it flap back you stop!
i would imagine the blade at 0' lagging and 180' leading. does this cause a movement of the mast toward the leading/lagging side?
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Yeah but....
To: vorticey
Yes, but the texts say that the blades flapping up will lead and the blade flapping down will lag. The opposite is true. The blade flapping down will lead and the blade flapping up will lag. That is why I said it is a point to ponder.
naturally in forward flight the advancing blade is flapping down and retreating flapping up to get the corect disc attitude. if you let it flap back you stop!
