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Old Dec 28, 2006, 07:25 AM
Mickey from Orlando. Really.
Joined Nov 2004
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Quote:
Originally Posted by leadfeather
Mickey,

With cyclical controls (assume very flexible flapping hinge), is the gyroscopic precession of the rotor disk from control inputs eliminated?

Dan
I think I understand the question, so my answer is no, the 90 degree lag is always there, regardless of how loose the flapping hinge is.
Precession doesn't really apply. Precession is often used to describe a rotor behavior, but this is an error. Precession is a gyroscope trying to maintain constant angular momentum and isn't the same.
If this doesn't answer, please expand on the question.
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Old Dec 28, 2006, 07:36 AM
Mickey from Orlando. Really.
Joined Nov 2004
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Quote:
Originally Posted by edi
Mickey,

your posts are just great!! Please keep them coming, I find them extremely instructive and well written.

A question about the relationship between flapping and coning: Doesn't flapping rotors allow for more coning automatically? In other words, if you don't allow coning, won't the flapping be gone, too?

Edi
Perfect conclusion! Yes this is entirely true, allowing one generally allows the other. However you can arrange flapping with lower coning by choice of RPM and blade weighting. So it's possible to have flapping with fairly low coning. But keep in mind, that if you have cyclic feathering (spindle tilt or swashplate) you don't need flapping in the first place. Cierva used flapping because he didn't have cyclic feathering.
This has been a problem with model gyrocopters for a long time, probably delaying their development. Modelers misunderstood this relationship and also did not understand that flapping and feathering accomplished the same thing. So older gyro models had flapping and feathering combined with high following rate (also misunderstood). What resulted was a very unflyable model.
Perhaps I'll deal with this combination next.
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Old Dec 28, 2006, 07:37 AM
edi
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OK, so what's feathering?
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Old Dec 28, 2006, 09:08 AM
Mickey from Orlando. Really.
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Quote:
Originally Posted by edi
OK, so what's feathering?
Twisting the blade so it rotates along the long axis. Tilting the spindle feathers the blades, so does moving a swashplate. Essentially you change the blades angle of attack with respect to the shaft as the blade rotates. Swashplate control does this mechanically with a feathering bearing. Tilting spindles do it by actually tilting the shaft with respect to the rotor plane. This is detailed in the post above about cyclic.
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Old Dec 28, 2006, 09:39 AM
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Quote:
Originally Posted by mnowell129
I think I understand the question, so my answer is no, the 90 degree lag is always there, regardless of how loose the flapping hinge is.
Precession doesn't really apply. Precession is often used to describe a rotor behavior, but this is an error. Precession is a gyroscope trying to maintain constant angular momentum and isn't the same.
If this doesn't answer, please expand on the question.
I think my question may not be clear enough.

Case 1) perfectly rigid rotor. Nose up input from the elevator will cause a rolling moment on the rotor due to gyroscopic precession...just plain physics.

Case 2) flexible flapping hinge. Nose up input from elevator causes cyclical input which causes blades to "fly" to new plane of rotation. Since the change of rotational plane of the rotor disk is accomplished by the blades acting on the air rather than the spindle torquing the rotor, will there still be gyroscopic precession?

I think the answer is probably yes, angular momentum is conserved and there will still be precession in case 2.

Dan
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Old Dec 28, 2006, 09:56 AM
Mickey from Orlando. Really.
Joined Nov 2004
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Quote:
Originally Posted by leadfeather
Case 1) perfectly rigid rotor. Nose up input from the elevator will cause a rolling moment on the rotor due to gyroscopic precession...just plain physics.

Case 2) flexible flapping hinge. Nose up input from elevator causes cyclical input which causes blades to "fly" to new plane of rotation. Since the change of rotational plane of the rotor disk is accomplished by the blades acting on the air rather than the spindle torquing the rotor, will there still be gyroscopic precession?

I think the answer is probably yes, angular momentum is conserved and there will still be precession in case 2.

Dan
Precession always gives me a headache. But I think in case 1, I agree with you. A rigid rotating body is a gyroscope and tries to maintain angular momentum by precessing.
This is clearly true when you use rudder to make an airplane do lomcevak by forcing a nose down precession with right rudder.
In case 2 I think you are off by 90 degrees. The cyclic is 90 degrees off from the spindle tilt. Agreed, the nose up tilt doesn't really apply a force to the blades directly. Agreed, the cyclic does cause the blades to fly to a new position. But the cyclic is applied at the sides not the front and back so the phase lag (or precession) is from the side to the front, causing a tilt up.
A rotor, even a "rigid" rotor really isn't a rigid body, i think this is the key.
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Old Dec 28, 2006, 11:22 AM
Mickey from Orlando. Really.
Joined Nov 2004
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Following rate.

Here's some reading about following rate. Topic after we talk about steering the rotor with indirect cyclic.
I'm posting here because we'll need it and I'm answering another question on RCU.

