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"Dihedral" or preset coning effects on rotor

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Old 10-03-2006, 09:06 PM
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ottogiro58
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Default "Dihedral" or preset coning effects on rotor

Hi Gang, I was wondering if anyone has experimented with built-in coning angle (effective dihedral) in these new flex plate rotor systems ?? I like 3-channel flying, and currently fly a 3-ch. twin (side-by-side) rotored gyro with "dihedraled" rotors. Each rotor is tilted up to give a difference of @ 15 degrees between the two. It has also been flown with "polyhedraled" rotor systems, that is a @ 15 degree difference between the two, and coning built into each rotor. It would seem that something like the PT Gyro could fly as a 3-channel, fixed rotor model using rudder and elevator if the blades were mounted at a "dihedral" angle. Building in vertical fin/rudder area above centerline/CG of model should aid rudder-only steering. Any comments or experimenters' viewpoints would be appreciated. Thanx !!! Charlie Anderson ottogiro58@aol.com
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Old 10-03-2006, 09:36 PM
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Default RE: "Dihedral" or preset coning effects on rotor

Hopefully Mickey will chime in. He has an excellent description of why coning is bad. Mickey????
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Old 10-03-2006, 10:15 PM
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Default RE: "Dihedral" or preset coning effects on rotor

Coning is definitely not helpful. It is de-stabilizing, not stabilizing.
Here's the reference(s).
http://www.rcgroups.com/forums/showp...4&postcount=64
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Old 10-04-2006, 10:35 AM
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Default RE: "Dihedral" or preset coning effects on rotor

Hi Mickey, Thanx for the input. You have designed some really nice machines. I hope there are some experimenters out there with the flex plate rotor system (non-heli head) who will try this idea. Very easy to test out with their existing setup. . In my book, the simpler, the better. No servo loads to be concerned with, lighter weight=better performance. My 3-channel gyro (.15 motor and 2lbs.) has good control down to 5 mph or less. It should not take much "dihedral" to allow rudder-only steering. Take on the challenge !!! Should be fun to try. Please post the results of your experiments. I would like to see more 3-channel gyroplanes (fixed rotor) flying. Thanx !!! Charlie Anderson PS Take a look at any picture of full-scale gyroplanes in flight, and you will see that they fly with a ton of coning angle. The blades flex upwards to achieve this.
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Old 10-04-2006, 01:35 PM
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Default RE: "Dihedral" or preset coning effects on rotor


Quote:
ORIGINAL: ottogiro58
PS Take a look at any picture of full-scale gyroplanes in flight, and you will see that they fly with a ton of coning angle. The blades flex upwards to achieve this.
I don't disagree with the fact that they cone up. What I disagree with is whether or
not it is intentional and desired or an undesired byproduct.
Coning is the result of flexible blades, low rotor speed, or other factors. Full sized
rotors cone because the blades can't be made stiff enough and the rotor speed is
not high enough to get them to flatten out. If you go read about the design of
full sized rotorcraft both autogyros and helicopters, you find references to coning
that are aimed at minimizing it, not creating it. Coning causes an induced roll in forward
flight, creates gust problems and creates coreolis accelerations in the blades that
require lead/lag hinges, vibration problems with controls, etc. etc.
It absolutely does not work like wing dihedral and doesn't provide any
roll stability, so I don't see why you would intentionally try to design more
coning angle in.
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Old 10-04-2006, 03:22 PM
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Default RE: "Dihedral" or preset coning effects on rotor

Hi Mickey, I am all for rotors with no coning or dihedral, as long as the gyroplane they are mounted to will steer well on rudder only. My aim is to simplify construction, have a single rotor gyro, and to fly with 3 channels. Any manner in which this goal can be acheived, is fine by me. I hope to see a 2-bladed 3-channel gyroplane developed. Anybody out there working on the solution ?? Charlie Anderson
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Old 10-04-2006, 04:26 PM
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Default RE: "Dihedral" or preset coning effects on rotor


