Dissymmetry of lift, flapping and blowback...
Baobab - you got it mostly correct apart from this bit
There is no precession or 'sensing' it is just flapping.
As the blade comes over the tail on the advancing side it begins to see more airspeed so it begins to flap up - by the time it is at the 3 o'clock position it has reached its maximum airspeed and therefore the maximum rate of flapping up.
From the 3 o'clock to the 12 o'clock the airspeed begins to reduce and so does the rate of flapping up until it runs out of momentum at the 12 o'clock position and the blade it at its highest point.
It is important to recognise that the blade has essentially the same airspeed at the 6 and 12 o'clock positions which is why, anywhere else in that arc, it has an increased airspeed and therefore wants to flap up.
however due to gyroscopic precession the increase in lift is sensed 90 degrees away
As the blade comes over the tail on the advancing side it begins to see more airspeed so it begins to flap up - by the time it is at the 3 o'clock position it has reached its maximum airspeed and therefore the maximum rate of flapping up.
From the 3 o'clock to the 12 o'clock the airspeed begins to reduce and so does the rate of flapping up until it runs out of momentum at the 12 o'clock position and the blade it at its highest point.
It is important to recognise that the blade has essentially the same airspeed at the 6 and 12 o'clock positions which is why, anywhere else in that arc, it has an increased airspeed and therefore wants to flap up.
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this subject is muddled unfortunately
Baobab:
You do need to be able to answer questions along the lines of Crabs answer if you need to pass exams however if you want to understand how this works it is useful to understand this:
Flapping (there are 2 definitions for this depending on reference plane) is NOT responsible for nullifying D of Lift) flapping, if it is employed, results in Attitude Change, it is this Attitude Change that provides the cue to the pilot to move the cyclic forward,holding the cyclic forward results in flatter pitch on the advancing side and coarser pitch on the retreating side - nullifying DoL. ... (no knee jerk new friend crab .. please)
Since the advancing side is running lower pitch and the retreating side coarser pitch - when the lever is raised this will have a proportionally greater effect on the advancing side (2 deg plus 1deg is bigger than 6 deg plus 1) so this will have an asymmetric effect resulting in a rate of Attitude Change for the disc. (Nose up)
Simples
You do need to be able to answer questions along the lines of Crabs answer if you need to pass exams however if you want to understand how this works it is useful to understand this:
Flapping (there are 2 definitions for this depending on reference plane) is NOT responsible for nullifying D of Lift) flapping, if it is employed, results in Attitude Change, it is this Attitude Change that provides the cue to the pilot to move the cyclic forward,holding the cyclic forward results in flatter pitch on the advancing side and coarser pitch on the retreating side - nullifying DoL. ... (no knee jerk new friend crab .. please)
Since the advancing side is running lower pitch and the retreating side coarser pitch - when the lever is raised this will have a proportionally greater effect on the advancing side (2 deg plus 1deg is bigger than 6 deg plus 1) so this will have an asymmetric effect resulting in a rate of Attitude Change for the disc. (Nose up)
Simples
Imagine the aircraft trimmed into straight and level flight at 60 kts - we are speed stable with a specific attitude and power selected to maintain that equilibrium.
Then raise the lever to add 2 degrees of pitch to all the blades - the advancing side has the highest airspeed and the retreating side the lowest - simple application of the lift formula tells you that the advancing side will flap up more due to higher V squared and the result (just like with blowback/flapback) is that the nose pitches up because that is what the disc has done.
Lowering the lever is the opposite - I believe it is called Instability with Angle of Attack.
Both effects are overcome by the pilot moving the cyclic in the appropriate direction to maintain the desired attitude and speed.
Note this is nothing to do with downwash on stabilisers or fuselage - those are separate effects dependent on aircraft type and design.
Many aircraft have adjustments applied within the control runs (mixing unit if you have one) that offsets these effects to reduce pilot workload.
Then raise the lever to add 2 degrees of pitch to all the blades - the advancing side has the highest airspeed and the retreating side the lowest - simple application of the lift formula tells you that the advancing side will flap up more due to higher V squared and the result (just like with blowback/flapback) is that the nose pitches up because that is what the disc has done.
Lowering the lever is the opposite - I believe it is called Instability with Angle of Attack.
Both effects are overcome by the pilot moving the cyclic in the appropriate direction to maintain the desired attitude and speed.
