off set gimble vs swash plate

[Vance]

I ment swash plate, but it woun't let me edit the title.

My experance is limited, so I would be gratefull for some input from those that have flown both. In the Robinson 22 it seemed like I was always having to do something with the cyclic just to fly straight and level. A Robinson 44 was better, but still required attention. I would not be comfortable flying either hands off. I have flown a Hiller hands off, but I imagine that has something to do with the paddles and the wood blades.

I have flown a couple of SparrowHawks, a modified RAF and even a stock RAF and all seemed fully capable of hands off flight. I came to believe that this was because of the offset gimble rotor head. I have not come to truly understand how this makes the aircraft stable. It seems to me that the ofsset provides a force that we counter with a spring that centers the stick when it is in trim. Am I understaning this correctly?

Would some kind of a centering spring make a Robinson 22 feel more stable?

Does the gyroplane feel more stable because it is a gyroplane or because of the offset gimble and trim spring?

To those that have flown gyroplanes with a swash plate rotro head, do they feel less stable than a gyroplane with an offset gimble?

The Air and Space and the J2 have three blade rotors with swash plate heads, do the three blades have an effect on stability? The Groen Brothers Hawk 4 has a teeter 2 blade rotor with a swash plate head, how does it compare to an offset gimble?

They have a nice example of a J2 and an exceptional example of an Air and Space 18C that I saw while passing through Liberal, Kansas and this is what set me to wondering.

I would be gratefull for any thoughts on this. I am on my way to Cincinati for a motorcycle trade show, so my attendance on the forum will be a little spotty for the next week and a half. I am going to spend tomorrow at Wright Patterson.

Thank you, Vance

PS I was reading a history book in an antique book store in Wichita, and found out that in the twenties there was a French company that refered to what we now call helocopters as "gyroplanes". Is this where the FAA got the name. I have heard other stories, but I belive that Giroplane was a trade name. I would find any information on this most interesting.

Thank you, Vance

[C. Beaty]

A gyro with swash plate (feathering cyclic) control feels exactly like a helicopter.

The component of rotor thrust fed back into the control system by a tilt head rotor telegraphs the pilot what the rotor is doing.

[GyroRon]

The J-2 needs to be flown nearly at all times. Hands off for a couple of seconds at a time is about all you can get away with....

[C. Beaty]

The J-2 needs to be flown nearly at all times. Hands off for a couple of seconds at a time is about all you can get away with....
On the other hand Ron, both of Dick DeGraw’s gyros are routinely flown for extended periods on cross country flights without ever touching the stick. Both have helicopter type rotor controls and both have spring detent stick locks.

[Udi]

Chuck - what are the mechanisms for airspeed and G-load stability with a helicopter rotor head?

Udi

[C. Beaty]

Mostly, Udi, lots of horizontal stabilizer. The lift slope of a fixed airfoil is steeper than that of a rotor.

When loaded by rolling into a turn, the angle of attack is increased equally on rotor and horizontal stabilizer but the percentage lift increase is greater on the stab.

As Raghu has pointed out in earlier posts, an inertialess rotor is stable vs. angle of attack. An increase of “G” load increases rotor speed which decreases flapping angle, tilting the rotor disc nose down. This is what causes gyro and helicopter response to diverge.

And of course, the same old rules apply with respect to rotor thrust line alignment versus CG.

A stabless RAF-2000 equipped with swash plate could only be flown by expert unicycle riders.

[Ga6riel]

does that make aerodynamicly less efficient stabs a better choice ?

[TomCarlisle]

The R-22s that I have flown have a trim spring that you ingage for straight an level flight. It makes it a lot easier on a long flight, but it has to be released for landing (hovering)

[brett s]

That R22 "trim" is just a bungee adding a bit of lateral pressure so you don't have to, I've landed with it still on too :)

[Vance]

Thank you Mr Beaty, I felt I had my answer untill you told Ron about Mr Degraw's stability with a swash palte head, but now I am clearly confused on a higher level.

