Helicopter Dynamics: Gyroscopic Precession
. . This thread reminds me of another phenomena applicable to helicopter flight theory - what we liked to call the Lu Zuckerman effect.
Remember the fun?
This effect more than adequately explains gyroscopic precession, centrifugal forces, acceleration vectors, pizo-electric effects, B214 vibrations, and a lot about the R22, etc, etc etc.
PS: it appears AC that landing gently means no gyroscopic forces, but presumably there is a rate of landing at which it would be impossible due to gyroscopic forces wouldn't it?
..... Lu Zuckerman effect again I think!
Question for the day...how do we get dynamic roll over?
Last edited by helmet fire; 15th Mar 2014 at 07:09.
Dynamic rollover comes from the missing 18 degrees of the R22 control orbit, multiplied by the Automatic Wind Balancing Lu Aerodynamic Interference Ningnong (shortened to awblain) to ensure that we totally ignore the realities of rollover and pretend that a big gyroscope is causing all the problems.
The Zuckerman effect it most certainly is. And it is still Groundhog Day.
The Zuckerman effect it most certainly is. And it is still Groundhog Day.
Wonderful thread.
Too much confusion about gyroscopes and centripetal force at the start. Well done to those who have introduced the concept of Angular Momentum. Fundamental properties such as this underpin the function of gyroscopes and other rotating devices.
Too much confusion about gyroscopes and centripetal force at the start. Well done to those who have introduced the concept of Angular Momentum. Fundamental properties such as this underpin the function of gyroscopes and other rotating devices.
Exactly!
So next time you know, when having dynamic roll over, ensure your cyclic reaction is either fore or aft, do not lower the lever and create a massive shift in angular momentum that will result in a pizo-electric effect that will surely blind you to the fact that 18 degrees was the critical missing factor that caused you to forget to pull back on the cyclic as an automatic reaction to everything because you get flung outwards from all turns and can only be saved by complete automation with two pilots and a vibration absorber from a B214.
Simple really.
So next time you know, when having dynamic roll over, ensure your cyclic reaction is either fore or aft, do not lower the lever and create a massive shift in angular momentum that will result in a pizo-electric effect that will surely blind you to the fact that 18 degrees was the critical missing factor that caused you to forget to pull back on the cyclic as an automatic reaction to everything because you get flung outwards from all turns and can only be saved by complete automation with two pilots and a vibration absorber from a B214.
Simple really.
Avoid imitations
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Cat v Pigeons
No gyroscopic precession? Well I've been lucky enough to fly several types of helicopters as well as some big piston-propellor fixed-wing aircraft. I can confirm that, certainly on the fixed-wing propellor types, they all suffered the effects of gyroscopic precession from the propellor.
The big difference between a stall turn left and one to the right was caused directly by a combination of the propellor slipstream effect and the gyroscopic precession. During the take-off run on the tail-wheel types you definitely get a pronounced yaw as you lift the tail, again caused by the nose-down torque applied to the rotating propellor being translated into a yaw.
Are we saying these gyroscopic forces don't apply to helicopters?
No gyroscopic precession? Well I've been lucky enough to fly several types of helicopters as well as some big piston-propellor fixed-wing aircraft. I can confirm that, certainly on the fixed-wing propellor types, they all suffered the effects of gyroscopic precession from the propellor.
The big difference between a stall turn left and one to the right was caused directly by a combination of the propellor slipstream effect and the gyroscopic precession. During the take-off run on the tail-wheel types you definitely get a pronounced yaw as you lift the tail, again caused by the nose-down torque applied to the rotating propellor being translated into a yaw.
Are we saying these gyroscopic forces don't apply to helicopters?
Basic rotor theory!
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Feeling sorry for blain:
I am not so sure you are right; we are talking about a disk with no airspeed, i presume, to isolate the effect we are discussing. Where the AoA is highest to cause a Rate of Attitude Change is surely where the Flapping Rate (ref'd against Mast Axis or previous TPP) is highest ie where the inertial path of the blade needs to be altered to achieve a new plane of rotation, this is (approximately) the same place in the cycle as the delta-Pitch (amount, not rate) is the greatest. This is approximately at 90deg to the place where we want the blade to have flapped up (or down). Or to use your terms (which are good) the blade is flown from low to high by the extra AoA, achieved by extra pitch. tbf i might be wrong (am I?) about this tho, without deeper thought. (so I don't think he is as wrong as you make out)
(and also; Max Rate of Pitch Change is 90deg before (and after) Max Pitch. IE at the Front (and Back) for an Attitude Change in the Pitching direction)
Yup it's great to see us (almost all) move away from Gyroscopic precession, which i think was achieved courtesy of pprune discussion rather than CFS?
