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Gyro PRECESSION



 
 
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  #1  
Old January 19th 09, 07:11 PM posted to rec.aviation.rotorcraft
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Posts: 6
Default Gyro PRECESSION

I've been reading up on gyro precession as it refers to gyroplanes. It
seems that is not how a rotor disc works on a gyroplane. The info I've
found so far states that: a two blade rotor system doesn't tilt by
precession, but that it's entirely aerodynamic and uses vane (pitch?).
How does gyro precession effect rotor blades on a helicopter? (I could
do some research on the www, but I would like to hear someone's
opinion here first)
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  #2  
Old January 20th 09, 03:10 AM posted to rec.aviation.rotorcraft
Steve R.[_2_]
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Posts: 31
Default Gyro PRECESSION

wrote in message
...
I've been reading up on gyro precession as it refers to gyroplanes. It
seems that is not how a rotor disc works on a gyroplane. The info I've
found so far states that: a two blade rotor system doesn't tilt by
precession, but that it's entirely aerodynamic and uses vane (pitch?).
How does gyro precession effect rotor blades on a helicopter? (I could
do some research on the www, but I would like to hear someone's
opinion here first)



Where did you get that from? I'm not sure I agree with it. The
experimental gyros I've looked at may not have a swash plate like a
helicopter does but rather, tilts the rotor hub relative to the plane of the
rotor disk. The net effect of that is to apply a cyclic pitch action to the
main blades and they react 90 degrees in the direction of rotation per
gyroscopic precession.

What info source are you looking at?

Steve R.

  #3  
Old January 20th 09, 07:35 PM posted to rec.aviation.rotorcraft
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Posts: 6
Default Gyro PRECESSION

On Jan 19, 9:10 pm, "Steve R." wrote:

Where did you get that from? I'm not sure I agree with it. The
experimental gyros I've looked at may not have a swash plate like a
helicopter does but rather, tilts the rotor hub relative to the plane of the
rotor disk. The net effect of that is to apply a cyclic pitch action to the
main blades and they react 90 degrees in the direction of rotation per
gyroscopic precession.

What info source are you looking at?

Steve R.


Hi Steve,

I got my info from an article written by: Don McCoy, via regalpony.ca
and found on asra.org.au

A summary, in his own words: (gyroplanes)

The joystick is moved to the right causing the rotor head axis to tilt
to the right and out of alignment with the rotor axis. The pivoting
motion of the teeter bolt causes the pitch angle of the blades to van.
during the rotation. increasing the lift at the front and reducing it
at the rear. The resulting large torque due to the aerodynamic effects
of the change in pitch is directed to the right and causes the rotor
axis (angular momentum of the rotors) to rotate to the right following
the rotor head axis. As the rotor axis comes back in line with the
tilted rotor head axis, the pitch angle ceases to vary and the lift
from the front and rear rotor blade equalises, the torque vanishes and
the rotors are tilted to the right to initiate the turn.

It's a long article. I'm happy with his explanation as per gyroplanes.
i.e.: a gryoplane uses an unpowered rotor (as you know) and the whole
body of the gyroplane hangs from the gimble head. But on a helicopter,
the rotor is powered and operates like a fan, the helicopter body is
firmly fixed to rotor head.

anyway, what's your opinion of how precession works on a helicopter?
  #4  
Old January 21st 09, 12:12 AM posted to rec.aviation.rotorcraft
Steve R.[_2_]
external usenet poster
 
Posts: 31
Default Gyro PRECESSION

wrote in message
...
On Jan 19, 9:10 pm, "Steve R." wrote:

Where did you get that from? I'm not sure I agree with it. The
experimental gyros I've looked at may not have a swash plate like a
helicopter does but rather, tilts the rotor hub relative to the plane of
the
rotor disk. The net effect of that is to apply a cyclic pitch action to
the
main blades and they react 90 degrees in the direction of rotation per
gyroscopic precession.

What info source are you looking at?

Steve R.


