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Old Dec 11, 2007, 05:10 PM
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Hmm. Should I delete this or not. But perhaps I'll entertain myself with some responses.
1) I've read every word of Juan De La Cierva "Wings Of Tomorrow The Story Of The Autogiro". It's very clear that you haven't because there are no references to coning being stabilizing. In fact to quote Cierva himself :
Quote:
Until more recent Autogiros, it was found that the high center of gravity with the low fin area formed by the fuselage and vertical surfaces rendered the machine laterally unstable at high speed. This has been corrected in the present machines by the use of substantial dihedral in the fixed wings and turned-up tips, giving the effect of high fin area.... When the rudder is used to turn the machine, a flat turn would result with attendant skidding if no up-turned tips were used."
This was Cierva himself talking about his machines that had coning. You're wrong Dave, read your own book.

2) You are so confused about what I'm saying we really can't have a conversation. All the designs that fly, fly. I'm not saying that they don't. I'm saying that the coning doesn't improve the designs, it's just something that happens.

3) There is no full scale information supporting your theory other than you pointing at videos and vaguely referring to books you haven't read. I've searched the web over and over trying to see if I could find some reason to agree with you and there is no support for your theory in the full scale business, only references to the need for roll trim due to coning. You're tantrums where you point at a gyrocopter that flies with coning insisting that its the cause of the stability have no scientific basis.

4) I've flown models with and without coning. I know you haven't because you'd probably rather have a porcupine rammed down your throat than fly one of my designs. The stability of both models was the same with or without coning.

5) When someone proves me wrong I admit it. I thought a small teetering rotor would be basically impossible, but Emilio has made one. I already admitted that I though this was impossible and that I was wrong.
Prove me wrong Dave and I'll admit it.
But of course you won't try to prove me wrong on the technical aspects of the argument, you'll call me names and send PM's and emails to your buddies and giggle and rant behind the scenes but never come up with any real science.

Have fun Dave, this post will probably provide you a whole month of entertainment. In a day or two I'll delete both your and my post because it doesn't add anything to the aerodynamics thread.
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Old Dec 11, 2007, 08:11 PM
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What ever you say Mickey.......
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Old Dec 12, 2007, 10:37 AM
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Well, at least to me this has made some things clearer. This spinning bit always makes my head go dizzy after a while and more than once I have found that there were bits I had thought I had understood them only to find out that I hadn't after all.

Right now, I think I have understood Mickey's explanation why coning isn't desirable. Still, these last posts made it even clearer to me and finally, they have pointed me to a book I do not have. Thanks also for the nice video with that Pitcairn. Whatever we think about coning, we can certainly agree that it shows very cool pictures of a very cool autogyro. So at least I have profited here
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Old Dec 12, 2007, 02:03 PM
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I hope J. Gordon Leishman will forgive for quoting him electronically. The credit goes to
Principles of Helicopter Aerodynamics, by J. Gordon Leishman, Cambridge University Press, 2000.

Dr. Leishman is an Associate Professor of Aerospace Engineering at the University of Maryland and a former aerodynamicist at Westland Helicopters.

There are two clear statements. Dissymmetry of lift causes the rotor to flap back, not roll. Coning causes the rotor to roll 90 degrees from the direction of flight.

If the direction of flight is sideways, like in a slip or gust created roll, the rotor will flap back due to dissymmetry of lift, creating a correction. The coned rotor will pitch nose up or down depending on rotor direction and roll direction but will not roll back the correct direction for a correction. Stability is the measure of a craft to return to level after a disturbance. A rotor tends to return to level because of lift dissymmetry by itself. Coning does nothing to cause a return to level other than pitch the nose up or down that does nothing to correct a roll disturbance.
If you consider further, a model with no controllable rudder will slip in the turns, if the rotor is coned then the nose will pitch up or down in the slip. This will make the model require more up elevator in the turn to one direction versus the other. This problem is reduced when the rudder is used to produce a coordinated turn with no slip. However consider that when you turn you increase the G load, this increases the coning and changes the roll trim yet again. It is to your advantage to have the least coning possible. The easiest way to do this is to increase the mass of the blades, the further out the better, thus tip weights will improve the flyability of the model, albeit at the cost of increase rotor spin up time.
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Old Dec 22, 2007, 07:11 AM
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Hi,

I have started reading this topic and I am at page 3 right now. Sometimes I have to re read some section to understand it.

