Helicopter Dynamics: Gyroscopic Precession
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
crab
Yes, but does it match the yaw rate? And what limits it?
Do you mean "rate of pitch change" or "pitch"?
To make the helicopter move left or right where are the maximum and minimum lift azimuths of the disk? Left and right, or fore and aft?
The pitch and roll rates achievable in modern, semi-rigid rotor systems are much higher than those for teetering or fully articulated rotors.
Hence the rate of pitch change (and therefore its resulting aerodynamic effects) are at maximums 90 degrees…
… are at maximums 90 degrees from the point where the initial change is made (ie from the TDC or BDC).
"
?"
Who cares???
A BK117 will roll fast enough to bang your head against the windows. But who cares? We are carrying people, and we care for our passengers.
Why are we stuck in Groundhog Day with awblain asking the same questions, which have already been answered logically, but this thread keeps on going.
Yes, a rotor system behaves LIKE a gyroscope but IT IS NOT A GYROSCOPE.
If it WAS a gyroscope, we would not need swash plate, cyclic feathering, stabiliser bars or such, just a small servo that pushes against the mast EXACTLY 90 degrees ahead of where we want the disc to point. But this doesn't happen.
6am, darn, that clock radio is playing "I've Got You, Babe" again.
Does it match the yaw rate
Who cares???
A BK117 will roll fast enough to bang your head against the windows. But who cares? We are carrying people, and we care for our passengers.
Why are we stuck in Groundhog Day with awblain asking the same questions, which have already been answered logically, but this thread keeps on going.
Yes, a rotor system behaves LIKE a gyroscope but IT IS NOT A GYROSCOPE.
If it WAS a gyroscope, we would not need swash plate, cyclic feathering, stabiliser bars or such, just a small servo that pushes against the mast EXACTLY 90 degrees ahead of where we want the disc to point. But this doesn't happen.
6am, darn, that clock radio is playing "I've Got You, Babe" again.
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
If it WAS a gyroscope, we would not need swash plate, cyclic feathering, stabiliser bars or such, just a small servo that pushes against the mast EXACTLY 90 degrees ahead of where we want the disc to point. But this doesn't happen.
That DOES happen, but it's not a small push. The last question above is the key.
If you did make a small push, you'd get an exceptionally small angular acceleration in response, rotating the plane of the disk in a direction 90 degrees away from the moment of your push.
Yes, but does it match the yaw rate? And what limits it?
Do you mean "rate of pitch change" or "pitch"?
To make the helicopter move left or right where are the maximum and minimum lift azimuths of the disk? Left and right, or fore and aft?
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
I told you it is control power.
the same thing
Where pitch changes most quickly is not where the lift is a maximum or minimum, it's where it has the average value.
that depends on your control orbit set up.
There's a cyclic control input to move to the left.
Is the maximum-minimum-lift/lowest-highest-blade-tip direction fore-and-aft or left-to-right?
Join Date: Oct 2007
Location: USA
Posts: 147
Likes: 0
Received 0 Likes
on
0 Posts
Go back a couple pages and read what heedm wrote. Yes, conservation of angular momentum must be accounted for and it is, much like conservation of mass, momentum and energy are incorporated into the Euler equations when discussing lift production. The gyro visualization is an over simplification and not directly applicable to rotor dynamics.
OK, let's say that the rotor system IS a great big gyroscope.
Here comes a helicopter, hovering over a hill. The pilot noses into the slope and touches down on the toes of the skids, and does nothing to the cyclic as he allows the machine to settle. The heels, being unsupported, sink, taking the rest of the aircraft and the disc with it. But a miracle happens!!
Instead of the tail going down and the nose pitching up, the aircraft rolls RIGHT!
He is puzzled, so he lifts to the hover again, puts the left skid uphill, and touches down on the left skid. Again, he allows the machine to want to settle to the right, but it pitches nose down!
Hmm...
Does this really happen?
NO!!!
Because?
it is not a gyroscope
Here comes a helicopter, hovering over a hill. The pilot noses into the slope and touches down on the toes of the skids, and does nothing to the cyclic as he allows the machine to settle. The heels, being unsupported, sink, taking the rest of the aircraft and the disc with it. But a miracle happens!!