Hi Steve,

I got my info from an article written by: Don McCoy, via regalpony.ca
and found on asra.org.au

A summary, in his own words: (gyroplanes)

The joystick is moved to the right causing the rotor head axis to tilt
to the right and out of alignment with the rotor axis. The pivoting
motion of the teeter bolt causes the pitch angle of the blades to van.
during the rotation. increasing the lift at the front and reducing it
at the rear. The resulting large torque due to the aerodynamic effects
of the change in pitch is directed to the right and causes the rotor
axis (angular momentum of the rotors) to rotate to the right following
the rotor head axis. As the rotor axis comes back in line with the
tilted rotor head axis, the pitch angle ceases to vary and the lift
from the front and rear rotor blade equalises, the torque vanishes and
the rotors are tilted to the right to initiate the turn.

It's a long article. I'm happy with his explanation as per gyroplanes.
i.e.: a gryoplane uses an unpowered rotor (as you know) and the whole
body of the gyroplane hangs from the gimble head. But on a helicopter,
the rotor is powered and operates like a fan, the helicopter body is
firmly fixed to rotor head.

anyway, what's your opinion of how precession works on a helicopter?



Hi Handel.barz,

Thanks for the quote. Some of what he's saying makes sense to me, some of
it doesn't. I don't understand where this "large torque" caused by the
pilots control input is coming from. The gyros rotor system is in a
perpetual state of autorotation and there is no torque involved, per say.
Moving the control as the author says will indeed cause an increase in the
pitch of one blade while decreasing it on the other. That will increase the
aerodynamic drag of the one blade while decreasing it on the other but
that's not applying a "torque" to the system one way or the other as far as
I can tell. The change in pitch alone will also cause a difference in lift
on opposite sides of the rotor disk and the blades will precess (sp?) and
change the plane of rotation of the rotor disk as a result. In my mind,
it's pretty much the same thing a helicopter does, only the cyclic pitch
change is achieved via a swashplate on the helicopter as apposed to changing
the attitude of the rotor hub itself relative to the rotor mast to do the
same thing.

The one thing you quoted that I'll definitely take exception to it the
sentence that says "the helicopter body is firmly fixed to the rotor head."
That may be true in some cases but definitely not in all of them. The
experimental gyros I've looked at (Air Command, Dominator, Little Wing,
Sport Copter, and others) all have a fixed pitch (no collective capability)
under slung teetering rotor systems. That is a very common rotor design for
a number of helicopters too with the exception that the helicopters have
collective pitch. Some of the helicopters that come to mind that also use
an under slung teetering rotor hub are the Jet Ranger, Huey's, the old Bell
47's, Robinson's, and Rotorway's Exec on the experimental side. Their rotor
hubs look a bit difference but they have similar characteristics and
restrictions while flying. For example, none of them handle sustained low
or negative G loads for very long.

FWIW! :-)
Steve R.

  #5  
Old January 21st 09, 02:45 AM posted to rec.aviation.rotorcraft
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Posts: 6
Default Gyro PRECESSION

On Jan 20, 6:12 pm, "Steve R." wrote:
wrote in message

...



On Jan 19, 9:10 pm, "Steve R." wrote:


Where did you get that from? I'm not sure I agree with it. The
experimental gyros I've looked at may not have a swash plate like a
helicopter does but rather, tilts the rotor hub relative to the plane of
the
rotor disk. The net effect of that is to apply a cyclic pitch action to
the
main blades and they react 90 degrees in the direction of rotation per
gyroscopic precession.


What info source are you looking at?


Steve R.


Hi Steve,


I got my info from an article written by: Don McCoy, via regalpony.ca
and found on asra.org.au


A summary, in his own words: (gyroplanes)


The joystick is moved to the right causing the rotor head axis to tilt
to the right and out of alignment with the rotor axis. The pivoting
motion of the teeter bolt causes the pitch angle of the blades to van.
during the rotation. increasing the lift at the front and reducing it
at the rear. The resulting large torque due to the aerodynamic effects
of the change in pitch is directed to the right and causes the rotor
axis (angular momentum of the rotors) to rotate to the right following
the rotor head axis. As the rotor axis comes back in line with the
tilted rotor head axis, the pitch angle ceases to vary and the lift
from the front and rear rotor blade equalises, the torque vanishes and
the rotors are tilted to the right to initiate the turn.