There are some things I don't understand but I will if I read more about it I think.

But if you look at the following picture you see something I don't get and will not get by myself.



The 16lb (pounds?, is it possible for the writer to convert this to metric? Or are there conversion tables available so I can do it myself?) force is said to be the centrifugal force. Should this force not be perpendicular to the axis of rotation? So both forces should point in a direction parallel with the plane of rotation for the non hinged blades. Now there is a torque introduced by the forces to the non hinged blades. I think this is wrong. Or does it have something to do with gyroscopic effects while tilting the mast? I think the only "torque" due to rotation is in the hinged situation forcing the blades to move to the plane of rotation, they will not go there, except in front and aft position, due to the lift forces.

It raises another question as I'm writing this. Will the down flapping blad get a position beneath the plane of rotation? There is only less lift but no "downlift" as far as I know.

Please help.

Regards,

Leo
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Old Dec 22, 2007, 08:07 AM
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Leo,
The picture that you posted is an illustration of lift and drag and the 16 lbs is the drag. It has nothing to do with centrifical force. Try <http://www.marylandmetrics.com/techindex.htm> for your conversions.
Gene
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Old Dec 22, 2007, 04:47 PM
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Quote:
Originally Posted by rchopper56
Leo,
The picture that you posted is an illustration of lift and drag and the 16 lbs is the drag. It has nothing to do with centrifical force.
The diagram is not about lift and drag, it is about centrifugal force. This diagram illustrates the different amount of centrifugal force required to apply cyclic to a rigid rotor or a flapping rotor.
The forces are in the correct direction because they are referenced to the first figure in the set of three. This is an exaggerated diagram to show the difference between a hinged and rigid rotor. What this shows is that if you want to tilt the rotor up, you must apply cyclic to the sides and to do so requires that you overcome the centrifugal force of the blades that are fore and aft to apply cyclic to the blades at the side (imagine a 4 four bladed rotor for the example).
If the rotor is rigid the centrifugal force tries to fight the input more than the flapping hinge rotor. Not shown is that if you have the flapping hinge at the rotor axis (like a teetering rotor) the centrifugal force opposing the control in put is zero.
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Old Dec 23, 2007, 01:39 AM
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Mickey,
Your right, I did not look at the illustration close enough.
Gene
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Old Dec 23, 2007, 07:57 AM
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Originally Posted by rchopper56
Mickey,
Your right, I did not look at the illustration close enough.
Gene
No sweat dude.
I just try to keep up with things here because once wild theories get started they can get out of hand....
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Old Dec 23, 2007, 12:56 PM
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Originally Posted by mnowell129
No sweat dude.
I just try to keep up with things here because once wild theories get started they can get out of hand....

Mickey,

I still think a centrifugal force acts only perpendicular to the axis of rotation and that exaggerated still is perpendicular.

Leo
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Old Dec 23, 2007, 02:57 PM
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Quote:
Originally Posted by autogyrofreak
Mickey,

I still think a centrifugal force acts only perpendicular to the axis of rotation and that exaggerated still is perpendicular.

Leo
Yes it does and it is drawn so in the diagram. In this diagram the shaft is not the axis of rotation. The axis of rotation is vertical. This diagram illustrates what happens when you try to tilt the shaft away from the axis of rotation. When you do that the centrifugal force tries to oppose the tilting of the shaft. That is what this diagram is trying to illustrate.

