Joined: Mar 29, 2006 Posts: 289 Location: Scotland
Posted: Mon Feb 16, 2009 5:05 pm Post subject: Gyroscopic Precession
OK, blame WG for this one.
Is it possible to have gyroscopic precession in a non-rigid rotor system, I know many instructors teach it, and even Wagtendonk (is that the spelling?) preaches it.
But just because it is a simple and accepted explanation doesn't mean it's right. _________________ Generally wrapped in rubber, be it in the air or on the water.
Joined: Feb 20, 2008 Posts: 1059 Location: New York
Posted: Mon Feb 16, 2009 5:38 pm Post subject:
Ummmm
There are fully-articulated,
There are semi-rigid,
and there are rigid.......
Can you define a non-rigid system for me? That's a new one on me unless I need to add it to my UK/US dictionary.
As for gyroscopic precession............
Gyroscopic Precession is the resultant action or deflection of a spinning object when a force is applied to this object. Regardless of the rotor system, if it spins and we try to tilt it, GP exists. Can't change physics.
AB _________________ "A Copter Pilot's Life has it's... ups and downs"
Bell 47-206, Schweizer 300/500, Citation 525
if its anything but a rigid rotor surely the accelerative torque produced by GP can't be translated to the head as the change of path stops at the flapping hinge. is it not people getting confused with phase lag in regards to fully articulated rotorheads.
Now i'm writing this i am thinking that with a teetering hinge you would have the rotation and two opposing forces so in theory...
...ok now i'm confusing myself... _________________ PPL (H)
R22
B206
If it moves i want a go
There are fully-articulated,
There are semi-rigid,
and there are rigid.......
And there are Robinsons
My guess is what HW means by a non-rigid system is something where the blades are free to flap independently. Like a 300, 500, 109 etc.
I'll ask a question thats partially related to the gyros thing which I could probably phrase better if I thought about it. But it shoud get the debate fuelled.
Quote:
Does applying a force to something cause it to move ?
Joined: Feb 20, 2008 Posts: 1059 Location: New York
Posted: Mon Feb 16, 2009 8:57 pm Post subject:
animalsticks wrote:
my take on it until veeany puts something
if its anything but a rigid rotor surely the accelerative torque produced by GP can't be translated to the head as the change of path stops at the flapping hinge. ............ok now i'm confusing myself...
Yes, you are confusing yourself. If he is referring to the system on the 300/500 etc, then he is referring to a fully articulated rotor system in which the blades are free to flap, lead/lag (or hunt) and feather.
A flapping hinge (horizontal hinge) is part of the rotor system to overcome the effects of Coriolis force.
A lead/lag or drag hinge (vertical hinge) is used to overcome di-symmetry of lift. Neither has anything to do with gyroscopic precession.
If it rotates, it will experience GP. NO way around it.
Veeany,
No necessarily. The rotating body must only "experience" the force being applied. The amount of GP is directly related to the actual deflection of the rotating body from the plan of rotation. It is a matter of Precession therory.
The physics lecture usually runs all day but here it is in a nutshell. (I am renaming it The Veeany Theorim... )
Precession is due to the fact that the resultant of the angular velocity of rotation and the angular velocity produced by the torque is an angular velocity about a line which makes an angle with the permanent rotation axis, and this angle lies in a plane at right angles to the plane of the couple producing the torque. The permanent axis must turn towards this line, since the body cannot continue to rotate about any line which is not a principal axis of maximum moment of inertia; that is, the permanent axis turns in a direction at right angles to that in which the torque might be expected to turn it. If the rotating body is symmetrical and its motion unconstrained, and if the torque on the spin axis is at right angles to that axis, the axis of precession will be perpendicular to both the spin axis and torque axis. Under these circumstances the period of precession is given by:
T_p = \frac{4\pi^2I_s}{QT_s} In which Is is the moment of inertia, Ts is the period of spin about the spin axis, and Q is the torque.
In general the problem is more complicated than this, however.
Got it all down pat now?
Oh, by the way....I didn't make that up. It's all valid theory.