It's a long article. I'm happy with his explanation as per gyroplanes.
i.e.: a gryoplane uses an unpowered rotor (as you know) and the whole
body of the gyroplane hangs from the gimble head. But on a helicopter,
the rotor is powered and operates like a fan, the helicopter body is
firmly fixed to rotor head.


anyway, what's your opinion of how precession works on a helicopter?


Hi Handel.barz,

Thanks for the quote. Some of what he's saying makes sense to me, some of
it doesn't. I don't understand where this "large torque" caused by the
pilots control input is coming from. The gyros rotor system is in a
perpetual state of autorotation and there is no torque involved, per say.
Moving the control as the author says will indeed cause an increase in the
pitch of one blade while decreasing it on the other. That will increase the
aerodynamic drag of the one blade while decreasing it on the other but
that's not applying a "torque" to the system one way or the other as far as
I can tell. The change in pitch alone will also cause a difference in lift
on opposite sides of the rotor disk and the blades will precess (sp?) and
change the plane of rotation of the rotor disk as a result. In my mind,
it's pretty much the same thing a helicopter does, only the cyclic pitch
change is achieved via a swashplate on the helicopter as apposed to changing
the attitude of the rotor hub itself relative to the rotor mast to do the
same thing.

The one thing you quoted that I'll definitely take exception to it the
sentence that says "the helicopter body is firmly fixed to the rotor head."
That may be true in some cases but definitely not in all of them. The
experimental gyros I've looked at (Air Command, Dominator, Little Wing,
Sport Copter, and others) all have a fixed pitch (no collective capability)
under slung teetering rotor systems. That is a very common rotor design for
a number of helicopters too with the exception that the helicopters have
collective pitch. Some of the helicopters that come to mind that also use
an under slung teetering rotor hub are the Jet Ranger, Huey's, the old Bell
47's, Robinson's, and Rotorway's Exec on the experimental side. Their rotor
hubs look a bit difference but they have similar characteristics and
restrictions while flying. For example, none of them handle sustained low
or negative G loads for very long.

FWIW! :-)
Steve R.


Hi Steve, thanks for the reply.

Yes, I know some of the workings of the rotorhead on a gyro are a
mystery to many people. Me too. I've been looking around and asking a
lot of questions where I can. I understand what McCoy is talking
about, mostly. (he's got a phd and I don't). However, since the hub
bar basically floats around in the rotor head and uses the rotor head
to define where it stops teetering, by tilting the rotor head the
teeter stops are moved out of alignment to the hub bar momentarily,
until the next teeter which is greater than the one revolution before.
That has to tip the blade up to get much more air suddenly and the
extra air is enough to change the angle of the rotor plane (disc). I
don't think anyone could force the rotor plane over by brute force
applied in such a small area. (I think I just repeated what McCoy said
only differently) But I'm satisfied with his definition.
As to helicopters, I know they morphed out of autogyros thanks to the
rotor head development of Cierva. But I don't understand how
gyroscopic precession can enter into the control of the helicopter
rotor plane when there is so much imput coming from the pilot. That
would mean his controls would be set in precession so a move to right
is really an input on the bottom of the rotor plane?
  #6  
Old January 21st 09, 07:14 AM posted to rec.aviation.rotorcraft
Steve R.[_2_]
external usenet poster
 
Posts: 31
Default Gyro PRECESSION

wrote in message
...
On Jan 20, 6:12 pm, "Steve R." wrote:
wrote in message

As to helicopters, I know they morphed out of autogyros thanks to the
rotor head development of Cierva. But I don't understand how
gyroscopic precession can enter into the control of the helicopter
rotor plane when there is so much imput coming from the pilot. That
would mean his controls would be set in precession so a move to right
is really an input on the bottom of the rotor plane?



If I'm reading you correctly, you're uncertain why the control inputs are
applied to the rotor blades ahead of where you really want the rotor to
react? Maybe I'm not following you on this but with regards to gyroscopic
precession, I'll offer this.