Quote:
ORIGINAL: ottogiro58

Hi Mickey, I am all for rotors with no coning or dihedral, as long as the gyroplane they are mounted to will steer well on rudder only. My aim is to simplify construction, have a single rotor gyro, and to fly with 3 channels. Any manner in which this goal can be acheived, is fine by me. I hope to see a 2-bladed 3-channel gyroplane developed. Anybody out there working on the solution ?? Charlie Anderson
Coning doesn't work like dihedral. It doesn't help the gyro turn with rudder.
Gyrocopters turn just fine with rudder. I can leave the ailerons alone and
fly with rudder as much as I like. I understand and don't disagree with your goal,
but coning is simply not the same as dihedral and doesn't work like dihedral.
I'm not sure where this theory came from but its incorrect.
Dihedral in an airplane works because the rudder causes a slip in the airplane.
The wing panel that is now yawed forward sees a higher angle of attack due
to the dihedral angle and produces more lift, creating a roll. This is dihedral based
turning with rudder. For lateral stability in an airplane, when a gust of wind pushes
a plane off level, again it starts to slip to one side or the other. Since the model is slipping
the dihedral presents a higher AOA to the low wing, causing it to lift up and come back to level.
Dihedral works by yawing one wing panel ahead of the other causing one panel to see a higher
AOA and produce more lift.
Coning means all blades fly at some angle < 90 degrees from vertical. Yawing the body of a gyrocopter
doesn't change anything to any of the blades with respect to each other. How can it? They are turning.
Yawing makes a gyro turn because the aft tilted shaft becomes tilted to one side because of the yaw.
This inputs cyclic in the direction of the yaw, causing the turn. Coning has no effect on this.

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Old 10-05-2006, 02:39 AM
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Default RE: "Dihedral" or preset coning effects on rotor

Hi Mickey.

Conical angle is critical on full size gyro for two reasons: (single rotor).

To avoid rolling to the side, the lift developped by each blade shall not apply any torque on the rotor axis. Then, a flapping articulation is required. There is two possibilities:

On 2 bladed teeter rotors, whether ther use cyclic pitch or not, the designer has to ajust a conical angle to limit the bending moment on the blade holder and blade root. Having 3° to 5° dihedral on the blade holder, the centrifugal force will help reducing this bending moment to an acceptable level. When the blade holder does not have this dihedral, the flexibility of the blades will make it in flight.

On individually hinged blades, the conical angle is the resulting of the blade equilibrium referenced to its flapping hinge axis. of course, increased blade inertia will decrease conical angle for an equal lift.


On scaled gyro, by chance, the forces and moment on blade holder are not an issue, these is plenty of way to solve that, so it is possible to use 2 blades rotors with no conical angle.

What is the remaining effect of conical angle? in fact, it had stability, but not on the airplane view. It act equivalent to the Bell flybar of an helicopter. If you increase flybar inertia, the helicopter will slow down in response and will be more stable. The conical angle act the same, except that the governing parameter is not an inertia, but its rotating speed. Thus, it will stabilize the gyro, but will not definitively act like dihedral on an airplane. The vibration problem comes usually from flapping, not directly from the conical angle. BUT, more the cone angle is, more is the flapping for a 90° hinge articulation. It is the initial reason why on full size gyro, the cone angle was as much as possible limited down to 5° by design on normal flight. The other way of reducing flapping is to put a delta angle on flaping hinge, i.e. incidence angle decrease when blade goes up and increase when go down. Add the same on drag hinge and you have the called autodynamic rotor head (that allows also vertical take off if prerotating speed is above normal flight rotating speed).

On a two lateral rotor configuration, the rolling effect of the gyro is suppressed by having the two rotors rotating in opposite directions. Whatever is the number of blades, there is no reason to hinge the blades as long as there is no structural problem on blade support, rotor hub and rotor axis. Most of the time, these rotor don't have any conical angle (and they don't need to have). BUT, to add stability of the whole gyro, a dihedral is given to the rotors axis, and then, it is acting like a traditional airplane for stabilizing on roll axis, but with no effect on yaw axis (i.e. an imput on rudder will not cause rolling the airplane unless the stub wing, if any, has also dihedral and enough surface). Dihedral can be suppressed to have a neutral gyro on roll axis, but may be a bit difficult to control it for beginners.