Most of my hours in an autogiro are in a SparrowHawk and a modified RAF. Mr Mayfield showed me an interesting style of flying where he briefly put preasure on the stick to initiate a manuver and then a short presure to return to straight and level. While I was trying to learn this, I felt that I didn't get much feed back from the rotor. I don't believe that this would work as well for me in most of the light helicopters I have flown. I began to try to feel the rotor feed back that people talk about with an offset gimbal, and it felt more like a steady preasure that could be overcome with a trim spring. We flew in what my instructors would call enviromental disturbances and I was not able to feel the feedback in the stick. Some of the thermals and gusts were quite strong.

Mr Beaty, I like your discription of a horizantal stabilizer's lift curve vs a rotor, but that still leaves me starving for understanding. I know that is part of why autogiros are more fun to fly in gusting conditions than a fixed wing with a low wing loading, but I am left trying to align my experance with my understanding of theory. I am not good at flying an RAF without a stabilizer so I am not able to imagine what it would be like if it was more sensitive to divergance. I tended to initiate an gentle rocking of the ship that was disquieting. This also occured in a ship with the new stabilator.

Is the stability of a giroplane coming from the offset gimbal, a horizantal stabilizer, or a combination of the two????

Thank you Ron, that is just the kind of experiance I was hoping for.

Tom and Bret, I left the trim on the 22 out for clarity. It still didn't feel stable like a Hiller. I felt like it just loaded the controls to one side, instead of swiming in the middle of the control slop. In their defense I never could get used to the funny stick either. Part of the alure of an autogiro for me is that relaxed quality of flight. I would like to better understand where it comes from. A Robinson 22 and to some extent a 44 always felt a little frantic to me. Understand, much of my flying is in 25 kt + gusting winds in Kansas and near thunderheads in Florida.

Thank you all, Vance

[StanFoster]

Vance: In regards to hands off flight.....the most impressed I have ever been was after a demonstration by Duane Hunn at Bensen days last year. We went up when there was some very good thermal activity....and his RAF was flown completely hands offs with nothing but a pinky touching the electric trim control to change speed. We could completely get into the pattern...make turns and line up for a landing without touching the cyclic once.

I was very impressed with that part of it....but still was not convinced on doing away with my stab on mine and going with a stabilator. I know what could happen should a severe downdraft occur with that large thrust offset.

If my SparrowHawk does anything close to what Duane demonstrated to me....I will be tickled pink.

Stan

[Vance]

Hello Stan, I flew in Jim's ship with a stabilator and no horzantal stabilizer. I found if I left it alone, it was very steady. I seemed to initiate the rocking motion. It would dampen out enviromental disturbences with a gental rocking.

My experiance with the SparrowHawk was that it was more tolerant of my flying skills and recovered more quickly than the RAF from my excursions. The RAF would rock about three times to the SparrowHawk's one.

If stability and serenity is what you are after, I believe you will be pleased with the SparrowHawk.

I feel that your build is the nicest I have seen. I appreciate you sharing it with us. It is an inspiration for me.

Thank you, Vance

[KenSandyEggo]

Ron and Chuck pretty much summed it up. The J-2 has to be flown all the time. There is very little feedback in the stick as compared to my old gyro. On my 600+ mile trip, I got the best results from resting my arm on my leg. Even then my hand would get tired and I'd take over the stick with my left hand for awhile. There is a friction lock for the stick. They call it a "trim." I guess I could try it sometime in flight. I've heard that almost no one uses it. It locks the stick, but can be easily over-ridden with hand pressure.

When I recently went for a ride with my step-son, the ex-Army helicopter pilot, I noticed that he generally flew the J-2 with both hands sort of cradling the stick. I guess that seemed most comfortable to him. He said the J-2 flies very similarly to a helicopter. He flew the J-2 around as though he'd been flying it for years.

[Vance]

Thank you Ken,

I am wondering if this type of feel is necasary with a swash plate or if this is part of the J2 design. I would be gratefull for help in understanding this.

Thank you, Vance

[C. Beaty]

Vance, the difference between helicopter and autogyro results from fixed vs. variable speed rotor, ignoring for the moment rotor thrust feedback in the stick.

