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HeliTorque :: View topic - Gyroscopic Precession
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HeliTorque Forum Index » Flight Dynamics

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Heliwhore
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PostPosted: Mon Feb 16, 2009 11:54 pm    Post subject: Reply with quote

Hmmm, another spanner in the GP wagon: does a gyroscope not require large mass (preferably located at the outer limits) and very high rotational velocity? Neither of which are exhibited by a helicopter.

Try spinning a bicycle wheel very slowly, and see if theres much precession on the go. Now a bike wheel has an AWFUL lot more mass around the circumference than a rotor system.

If there is any GP happening, it is so far out-shaddowed by aerodynamic forces as to be barely noticeable.
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PostPosted: Tue Feb 17, 2009 12:00 am    Post subject: Reply with quote

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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.


As we are not looking at precession resulting from a force exerted by the hub/rotor shaft, then I think thats a fair definition.

Edit: I was just off to bed, but this is bugging me. Surely a gyro requires the whole unit to be rigid, so that a force can be precessed to 90 deg around the gyro. Now apart from the fact that a 2 blade-er does not have a blade at 90 degrees to the force at the time, I'm still gonna stand by " the things as floppy as a month old carrot". Therefore how can it precess any force to any other area of the plane of Rot.

Now I'm going to bed.....
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Last edited by Heliwhore on Tue Feb 17, 2009 12:30 am; edited 1 time in total
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PostPosted: Tue Feb 17, 2009 12:30 am    Post subject: Reply with quote

veeany wrote:
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.


Correct Vee.
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PostPosted: Tue Feb 17, 2009 12:36 am    Post subject: Reply with quote

Heliwhore wrote:
Hmmm, another spanner in the GP wagon: does a gyroscope not require large mass (preferably located at the outer limits) and very high rotational velocity? Neither of which are exhibited by a helicopter.

Try spinning a bicycle wheel very slowly, and see if theres much precession on the go. Now a bike wheel has an AWFUL lot more mass around the circumference than a rotor system.

If there is any GP happening, it is so far out-shaddowed by aerodynamic forces as to be barely noticeable.


No it does not. Mass is not important. Lighter rotating objects or heavier ones exhibit GP proportionately. Any rotating object such as a bicycle wheel does exibit GP but the amount and resultant force is directly proportional to the force applied. In a slowly rotating bike wheel, the wheel will push away easier than at a faster speed since it does not have enough velocity to resist the force applied.

As speed increases, the rotating object becomes more "rigid in space" and will resist any force applied which would tend to offset its' rotational plane.

Also, if you pedalel to slow, you'll fall off the bike. Laughing Laughing

ADDED:
Think of it this way....
A normal bike tire is fat with a lot of mass, but a racing tire is thin with half the mass. Both will exhibit GP in proportion to the force applied and react the same way, as long as the velocity is sufficient.
Look at a rotor system as the thin racing tire.
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Last edited by afterburner on Tue Feb 17, 2009 12:43 am; edited 1 time in total
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PostPosted: Tue Feb 17, 2009 12:43 am    Post subject: Reply with quote

Thanks AB, I think you just proved my argument.
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PostPosted: Tue Feb 17, 2009 12:45 am    Post subject: Reply with quote

Heliwhore wrote:
Thanks AB, I think you just proved my argument.


See the bottom remark which I ADDED which might help to further draw a better picture of GP and MASS.
The more velocity, the more rigid the object becomes.

AB

PS: Sent you also a PM re: above.
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PostPosted: Tue Feb 17, 2009 12:52 am    Post subject: Reply with quote

Hmmm, I'm not sure I've said that GP doesn't exist at all in this. I can't prove that either way. I'm not that smart.

What I did say is that at low mass and velocity the effects are small, that I can prove. So cheers, you're adding to my campaign.
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PostPosted: Tue Feb 17, 2009 1:26 am    Post subject: Reply with quote

Heliwhore wrote:
Hmmm, I'm not sure I've said that GP doesn't exist at all in this. I can't prove that either way. I'm not that smart.
What I did say is that at low mass and velocity the effects are small, that I can prove. So cheers, you're adding to my campaign.


Not sure what we are saying anymore.... Laughing
but glad I'm helping. Laughing ... I think. Rolling Eyes

Heliwhore wrote:
Thanks AB, I think you just proved my argument.

I did. Great. What the heck was it anyway??? Laughing

Got a cold, mind tired, going to bed.

Sickly
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PostPosted: Tue Feb 17, 2009 2:20 am    Post subject: Reply with quote

Quote:
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.


Well, AB, if you know more than these hugely experienced and qualified test pilots, one of whom has been the chief Sikorsky test pilot, and is now with Bell, you should write some books like they have. And get the publishers to put their books in the fiction section and put yours into fact.

Yes, GP fits the idea well enough to convince the punters. But why, then, would several helicopters not obey the theory? Why would they have 72 degrees of lead angle, if a gyroscopologist demands that it be 90 degrees to fit the theory?