Note this is nothing to do with downwash on stabilisers or fuselage - those are separate effects dependent on aircraft type and design.
Many aircraft have adjustments applied within the control runs (mixing unit if you have one) that offsets these effects to reduce pilot workload.
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convergence...
Crab - I think you are coming around to my way of thinking?
"the advancing side has the highest airspeed" and therefore must have the lowest angle of attack because of the cyclic position "and the retreating side the lowest {airspeed}" and because of the forward cyclic a higher angle of attack...
pretty eh?
"the advancing side has the highest airspeed" and therefore must have the lowest angle of attack because of the cyclic position "and the retreating side the lowest {airspeed}" and because of the forward cyclic a higher angle of attack...
pretty eh?
I don't think I have ever questioned the fact that AoAs are always bigger on the advancing side than the retreating side - most basic textbooks have diagrams of the AoA distribution across the disc.
You do seem a bit hung up on the cyclic causing dissymmetry of lift though - what about a helicopter in a still air hover with all the AoA equal around the disc - no advancing or retreating side.
Then a gust of wind comes from the 12 o'clock - suddenly you have a difference in airspeed between the right side and the left side - guess what? the disc will flap back as it is aerodynamic effect that has caused this - BUT it is CORRECTED with cyclic (just like inflow roll).
The rotor is affected by aerodynamic inequalities (airspeed or inflow angle) and we use the cyclic to put the disc back where we want it. Now there will be transient effects caused by accelerations whilst moving the controls (this is what Frank Robinson was alluding to I believe) but cyclic movement doesn't cause flapback and inflow roll - it is the dissymmetry of lift which is a result of making the helicopter move in our chosen direction.
You do seem a bit hung up on the cyclic causing dissymmetry of lift though - what about a helicopter in a still air hover with all the AoA equal around the disc - no advancing or retreating side.
Then a gust of wind comes from the 12 o'clock - suddenly you have a difference in airspeed between the right side and the left side - guess what? the disc will flap back as it is aerodynamic effect that has caused this - BUT it is CORRECTED with cyclic (just like inflow roll).
The rotor is affected by aerodynamic inequalities (airspeed or inflow angle) and we use the cyclic to put the disc back where we want it. Now there will be transient effects caused by accelerations whilst moving the controls (this is what Frank Robinson was alluding to I believe) but cyclic movement doesn't cause flapback and inflow roll - it is the dissymmetry of lift which is a result of making the helicopter move in our chosen direction.
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getting there...
"the fact that AoAs are always bigger on the advancing side" I am sure you intended to write; smaller
I think we are getting there.
Using your example of the gust: I say the attitude change caused by the DoL is the cue to the pilot to move the stick to the position where the DoL is cancelled by assymetric pitch.
Most of these effects are generally not noticed(able) by the pilot since all the time he is making continuous corrections to the position of the cyclic such that there are no (enduring) dissymetries from any cause (Cliff recirculation, greater induced flow at the rear, or speed differences between advancing side and retreating side). If the pilot fails to hold the stick where it is required then an Attitude change will result. Most pilots are able to react such that attitude does not significantly change even in quite extreme gustiness, but gusts aren't really the point - the point is for any steady state condition the pilot is holding the cyclic in a position where all half cycle dissymetries are cancelled by 'Dissymetry of Pitch'
In your example the pilot is allowing the Dissymetry to last sufficiently for the rate of attitude change to result in a changed attitude - which of course he (eventually) will have to rectify, also with the cyclic.
The points about inertial effects and non teetering heads whilst true are partially a red herring since it is the cyclic which must be moved to counter-match any attitude changing effects which last for half a cycle.
I think we are getting there.
Using your example of the gust: I say the attitude change caused by the DoL is the cue to the pilot to move the stick to the position where the DoL is cancelled by assymetric pitch.
Most of these effects are generally not noticed(able) by the pilot since all the time he is making continuous corrections to the position of the cyclic such that there are no (enduring) dissymetries from any cause (Cliff recirculation, greater induced flow at the rear, or speed differences between advancing side and retreating side). If the pilot fails to hold the stick where it is required then an Attitude change will result. Most pilots are able to react such that attitude does not significantly change even in quite extreme gustiness, but gusts aren't really the point - the point is for any steady state condition the pilot is holding the cyclic in a position where all half cycle dissymetries are cancelled by 'Dissymetry of Pitch'
In your example the pilot is allowing the Dissymetry to last sufficiently for the rate of attitude change to result in a changed attitude - which of course he (eventually) will have to rectify, also with the cyclic.