Norbert.
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Old 10-05-2006, 06:16 AM
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Default RE: "Dihedral" or preset coning effects on rotor


Quote:
ORIGINAL: NMOSSON

On 2 bladed teeter rotors, whether ther use cyclic pitch or not, the designer has to ajust a conical angle to limit the bending moment on the blade holder and blade root. Having 3° to 5° dihedral on the blade holder, the centrifugal force will help reducing this bending moment to an acceptable level. When the blade holder does not have this dihedral, the flexibility of the blades will make it in flight.
I agree, coning is going to occur and you must design for it to reduce blade root stresses. But designing for
something that is unavoidable to avoid stress is not the same as designing something because it is an
aerodynamic advantage.
Quote:
ORIGINAL: NMOSSON
What is the remaining effect of conical angle? in fact, it had stability, but not on the airplane view. It act equivalent to the Bell flybar of an helicopter. If you increase flybar inertia, the helicopter will slow down in response and will be more stable. The conical angle act the same, except that the governing parameter is not an inertia, but its rotating speed. Thus, it will stabilize the gyro, but will not definitively act like dihedral on an airplane.
I still don't see where coning adds any stability. The little bit of rotor speedup due to coning adds some inertia to the rotor but also increases it's sensitivity to control input.

Quote:
ORIGINAL: NMOSSON
The vibration problem comes usually from flapping, not directly from the conical angle.
Not true. In a coned rotor the center of mass moves off the center of rotation when the rotor tilts for control.
Art Young's (inventor of the Bell 47) contribution to two bladed rotors was the underslung rotor head where
the teeter axis is above the center of the hub to get the teeter axis more aligned with the center of mass to avoid
the 2 per rev vibration resulting from coning.
The fact that coning causes vibration is an accepted truth in rotor design. Virtually all full sized two bladed teetering
gyrocopter rotors have this underslung design for just this reason.
Research will prove this to you.

Quote:
ORIGINAL: NMOSSON
BUT, to add stability of the whole gyro, a dihedral is given to the rotors axis, and then, it is acting like a traditional airplane for stabilizing on roll axis, but with no effect on yaw axis (i.e. an imput on rudder will not cause rolling the airplane unless the stub wing, if any, has also dihedral and enough surface). Dihedral can be suppressed to have a neutral gyro on roll axis, but may be a bit difficult to control it for beginners.

Norbert.
I agree dihedral on a model with two outboard rotors adds stability. But yaw does cause roll. The rotor that is advanced by yaw sees an increased
angle of attack due the the dihedral mounting. The inboard canted shaft, when yawed, tilts more backward increasing the angle of attack. This rotor
produces more lift, causing a roll. Therefore rudder only two rotor models roll quite nicely with yaw. Witness the Whopper design on RCGroups.

N.B.
My apologies if my reply is direct. I believe the modelling community deserves to know the real theory of rotor design.
I have been studying this topic for some time ( you will find my comments from 1995 in the technical section of www.autogyro.com)
and many misconceptions about rotorcraft have persisted for some time, even in the face of direct evidence to the contrary.
There are many published references that state or mathematically show how coning is a necessary, but detrimental condition for
rotor craft and ZERO credible published references that state that coning has any advantage.
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Old 10-05-2006, 07:57 AM
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Default RE: "Dihedral" or preset coning effects on rotor

Some quotes from :
http://www.magnigyro.com/USA/SMOOTH....or%20coning%22
Emphasis mine.

"Magni rotor systems have a very shallow coning angle. This allows a very short teeter height - height of the teeter bolt above the
plane of the hubbar. The teeter height should be such that it matches the vertical CG of the CONED rotor. When the rotor cones
under flight loads, the CG of each blade is higher than the hubbar - this should be precisely the teeter height offset built into the rotor
head teeter towers or block. If the CG of the coned rotor does not match the vertical position of the teeter axis - the bolt about which
the rotor teeters, cyclic out-of-balance will occur because the spanwise CG will not be aligned with the spindle axis when cyclic
control is applied. If the CG of the coned rotor is precisely aligned vertically with the teeter axis, the spanwise CG (and other critical
centers) will remain centered over the spindle axis. Misalignment of the coned rotor CG with the Teeter axis (bolt) will result in rotor
shake
- most noticeable when moving the cyclic control or when flying through turbulence. The amount of misalignment is
proportional to the amount of rotor induced vibration."