Of course (as blain (rightly) says) these are not incompatible theories, there is still an inertial phenomenon (angular momentum) that needs to be accounted for. It's just that the angular momentum is not (mostly) (rigidly) coupled to the rotor mast (as the De Cierva reference makes clear). So a damped resonant free flapping wing becomes the elegant way to think about the blade, as you say. (finding hairs on eggs)
Originally Posted by [email protected]
AnFI, read awblains comment about max pitch being the point of max lift and you will see he is wrong and that is why I emphasised the AoA (which is max at the same point as max rate of pitch change) as being the main issue.
(and also; Max Rate of Pitch Change is 90deg before (and after) Max Pitch. IE at the Front (and Back) for an Attitude Change in the Pitching direction)
Yup it's great to see us (almost all) move away from Gyroscopic precession, which i think was achieved courtesy of pprune discussion rather than CFS?
Of course (as blain (rightly) says) these are not incompatible theories, there is still an inertial phenomenon (angular momentum) that needs to be accounted for. It's just that the angular momentum is not (mostly) (rigidly) coupled to the rotor mast (as the De Cierva reference makes clear). So a damped resonant free flapping wing becomes the elegant way to think about the blade, as you say. (finding hairs on eggs)
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busdriver,
I was thinking about what I understand is called "servo transparency".
I would say that the high angular momentum be especially resistant to redirection in the more extreme conditions with more lift, although that would only be the case if the rotor speed was higher. Just having higher pitch doesn't change the angular momentum. I would say that this would apply to any manufacturing nations' rotational sense or any change of direction.
I'm assuming you're talking about lack of left roll control authority in a high G right turn (assuming a counter clockwise rotating rotor system) which is ironically the exact situation I was going to bring up to counter your gyro discussion. How would you explain this in a rotational dynamics concept?
I would say that the high angular momentum be especially resistant to redirection in the more extreme conditions with more lift, although that would only be the case if the rotor speed was higher. Just having higher pitch doesn't change the angular momentum. I would say that this would apply to any manufacturing nations' rotational sense or any change of direction.
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ShyTorque,
I agree that the flapping makes things much easier on the mechanical components, letting the air take more of the forces that hammer from a wrenched propellor to the crankshaft bearings of a piston engine.
However, if you make a toy gyroscope with the ability to flap when torqued, it would behave just as the regular toy you're familiar with. I suggest that it's the exploitation of the power of the airflow around a rotor that makes the difference, not some sort of cancellation of the couples and angular momenta involved because there's a hinge.
Do you feel "gyroscopic" forces when a hand blender is in soup? Not like you do in air - because the coupling of the rotating blades to the soup is very strong.
The difference is that helicopter rotor blades have flapping hinges, which alleviate the feedback into the rotational axis (the crankshaft of your aeroplane engine, or the main rotor shaft of a rotary winged aircraft) by allowing flapping to equality. Cierva discovered the need for these flapping hinges during flight trials, after he scaled up his small model aircraft.
However, if you make a toy gyroscope with the ability to flap when torqued, it would behave just as the regular toy you're familiar with. I suggest that it's the exploitation of the power of the airflow around a rotor that makes the difference, not some sort of cancellation of the couples and angular momenta involved because there's a hinge.
Do you feel "gyroscopic" forces when a hand blender is in soup? Not like you do in air - because the coupling of the rotating blades to the soup is very strong.
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However, if you make a toy gyroscope with the ability to flap when torqued, it would behave just as the regular toy you're familiar with. I suggest that it's the exploitation of the power of the airflow around a rotor that makes the difference, not some sort of cancellation of the couples and angular momenta involved because there's a hinge.
I'm not prepared to try to advise you any further because you obviously aren't prepared to listen and believe you know better than any professional, in any case.
This is a case of Lu Zuckerman all over again. So many incorrect perceptions of basic principles.
