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Old Jan 12, 2008, 09:02 PM
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Joined Feb 2002
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Hi Mickey. I should preface this by saying I am a Sikorsky engineer, working in the loads group. This has been an informative post for me, and I learned some things about autogyros that I didn't know. Some comments:

1) On the issue of phase lag, you mentioned early on rigid rotors have slightly less than 90 deg phase lag. This of course depends on the flexibility of the rotor (where the virtual flapping hinge is). For some full scale rigid rotors it can be less than 40 deg.

2) That flapping hinges only alleviate control loads is untrue. They alleviate blade flapwise moment (you call it torque) by an incredible amount, just like lead lag hinges do for edgewise moment.

3) High servo rate is not needed for a servo tab rotor (like Kaman helicopters). The swashplate angle does not change 1/rev as you imagine, its actually the same as conventional swashplate. The control rods go to torque tubes in the blades which go out to the tabs at 3/4 rotor radius. The tabs go up and down 1/rev because the rotor is turning. The main advantage is that it reduces control loads. Now if you wanted to put servos in the blades, then they would have to be fast. Also, I was under the impression they did not flex the blades. The blades are on a feathering bearing and have a feathering spring which tries to center the feathering at mid collective. This prevents the tabs from having to be up for positive collective, which would reduce lift coefficient of the airfoils in the tabbed region.

3) Following rate...What you are describing as following rate seems to me to be control power. In one post you suggested it's how quickly the rotor reaches steady state. The reason I disagree with this is that given a small cyclic input the rotor will flap only a small amount, and if it reached that state instantaneously, that is not a controllability problem. Now if a small amount of cyclic input produced a large moment that would be a problem. I experienced this first hand when I made a flybarless CP piccolo with a rigid hub and flapping hinges, back in the day. Version 1 had no tip weights and was unflyable; too much control power. 5-7g of lead in the LE of each tip did the trick, and it flew excellently.

4) I've tried flybarless on the eco 8 with heavy blades. The main difference I found, is that the helicopter felt less damped. After putting in a roll input it would continue to roll after you neutralize control, for about 1s. While it was certainly flyable, it took more concentration, so I stuck the flybar back on. With the piccolo I di not get this feeling, I suspect due to the flapping hinges.

5) Gyroscopic precession: Yeah I know its a headache, and many confuse it as being the source of the 90 degree phase lag. On rigid rotors, you will get a roll moment when the aircraft has a pitch rate and vice versa. This effect is of course separate from the whole asymmetry of lift issue.

The reason I've been reading all this is I'd like to get into autogyros. Maybe I can turn the scraps from my old flybarless piccolo into one. My goal is to some day make a cartercopter type model, but currently I have too many other projects.

Thanks again for the informative posts.
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Old Jan 12, 2008, 10:00 PM
Mickey from Orlando. Really.
Joined Nov 2004
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First thing, great to have you commenting.
Let me try to clarify some things that may have come across incorrectly.
Quote:
Originally Posted by Erez-E
1) On the issue of phase lag ...For some full scale rigid rotors it can be less than 40 deg.
Thanks for the correction. I had one figure of 68 degrees for the Lockheed Cheyenne but don't have access to other data. My only research library is my own collection and the internet. There are a couple papers I'd like to get but can't justify hunting them down. I knew the phase angle varied with what is essentially the end fixity coefficient but didn't have the patience to wade through the calculations. I find Leishman's book helpful but I don't use that math everyday anymore so it's tiresome and slow for me.
Quote:
Originally Posted by Erez-E
2) That flapping hinges only alleviate control loads is untrue. They alleviate blade flapwise moment (you call it torque) by an incredible amount, just like lead lag hinges do for edgewise moment.
I think the point that I was trying to make for the tilting spindle cyclic case was that flapping hinges aren't necessary to resolve asymmetric lift and thus their only useful purpose in a tilting spindle model is to reduce the flapping moment, which in a tilting spindle feeds right into the servos. Make sense?