And
"How does Magni keep the rotor coning angle this shallow, and therefore allow shorter teeter heights? I'd like to say it is a trade
secret, but almost anyone can tell you it is done by using shorter diameter rotors. ...
Why don't all gyroplanes use shorter rotors - now here is the BIG secret! Also, not so much of a secret, just ask any rotor blade
manufacturer. Shorter rotors spin faster - they also spin faster to carry the heavier weights of 2-seat gyroplanes. ...
Ideally, the center of lift on the blade should align chordwise with the
feathering axis of the blade - this is to balance stick forces. If the center of lift on the blade starts moving aft, most blades will twist
somewhat, torsionally from tip to root. If/when this happens at the tip of the blade, the rotor naturally speeds up to handle the same
gyroplane weight load but with less blade angle of attack. To avoid this possible
situation, because most rotor blades can twist torsionally from tip to hub, most gyroplane manufacturers keep the rotor diameters
exceptionally long to keep blade tip speeds low enough to avoid compressibility speeds where blade twisting might become
significant.
Magni shorter rotors are allowed to spin faster because the construction of each fiberglass blade is designed to resist torsional twisting.
If the blades cannot twist from tip to root, there is little danger of blade runaway and higher rotor RPMs can be allowed. Higher rotor
RPM means shallower coning height and shorter teeter heights with all the paybacks noted above. The Magni rotors are also very
heavy
and very stiff, reducing the coning angle further."

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Old 10-06-2006, 02:58 AM
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Default RE: "Dihedral" or preset coning effects on rotor

Hi Mickey.

Don't worry, I am not offensed by your "direct style writing!". It is always a pleasure to exchange ideas with you.

Redarding vibrations, there is only two way of generating them on a rotating element: Either from an unbalance weight, or from dissymetry of external forces applied to the rotating parts.

On an autogyro rotor, dissymetry of external forces, i.e. lift forces dissymetry, exists only on rigid rotors used with two rotor gyros (either syde by side or coaxial). The vibrations can be kept enough low if the rotating speed and blade passing frequency is far enough from natural frequencies of the aircraft structure and rotor pylon(s) to avoid resonance effect. Most of the time, they use small diameter fast runing 4 blades rotors, and thats perfect to have the excitation frequency far away from the low aircraft natural frequencies.

On single rotor autogiro, it is not possible to fly with a rotor having lift dissymetry, the craft will always bank on one side (it was the problem that Senor De La Cierva face to on its early autogiros). To correct this, flapping is required. BUT, flapping is also modifying rotor geometry when rotating, and even for a perfectly statically and dynamically balanced rotor, vibrations will occurs because the CG of the rotating rotor in flight will not stay on the rotating axis, it will slip from its original position.


The conical angle will move the CG along the rotating axis, it will not lead to slip it from the axis. if you test a rotor in a wind tunnel suc hthat the wind speed is parallel to rotating axis, i.e. you supress the flapping, no vibration will occur, the CG will stay on the rotating axis. If the wind direction is no longer paralllel to rotating axis, the flapping resulting from blade lift equilibrium will change the cone axis and then the complete rotor CG will move along this axis and deviate from rotating axis. An unbalance weigh is created and it will generate vibrations at rotating speed harmonics and blade passing frequency. To keep this offset as low as possible, there is various ways: Increasing inertia will decrease conical angle and then, distance between the rotor flying CG and rotating axis. increasing the blade number is also a solution to drop down the flying CG (up to 5, after, there is no significant amelioration). You can also add delta angle to reduce the flapping. These possibilities act on the cause itself, but you ca nalso play on the results, i.e. finding a solution to limit vibration amplitude and/or frequency, by adding damper devices on the blade hinges and on rotor head support.