Instead of the tail going down and the nose pitching up, the aircraft rolls RIGHT!
He is puzzled, so he lifts to the hover again, puts the left skid uphill, and touches down on the left skid. Again, he allows the machine to want to settle to the right, but it pitches nose down!
Hmm...
Does this really happen?
NO!!!
Because?
it is not a gyroscope
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
Thirteen years on, that statement from heedm is still correct.
However, being an oversimplication, and it isn't really, is much better than being flat wrong and denying, somewhat absurdly and just because someone told you to, that all the standard rotational dynamics that everyone knows is
not helpful in explaining how rotors respond.
It catches the direction of everything that happens, and the powers, forces and timescales involved. It even explains the orientation of the cyclic feathering motion, without introducing an arbitrary right angle from getting the difference between acceleration and speed mixed up.
"Sikorsky eventually found that it all worked if you did that" isn't much of an explanation, and I'll bet Sikorsky's surely excellent knowledge of rotational mechanics based on his Russian technical education lead to him getting the phase right in his control systems, if not intuitively then consciously.
However, being an oversimplication, and it isn't really, is much better than being flat wrong and denying, somewhat absurdly and just because someone told you to, that all the standard rotational dynamics that everyone knows is
not helpful in explaining how rotors respond.
It catches the direction of everything that happens, and the powers, forces and timescales involved. It even explains the orientation of the cyclic feathering motion, without introducing an arbitrary right angle from getting the difference between acceleration and speed mixed up.
"Sikorsky eventually found that it all worked if you did that" isn't much of an explanation, and I'll bet Sikorsky's surely excellent knowledge of rotational mechanics based on his Russian technical education lead to him getting the phase right in his control systems, if not intuitively then consciously.
Maximum pitch, associated with maximum (but constant) lift, occurs 90 degrees away from maximum rate of change of pitch (and lift).
Where pitch changes most quickly is not where the lift is a maximum or minimum, it's where it has the average value.
Where pitch changes most quickly is not where the lift is a maximum or minimum, it's where it has the average value.
When the blade gets to its high/low point the pitch change starts in the opposite direction.
Maybe it does, but where is it?
There's a cyclic control input to move to the left.
Is the maximum-minimum-lift/lowest-highest-blade-tip direction fore-and-aft or left-to-right?
There's a cyclic control input to move to the left.
Is the maximum-minimum-lift/lowest-highest-blade-tip direction fore-and-aft or left-to-right?
awblain, you are still ducking the BIG points:
1. If the disc was really a gyroscope, we would not need a swash plate or cyclic feathering to tilt the disc. One shove (of variable size) at exactly 90 degrees to the intended direction of travel will do it. Don't need any feathering.
2. If it really was gyroscope, slope landings would never be possible. The downhill skid would never drop down, because the precession would only result in a nose down or nose up pitch.
Your response please?
1. If the disc was really a gyroscope, we would not need a swash plate or cyclic feathering to tilt the disc. One shove (of variable size) at exactly 90 degrees to the intended direction of travel will do it. Don't need any feathering.
2. If it really was gyroscope, slope landings would never be possible. The downhill skid would never drop down, because the precession would only result in a nose down or nose up pitch.
Your response please?
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
If you understood "gyroscopes", then you wouldn't ask question 1. That's what's happening - think harder about my question about lift. I'm not going to repeat myself for the third or fourth time.
If you think the gentle reaction force on a skid tip matches the lift force that raises a helicopter off the ground, and acts further from the center of mass, then you need to reconsider. What lowers the skids to the ground? Reducing lift and allowing the aircraft to settle. The skids don't provide a couple to the rotating part, but to the stationary part. That has no effect on the rotating parts.
You seem to be obsessed with a spinning top toy that has a huge angular momentum to mass ratio and no couples acting apart from an azimuthal drag and a gravitational turning force. That's why I again suggest that you drop this toy "gyroscope" fixation and consider instead the physical reality of the rotating objects.
This guy's description is broadly correct: Pitch Control
You can argue yourself blue, with a theology that rotors don't behave like any other rotating object, but physical reality is still going to be what it is.