Quote:
Originally Posted by Erez-E
3) High servo rate is not needed for a servo tab rotor (like Kaman helicopters). The swashplate angle does not change 1/rev as you imagine, its actually the same as conventional swashplate. The control rods go to torque tubes in the blades which go out to the tabs at 3/4 rotor radius. The tabs go up and down 1/rev because the rotor is turning. The main advantage is that it reduces control loads. Now if you wanted to put servos in the blades, then they would have to be fast. Also, I was under the impression they did not flex the blades. The blades are on a feathering bearing and have a feathering spring which tries to center the feathering at mid collective. This prevents the tabs from having to be up for positive collective, which would reduce lift coefficient of the airfoils in the tabbed region.
I must have mis-spoken again. I wasn't trying to say that the Kaman system worked other than you describe, someone here suggested that you might drive the servo tab directly not via swashplate and rods, but right on the blade. My comment was that the servo would have to be a very high speed servo to do that. I did find some mention somewhere that a piezo based servo had been experimented with to do this, but couldn't find out if it was successful. My impression is that the piezo actuators are fast enough but might not have the necessary power. Thanks for the info on the centering spring on a feathering bearing. I had read somewhere that early on blade flexibility was used to get the job done, but maybe I mis-interpreted what I was reading. If you have a reference I'd like it.
Quote:
Originally Posted by Erez-E
3) Following rate...What you are describing as following rate seems to me to be control power. In one post you suggested it's how quickly the rotor reaches steady state. The reason I disagree with this is that given a small cyclic input the rotor will flap only a small amount, and if it reached that state instantaneously, that is not a controllability problem. Now if a small amount of cyclic input produced a large moment that would be a problem. I experienced this first hand when I made a flybarless CP piccolo with a rigid hub and flapping hinges, back in the day. Version 1 had no tip weights and was unflyable; too much control power. 5-7g of lead in the LE of each tip did the trick, and it flew excellently.
I want to discuss this further but I'm too tired tonight. Following rate came from a text I have with a reference to paper by Stanley Hiller. One of those papers I want to read but can't get my hands on. It was Hiller that suggested that if the flap response to input was too fast it was out of the range of human controllability. He coined the term following rate to describe this, as in the rotors rate of following the control input. I think we are describing the same thing with different terms. My understanding is that a teetering (Bell) rotor has low control power but a high following rate.
Quote:
Originally Posted by Erez-E
5) Gyroscopic precession: Yeah I know its a headache, and many confuse it as being the source of the 90 degree phase lag. On rigid rotors, you will get a roll moment when the aircraft has a pitch rate and vice versa. This effect is of course separate from the whole asymmetry of lift issue.
Thanks for the backup. Rumor has it that the top heli 3D pilots mix control in to cancel out precession in the tumbling maneuvers.
Quote:
Originally Posted by Erez-E
The reason I've been reading all this is I'd like to get into autogyros.
Good luck, they are maddening but fun. It's a whole new world when the rotor speed lags aircraft speed. It's like flying a turbine powered glider in that you have a sluggish aircraft that's way behind the throttle.

Quote:
Originally Posted by Erez-E
Thanks again for the informative posts.
You're welcome. I'm relieved you didn't find a couple hundred glaring errors.
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Old Mar 09, 2008, 09:09 AM
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DC control

Ive had this question on my mind for sometime. Hope it doesn't sound too dumb.

Is DC control and weight shift control essentially the same thing for an autogyro?
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Old Mar 09, 2008, 10:48 AM
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Not sure but think you are right. By controlling the tilt of the head you create an offset/arm between the lift force and CG resulting in a torque to tilt the craft. If you shift the weight you would get the same result I guess.
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Old Mar 09, 2008, 02:34 PM
Mickey from Orlando. Really.
Joined Nov 2004
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I don't think weight shift and DC are the same thing. Consider a full sized "bensen" type with DC control. There's nothing to weight shift against, you have to make the rotor move to control.
However with a three bladed model with stiff flapping hinges I believe there is a component of weight shift involved. But you'd have to be careful in determining the exact amount, because when you weight shift, you use the weight shift to tilt the shaft as well which is putting in cyclic.
Probably hard to determine just how much is cyclic and weight shift without a careful study.
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Old Mar 11, 2008, 06:00 AM
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Please delete.
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Old Mar 11, 2008, 06:26 AM
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There are two pictures below. The first is a Benson with an over head bar connected in order to tilt the spindle. This looks like weight shift to me, the whole machine swings under the rotor.