When a helicopter encounters an upward gust, the angles of attack of both advancing and retreating blades are increased equally. Since the advancing blade has higher airspeed than the retreating blade, its lift increase is greater which tends to tilt the rotor disc nose up.

When a helicopter is flying along straight and level, pulling the stick back increases the angle of attack of the advancing blade and decreases the angle of attack of the retreating blade by the same amount but because of the airspeed differential, the advancing blade develops a greater increment of lift change. This causes the rotor to tilt more nose up than commanded.

Both effects cause a helicopter to be unstable vs. angle of attack.

An autorotating rotor is free to change rpm in response to load. An inertialess rotor would instantly respond to an upward gust by increasing its rpm, decreasing the airspeed differential and the resulting flapping angle, tilting the rotor disc nose down and keeping it aligned with the relative wind.

A real autogyro rotor of course has inertia but gust response is more nearly like an airplane than like a helicopter.

A gimbaled rotorhead produces a force in the control system that makes flying more pleasant and guides the pilot in the avoidance of disturbances. An upward gust produces a nose down force in the stick, the correct direction for stability.

But a gimbaled rotorhead as well as other rotor stabilizing devices only produce the illusion of stability and serve to mask actual instability. The only real test of stability is the behavior of a gyro with the stick locked which mimics the behavior of swashplate controlled rotors.

I built a gyro from junk parts back in the ‘80s that used a layout similar to RAF-2000s and early Dennis Fetters AirComands. This gyro was viciously unstable but non-the less, was pleasant to fly in anything but severe turbulence because of the gimbal rotorhead. Experience teaches pilots to fly such machines with fingertip pressure on the stick and to let the gimbal do its work.

It was quite an eye-opener vis-à-vis stability. Ernie Boyette, David Sease and I flew this machine for many hours until I fitted it with a collective pitch rotorhead similar to that of a Bell-47 but without the gyro stabilizer bars. After that, I was the only one damn fool enough to fly it on a regular basis.

It required my full attention just to keep it right side up.

This machine led directly to the rediscovery of CLT.

I had flown other gyros with helicopter type cyclic control that were reasonably well behaved; first, my old Bensen and later, one purposely built for swashplate cyclic control. The Bensen was very nearly CLT and had an adequate horizontal stabilizer. The second didn’t have a horizontal stabilizer but had the CG above the propeller thrust line. Both had 3-blade, hingeless rotors.

Nearly anyone reasonably competent with machinery can scale a Bensen up, provide a more aesthetic arrangement of the airframe tubes, mount a fiberglass cabin, spend $5,000 on instruments, carpets, wheel spats, etc., but without understanding the physics of flight is just putting lipstick on a hog.

[Victor Duarte]

Vance, thanks for this thread.
Chuck,
thank you for this very enlighting post.
I remember your hingeless rotorhead, clever design.
A friend that built a cyclic controled teetering rotor also told me this rotor was unstable (too sensitive) and he felt that the cyclic control wouldn't let the rotor autorotate as well as a gimbal head.
can i ask a question : if money, space , technology weren't issues, what kind of rotorhead would you build ?
thank you.

[Vance]

Thank you Chuck, now I am realy confused.

In my experance in Arizona thermals in a modified RAF and a SparrowHawkwith an offset gibmal rotorhead, when I would come across a strong thermal there was very little feel in the stick. Down drafts would cause a rapid decay in rotor rpms, but again little stick feel. Am I just insensitive?

I thought that three blade hingless rotors were impractical. I would be gratefull if you would share the details of managing rotor stress. In my confused way I thought that hinges were important to manage blade root stress and an extra set of hinges were desierable with more than two blades. I have seen it done with things that flex, but that still seems like a hinge to me. Is a Kaman servoflap rotor considered hingeless?

Victor, I feel like I should understand these things by now, but my understanding has moved backward. I have had experance to contradict my understanding of the theory and thoughts to contradict my experance, so I must go back and form an unshakeable understanding.

Thank you all for your patience. I only have the one speed and I tend to be traped by canundrums.

