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"Dihedral" or preset coning effects on rotor - 10/4/2006 3:06:47 AM   
ottogiro58


 

Posts: 164
Joined: 12/29/2003
From: Silver Springs, FL, USA
Status: offline 		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 ttogiro58@aol.com">ottogiro58@aol.com

< Message edited by ottogiro58 -- 10/4/2006 6:53:02 PM >
       Post #: 1

RE: "Dihedral" or preset coning effects on r... - 10/4/2006 3:36:50 AM   
Scotth


 

Posts: 31
Joined: 11/12/2002
From: Long Beach, CA,
Status: offline 		Hopefully Mickey will chime in. He has an excellent description of why coning is bad. Mickey????

(in reply to ottogiro58)
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RE: "Dihedral" or preset coning effects on r... - 10/4/2006 4:15:54 AM   
mnowell129


 

Posts: 468
Joined: 8/6/2004
From: longwood, FL, USA
Status: offline 		Coning is definitely not helpful. It is de-stabilizing, not stabilizing.
Here's the reference(s).
http://www.rcgroups.com/forums/showpost.php?p=5689254&postcount=64
mick

(in reply to Scotth)
       Post #: 3

RE: "Dihedral" or preset coning effects on r... - 10/4/2006 4:35:18 PM   
ottogiro58


 

Posts: 164
Joined: 12/29/2003
From: Silver Springs, FL, USA
Status: offline 		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.

< Message edited by ottogiro58 -- 10/4/2006 4:40:57 PM >

(in reply to mnowell129)
       Post #: 4

RE: "Dihedral" or preset coning effects on r... - 10/4/2006 7:35:25 PM   
mnowell129


 

Posts: 468
Joined: 8/6/2004
From: longwood, FL, USA
Status: offline
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.

(in reply to ottogiro58)
       Post #: 5

RE: "Dihedral" or preset coning effects on r... - 10/4/2006 9:22:35 PM   
ottogiro58


 

Posts: 164
Joined: 12/29/2003
From: Silver Springs, FL, USA
Status: offline 		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

(in reply to mnowell129)
       Post #: 6

RE: "Dihedral" or preset coning effects on r... - 10/4/2006 10:26:07 PM   
mnowell129


 

Posts: 468
Joined: 8/6/2004
From: longwood, FL, USA
Status: offline
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.


(in reply to ottogiro58)
       Post #: 7

RE: "Dihedral" or preset coning effects on r... - 10/5/2006 8:39:50 AM   
NMOSSON



Posts: 45
Joined: 5/26/2003
From: Le Havre, FRANCE, METROPOLITAN
Status: offline 		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.

_____________________________

Norbert MOSSON
The autorotating brain....

(in reply to mnowell129)
       Post #: 8

RE: "Dihedral" or preset coning effects on r... - 10/5/2006 12:16:13 PM   
mnowell129


 

Posts: 468
Joined: 8/6/2004
From: longwood, FL, USA
Status: offline
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.

(in reply to NMOSSON)
       Post #: 9

RE: "Dihedral" or preset coning effects on r... - 10/5/2006 1:57:37 PM   
mnowell129


 

Posts: 468
Joined: 8/6/2004
From: longwood, FL, USA
Status: offline 		Some quotes from :
http://www.magnigyro.com/USA/SMOOTH.pdf#search=%22rotor%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."



< Message edited by mnowell129 -- 10/5/2006 1:58:49 PM >

(in reply to mnowell129)
       Post #: 10

RE: "Dihedral" or preset coning effects on r... - 10/6/2006 8:58:47 AM   
NMOSSON



Posts: 45
Joined: 5/26/2003
From: Le Havre, FRANCE, METROPOLITAN
Status: offline 		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.

_____________________________

Norbert MOSSON
The autorotating brain....

(in reply to mnowell129)
       Post #: 11

RE: "Dihedral" or preset coning effects on r... - 10/6/2006 4:57:27 PM   
mnowell129


 

Posts: 468
Joined: 8/6/2004
From: longwood, FL, USA
Status: offline
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

< Message edited by mnowell129 -- 10/9/2006 11:33:25 AM >

(in reply to NMOSSON)
       Post #: 12

RE: "Dihedral" or preset coning effects on r... - 10/9/2006 2:47:11 AM