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HomeAircraftHelicopterHelicopter Main Discussion › Theory discussion on Gyroscopic Precession
04-23-2009 06:52 AM  9 years agoPost 1
psawya

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Woodinville, WA

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There was a post that asked a question about multiblade rotor heads that got sidetracked, some would say hijacked, partly my fault. As such I decided to continue this debate in an entirely new thread. For reference here is the original post:

http://runryder.com/t508224p1/

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04-23-2009 07:21 AM  9 years agoPost 2
psawya

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Woodinville, WA

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F6, you glossed over this:

Quote
When the blades deviate from that steady state hover . . . then GP happens.

I think what he was trying to say is that there needs to be some force acting on the rotor disc for GP to happen.
More appropriately what should be said is "there needs to be some force acting on the rotor disc for the effects of GP to be shown"

The gyroscopic precession is always present in a spinning gyro, it takes an external force for gyroscopic precession to manifest itself into a motion. Without an external force the gyroscopic precession is "in balance" so to say. When an external force attempts to move the gyro off it's axis the effect of gyroscopic precession to the external force is a movement 90 degrees from the external force. GP is not the external force, nor is it the result of the external force. The result of the blade movement 90 degrees from the input is the combination of the external force and gyroscopic precession.

Best Regards,
Pete

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04-23-2009 04:06 PM  9 years agoPost 3
Jlerch

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Parrish, Florida

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Watch at YouTube

If you want to know more, watch and learn via the MIT open course learning tools

Watch at YouTube

James Lerch - Tampa Bay FL

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04-23-2009 08:19 PM  9 years agoPost 4
S Bell

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Nova Scotia Canada

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That won't apply with a helicopter rotor, tilt a spinning helicopter fuse and it will follow the applied force or it will fall and cut your legs off

If you removed GP from the rotor how long would it take for the blade to reach its highest point after maximum cyclic pitch change? That's what you have to ask. Action and reaction takes time just like with a non spinning wing on an airplane. It just happens to directionally cooperate or agree with gyroscopic laws and that's why it is often explained that way. This is why rotors flap aft with increase in forward speed if nothing else is done to correct the tendency. Spinning Frisbees don't do that.

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04-23-2009 11:38 PM  9 years agoPost 5
Awesome

rrApprentice

Ireland

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That's a brilliant lecture.

I remember learning gyroscopic precession and rigidity but it was not very interactive like that lecture.

Is there nore available on-line?

Thanks for posting Pete.

Kieran

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04-24-2009 12:23 AM  9 years agoPost 6
GimbalFan (RIP)

rrProfessor

Big Coppitt Key, FL

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This post by Mark C a few years back was helpful:
"Let's start with a pendulum. Yes, a simple pendulum swinging back and forth back and forth and no, I'm not trying to hypnotize you.

"Now as the pendulum swings past the bottom, imagine that you tap it with a small hammer. Now imagine the path that the pendulum takes. It's path is constrained to an arc and the forces you applied at a right angle turn the path of the arc. If you are holding the pendulum in your right hand and it is swinging away from you as you tap it with your left hand the arc will have turned clockwise (looking down at the arc).

"It is all that simple. In your mind extend the path of the arc so that the pendulum swings in a complete circle. The top coming toward you and the bottom going away.

"Now add another pendulum 180 degrees opposite of that one. Now let the string become a shaft and the 2 pendulums are spinning about a hard axle. You still with me?

"Now if instead of tapping the pendulum with a hammer as it crosses the bottom you use 2 hammers and tap the axle. Yea, I know gotta be in space to do this but stick with me here. You tap the top of the left side of the axle and the bottom of the right side right as one of the pendulums cross the bottom. This is the same as tapping the pendulum itself from the left. Notice I use hammers because I want you for now to only think in terms of a momentary event and the resultant changes that occur at that moment. The path of the pendulums change the same way and the path of their arcs change and the pendulums, shafts and axle have now rotated clockwise slightly to align themselves with the new path of the arcs. Simple.

"Now keep in mind that once the assembly has rotated to align itself on the new path it stops since the force was only a momentary tap of the hammers. Trust me. Try this with the pendulum. It will keep swinging on it's new path the same way the pendulums are now rotating about their new axis.

"Now here is where I may loose you. Doesn't take a leap of faith. Takes a leap of 3 dimensional thinking in continuous terms. Lets go for it.

"Imagine that there are now four pendulums on the axle all 90 degrees apart. Now as each pendulum passes the bottom of the arc heading away from you you tap the axle with the hammers. Each time you tap the axle (lightly) the whole assembly rotates and stops and you find yourself chasing the axle around in a clockwise circle in order to keep tapping it. You still with me?

"Then lets go for complete continuity. The pendulums are now a solid wheel spinning (we are back on earth) and you support the axle on your fingertips. Now you remove your left fingertip and now there is a constant force that is applied to the axle. Gravity down on the left and your fingertip up on the right. Now with this "constant tap of the hammers" or constant steady force applied the assembly begins to spin clockwise on your right fingertip.

"I can see it.. Can you?"

op-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-t

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04-24-2009 12:36 AM  9 years agoPost 7
predatorman

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Falkland Islands

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AIRWOLF!!! where are you when I need you

I will try to think about this....when going to sleep

Quality takes........time!

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