Helicopter Dynamics: Gyroscopic Precession
Awblain - Cierva couldn't understand why his models flew successfully but the full size aircraft rolled over - eventually he realised that the rattan that he made the model's blades from was flexible, whereas his carefully braced full size rotor wasn't.
I think the blades tilted independently from the hub rather than the other way round
Carry on with your angular momentum theories but I think you have been pipped to the post for this year's Nobel prize by the guys validating the Big Bang theory analysing ripples within ripples of decaying light. - Their work is about as much use to a helicopter pilot as your theories of blade movement
I think the blades tilted independently from the hub rather than the other way round
Carry on with your angular momentum theories but I think you have been pipped to the post for this year's Nobel prize by the guys validating the Big Bang theory analysing ripples within ripples of decaying light. - Their work is about as much use to a helicopter pilot as your theories of blade movement
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Crab: "so you accept that your statement above was complete tosh then(?Ed)" Er, no.
Me:"there are 2 things (i can think of) that control force is needed for: 1 to overcome the moment of CoP at Arm from Pitch Axis and 2 to rotate the blades about their Pitch Change Axis - which is bigger if there is a larger polar moment of inertia around the pitch change axis , as you might well find for a larger chord blade (H500 compared to Gazelle for instance). I never really bought the story about the CoP not moving for symmetrical aerofoils anyway ... you can certainly feel it move in the (non hydraulic) H500 when you 'pull a bit'." (modified to the CoP change may just be inboard blade section stalling a little)
"1 to overcome the moment of CoP at Arm from Pitch Axis" covers: "Servo transparency is all about the amount of hydraulic power available in the jacks to oppose the aerodynamic forces trying to return the blade to flat pitch."
and I agree on your call of 'bull****' on :"Aerospatiale claimed it was a design feature to prevent pilots overstressing the aircraft"
and I agree that the ripples in Space-Time are far more interesting
and while we are talking tosh i think one of the really interesting phenomena is the business of blade stall at full RRPM by pulling to hard (needing to) lots of accidents from that.
Me:"there are 2 things (i can think of) that control force is needed for: 1 to overcome the moment of CoP at Arm from Pitch Axis and 2 to rotate the blades about their Pitch Change Axis - which is bigger if there is a larger polar moment of inertia around the pitch change axis , as you might well find for a larger chord blade (H500 compared to Gazelle for instance). I never really bought the story about the CoP not moving for symmetrical aerofoils anyway ... you can certainly feel it move in the (non hydraulic) H500 when you 'pull a bit'." (modified to the CoP change may just be inboard blade section stalling a little)
"1 to overcome the moment of CoP at Arm from Pitch Axis" covers: "Servo transparency is all about the amount of hydraulic power available in the jacks to oppose the aerodynamic forces trying to return the blade to flat pitch."
and I agree on your call of 'bull****' on :"Aerospatiale claimed it was a design feature to prevent pilots overstressing the aircraft"
and I agree that the ripples in Space-Time are far more interesting
and while we are talking tosh i think one of the really interesting phenomena is the business of blade stall at full RRPM by pulling to hard (needing to) lots of accidents from that.
Avoid imitations
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"Aerospatiale claimed it was a design feature to prevent pilots overstressing the aircraft"
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Pressure relief - adds plausibility but still sounds like they designed something requiring excessive torsional forces via the pitch links. AS350 and AS365 (great coast gaurd demo on youtoob somewhere, EC155 too?
The PRV will be there to protect the hydraulic system from over-pressure - not as a fail safe for avoiding RBS.
AnFI - excessive torsional force is exactly what you get when you force a blade to go somewhere it doesn't want to be - ie at high AoA - they just got the maths wrong when they estimated how powerful the jacks needed to be to control that force - simple under-engineering.
Why do you think the bigger and heavier R44 has hydraulic boost where the R22 doesn't?
More powerful engine equals higher AUM and/or smaller disc equals higher disc loading equals higher aerodynamic backloads equals more powerful control systems.
AnFI - excessive torsional force is exactly what you get when you force a blade to go somewhere it doesn't want to be - ie at high AoA - they just got the maths wrong when they estimated how powerful the jacks needed to be to control that force - simple under-engineering.
Why do you think the bigger and heavier R44 has hydraulic boost where the R22 doesn't?
More powerful engine equals higher AUM and/or smaller disc equals higher disc loading equals higher aerodynamic backloads equals more powerful control systems.