The points about inertial effects and non teetering heads whilst true are partially a red herring since it is the cyclic which must be moved to counter-match any attitude changing effects which last for half a cycle.
Yes, I did mean to write smaller regarding AoA on the advancing side.n Doh!
Wrt the rest - you are just arguing from an unconventional standpoint - I am explaining why there is a dissymmetry of lift and you are stating that the pilot negates it with cyclic.
Both viewpoints are valid but pretty much everyone knows that you control a helicopter with the cyclic and collective - you are overcoming the aerodynamic factors using the controls.
Why you believe that your standpoint gives a deeper understanding of helicopter P of F is a mystery.
PS did you agree with my comments about advance angle and control orbits on the other thread?
Wrt the rest - you are just arguing from an unconventional standpoint - I am explaining why there is a dissymmetry of lift and you are stating that the pilot negates it with cyclic.
Both viewpoints are valid but pretty much everyone knows that you control a helicopter with the cyclic and collective - you are overcoming the aerodynamic factors using the controls.
Why you believe that your standpoint gives a deeper understanding of helicopter P of F is a mystery.
PS did you agree with my comments about advance angle and control orbits on the other thread?
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Why so important?
Yes I do agree.
Why so important?
Because the fundamental beauty of the helicopter is not being well understood and that is sad. It is sad when helicopter pilots refer to helicopters as angry palm trees and don't really know what they are doing.
It guides the pilot's style of control if he understands what he is controlling and why: Large displacements of cyclic to deal with Dissymetries which are big (and generally slow to develop) and small ultra fine displacements to create rates of attitude change to adopt required attitudes.
It is a major change in thinking to think of Flapback as the cue to the pilot to move the cyclic to counter any need for 'Flapping to Equality', as it is to think of the pilot function as selecting cyclic position such that effective lift is equal around the disk - don't YOU agree? The feedback mechanism is elegant and helps the pilot with the 'feel' of the function.
By not understanding how these fundamentals work for the pilot we run the risk of creating helicopters which must handle the way we mis-understand they should work. eg whether a cyclic commands a new attitude or a rate of attitude change.
Incidentally the various definitions of flapping don't help much and cause confusion - eg if you use one common definition that Flapping is the movement about the Flapping hinge (ie Angular change measured against the Hub Plane) it is most unhelpful since a (nil wind) hovering helicopter will show flapping in any chosen direction depending on CoG. And a helicopter in forward flight, if the fuselage is relatively horizontal, will show flapping in the opposite direction to that prescribed by the (flawed) classical Dissymetry of Lift argument.
When the Earth was the center of the universe by Papal decree astronomers had to go through agonisingly convoluted complication with their clear results in order not to fall fowl of the established body of opinion and avoid having their heads chopped off - we should be smarter than that in 'helicoptering'.
Why so important?
Because the fundamental beauty of the helicopter is not being well understood and that is sad. It is sad when helicopter pilots refer to helicopters as angry palm trees and don't really know what they are doing.
It guides the pilot's style of control if he understands what he is controlling and why: Large displacements of cyclic to deal with Dissymetries which are big (and generally slow to develop) and small ultra fine displacements to create rates of attitude change to adopt required attitudes.
It is a major change in thinking to think of Flapback as the cue to the pilot to move the cyclic to counter any need for 'Flapping to Equality', as it is to think of the pilot function as selecting cyclic position such that effective lift is equal around the disk - don't YOU agree? The feedback mechanism is elegant and helps the pilot with the 'feel' of the function.
By not understanding how these fundamentals work for the pilot we run the risk of creating helicopters which must handle the way we mis-understand they should work. eg whether a cyclic commands a new attitude or a rate of attitude change.
Incidentally the various definitions of flapping don't help much and cause confusion - eg if you use one common definition that Flapping is the movement about the Flapping hinge (ie Angular change measured against the Hub Plane) it is most unhelpful since a (nil wind) hovering helicopter will show flapping in any chosen direction depending on CoG. And a helicopter in forward flight, if the fuselage is relatively horizontal, will show flapping in the opposite direction to that prescribed by the (flawed) classical Dissymetry of Lift argument.