First of all, as a spinning object, the rotor system, be it on a helicopter
or gyroplane, is a gyroscope. As such, it acts like one. One of the
properties of a gyroscope is the property of rigidity in space. That is, it
wants to maintain it's plane of rotation and resists deviating from that
plane. If a force is applied to the gyroscope that is strong enough to
force it out of it's plane of rotation, that force will be reacted to at a
point 90 degrees in the direction of rotation from the point where the force
was applied. That is the definition of gyroscopic precession.

In a rotorcraft, all cyclic blade movements that are used to change the
attitude of the rotor disk in pitch or roll, must be applied 90 degrees
ahead of where the pilot really wants the rotor to move. If the rotor is
spinning clockwise as viewed from above, or moving from the pilots left to
right as seen from the cockpit, and the pilot wants to roll the aircraft to
the left, the actual cyclic movement applied to the rotor blades should have
each blade reach it's maximum pitch straight ahead and minimum pitch back
over the tail. The rotor will react to this at a point, 90 degrees in the
direction of rotation. That will have the forward blade climb to it's
maximum point on the right side of the bird, and the rearward blade descend
to it's minimum point on the left side of the bird and she rolls left.
However the cyclic pitch movements are achieved, the result is the same.

I hope that makes sense! :-)

Steve R.

  #7  
Old January 21st 09, 07:28 PM posted to rec.aviation.rotorcraft
[email protected]
external usenet poster
 
Posts: 6
Default Gyro PRECESSION

On Jan 21, 1:14 am, "Steve R." wrote:
wrote in message

...

On Jan 20, 6:12 pm, "Steve R." wrote:
wrote in message


As to helicopters, I know they morphed out of autogyros thanks to the
rotor head development of Cierva. But I don't understand how
gyroscopic precession can enter into the control of the helicopter
rotor plane when there is so much imput coming from the pilot. That
would mean his controls would be set in precession so a move to right
is really an input on the bottom of the rotor plane?


If I'm reading you correctly, you're uncertain why the control inputs are
applied to the rotor blades ahead of where you really want the rotor to
react? Maybe I'm not following you on this but with regards to gyroscopic
precession, I'll offer this.

First of all, as a spinning object, the rotor system, be it on a helicopter
or gyroplane, is a gyroscope. As such, it acts like one. One of the
properties of a gyroscope is the property of rigidity in space. That is, it
wants to maintain it's plane of rotation and resists deviating from that
plane. If a force is applied to the gyroscope that is strong enough to
force it out of it's plane of rotation, that force will be reacted to at a
point 90 degrees in the direction of rotation from the point where the force
was applied. That is the definition of gyroscopic precession.

In a rotorcraft, all cyclic blade movements that are used to change the
attitude of the rotor disk in pitch or roll, must be applied 90 degrees
ahead of where the pilot really wants the rotor to move. If the rotor is
spinning clockwise as viewed from above, or moving from the pilots left to
right as seen from the cockpit, and the pilot wants to roll the aircraft to
the left, the actual cyclic movement applied to the rotor blades should have
each blade reach it's maximum pitch straight ahead and minimum pitch back
over the tail. The rotor will react to this at a point, 90 degrees in the
direction of rotation. That will have the forward blade climb to it's
maximum point on the right side of the bird, and the rearward blade descend
to it's minimum point on the left side of the bird and she rolls left.
However the cyclic pitch movements are achieved, the result is the same.

I hope that makes sense! :-)

Steve R.


Hey, thanks. I'm begining to get the right idea.
I found this tid bit to add to it:
" . . . the control rigging in the helicopter compensates for
gyroscopic precession. In helicopters, the controls are rigged is such
a way that when forward cyclic is applied, the helicopter moves
forward, likewise for aft, etc. To accomplish this, the pitch horn is
offset 90 to the rotor blade. The controls still tilt the swashplate
in the same direction as the control input is made, but due to the
pitch horn placement, the input to the blade occurs 90 earlier in the
plane of rotation . . . "
So as the pilot pushes the stick left the input is really hitting the
swash plate 90 degrees before left or at the top. (if the blades are
rotating counter clock wise).
right?
  #8  
Old January 22nd 09, 02:21 AM posted to rec.aviation.rotorcraft
Steve R.[_2_]
external usenet poster
 
Posts: 31
Default Gyro PRECESSION

wrote in message
...
On Jan 21, 1:14 am, "Steve R." wrote:
wrote in message

...