_________________ "A Copter Pilot's Life has it's... ups and downs"
Bell 47-206, Schweizer 300/500, Citation 525
Joined: Aug 23, 2005 Posts: 266 Location: On a course.... golf course
Posted: Mon Feb 16, 2009 10:32 pm Post subject:
Precession is only a convenient way to explain the phenomenon of phase lag / advance angle.
If GP were TRULY the cause of it, then advance angle would always be 90 degrees.
It ain't. in some machines it is 72 degrees, in others it is 84, in most it is very close to 90, and it is simply the rotational distance it takes for the blade to experience the increase in pitch angle applied by the swash plate, to start moving up as a result of the accelerative force, to have its angle of attack reduced by the increased induced flow, and to peak out at maximum upward deflection before flapping down again on the other side.
But as an approximation to the dumbos in the street, a gyroscope is a good place to start. It ISN'T the cause. Look up Shawn Coyle or Nick Lappos for a more detailed debunking of the GP theory.
Or Ray Prouty The Article is entitled something like "Is the Rotor A Gyro"
I'll put some of this in detail in the next few days but for now think about these points.
Can the behaviour of a rotor system be described pretty much by how it reacts to the aerodynamic forces acting up it.
If the wind blows on a rotor disc from the front of the aircraft (in the hover or on the ground) which way will the disc flap and why ?
Where is the most lift force generated in this example ? Would the biggest lift force result in the biggest amount of flap or the biggest rate of flap ?
Could it be that the gyroscopic properties of a spinning rotor system are overwhelmed by the aerodynamic ones which work in a similar sense ?
Could it also be that some member(s) of the forum believe that Wikipedia should be quoted without question ?
i just posted saying "that's what i read on wikipedia" then saw veeanys post... back pedal back pedal!!! _________________ PPL (H)
R22
B206
If it moves i want a go
Joined: Mar 29, 2006 Posts: 289 Location: Scotland
Posted: Mon Feb 16, 2009 11:07 pm Post subject:
Nice one AC.
A non-rigid rotor system?, teetering head system perchance? Especialy one with coning hinges as well? Things as floppy as a month old carrot. The only rigidity there would be in the lead lag, and even that is there whilst one blade leads, the other lags.
Another question, is rigidity not one of the fundamental requirements of a gyroscope? _________________ Generally wrapped in rubber, be it in the air or on the water.
Joined: Aug 23, 2006 Posts: 25 Location: Hall Green
Posted: Mon Feb 16, 2009 11:07 pm Post subject:
veeany wrote:
If the wind blows on a rotor disc from the front of the aircraft (in the hover or on the ground) which way will the disc flap and why ?
Where is the most lift force generated in this example ? Would the biggest lift force result in the biggest amount of flap or the biggest rate of flap ?
At constant RRPM would "amount" and "rate" not be directly proportional?
Joined: Feb 20, 2008 Posts: 1059 Location: New York
Posted: Mon Feb 16, 2009 11:42 pm Post subject:
Ascend_Charlie wrote:
But as an approximation to the dumbos in the street, a gyroscope is a good place to start. It ISN'T the cause. Look up Shawn Coyle or Nick Lappos for a more detailed debunking of the GP theory.
I don't believe that you are trying to sell us on "debunking" the GP theory which has been part of helicopter development since the first copter flew!
I have read some of Coyle's and Lappos' works and look upon them more as fiction than fact.
Besides, I do not look upon myself as a "dumbo in the street" but rather knowledgeable, reasonable, and open minded (most of the time).
However, as far as this goes, I'll stay with the present GP theory, and so will my students.
No offense meant, and none taken.
AB
ADDED:
Read my post on the Precession Theorem which clearly shows GP is the proper explanation for the aerodynamic phenomenon.
_________________ "A Copter Pilot's Life has it's... ups and downs"
Bell 47-206, Schweizer 300/500, Citation 525
I believe the rigidity relating to gyroscopes refers the gyroscopes desire for its spin axis to remain rigid in space ( aligned where it is already) and not whether its a rigid disc.
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