When the Earth was the center of the universe by Papal decree astronomers had to go through agonisingly convoluted complication with their clear results in order not to fall fowl of the established body of opinion and avoid having their heads chopped off - we should be smarter than that in 'helicoptering'.
I have read your last post several times and still don't understand what your standpoint is or how relevant it is to pilot training.
I don't know your experience but I have instructed for over 20 years most things from basic effects of controls through NVG, IF, formation, USLs, SAR, and many more and given that instruction to basic students all the way to the operational front-line including teaching trainee instructors.
In all that time and amongst the many, many pilots I have flown with, not one has failed to grasp how to fly a helicopter based on the theory of P of F that is in mainstream use. Control action is a function of demonstration and practice, not whether or not you know what is happening at the pitch links in great detail.
I just don't understand why you feel you have some magic secret ingredient that will suddenly open the eyes of students and make them fly better.
I have been brought on in my career by some truly outstanding individuals and I have tried to pass on their attitudes and knowledge - I don't know what you measure your instructional success by but the way we teach in the UK Mil (and in most other places in the world) seems to work just fine.
Do please try to explain your philosophy of instruction because you advocate a different direction without validating its quality.
I don't know your experience but I have instructed for over 20 years most things from basic effects of controls through NVG, IF, formation, USLs, SAR, and many more and given that instruction to basic students all the way to the operational front-line including teaching trainee instructors.
In all that time and amongst the many, many pilots I have flown with, not one has failed to grasp how to fly a helicopter based on the theory of P of F that is in mainstream use. Control action is a function of demonstration and practice, not whether or not you know what is happening at the pitch links in great detail.
I just don't understand why you feel you have some magic secret ingredient that will suddenly open the eyes of students and make them fly better.
I have been brought on in my career by some truly outstanding individuals and I have tried to pass on their attitudes and knowledge - I don't know what you measure your instructional success by but the way we teach in the UK Mil (and in most other places in the world) seems to work just fine.
Do please try to explain your philosophy of instruction because you advocate a different direction without validating its quality.
Avoid imitations
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I've been trying to follow this thread but it seems I have have no real idea how a helicopter flies, or how to fly one. I did have few lessons in the late 1970s but it's obviously much more complicated than I thought.
So where can I get some proper lessons, where someone can explain all this properly, I wonder?
So where can I get some proper lessons, where someone can explain all this properly, I wonder?
Hey, Arm, we went to the same school but you musta forgotten the words to "How to Phly a Phantom " - "arrange the knobs and dials in a pleasing and eye-catching manner".
I'll let you off this time, but the Pedant Police will be called in for any future violations.
I'll let you off this time, but the Pedant Police will be called in for any future violations.
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simpler really
Shy: "much more complicated than I thought."
- simpler than you thought - i think!
Since all these effects really become one.
and it's really just AscendCharlie's broomstick - it works anyway even if you don't actually know what you're doing.
- simpler than you thought - i think!
Since all these effects really become one.
and it's really just AscendCharlie's broomstick - it works anyway even if you don't actually know what you're doing.
Last edited by AnFI; 11th Sep 2013 at 13:28.
Hmmm. AnFI - I think we are now stuck in the same feedback loop that we have been on other threads - where I ask you to clarify your instructional qualifications and experience and explain your viewpoint clearly and then you answer with obtuse comments like the ones above.
PM me if you don't want to put it on here.
PM me if you don't want to put it on here.
Avoid imitations
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Shy: "much more complicated than I thought."
- simpler than you thought - i think!
Since all these effects really become one.
and it's really just AscendCharlie's broomstick - it works anyway even if you don't actually know what you're doing.
- simpler than you thought - i think!
Since all these effects really become one.
and it's really just AscendCharlie's broomstick - it works anyway even if you don't actually know what you're doing.
You have gone very quiet AnFI!
Still waiting....................................
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Holy **** ANFI - I am not sure what I have being doing all these years sitting on my furry seat cover with no idea what the bades are doing.
All I have been doing is rotating the nose, pulling up a bit of collective and putting the cyclic into like a, sort of, 3rd gear position like a car. Can you please explain why I have to do this. Is it because of the "Charlie Broomstick" effect??
DB
All I have been doing is rotating the nose, pulling up a bit of collective and putting the cyclic into like a, sort of, 3rd gear position like a car. Can you please explain why I have to do this. Is it because of the "Charlie Broomstick" effect??
DB