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RetreatingBladeStall?
R22/R44.... bigger chord = longer Arm for CoP from Pitch Change Axis. AND more Polar Moment of Inertia for pitch change per rev.
EG Hughes500 vs B206. Hydraulic vs not Hydraulic. Pertinent difference is chord, (affecting both factors)
R22/R44.... bigger chord = longer Arm for CoP from Pitch Change Axis. AND more Polar Moment of Inertia for pitch change per rev.
EG Hughes500 vs B206. Hydraulic vs not Hydraulic. Pertinent difference is chord, (affecting both factors)
Utter Bolleaux!
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c'mon crab, you can't be that unable to read with an open mind:
the Aerodynamic force on a blade acts at the Center of Pressure, if it is not co-incident with the Pitch Change Axis (almost the same thing as the Longditudinal Axis of a Blade) this will result in a Torsional Force about this Axis, this force will have to be resolved by the Pitch Link (and thereby the control rods and hydraulics if there are any). The distance of the CoP from the Pitch Change Axis is the 'Arm' and the Magnitude does indeed depend on the Lift being generated. The force from this factor needing to be supplied by "Control Forces' (or Hydraulics) will be the product of Magnitude and Arm.
The second force that Controls (or Hydraulics) need to supply is the Force required to change the pitch every revoloution.
One variable that both factors are dependant on is the Chord.
OK? If not re-read. (althought the Boulleux jape is irresistible.)
the Aerodynamic force on a blade acts at the Center of Pressure, if it is not co-incident with the Pitch Change Axis (almost the same thing as the Longditudinal Axis of a Blade) this will result in a Torsional Force about this Axis, this force will have to be resolved by the Pitch Link (and thereby the control rods and hydraulics if there are any). The distance of the CoP from the Pitch Change Axis is the 'Arm' and the Magnitude does indeed depend on the Lift being generated. The force from this factor needing to be supplied by "Control Forces' (or Hydraulics) will be the product of Magnitude and Arm.
The second force that Controls (or Hydraulics) need to supply is the Force required to change the pitch every revoloution.
One variable that both factors are dependant on is the Chord.
OK? If not re-read. (althought the Boulleux jape is irresistible.)
So a clever designer ensures that the CoP doesn't vary much and puts the pitch change axis (feathering axis) coincident with the aerodynamic centre of the blade such that the pitching moments, CMo, are zero.
The reasons for having a bigger blade chord will be to do with the critical mach number of the aerofoil, the amount of camber and the need for a sensible rotor solidity ratio based on the number of blades.
According to your logic, having a longer lever (distance between CoP and pitch change link) means needing more powerful controls yet the reverse is true - if your lever is longer, you need less force to feather the blade as your force is acting at an increased arm from the CoP whereas the aerodynamic backloads are always a small distance from the CoP (or even zero distance).
As I said before - utter bolleaux spouted in embarrasment because your initial suggestion that it was the assymetric aerofoil that was responsible for servo transparency was totally wrong.
The reasons for having a bigger blade chord will be to do with the critical mach number of the aerofoil, the amount of camber and the need for a sensible rotor solidity ratio based on the number of blades.
According to your logic, having a longer lever (distance between CoP and pitch change link) means needing more powerful controls yet the reverse is true - if your lever is longer, you need less force to feather the blade as your force is acting at an increased arm from the CoP whereas the aerodynamic backloads are always a small distance from the CoP (or even zero distance).
As I said before - utter bolleaux spouted in embarrasment because your initial suggestion that it was the assymetric aerofoil that was responsible for servo transparency was totally wrong.
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Gosh Crab you are hard work.
You have mis-read what I wrote and mis quote it.
The bigger distance from Pitch Change Axis (not Pitch Link) to CoP causes a greater moment that needs to be resisted- bigger Chords allow for bigger moments - I don't propose to write it all again - having done so perfectly clearly the first time.
Yes a designer would attempt to leave the CoP near (and just aft of) the pitch change axis. I have never bought the textbook 'arguement' that the CoP does not move with a Symetrical Aerofoil. In any event it is probably safe to assume that the CoP is behind the Pitch Change Axis in the Gazelle as it is for the H500. The smaller Chord size of the H500 means that hydraulics are not neccessary. Stop being rude to me and just learn (again!!).
You have mis-read what I wrote and mis quote it.