If you think the gentle reaction force on a skid tip matches the lift force that raises a helicopter off the ground, and acts further from the center of mass, then you need to reconsider. What lowers the skids to the ground? Reducing lift and allowing the aircraft to settle. The skids don't provide a couple to the rotating part, but to the stationary part. That has no effect on the rotating parts.
You seem to be obsessed with a spinning top toy that has a huge angular momentum to mass ratio and no couples acting apart from an azimuthal drag and a gravitational turning force. That's why I again suggest that you drop this toy "gyroscope" fixation and consider instead the physical reality of the rotating objects.
This guy's description is broadly correct: Pitch Control
You can argue yourself blue, with a theology that rotors don't behave like any other rotating object, but physical reality is still going to be what it is.
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?
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?
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
crab,
You really believe that that provides a more transparent explanation than imposing a differential lift along a line 90 degrees away from the desired direction into which to reposition the disk?
You can certainly meld together all these changing quantifies to explain what's going on, and need to track the three dimensional paths of the blades, and deal with at least one phase-lag term; but why not just embrace the idea that it's just like any other rotating dynamical system and identify the torques and responses instead?
With no advance angle on either the jacks or the pitch change horn, the swash plate would have to be tilted to the right to enable the disc to tilt forwards (in your example the left cyclic would result in an aft tilt of the disc with the maximum rate of pitch change/AoA/flap down in the 9 o'clock position) On most helicopters the jacks/pitch change rods are organised so that the disc follows the swash plate tilt because the pitch changes are made 90 degrees ahead of the desired disc attitude.
You can certainly meld together all these changing quantifies to explain what's going on, and need to track the three dimensional paths of the blades, and deal with at least one phase-lag term; but why not just embrace the idea that it's just like any other rotating dynamical system and identify the torques and responses instead?
The link you provided says exactly the same as I have and he dismisses precession in one line as an alternative way of explaining HOW the disc behaves not WHY.
Knock yourself out if you want to try and change everyone's detailed and satisfactory understanding of how a rotor system behaves - you will probably find that no-one cares or listens. There have been many very clever people working in the design and construction of helicopters for many years and you think you know better - crack on and prove it
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.
Knock yourself out if you want to try and change everyone's detailed and satisfactory understanding of how a rotor system behaves - you will probably find that no-one cares or listens. There have been many very clever people working in the design and construction of helicopters for many years and you think you know better - crack on and prove it
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.
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
If people don't want a natural explanation in terms of rotational motion, then fine… but if you need to introduce at least one phase term, have a picture of blades flapping in three dimensions and engage in a seemingly theologically fight against using the concept of angular momentum, then I think you're missing out on the chance to have an easier picture of what's going on above your head.
All the dynamic wagging blade stuff's a fine description if you get all the shifts and movements correct, but can you really sit down and convince someone who's never thought about it that it's correct and helpful as a description compared with the "gyroscope" description of motion, once you get over the counterintuitive aspects of the directionality of angular momentum. While all that detailed motion around the hub stuff is clearly important to designers, does it really help an operator to understand what's happening?
In particular, AC seems to have the idea that a small torque would move a big rotating object dramatically, or stop it turning: this is at gross variance with reality, and I suggest isn't providing a good mental model of the important issues in who the machine is working.
One thing that comes to mind is running out of control authority in fast turns. How does that get explained in the dynamical blade picture? It seems more natural to describe that again using collective properties of the disk.
All the dynamic wagging blade stuff's a fine description if you get all the shifts and movements correct, but can you really sit down and convince someone who's never thought about it that it's correct and helpful as a description compared with the "gyroscope" description of motion, once you get over the counterintuitive aspects of the directionality of angular momentum. While all that detailed motion around the hub stuff is clearly important to designers, does it really help an operator to understand what's happening?
In particular, AC seems to have the idea that a small torque would move a big rotating object dramatically, or stop it turning: this is at gross variance with reality, and I suggest isn't providing a good mental model of the important issues in who the machine is working.
One thing that comes to mind is running out of control authority in fast turns. How does that get explained in the dynamical blade picture? It seems more natural to describe that again using collective properties of the disk.