On teeter two blade rotors, an additional possibility is to offset the teeter axis to have the flying rotor CG on it. From a theoretical point of view, this can suppress unbalanced excitation caused by the flapping, however, it is very difficult to match the right position, and this may occurs for a particular airborn weight and load factor. But, on two bladed rotors, there is one axis of negligible inertia, and them it is impossible to balance them dynamically, so the remaining geometrica lunbalance will always be higher than a 3 or 4 bladed rotor. If you add also that there is no drag hinge and this generates exitation forces on the teeter head, parallel to the teeter axis, at twice the rotating speed, a two bladed teeter rotor will always create vibrations. Some researchs are on going to limit the effect by adding damping on the teeter axis and rotor head articulations.

On Magni gyros, the rotor is exceptionnally heavy compared to most of gyros in the market. The main reason is a safety issue: a rotor with high inertia will take its time to change its rotating speed while flight conditions change quickly. This avoid fast dropping of rotating speed if the pilot unload the rotor (and prevent from having the rotating speed going down to minimum speed required to maintain autorotation), and help also a lot to do good a perfect flare at landing. The design of the Magni rotor is one of the best: the blade holder in incorporating a dihedral angle that correspond to the flight conical angle (at normal flying weight). Then, the blades does not have any bending stress (and no torsion induced by bending stress). The used airfoil is slightly reflexed airfoil and neutral from moment point of view. The blade CG axis and root position are just corresponding to the lift force application point, then there is no torsion at all created into the blade structure. And, of course, the teeter axis offset is such that the rotor flying CG is on it. However, vibration (and not a low level) remains at blade passing frequency (twice rotating speed) because there is no drag hinge on such design.
For your info, on Magni M16, prerotation up to 280 RPM, Nose wheel up at 300 RPM, lift of at 45 km/h and 350 RPM, cruise speed at 370 RPM & 90 km/h. Landing flare at 40 km/h. A friend will try to instrument it to do in flight balancing and vibration analysis.

Take care

Norbert.
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Old 10-06-2006, 10:57 AM
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Default RE: "Dihedral" or preset coning effects on rotor

Quote:
ORIGINAL: NMOSSON

On single rotor autogiro, it is not possible to fly with a rotor having lift dissymetry, the craft will always bank on one side (it was the problem that Senor De La Cierva face to on its early autogiros). To correct this, flapping is required.
Not true. Cyclic pitch is required. Cyclic pitch can be implemented with flapping (Cierva,Pitcairn,Kellet) or
by feathering (Wilford,Reiseler and Kreiser http://avia.russian.ee/helicopters_eng/wilford-r.html).
Flapping is cyclic pitch and the interchangeability of flapping and feathering are
well documented many places in the literature. There are many examples of successful rigid rotors in
full size (Lockheed) and models (Lodge).


Quote:
ORIGINAL: NMOSSON
The conical angle will move the CG along the rotating axis, it will not lead to slip it from the axis.
This is only true when the rotor has no tilt. When the coned rotor tilts the CG of the rotor DOES
slip away from the rotation axis and cause a severe vibration. The underslug teeter hub attempts
to reduce this by trying to match the coned rotor CG with the teeter bolt. But this only occurs
at one load setting and RPM. At other RPM and load settings (2 vs 1 person perhaps) the vibration
with rotor tilt returns.

Quote:
ORIGINAL: NMOSSON
On teeter two blade rotors, an additional possibility is to offset the teeter axis to have the flying rotor CG on it. From a theoretical point of view, this can suppress unbalanced excitation caused by the flapping, however, it is very difficult to match the right position, and this may occurs for a particular airborn weight and load factor.
So you make my point and confirm my original assertions!
--Coning creates an imbalance in a teetering head.
--You must design the head to minimize the stresses produced by coning,
not because coning is advantageous.