The bigger distance from Pitch Change Axis (not Pitch Link) to CoP causes a greater moment that needs to be resisted- bigger Chords allow for bigger moments - I don't propose to write it all again - having done so perfectly clearly the first time.
Yes a designer would attempt to leave the CoP near (and just aft of) the pitch change axis. I have never bought the textbook 'arguement' that the CoP does not move with a Symetrical Aerofoil. In any event it is probably safe to assume that the CoP is behind the Pitch Change Axis in the Gazelle as it is for the H500. The smaller Chord size of the H500 means that hydraulics are not neccessary. Stop being rude to me and just learn (again!!).
AnFI, you wrote this
not really a misquote, is it? Maybe you should read exactly what you post rather than what you think you post.
This
is another fatuous AnFI statement - just because the chord is bigger doesn't mean the CoP will be further away from the pitch axis - as I wrote before, selecting the location of the pitch change axis is what will determine the pitching moments of the blade.
Perhaps you ought to learn what an aerodynamic centre is!
I think the 'textbook' indicates that the CoP moves little enough not to be an issue because the arm is so small - the bigger movements come with cambered blades and that can be minimised along the length of the blade by aeroelastic tailoring (something else you probably haven't heard of).
this force will have to be resolved by the Pitch Link (and thereby the control rods and hydraulics if there are any).
This
bigger Chords allow for bigger moments
Perhaps you ought to learn what an aerodynamic centre is!
I have never bought the textbook 'arguement' that the CoP does not move with a Symetrical Aerofoil
6am
*click*
It's still groundhog day here in rotor heads, but at least we are off precession and onto CoP.
We have had a change of cast - previously Lu Zuckerman was played by Awblain, but now Anfi* has taken over Lu's role.
*Anfi does NOT stand for Always No F****g Idea.
*click*
It's still groundhog day here in rotor heads, but at least we are off precession and onto CoP.
We have had a change of cast - previously Lu Zuckerman was played by Awblain, but now Anfi* has taken over Lu's role.
*Anfi does NOT stand for Always No F****g Idea.
Nor Another nugatory Flying idea
nor Another numpty Formulating incoherence
nor Aviation newbie Fomenting inaccuracies
nor Another numpty Formulating incoherence
nor Aviation newbie Fomenting inaccuracies
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Crab your last post consists ONLY of insults - APPEAL TO MODS PLEASE !
Rude and wrong - why do it?
Bigger chords cause bigger moments that need to be resolved, BY CONTROL FORCES.
10% of Chord is bigger for a bigger chord than a smaller chord. (self evident)
The Bell206 vs H500 are a good example for you. (Hydraulics being required for the bigger chord).
The alternative soloution to resolve larger torsional forces is with 'control gearing' ie length of pitch horns etc but that results in too larger control travel required that may not be able to be accomodated in the cockpit.
The full text that you selectively quote from is this: "the Aerodynamic force on a blade acts at the Center of Pressure, if it is not co-incident with the Pitch Change Axis (almost the same thing as the Longditudinal Axis of a Blade) this will result in a Torsional Force about this Axis, this force will have to be resolved by the Pitch Link (and thereby the control rods and hydraulics if there are any)." You are willfully misquoting it and insulting me - it is outrageous!
Perhaps you could win a discussion by addressing the point with reasoned arguement rather than pathetic and purile insult?
Are you without honour?
Or re-read my posts and you'll see that you are now re-gurgitating what I wrote initially anbout unsymetrical aerofoils, you are hard work but at least I am 'sincerely trying'.
Same old story with you, argue automatically, read text book, find you are wrong and then try to own the arguement by insult
... and AC don't you start, why don't you explain it to Crab instead.
Rude and wrong - why do it?
Bigger chords cause bigger moments that need to be resolved, BY CONTROL FORCES.
10% of Chord is bigger for a bigger chord than a smaller chord. (self evident)
The Bell206 vs H500 are a good example for you. (Hydraulics being required for the bigger chord).
The alternative soloution to resolve larger torsional forces is with 'control gearing' ie length of pitch horns etc but that results in too larger control travel required that may not be able to be accomodated in the cockpit.
The full text that you selectively quote from is this: "the Aerodynamic force on a blade acts at the Center of Pressure, if it is not co-incident with the Pitch Change Axis (almost the same thing as the Longditudinal Axis of a Blade) this will result in a Torsional Force about this Axis, this force will have to be resolved by the Pitch Link (and thereby the control rods and hydraulics if there are any)." You are willfully misquoting it and insulting me - it is outrageous!