The whole point of this diagram is to show that the centrifugal force of the blades continues with respect to the rotor axis as you attempt to move the shaft. Only after cyclic causes the rotors to tilt to a new plane of rotation does the centrifugal force of the rotor become re-aligned with and perpindicular to the shaft.
During input of cyclic pitch the shaft becomes non aligned with the axis of rotation until the rotor axis of rotation moves to the axis of the shaft. This diagram is trying to show that the rotor CF opposes the shaft movement and that the rigid rotor opposes the shaft more than a flapping rotor. The point is that flapping hinges reduce the control loads and are, once you use cyclic pitch to control assymmetry of lift, the only reason you need flapping hinges.
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Old Dec 24, 2007, 10:57 PM
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Mickey,

You talk a lot about cyclic pitch control, about how Cierva knew he
needed it but his implementation with cables was not practical. I
think one of his ideas involved adding "ailerons" to every blade,
either traditional trailing edge control surfaces or rotating blades'
tip sections around the center of pressure.

I've been thinking about a similar implementation, but with
independent servos, one servo per blade, and a microprocessor that
would translate nick/roll from the RX and current angular position
into a deflection angle. Do you know of any such implementations?

Ari.
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Old Dec 24, 2007, 11:04 PM
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Quote:
Originally Posted by iter
Mickey,

You talk a lot about cyclic pitch control, about how Cierva knew he
needed it but his implementation with cables was not practical. I
think one of his ideas involved adding "ailerons" to every blade,
either traditional trailing edge control surfaces or rotating blades'
tip sections around the center of pressure.

I've been thinking about a similar implementation, but with
independent servos, one servo per blade, and a microprocessor that
would translate nick/roll from the RX and current angular position
into a deflection angle. Do you know of any such implementations?

Ari.
The only thing I know is the Kaman full sized stuff that uses a servo flap to implement cyclic. It is still mechanical in nature however and depends on blade flex to work.

I've considered this approach but even the fastest of servos isn't fast enough for a small model.
For example at 500 rpm, this is 8 revs/sec or .12 seconds/rev. The servo would have to be 4 times that fast or .03 seconds. I don't know of any that fast, let me know if you know of some. It is an interesting approach however.
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Old Dec 24, 2007, 11:42 PM
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I was just reading up on servo refresh rates because I realized after posting that this will be a limiting factor. "Analog" servos are designed for 50Hz refresh rate (PPM packets are nominally .02 sec long), but you can push many of them to as short as .005 sec. I've never used digital servos, but I hear that some will go as high as 2kHz, or update every .0005 sec. this works out to 240 updates/revolution in your example if my arithmetic is correct.

The other question is whether it makes sense to put a control surface on the blade or twist the whole blade on a feathering hinge. From what I understand in this thread, the twisting forces should be low enough for a servo to cope.

Ari.
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Old Dec 25, 2007, 10:07 AM
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Quote:
Originally Posted by iter
I was just reading up on servo refresh rates because I realized after posting that this will be a limiting factor. "Analog" servos are designed for 50Hz refresh rate (PPM packets are nominally .02 sec long), but you can push many of them to as short as .005 sec. I've never used digital servos, but I hear that some will go as high as 2kHz, or update every .0005 sec. this works out to 240 updates/revolution in your example if my arithmetic is correct.
I think you may have to be careful. Refresh rate is more about holding power and positioning. Lets use 1000 rpm for a rpm or 16 rev/sec or .06 sec/rev. Let's say you are going to put in full nose down cyclic, then the rotor goes from the tail with no pitch, to full up pitch at the advancing side to back to 0 pitch at the front to full down pitch at the retreating side. So the servo has to travel from lock to lock in 0.03 seconds. I don't know of a servo that will do that, even the digitals. The fastest I've seen is in the .09 range.
Second problem, even if the servo can keep up it's going to get hot. They aren't designed to run continuously....

Quote:
Originally Posted by iter

The other question is whether it makes sense to put a control surface on the blade or twist the whole blade on a feathering hinge. From what I understand in this thread, the twisting forces should be low enough for a servo to cope.

Ari.
Welcome to the dark side. Once you put a feathering hinge on it doesn't take much of a nudge to see that the easiest way to put cyclic in is with a swashplate or steering spider, etc.. The response time is infinitesimal and the servo only runs when you change control, not every time around.
The more you think about this whole problem. You tend to effectively re-confirm that helicopters and modern gyrocopters (the larger enclosed cabin types) have swashplates and feathering bearings for good reasons. Other than complexity there's really not much downside to doing cyclic with a swashplate and feathering bearings.
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