Still there is no evidence that coning is an advantage and no indication that it adds stability or
provides a roll couple with yaw. I still assert that coning is an unwanted byproduct and has
no inherent advantage, but merely a nuisance to be designed around.

mickey
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Old 10-08-2006, 08:47 PM
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Default RE: "Dihedral" or preset coning effects on rotor

Much thanks to my fellow aviators for their enlightening ideas and comments. Very interesting. Charlie Anderson
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Old 10-23-2006, 06:17 PM
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Default RE: "Dihedral" or preset coning effects on rotor

I wonder why we make those flapping hinges to begin with? Hmmmmmm......? They work for all of us just fine. No helicotper parts to order.....just some common sense and elbow grease.

Norbert it was a LOT of fun hearing from you again! Keep up the good work buddy!

Dave
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Old 10-24-2006, 08:18 PM
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Default RE: "Dihedral" or preset coning effects on rotor


Quote:
ORIGINAL: imsofaman

I wonder why we make those flapping hinges to begin with? Hmmmmmm......? They work for all of us just fine. No helicotper parts to order.....just some common sense and elbow grease.

Dave
Dave,
Why can't you make your point without being negative about something else? You have to order all kinds of parts for your models, wheels, wire, stuff that you don't make by hand. So I have to order parts for mine, just different parts.

It's not entirely common sense. You need flapping hinges to relieve your servo loads. I keep trying to help the autogyro community understand this but it seems to no avail. You don't need flapping hinges on a gyro. The Wilford gyro proved this in the 1930's about the same time Cierva used flapping hinges. It's a matter of choice. Flapping hinge designs have their advantages and their disadvantages. I'm willing to concede that flapping hinge designs are easier to make by hand, but they are not superior in every single category across the board.
Common sense doesn't always work. Ever watch a gyroscope turn in a circle when held by one end on a string instead fall to the ground? Common sense says the gyroscope will just fall down but it doesn't. Gyroscopic precession is real whether you want to believe in it or not.
Why am I even wasting time trying to provide information...
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Old 10-24-2006, 09:41 PM
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Default RE: "Dihedral" or preset coning effects on rotor

To begin with....I am not being negative, I am playing around, but still trying to make a point that a gyro can fly with simple equipment. If I were negative, you would be the first to know. Right now...I am serious with all kidding to the side...........

You know, there a lot of new gyro pilots out there and there even more future gyro pilots that are being confused by your relentless and forceful views on coning...cyclitic control and what ever else you deem as "the only way" for a gyro can be controled or can fly. You can refer to this and that with quotes and whatever. But sometimes we need to be gracious by politely stating our points.....with out sounding like a bully. You can easily make your point without point blank saying "You are wrong" You views on our hobby are simply YOUR views and NOT the gyro bible for everyone to follow. We all have different opinions and we are in America.....we have the freedom to say and think what we want.


Yeah.....the helicopter parts comment was below the belt. I am sorry for that. I apologize.

Now....getting back to subject at hand about coning, flapping hinges and the like.....Cierva around 1927 made the first fully articulated rotor hub. The hinges flapped up and down and had a drag link which allowed the rotors to also move slightly forward and aft to take the stress off of the huge blades. One of the advantages of the flapping hinge as opposed to the rigid blades is the rigid blades acted as a gyroscope which was the demise in his first unsusessful machines. Here is his quote on this subject.

It is sufficiant to say that is completely eliminated by the articulation of the rotor blades, which must be overcome in order to control the craft. The Autogiro responds as readily to the controls as the most maneuverable airplane. If gyroscopic effect were produced by the revolution of the rotor, it would be practically impossible to bank the craft for a turn or to bring its nose down or up for a landing.

So....Cierva was saying, due to the coning effect, he was able to bank and land his gyro with full control. Without the coning effect, it was uncontrollable and has the tendency to slice to the side thus were his fist failed attempts. The coning effect is actually the dihedral. You need this...unless you have cyclitic control like your gyro. Then coning is not a good thing. TWO DIFFERENT ANIMALS! Look at the twin rotor gyros....each rotor head is tilted slightly.....the rotors are rigid to the head.....but being that both rotor heads create dihedral, you can turn with a rudder.

Cyclitic control...coning is bad.

Fully Articulated (flapping hing) rotor head....coning is good and is a must.