Perhaps you could win a discussion by addressing the point with reasoned arguement rather than pathetic and purile insult?
Are you without honour?
Or re-read my posts and you'll see that you are now re-gurgitating what I wrote initially anbout unsymetrical aerofoils, you are hard work but at least I am 'sincerely trying'.
Same old story with you, argue automatically, read text book, find you are wrong and then try to own the arguement by insult
... and AC don't you start, why don't you explain it to Crab instead.
The Bell206 vs H500 are a good example for you. (Hydraulics being required for the bigger chord).
As for the insults, they are more an "old timers" joke...... We have not had this much fun since THE Lu Zukerman (of the famous Lu Zuckerman Effect) unfortunately departed this earth so that he could explain cloud dynamics, universal mechanical laws and what is wrong with corriolis and the R22 to the big Senior Pilot in the sky. Who I am convinced would have enjoyed the experience.
Luckily, we have the Zuckerman Effect as a legacy of his particular genius, and our focus on this thread is more nostalgic than anything else. Don't take it personally.
Bigger chords cause bigger moments that need to be resolved, BY CONTROL FORCES.
the Aerodynamic force on a blade acts at the Center of Pressure, if it is not co-incident with the Pitch Change Axis...etc
just because the chord is bigger doesn't mean the CoP is further from the pitch change axis! If the CoP is a long way from the pitch change axis it will cause a bigger moment but it is NOT dependent on chord length.
In what way are you 'sincerely trying'? You made a comment on a thread that was wrong (about symmetrical aerofoils) and since then you have kept moving the goalposts about where the argument is - generally banging on about chord length as if was even vaguely relevant to the discussion at the time about servo transparency.
Maybe your fawning acolytes, in whichever small pond you think you are a big fish, will hang on your every word (but you won't tell us which pond or give any information regarding your experience) but I don't. I have still to be convinced you are nothing but a troll.
If you are not then you really do need to develop a sense of humour or a thicker skin
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AnFi:
"10% of Chord is bigger for a bigger chord than a smaller chord. (self evident)"
would you argue that it is not?
You are in condradiction with yourself over several points you have made.
(and furthermore you have been wrong historically on many points on which you have now shifted your view: Cliff recirculation drawing a/c towards cliff, Vortex ring being unrecoverable, Diss o lift being resolved by Flapping, Private pilots should monitor instruments more to avoid IIMC, and the grammer of my handle)
Helmet
re 205 vs 412 I don't know it's only a generalisation (since as Crab points out one designer may place it's CoP closer to the Pitch Change Axis in absoloute terms and in relative terms. Also the Control Gearing may be different. There are clearly significant moments to resolve in the symetrical Gazelle example ( but I have never really bought the theory that the CoP doesn't move much for Symetrical aerofoils). Does the 412 have dual hydraulics against single in the 205?
"10% of Chord is bigger for a bigger chord than a smaller chord. (self evident)"
would you argue that it is not?
You are in condradiction with yourself over several points you have made.
(and furthermore you have been wrong historically on many points on which you have now shifted your view: Cliff recirculation drawing a/c towards cliff, Vortex ring being unrecoverable, Diss o lift being resolved by Flapping, Private pilots should monitor instruments more to avoid IIMC, and the grammer of my handle)
Helmet
re 205 vs 412 I don't know it's only a generalisation (since as Crab points out one designer may place it's CoP closer to the Pitch Change Axis in absoloute terms and in relative terms. Also the Control Gearing may be different. There are clearly significant moments to resolve in the symetrical Gazelle example ( but I have never really bought the theory that the CoP doesn't move much for Symetrical aerofoils). Does the 412 have dual hydraulics against single in the 205?
Last edited by AnFI; 25th Mar 2014 at 22:50.
205/huey : symmetrical aerofoil
412 : asymmetrical aerofoil. Huge loads, need hydraulics to make it happen. No hyd, no happen.
The reason symmetrical foils were picked originally was to minimise the forces needed to move the pitch (and for feedback) because otherwise the grips needed to be BIG and HEAVY, things which are not good for uptycopters with wooden blades and piston engines in the 40s/50s.
As technology improved, grips were made from lighter and stronger materials, and the designers could then progress from inefficient symmetrical shapes to more efficient funny shapes. But with the funnies came the need for stronger hydraulics and grips etc. With no hydraulics, it doesn't have the control power to move the pitch.