So I'm out. I've got helicopters to fly.
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blain,
servo transparency is a case of the hydraulic servos being incapable of handling the change in load when a rotor system approaches retreating blade stall due to load factor. What I mentioned about coning roll is very real and I've demonstrated it to my students on a routine basis, in an aircraft that has no issues with servo transparency.
In summation and my final foray into this thread: Angular momentum must of course be conserved, but to claim that a non-rigid rotor system behaves like a gyro violates all sort of assumptions that gyroscopic precession is based on (heedm said the same thing explicitly). If we want to simplify things for students, the aerodynamic forces are the predominate factor and what must be addressed for the non-designer level of discussion. If you want to delve deeper, you need to be prepared to understand Prouty's book, and all the equations that go with it and the fact that there are a multitude of factors that play into rotor dynamics and they don't always play nice.
servo transparency is a case of the hydraulic servos being incapable of handling the change in load when a rotor system approaches retreating blade stall due to load factor. What I mentioned about coning roll is very real and I've demonstrated it to my students on a routine basis, in an aircraft that has no issues with servo transparency.
In summation and my final foray into this thread: Angular momentum must of course be conserved, but to claim that a non-rigid rotor system behaves like a gyro violates all sort of assumptions that gyroscopic precession is based on (heedm said the same thing explicitly). If we want to simplify things for students, the aerodynamic forces are the predominate factor and what must be addressed for the non-designer level of discussion. If you want to delve deeper, you need to be prepared to understand Prouty's book, and all the equations that go with it and the fact that there are a multitude of factors that play into rotor dynamics and they don't always play nice.
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Originally Posted by busdriver
servo transparency is a case of the hydraulic servos being incapable of handling the change in load when a rotor system approaches retreating blade stall due to load factor.
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Perhaps you should write to the world's helicopter manufacturers and advise them they've all got it wrong.
Angular momentum must of course be conserved, but to claim that a non-rigid rotor system behaves like a gyro violates all sort of assumptions that gyroscopic precession is based on (heedm said the same thing explicitly).
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If the forces on climbing blades explanation gives you a good and helpful picture of how it all works, and as long as you're really sure about the origin of all the 90 degrees-es, and don't swap amplitude for rate of change arbitrarily to make it work, then that's excellent; but does it really give any useful understanding of limits to what can and can't be done when flying it? I fear from some of the replies here that it is rote-learned to pass tests. It's certainly needed by designers to make it all work, as it's the way to get the whirling bits to be the right size to cope with all the lifting and reaction forces. In particular, hinges don't the angular momentum of the blades at all, and I fear that's not what many would say if questioned about it.
All I disagree with in this series of posts is the often seemingly theological tone to the rejection of a role for adding angular momenta together, without emphasizing that any differences from a picture of a toy gyroscope are all there for interesting reasons, and that understanding them could even be beneficial. All the various sorts of rotor designs, and the hockey puck, and a bike wheel, and a hurricane, and a toy gyroscope are all governed by the same principles.
When an explanation based on irrefutable physical principles seems to break down, or someone tells you they doesn't apply to your case, then there's an opportunity for gaining more understanding. The physical principles do apply, but there are other things involved.
I hope people have learned things here. I certainly have, and perhaps shouldn't have jumped straight in with a "rubbish", after "it's not a gyroscope", rather than
interpreting that as "it's not like a toy gyroscope".
AnFI - the classic example of servo transparency is on the Gazelle when experiencing jackstall - guess what? ....the Gazelle has symmetrical aerofoils.
The Lynx has two main aerofoil shapes along the length of the blade - a high camber section inboard and a reflex trailing edge further outboard - these combine to minimise the pitching moments caused by the use of non-symmetrical aerofoils. Guess what?...the Lynx doesn't suffer from servo transparency and is next to impossible to get into RBS.
Awblain - the conservation of AM is observed as changes in the RRPM as the blade flaps and the C of G of each blade moves towards or away from the rotor hub. The blades accelerate and decelerate constantly (hence the need for dragging hinges, which ISTR was the clever bit of Cierva's design).
The Lynx has two main aerofoil shapes along the length of the blade - a high camber section inboard and a reflex trailing edge further outboard - these combine to minimise the pitching moments caused by the use of non-symmetrical aerofoils. Guess what?...the Lynx doesn't suffer from servo transparency and is next to impossible to get into RBS.