Dave.
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Old 11-15-2006, 10:51 PM
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Default RE: "Dihedral" or preset coning effects on rotor

[link=http://www.youtube.com/watch?v=Y4MwxHCzGbY]Modern Marvels The Autogyro[/link]
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Old 05-27-2008, 08:48 PM
  #18
ottogiro58
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Default RE: "Dihedral" or preset coning effects on rotor

Hi Gang, Looks like the new micro foamie 2-blader gyrocopters demonstrate the effectiveness of the use of coning..... boy, do they cone a lot !!! Charlie Anderson
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Old 06-06-2008, 12:53 AM
  #19
John235
 
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Default RE: "Dihedral" or preset coning effects on rotor

Quote:
ORIGINAL: imsofaman
It is sufficiant to say that is completely eliminated by the articulation of the rotor blades, which must be overcome in order to control the craft. The Autogiro responds as readily to the controls as the most maneuverable airplane. If gyroscopic effect were produced by the revolution of the rotor, it would be practically impossible to bank the craft for a turn or to bring its nose down or up for a landing.

So....Cierva was saying, due to the coning effect, he was able to bank and land his gyro with full control. Without the coning effect, it was uncontrollable and has the tendency to slice to the side thus were his fist failed attempts. The coning effect is actually the dihedral. You need this...unless you have cyclitic control like your gyro. Then coning is not a good thing. TWO DIFFERENT ANIMALS! Look at the twin rotor gyros....each rotor head is tilted slightly.....the rotors are rigid to the head.....but being that both rotor heads create dihedral, you can turn with a rudder.
Where in the quote did Cierva refer to the coning of the autogyro? It appears to me that the quote refers to the use of flapping hinges, rather than coning. I haven't read Cierva's book, but I believe the flapping hinges were required to solve the problem of lift dissymmetry, not to deliberately increase coning.

My view of this issue is that flapping hinges have several advantages. Mainly it works as a type of stabiliser to regulate the dissymmetry of lift. Dissymmetry of lift can be solved with cyclic controls but I believe that some form of blade articulation (flexible blades, flapping hinge, head damping or teeter hinge etc) is still needed to for reasons such as stability, to reduce vibrations, and to reduce the control forces. I don't think there is really any dispute about these benefits.

The advantages of coning is where the disagreement seems to be. There are obviously some disadvantages, but there may also be advantages. One thing that is totally clear to me is that coning does not provide yaw-to-roll coupling in the same way that dihedral does on model aircraft without ailerons. The question in my mind is how coning affects longitudinal and lateral stability of rotorcraft. It occurs to me that coning will increase stability on these two axis, which is similar to the use of dihedral to increase lateral stability on full size aircraft. I think its possible that stability provided by coning could be a help with model autogyros that do not use cyclic control. I expect the mechanism for yaw-to-roll coupling in autogyros without cyclic control has a more involved explaination than what I would consider as common sense.


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Old 06-06-2008, 06:56 AM
  #20
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Default RE: "Dihedral" or preset coning effects on rotor


Quote:
ORIGINAL: John235
Where in the quote did Cierva refer to the coning of the autogyro? It appears to me that the quote refers to the use of flapping hinges, rather than coning. I haven't read Cierva's book, but I believe the flapping hinges were required to solve the problem of lift dissymmetry, not to deliberately increase coning.
He didn't. We hashed this to death on RCGroups. Coning has no advantages. It doesn't provide yaw/roll coupling, nor does it
create any stability laterally or longitudinally.

Quote:
ORIGINAL: John235
My view of this issue is that flapping hinges have several advantages. Mainly it works as a type of stabiliser to regulate the dissymmetry of lift. Dissymmetry of lift can be solved with cyclic controls but I believe that some form of blade articulation (flexible blades, flapping hinge, head damping or teeter hinge etc) is still needed to for reasons such as stability, to reduce vibrations, and to reduce the control forces. I don't think there is really any dispute about these benefits.
This is all true, and as you point out it has nothing to do with coning.