On Jan 20, 6:12 pm, "Steve R." wrote:
wrote in message


As to helicopters, I know they morphed out of autogyros thanks to the
rotor head development of Cierva. But I don't understand how
gyroscopic precession can enter into the control of the helicopter
rotor plane when there is so much imput coming from the pilot. That
would mean his controls would be set in precession so a move to right
is really an input on the bottom of the rotor plane?


If I'm reading you correctly, you're uncertain why the control inputs are
applied to the rotor blades ahead of where you really want the rotor to
react? Maybe I'm not following you on this but with regards to gyroscopic
precession, I'll offer this.

First of all, as a spinning object, the rotor system, be it on a
helicopter
or gyroplane, is a gyroscope. As such, it acts like one. One of the
properties of a gyroscope is the property of rigidity in space. That is,
it
wants to maintain it's plane of rotation and resists deviating from that
plane. If a force is applied to the gyroscope that is strong enough to
force it out of it's plane of rotation, that force will be reacted to at a
point 90 degrees in the direction of rotation from the point where the
force
was applied. That is the definition of gyroscopic precession.

In a rotorcraft, all cyclic blade movements that are used to change the
attitude of the rotor disk in pitch or roll, must be applied 90 degrees
ahead of where the pilot really wants the rotor to move. If the rotor is
spinning clockwise as viewed from above, or moving from the pilots left to
right as seen from the cockpit, and the pilot wants to roll the aircraft
to
the left, the actual cyclic movement applied to the rotor blades should
have
each blade reach it's maximum pitch straight ahead and minimum pitch back
over the tail. The rotor will react to this at a point, 90 degrees in the
direction of rotation. That will have the forward blade climb to it's
maximum point on the right side of the bird, and the rearward blade
descend
to it's minimum point on the left side of the bird and she rolls left.
However the cyclic pitch movements are achieved, the result is the same.

I hope that makes sense! :-)

Steve R.

Hey, thanks. I'm begining to get the right idea.
I found this tid bit to add to it:
" . . . the control rigging in the helicopter compensates for
gyroscopic precession. In helicopters, the controls are rigged is such
a way that when forward cyclic is applied, the helicopter moves
forward, likewise for aft, etc. To accomplish this, the pitch horn is
offset 90 to the rotor blade. The controls still tilt the swashplate
in the same direction as the control input is made, but due to the
pitch horn placement, the input to the blade occurs 90 earlier in the
plane of rotation . . . "
So as the pilot pushes the stick left the input is really hitting the
swash plate 90 degrees before left or at the top. (if the blades are
rotating counter clock wise).
right?



Yes, sounds right, although I don't think I would have worded it quite like
that. I wouldn't say that the control rigging "compensates" for gyroscopic
precession, rather, I'd say the control rigging simply allows for gyrocopic
precession. That may be just a matter of semantics but in my mind, the word
"compensate" implies countering the effect (gyrocopic precession) and that's
not what's really going on. At least not the way I like to think about it.
:-) The 90 degree offset is built into the design of the rotor system and
the control links that run from the swashplate up to the rotor blade pitch
control arms. If you watch the swashplate tilt when the pilot moves the
cyclic control, you'll see that the swashplate actually tilts in the same
direction that the pilot moves the control. If the pilot moves it forward,
the swashplate tilts forward. Backward and the swashplate tilts backwards.
Likewise for left and right control movements and everything in between.
The main rotor system will follow in the direction that the swashplate
tilts. Everything that happens from the swashplate up, happens
automatically and the 90 degree lead is, as the man said, built into the
control rigging.

Steve R.

 




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