The second picture is a DC Benson. But isn't the same thing happening? The only difference is that the swinging of the craft under the rotor is accomplished through a system of linkages.
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Old Mar 11, 2008, 10:14 AM
Mickey from Orlando. Really.
Joined Nov 2004
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Weight shift works by moving the center of mass first then the rotor follows. In the Bensen you move the yoke while the body remains in the same attitude, the rotor then tilts to a new position and the body swings behind. If weight shift were being used the rotor/yoke would remain in a fixed attitude and you would move the whole mass of the fuse first then the rotor would tilt to follow.
You're tricking yourself into thinking that the rotor is a solid immovable object that you can push against to affect moving the mass of the fuse. It isn't, the rotor is teetering so you can't apply any torque to it so there is nothing to push against to move the center of mass of the fuse.
It would be weight shift if you moved the pilot around like a hang glider but when everything is fixed there's no way to move the mass around.

And both of these are DC bensens, the first one has a big gimble on the top that tilts the head, the second one is the later design that does away with the expensive ball joint and uses the current mechanism. Both of these are described in Bensen's book. Bensen describes how these are identical. However the control movement was backwards on the earlier one and there were no control feedback forces to tell the pilot where the rotor was. Bensen crashed once not knowing the rotor postion, so the later design incorporates a feedback spring to give pilot feedback.
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Old May 02, 2008, 03:02 PM
edi
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Coning again

Now I have finally a well-behaved Cierva C.6 in the air I am trying to understand what were the problems before. My first try had been a hub flexible enough to allow for some flapping up and down. I do not know whether it would have worked properly - the problem was that the blades could flap down and hack the aft fuselage to pieces. So I reinforced the hub to make it more rigid; my balsa blades had a coat of fiberglass to suppress any coning and the differential lift should only result in an up-elevator reaction.

To my suprise I had an autogyro which was almost uncontrolable due to a distinct tendency to roll to the left. I put in cheapskate delta hinges, too floppy in the first try, got quite a bit of coning which resulted into a roll tendency into the advancing blade. (That bit is clear to me.) I reduced the floppiness, have *slight* coning now and a perfectly well-behaved plane.

Now my interpretation is that the coning results in a tendency to roll into the advancing blade which now nicely counterbalances the tendency to roll into the retreating blade. I am still not sure where that former tendency comes from. My explanation would be that the induced drag of the blade tips plays a roll here since the plane will fly through the turbulent air. That way, the front half of the rotor disc will lose some lift the back half will not lose (since it's moving away from the turbulent air). With precession that would mean the left (retreating) side will have less lift and the plane would roll into this direction which is what I had experienced.

Now why do not all gyros have this problem? My Cierva has a mast angle of only five degrees which could be unlucky enough to make the front part of the rotor disc pass through its own drag.

But maybe all this is nonsense and there's a better explanation? What do you think?
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Old May 02, 2008, 04:14 PM
Mickey from Orlando. Really.
Joined Nov 2004
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I'm sure turbulence, etc play some minor part, but the big picture is just plain old physics. A very rigid rotor will cause the advancing blade to rise later in the cycle (this is called the phase lag) (how much later depends on how rigid it is), so the effect is to get a roll to the retreating side and a nose up pitch.
When the rotor cones, the blade over the nose has more pitch with respect to the oncoming air and sees more lift, it responds by rising up later in the cycle (the phase lag amount) (on the retreating side), causing a roll to the advancing side.

When you have a rotor that is uncontrolled, getting the flapping stiffness correct so the phase lag is close to 90 degrees so you don't get the advancing blade roll to the retreating side, but not so floppy that you get excessive coning and get roll the other way from coning induced roll is the great balancing act and probably the reason getting one to fly is so difficult. There are some small single uncontrolled rotor models (albeit with big aerodynamic damper fuselages) but very few large single uncontrolled rotor models around, probably for this reason.

The modern solution of course is to use cyclic pitch, either by tilting the spindle or using a swashplate. This puts in the correct amount of nose down and/or left right to get the rotor to trim regardless of how much coning.