Awblain - the conservation of AM is observed as changes in the RRPM as the blade flaps and the C of G of each blade moves towards or away from the rotor hub. The blades accelerate and decelerate constantly (hence the need for dragging hinges, which ISTR was the clever bit of Cierva's design).
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Crab: "AnFI - the classic example of servo transparency is on the Gazelle when experiencing jackstall - guess what? ....the Gazelle has symmetrical aerofoils." Yup good point. 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'.
As for the Lynx that probably has suitably powerful hydraulics to be 'not bovvered' anyway.
"the clever bit of Cierva's design" surely the flapping hinge?
As for the Lynx that probably has suitably powerful hydraulics to be 'not bovvered' anyway.
"the clever bit of Cierva's design" surely the flapping hinge?
Jack stall or servo transparency ...... they were actually also contributing factors of the Zuckerman Effect if I recall properly.
This is scary. And now I can't stop that song AC.....
One last crack at it..... The rigidity of a system is a fundamental pre-requisite for rotating bodies that exhibit gyroscopic principles. Conventional helicopter rotor systems are not rigid systems, they are individual wings moving individually, flapping, feather and lead lagging individually and do not form a disk.
I know that the powerful pull of the Zuckerman Effect will inevitably outweigh this principle, and many others besides.
".....Well I don't know if all that's true
'Cause you got me, and baby I got you...."
This is scary. And now I can't stop that song AC.....
One last crack at it..... The rigidity of a system is a fundamental pre-requisite for rotating bodies that exhibit gyroscopic principles. Conventional helicopter rotor systems are not rigid systems, they are individual wings moving individually, flapping, feather and lead lagging individually and do not form a disk.
I know that the powerful pull of the Zuckerman Effect will inevitably outweigh this principle, and many others besides.
".....Well I don't know if all that's true
'Cause you got me, and baby I got you...."
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I think pprune has moved on a long way in 10 years, this used to be a GP place, next person who wades in trying to tell everyone that you change the pitch on the sides to make it go back and forth because gyroscopic precession says so is going to be killed. (thank goodness)
Now we just need the ground exam syllabus to catch up.
CUE: Expert from DGAC: "I got yooou baaaaaaabe"
that song sounds particularly bad in Japanese Karaoke bars
Now we just need the ground exam syllabus to catch up.
CUE: Expert from DGAC: "I got yooou baaaaaaabe"
that song sounds particularly bad in Japanese Karaoke bars
With unsymetrical aerofoils the Center of Pressure moves along the Chord with different Angles of Attack, this is likely to be the cause of 'Servo Transparency'.
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. Aerospatiale claimed it was a design feature to prevent pilots overstressing the aircraft
As for Cierva - if you read deeper you will know that while the flapping hinges prevented the undemanded rollover - the dragging hinges prevented the lead-lag loads destroying the blades at the root - therefore equally clever
Back to the Lu Z phenomenon.......................
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The rigidity of a system is a fundamental pre-requisite for rotating bodies that exhibit gyroscopic principles.
Are ice skaters, slingshots, bolases and twirled pizza bases rigid systems?
How do they respond to torques being applied?
the conservation of AM is observed as changes in the RRPM as the blade flaps and the C of G of each blade moves towards or away from the rotor hub.
Does the fractional reduction in length from flap, by a small amount, match the fractional change in angular speed? I don't reckon it does, as the fractional speed variation is by more than the length change: which is fine, as the torque from the hub takes up the slack. There's also the issue of phasing the minimum/maximum length and maximum speed.
I'm going to keep with the conservation of angular momentum for the whole rotor-airflow system. I suggest that the angular momentum of a blade changes markedly around the circle, with couples from the hub and the airflow (which are not in the same direction) making it so, and indeed, making it flap.
As for Cierva - if you read deeper you will know that while the flapping hinges prevented the undemanded rollover - the dragging hinges prevented the lead-lag loads destroying the blades at the root - therefore equally clever
Does the fractional reduction in length from flap, by a small amount, match the fractional change in angular speed? I don't reckon it does
Lu Zuckerman was the same, kept on pushing his own (wrong) theories.
6am
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