Quote:
ORIGINAL: John235
The advantages of coning is where the disagreement seems to be. There are obviously some disadvantages, but there may also be advantages. One thing that is totally clear to me is that coning does not provide yaw-to-roll coupling in the same way that dihedral does on model aircraft without ailerons. The question in my mind is how coning affects longitudinal and lateral stability of rotorcraft. It occurs to me that coning will increase stability on these two axis, which is similar to the use of dihedral to increase lateral stability on full size aircraft. I think its possible that stability provided by coning could be a help with model autogyros that do not use cyclic control. I expect the mechanism for yaw-to-roll coupling in autogyros without cyclic control has a more involved explaination than what I would consider as common sense.
The disagreement is from those that don't understand how dihedral works on an airplane in the first place and that it works
by virtue of yaw/roll coupling. Since the yaw/roll coupling on a rotor is 90 degrees out of phase it can't possibly work like
dihedral.
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Old 06-06-2008, 08:41 AM
  #21
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Default RE: "Dihedral" or preset coning effects on rotor


Quote:
ORIGINAL: mnowell129
The disagreement is from those that don't understand how dihedral works on an airplane in the first place and that it works
by virtue of yaw/roll coupling. Since the yaw/roll coupling on a rotor is 90 degrees out of phase it can't possibly work like
dihedral.
Now you mention it, I can see that 90 degrees phase shift is the going to prevent coning from helping stability. That answers the question I wrote about. Thanks!

After I wrote the post I thought more about the yaw-to-roll coupling in the autogyro. I have read that some people say they have built autogyros that fly fine without cyclic control for the lateral axis, but I also read other comments that rudder-only control it isn't really workable, although I don't recall the reasoning from each side. I think it the yaw-to-roll coupling will exist anyway, but it isn't related to coning. After acknowledging the tendency for the path of the rotor blades to tilt backwards due to the incoming airstream, if the autogyro is yawed relative to the incoming stream, this same effect should cause the rotor plane to tilt in the same direction as the yaw. If I have my thinking straight, this roll-to-yaw coupling should allow the autogyro to make turns using rudder only, without the need for lateral cyclic control.

Thanks for your excellent thread on autogyro aerodynamics over at RC groups. I have been reading as much as I can since I am putting together my own design autogyro using some Trex heli components. I am doing it because I like a challenge, so I don't mind if it takes some design revisions to get successful flight.
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Old 06-06-2008, 09:12 AM
  #22
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Default RE: "Dihedral" or preset coning effects on rotor

Quote:
ORIGINAL: John235
Now you mention it, I can see that 90 degrees phase shift is the going to prevent coning from helping stability. That answers the question I wrote about. Thanks!
You're welcome.

Quote:
ORIGINAL: John235
After acknowledging the tendency for the path of the rotor blades to tilt backwards due to the incoming airstream,
This is the critical knowledge, once you get this, things make sense.


Quote:
ORIGINAL: John235
If I have my thinking straight, this roll-to-yaw coupling should allow the autogyro to make turns using rudder only, without the need for lateral cyclic control.
This is true in theory, but the rotor response to this takes a long time, longer than a human finds comfortable.
This is why the jump from a two rotor to single rotor is so dramatic for the pilot, the dual rotor self corrects because
the rotors are set with dihedral with respect to each other and correct. This single rotor does not and demands
constant attention to stay level.
What you effectively have is a very sluggish control system that works under ideal circumstances, but fails
to provide emergency maneuvering power, resulting in a crash.
Virtually all the yaw controlled single rotor gyros also have a significant lateral surface and turn by skidding
rather than rolling. You get some roll but it's likely due to the vertical surface being above the CG
causing a roll, not the rotor's response to the yaw.
Keep in mind that due to aft tilt of the rotor, yaw causes a cyclic input to the wrong side. The inward tilt of
dual rotor models corrects for this so that when the rotor is yawed it introduces nose up cyclic on the forward
yawed rotor to make it provide more lift, thus generating the desired roll response.
As an aside that rotor will also speed up and thus make the model overshoot in roll, thus producing the
characteristic dutch roll like behavior of the dual rotor models.

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