Trying to use gyro theory and precession to explain rotor behaviour because it is easier to understand is exactly what has gone wrong in the past. The two are not the same yet in some countries, many pilots are told it is precession because the authorities think it is an easier way of explaining a complex subject. Every pilot in the British military, and a whole lot more across the world, are taught the aerodynamic explanation and understand it completely.
Offering the wrong explanation for something, just because it is easy to understand,
is not progress.
Offering the wrong explanation for something, just because it is easy to understand,
is not progress.
Crab, you have identified what this boils down to.
During initial training I was taught the whole precession theory, but figured out a while ago that it was in error. And the "flying to position" theory really isn't that complicated or difficult to understand.
The gyroscopic precession theory doesn't want to die because on the surface it makes sense, despite being incorrect. This issue boils down to "yeah but, it makes more sense and is easier to explain" yet it needs to discredit the aerodynamic explanation in order to sound plausible.
During initial training I was taught the whole precession theory, but figured out a while ago that it was in error. And the "flying to position" theory really isn't that complicated or difficult to understand.
The gyroscopic precession theory doesn't want to die because on the surface it makes sense, despite being incorrect. This issue boils down to "yeah but, it makes more sense and is easier to explain" yet it needs to discredit the aerodynamic explanation in order to sound plausible.
Join Date: Dec 2007
Location: Pasadena
Posts: 633
Likes: 0
Received 0 Likes
on
0 Posts
yet it needs to discredit the aerodynamic explanation in order to sound plausible.
Do you analyze some problems using forces and some by considering energy? Both are appropriate, and compatible, but in some cases one might be more useful than the other. Look at a roller coaster as an example: how best to decide how tough the track needs to be at a point? - watch instantaneous accelerations. How to picture how the whole thing works? - watch the conversion of energy.
I fail to understand the theological nature of the passions. Angular momentum exists and changes in the way that it does. Why fight it?
Could it just be the hours and hours invested in being able to answer quiz questions about all the flapping, flying and phases? It still strikes me as a tutorial artifice developed to avoid having to address the issue of angular momentum in class.
Every pilot in the British military, and a whole lot more across the world, are taught the aerodynamic explanation and understand it completely.
If you're happy with that explanation, fine. But, if nothing else, keeping the direction of the change in angular momentum will help to keep track of all the phase terms.
Join Date: Oct 2007
Location: USA
Posts: 147
Likes: 0
Received 0 Likes
on
0 Posts
The simple answer is that the predominate force in rotor dynamics is aerodynamic and it is easier to explain using aerodynamic principles and end up with a more correct layman's explanation than the opposite. ie. You start flying the blade up at the tail to produce a left roll.
The more complex equations account for conservation of angular momentum because they have to, but gyroscopic precession is an over simplification of the effects of conservation of angular momentum because with the exception of a teetering rotor head, the system does not act as a rigid body which is an essential assumption of gyro behavior.
Further:
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 can explain this quite well within an aero forces construct. Simply put, as I increase the G on the head, coning increases. As a result of increased coning the aft portion of the disc sees an increase in induced flow/drag and a subsequent loss of lift while the front portion of the disc sees a decrease in induced flow/drag and an increase in lift. The result is the blades passing through the decreased lift region (aft) will tend to flap down and those passing through the increased lift region (front) will tend to flap up. To counter this pilot would have to apply increased left cyclic. As a result a sustained high G right turn in an American helicopter will tend to cause the cyclic to "migrate" left as G increases thanks to increasing coning, which will of course eat up total amount of left cyclic available and hence a reduction in control authority. The key to "fixing" this is to reduce the coning to restore control authority which can be done by either reducing collective or unloading the disc by adding forward cyclic or a small contribution of both.
The more complex equations account for conservation of angular momentum because they have to, but gyroscopic precession is an over simplification of the effects of conservation of angular momentum because with the exception of a teetering rotor head, the system does not act as a rigid body which is an essential assumption of gyro behavior.
Further:
One thing that comes to mind is running out of control authority in fast turns. How does that get explained in the dynamical blade picture? It seems more natural to describe that again using collective properties of the disk.