Note that you still want to get the coning as low as possible because the roll you get because of it will make the trim change at different speeds, different G loadings (Iike when you pull up to flare for a landing, having the roll trim change is not pleasant), and different aircraft weights.

As a footnote my original designs didn't have the head stiffener, they took 10 of roll trim to fly level and they changed trim under different loads. It really was a bit of a challenge. I added the head stiffener (and tip weights) and got the rotor to flatten out and now it flies with level aileron trim and doesn't change trim when you pull up, flare etc.

There are two ways to get the coning under control without resorting to stiff flapping hinges (which as we know cause other bad problems).
1) Increase the rotor RPM
2) Add mass as far out on the blades as possible.In other words tip weights.
Both of these increase the centrifugal force (or centripital if you are an academic purist, but don't bother me with it because I know the difference and I'm using the common term) so the blades run flatter.

good luck.
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Old May 02, 2008, 05:16 PM
edi
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Thanks for sorting it out for me. With a scale gyro, I'm stuck with the number of blades the original had and as I understand it a 4-blade rotor will always rotate at slower rpm than a 2-blade or 3-blade rotor. But as I am very happy with the this baby is flying now I won't change anything, of course.
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Old May 02, 2008, 09:51 PM
Mickey from Orlando. Really.
Joined Nov 2004
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Quote:
Originally Posted by edi
. But as I am very happy with the this baby is flying now I won't change anything, of course.
Smart move. Now you understand the beginner sticky advice that says "build a proven design first"....
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Old May 03, 2008, 06:54 AM
edi
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Sure I do, I only never found such an approach very interesting. Just imagine: You build a proven design and it flies. Not very thrilling, since it's a proven design, right? Then again, say you build a proven design and it does *not* fly. Obviously you've fouled it up while others have succeeded. How depressing.
On the other hand, you can start something different: If it flies - cool! If not - well, nobody has really expected it to work. With this philosophy, I have not only learned to fly (basically) on a triplane, but I also have experienced some spectacular crashes.
Having said that, I find myself much in favour of autogyros *without* direct contol and with stubby wings now. They keep things easier (no high-load servos or stuff), look cooler (IMHO) and above all, it's much easier to see whether the plane banks.
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Old Oct 09, 2008, 05:59 AM
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Gyrocopter Aerodynamics very useful to me.
Thanks

jim
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Old Jan 06, 2009, 10:27 AM
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Mickey,

now I'm confused.

Quote:
Originally Posted by mnowell129
Once you see that a teetering rotor cannot put a torque on the shaft you can see the negative G problem. The copter with a teetering head just hangs from the positively loaded rotor. To steer, you tilt the rotor and the body slings along. If you accidently push over to 0 G the rotor can no longer do anything to move the body. How can it as the rotor is making no thrust and the teeter prevents the rotor from putting any force on the shaft? Further once you go to negative G's and the rotor is loaded down, not up, when the rotor is tilted back it puts a nose DOWN force on the body, thus creating a control reversal.
I haven't studied the problem carefully but I think what happens is the pilot applies too much power, over corrects with nose down. The rotor goes to 0 G. The pilot sensing his overcontrol applies nose up as the rotor continues on to negative G. Once the rotor is negatively loaded but in a nose up condition the rotor pushes the nose down. The pilot pulls back aggressively worsening the nose down force. The rotor slows, stops and/or folds and we know what happens next.
Quote:
Originally Posted by mnowell129
....In the Bensen you move the yoke while the body remains in the same attitude, the rotor then tilts to a new position and the body swings behind. If weight shift were being used the rotor/yoke would remain in a fixed attitude and you would move the whole mass of the fuse first then the rotor would tilt to follow.
You're tricking yourself into thinking that the rotor is a solid immovable object that you can push against to affect moving the mass of the fuse. It isn't, the rotor is teetering so you can't apply any torque to it so there is nothing to push against to move the center of mass of the fuse......
To me it looks as if, in the second part of your first post, youre using weight shift control (body moving against a fixed rotor), while you're using DC control (rotor moving against a fixed body) in the second post. If I'm using DC control in a 0g situation to push the yoke back while the rotor is still rotating, why doesn't this get me out of the 0g condition?

Jochen
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