This is from Radio Control Helicopter Models by John Drake, published by Argus Books.
With apologies to John Drake and Argus books. I'm not trying to steal intellectual property from them but the dang book is out of print and I can't tell you to go buy a copy.
Maybe if there is enough demand you can force a reprint.
You can still find copies on www.abebooks.com
http://www.abebooks.com/servlet/Sear...er+models&x=42
and amazon and ebay.
mick
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Old Dec 28, 2006, 12:47 PM
FPV really is fun.
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Flybar time?
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Old Dec 28, 2006, 12:48 PM
Mickey from Orlando. Really.
Joined Nov 2004
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Will get to it. Seems like this thread will have many topics.
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Old Dec 28, 2006, 02:03 PM
FPV really is fun.
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Cool. Thanks for putting all this info out there. You have cleared up a few things for me. Good reading, too.
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Old Dec 29, 2006, 04:45 PM
Mickey from Orlando. Really.
Joined Nov 2004
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TURN the page

Ok so now we get to turning with the rudder (or not). We've already shown how a tilting spindle or swashplate can create cyclic pitch. This in turn tilts the rotor in a new direction. The tilted rotor thrust no longer goes through the CG and the model tilts in the intended direction providing pitch and roll control.
What about trying to turn with rudder.
In the first set of figures we have our happy little fixed mast gyro flying along. We have a side, top and rear view of our gyro, mostly showing our aft tilted main shaft. Now lets apply some right rudder as shown in the second set of figures. The side view looks pretty much the same but hold on. Our rear view looks pretty ugly. What happens is that our aft tilt mast, when yawed to the right, tilts the mast to the left. You can prove this to yourself by sticking a straw in a holiday cupcake at the aft mast angle and rotating your cupcake while viewing from the rear. Sure enough that aft tilt turns into side tilt when yawed. Oops. If you've followed along to this point this looks just like what it is, left cyclic pitch. I've even drawn what happens to the blade in the fore and aft position. This is a problem because this is left cyclic with right rudder.
Lets go further to the last picture. Remember asymmetric velocity. What used to be the peak advancing blade speed at A is now at A'. The rotor tries to flap up at B'. This is good because now we have a little roll component in the direction we want to turn. But wait, there's more. Because you have a fixed mast, you have to have flapping hinges to correct for asymmetric velocity. Guess what else you got. Yes, coning! So now the blades are coned and the blade with the highest angle of attack due to coning is now in the flight direction B'. But we now know that the phase lag of cyclic takes place and the maximum tilt of the rotor due to coning now takes place at C'. Double oops. Now we have left cyclic due to mast tilt, we have left coning induced roll and only a mild amount of asymmetric velocity flap to roll to the right. Which way it will roll is anybody's guess and could just as well be left as right depending on rotor speed, coning angle, flapping hinge stiffness, yada, yada, yada....
But wait. What if our rotor direction was CCW instead of CW as viewed from the top. Now our coning induced roll would peak at A' and add to the roll due to asymmetric velocity. Now we got a chance of turning. But guess what, we are going to roll better to the advancing blade not the retreating blade. This has to be confusing to the uninformed observer. The silly gyro copter will turn quite nicely into the advancing blade against all intuition and won't turn worth a hoot against the supposedly lower lift retreating blade.
Is there any wonder why those floppy hinged, rudder elevator controlled gyros didn't fly worth a D&^^ ?
So does anyone really want to try a rudder only controlled gyrocopter any more?
Here's the good news. If you do the same little mental experiment with elevator you will see that up elevator tilts the shaft back and provides up cyclic and makes the rotor tilt back, just as you desire. So at least the elevator works like we want.

So what's the simplest, highest likelyhood of success single rotor layout? At a minimum you need roll control on a tilting spindle and elevator control. Rudder control doesn't hurt because as we've seen here the gyrocopter does bad things when it skids like it would in an un-coordinated turn. So a little rudder to keep things coordinated wouldn't be a bad thing.

Next up. Is your rotor stable as well as controllable? Turns out your rotor can be too controllable.

anybody still out there or have I lost you all by now?

mickey
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Old Dec 29, 2006, 05:46 PM
iPhly R/C with iPhone
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Mickey, thanks! I was pondering an actuator-controlled, 3-channel gyro based on the Penni head, now I know it won't work! Your post is informative as ever.

Ari.
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Old Dec 29, 2006, 06:52 PM
Winging it >
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This matches up perfectly with what I've seen on the GyroStick project I'm working on. I was hoping there was a diifferent answer and there was a way to use the simple Slow Stick rudder for steering. Oh well, time for plan B.

Thanks again for the lesson Mickey!

Dan
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Old Dec 29, 2006, 07:11 PM
Mickey from Orlando. Really.
Joined Nov 2004
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Ari,Dan,
A little theoretical knowledge can go a long way. I'm sad that you guys have time invested (at least mentally in Ari's case) in something that might not work. I hope you don't see me as being negative about this, I'm just trying to explain why things the way they are.
There are some folks that will go ahead and try something even though there is fairly convincing scientific theory that it won't work. I'm just one of those that would rather try something that the theory says has a good chance of working rather than not.
Have a good weekend.
mickey
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Old Dec 29, 2006, 07:11 PM
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Mickey,
yr. post no. 41 in this thread explains perfectly one or two problems i had to go through with the slopter-1.0. I cannot claim full understanding of everything, but each little step of understanding counts.
Thanks. Please go on.
Stephan
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