But the cheapies R22, 44, B206 stick with the symmetrics as it is cheaper and easier.
Still groundhog day, but we are off precession, and that is looking good for Bill Murray and Andie McDowell.
412 : asymmetrical aerofoil. Huge loads, need hydraulics to make it happen. No hyd, no happen.
The reason symmetrical foils were picked originally was to minimise the forces needed to move the pitch (and for feedback) because otherwise the grips needed to be BIG and HEAVY, things which are not good for uptycopters with wooden blades and piston engines in the 40s/50s.
As technology improved, grips were made from lighter and stronger materials, and the designers could then progress from inefficient symmetrical shapes to more efficient funny shapes. But with the funnies came the need for stronger hydraulics and grips etc. With no hydraulics, it doesn't have the control power to move the pitch.
But the cheapies R22, 44, B206 stick with the symmetrics as it is cheaper and easier.
Still groundhog day, but we are off precession, and that is looking good for Bill Murray and Andie McDowell.
Oh dear AnFI - you are really clutching at straws now - your only argument in this debate is that 10% of a bigger chord is larger than 10% of a smaller chord!!!! really ground breaking and earth stopping stuff, you should write a book about it!
As to the rest, I concede the grammar of your handle but the rest of the items are figments of your imagination:
Cliff recirculation - have you ever hovered next to a cliff? I get to do it a lot and there is no discernible tendency for the aircraft to move towards the cliff, regardless of what theory might predict.
Vortex ring - it is theoretically possible to recover from it just using power but you need a lot of it to overcome the massive rotor drag. Otherwise, the standard recovery of getting speed on works fine.
Disymmettry of lift resolved by flapping - you argued your self blue in the face about this one but never made your point clear - inequality of lift results in flapping to equality, what more is there to say?
Private pilots should monitor instruments to avoid IIMC - now you really are making stuff up, I clearly advocated that pilots should make weather decisions early and turn back or land to avoid IIMC - once in it then instrument scan is your only chance of salvation.
Maybe you should just get out more and talk to a few more pilots who have actually done jobs with helicopters and have some real world experience.
As to the rest, I concede the grammar of your handle but the rest of the items are figments of your imagination:
Cliff recirculation - have you ever hovered next to a cliff? I get to do it a lot and there is no discernible tendency for the aircraft to move towards the cliff, regardless of what theory might predict.
Vortex ring - it is theoretically possible to recover from it just using power but you need a lot of it to overcome the massive rotor drag. Otherwise, the standard recovery of getting speed on works fine.
Disymmettry of lift resolved by flapping - you argued your self blue in the face about this one but never made your point clear - inequality of lift results in flapping to equality, what more is there to say?
Private pilots should monitor instruments to avoid IIMC - now you really are making stuff up, I clearly advocated that pilots should make weather decisions early and turn back or land to avoid IIMC - once in it then instrument scan is your only chance of salvation.
Maybe you should just get out more and talk to a few more pilots who have actually done jobs with helicopters and have some real world experience.
If I may offer some All noise Fcukall Insight to this discussion, regarding hydraulics needed for control of rotor systems ...
A fully articulated head on a 12,800 and a 13,500 lb pound helicopter can be flown without hydraulics (it is easier with, and the tail's a bit of a pain wihtout hydraulic boost) for extended periods by using the Kaman Servo Flap (SH-2F and SH-2G) style of blade design. The down side to that design choice is more parts, and the attendant increased maintenance and change of dynamicly loaded parts 12,800 lb with greater frequency. (Pitch change rods, flap attachment hardware, etcetera ... ). Based on the pictures, it looks like they did the same in the K-Max. Have not flown that (12,000 max GW IIRC).
From memory, those blades are not symmetrical, but I may be misunderstanding how that is being used in this conversation.
A fully articulated head on a 12,800 and a 13,500 lb pound helicopter can be flown without hydraulics (it is easier with, and the tail's a bit of a pain wihtout hydraulic boost) for extended periods by using the Kaman Servo Flap (SH-2F and SH-2G) style of blade design. The down side to that design choice is more parts, and the attendant increased maintenance and change of dynamicly loaded parts 12,800 lb with greater frequency. (Pitch change rods, flap attachment hardware, etcetera ... ). Based on the pictures, it looks like they did the same in the K-Max. Have not flown that (12,000 max GW IIRC).
From memory, those blades are not symmetrical, but I may be misunderstanding how that is being used in this conversation.