GP is for convenience. If you want to turn it into the Fourth Epistle According to Afterburner, then go ahead, but it isn't the absolute truth. Rolling Eyes
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PostPosted: Tue Feb 17, 2009 2:46 am    Post subject: Reply with quote

afterburner wrote:
No offense meant, and none taken. AB

I meant no offense, but it seems you do.

ascend charlie wrote:
Well, AB, if you know more than these hugely experienced and qualified test pilots, one of whom has been the chief Sikorsky test pilot, and is now with Bell, you should write some books like they have.

Glad you brought it up. I have. Three actually. All training manuals.

Quote:
GP is for convenience. If you want to turn it into the Fourth Epistle According to Afterburner, then go ahead, but it isn't the absolute truth. Rolling Eyes

That wasn't necessary. I made no attack upon you so why do you do it to me? I just don't agree with your authors. If you wish to rewrite aerodynamics as it applies to GP, fine.

Why do some have 72 deg and others 90 deg.....Could it be that they are fully articulated systems whereby the blades can "hunt" ? This would account for the difference of your 18 deg. However in semi-rigid and rigid rotor systems, the 90 deg principle accurately applies. Regardless of any incidental variance, the basis of theory still holds true.

However, I have no intention of turning this into a heated debate. Nothing else I wish to say on the subject.

Good nite.
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PostPosted: Tue Feb 17, 2009 9:06 am    Post subject: Reply with quote

Gents

This is unfortunately usually one of those subjects that causes lines to be drawn in the sand and neither side can see the others point.

There are usually two or three schools of thought on Gyroscopic Precession as it applies to a rotor system.

I AM NOT HAVING A GO AT ANYONE WHO IS DESCRIBED BELOW but I have been watching this for years and it usually goes something like

US Trained pilots seem to believe that gyroscopic precession is solely repsonsible for how a rotor system flaps.

UK and Aus Trained Piliots either seem to either favour the above or favour a purely aerodynamic description for the action of the rotor system.

It seems to come down to whatever you were taught when you trained which you then quote as gospel for the rest of your life, there are some exceptions to this rule and they are perhaps the idiots like me who spend their time trying to teach the truth and question just about everything until they can prove it.

A small amount of thinking about it does raise some quesitons which seem to cause trouble for both camps, and its at that point the blind following of a concept which may not fully explain things starts to fall apart and all kinds of obscure explanations are dragged in to try and justify what really happens (usually from those who are blind followers of the purely gyroscopic theory).

Before we go any further lets look some definitions of aerodynamics.

Aerodynamics
From Nasa
What is aerodynamics? The word comes from two Greek words: aerios, concerning the air, and dynamis, which means force. Aerodynamics is the study of forces and the resulting motion of objects through the air.

From Wikipedia
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object.

Gyrodynamics
From Most of the mechanical engineering websites
The study of rotating bodies, especially those subject to precession.

Put simply aerodynamics has got nothing to do with gyroscopic precession it is do with motion caused by moving through the air.

If any gyroscopic tendancies are exhibited by a rotor they are due to the angular velocity of the rotor system and the mass distribution about the spin axis. They will still be present in the absence of aerodynamic forces.

Anyone want to put up some clinical defintions of rigidity and precession as they apply to a gyroscope.

I do have a plan for where this is going but I think its good to get all the opposing theories and ideas out on the table before we go any further.
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PostPosted: Tue Feb 17, 2009 4:24 pm    Post subject: Reply with quote

i am finding this sooooo interesting - handbags at dawn, who cares, all these theories are great and relevant whether 100% accurate,not accurate or somewhere inbetween - think edison and the light bulb(is that right?) each time he got it wrong and he eventually said he found a thousand ways to not make a light bulb- till he found a light bulb.

can't wait to find out the correct answer - and we will!

Very Happy
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PostPosted: Tue Feb 17, 2009 9:16 pm    Post subject: Reply with quote

You know, I've been looking for any information on a gyroscope that isn't a rigid (and by that I mean solid) structure, and so far I've come up with zip.

We may have to conduct our own experiment here. (breaks out the mad scientist costume)
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PostPosted: Tue Feb 17, 2009 9:30 pm    Post subject: Reply with quote

HW

Lets start with a 206 and work towards a more floppy solution then !

Anyone got any definitions for me yet ?

I am building a model to show how these things behave but I may be some time.

GS
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PostPosted: Tue Feb 17, 2009 9:34 pm    Post subject: Reply with quote

Quote:
I do have a plan for where this is going but I think its good to get all the opposing theories and ideas out on the table before we go any further.


I have a funny feeling we have expressed our opposing theories.

At this point i'd like to state my reasons for bringing this topic up. It's not that I'm saying GP doesn't exist in rotor systems, as I said earlier, I'm not smart enough to prove it either way. It is also a subject that has been argued by much better informed and possibly better minds than ours many times over.

HOWEVER

The thing is, Veeany has edged on the reasons that its important we argue these topics. We are all guilty of passing on the information that was passed on to us, simply because the answer fits, and it makes our life/job easier. Occasionally we need to stop and think about the other side of the story, and even if we end up rejecting the opposing view, at least we are rejecting it for it's own merits.

I'm now looking forward to seeing where Veeany is heading with this, I still find this a fascinating subject, even after watching the arguments for years.....
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