Thank you, Vance

[C. Beaty]

I said hingeless, Vance, not rigid.

A perfectly rigid rotor, say a 3-blade propeller with feathering bearings would work just fine if the CG was at the center of the rotor and not suspended underneath. Feathering cyclic control would prevent it from behaving like a boomerang.

Rotor hinges solve a problem in kinematics when the rotor hub is rotating about a different axis than the rotor disc.

In my 3-blade hingeless rotor, the hub was connected to the rotorhead via rubber bushings which permitted the hub to align itself with the rotor disc, rendering hinges unnecessary.

Victor, if time and money were of no concern, I’d have another look at hingeless rotors with partial power to the rotor.

I have a large yellow streak and don’t fly strange contraptions higher than I’d care to fall without instrumented strain gauge testing.

[Vance]

Thank you Chuck, I stand corected.

I would think that the centripital force would be hard on the rubber bushings. How did you manage this? How much lead lag movement does a three blade rotor need? How much flaping movement? I am afraid I am still very muddled here. I know this has been discused before and I apoligise for my confusion. I am gratefull for your kindness.

Thank you, Vance

[C. Beaty]

Here’s a photo of my first 3-blade hingeless rotor, Vance.

http://www.rotaryforum.com/forum/attachment.php?attachmentid=16693&d=1132263068

The hub was a laminated triangular aluminum plate with a feathering bearing/grip attached to each apex.

The rubber bushings do not resist centrifugal force. They attach the hub to the rotorhead and have only to support the weight of the gyro. In this instance, I used 3 Chevrolet engine mounts.

Blade flapping exists only in the mind of the beholder. Viewed from the axis of the rotor plane, blades do not flap but undergo a cyclic pitch variation. Viewed from the axis of the rotorhead, blades flap but do not undergo a cyclic pitch variation.

It’s like viewing a spinning bicycle wheel off axis. The valve stem moves nearer and away from the viewer. That’s flapping in the same sense a rotor is said to be flapping.

[Vance]

Thank you Mr Beaty, you are a most remarkable fellow. You question things in a way that leads to discovery and harnessing of the fundimental order of things. I am so often confused by what I hear and do not have the tools for exploration.

As usual I am left more puzzeled than before. I thought I understood the reasons for a teeter rotor and flaping hinges.

Why did you use a swash plate insted of an offset gimbal?

Thank you, Vance

[C. Beaty]

Vance, the best way to understand a rotor is to make up a scale model and run it in front of a box fan.

All you need is a foot or so of yardstick and a stick of welding rod. Don’t omit the teeter bearing.

One way of understanding the equivalence of flapping and feathering is to make another teetering rotor but instead of yardstick blades, make something that looks like a barbell using a piece of welding rod with weights brazed or silver soldered to each end. Give it a spin and you can see the barbell rotor rotates about an axis independent of the rotorhead axis. The teeter hinge plus 2-blade rotor form a universal joint.

[Vance]

Thank you Mr Beaty, I made a model and it helped a lot. When it got up to speed it was like power steering. I could tilt it this way and that with the slightest effort. I used model helicopter blades and made a teeter head. It is still a mystery to me.

That still leaves me with the question, Why did you use a swash plate instead of an offsett gimble on your three blade rotor head?

Thank you, Vance

[C. Beaty]

That still leaves me with the question, Why did you use a swash plate instead of an offsett gimble on your three blade rotor head?

Thank you, Vance
For the same reason, Vance, that people climb mountains or go over Niagara Falls in a barrel.

[mceagle]

Good answer Chuck. Can appreciate that!

[Vance]

Thank you Mr Beaty, for completing the circle of confusion. I suspect you were investigating something, but I can understand you wanting to keep your reasoning private. You don't seem like a person who does anthing without a lot of thought.
Thank you, Vance

[C. Beaty]

I’ve done many things with insufficient forethought, Vance.

Like trying to fly a 1-blade main rotor.

With cyclic pitch change, the single blade follows the stick at normal rate; the counterweight doesn’t.

[Vance]

wow! that must have been something to see.

I try to make mistakes on a higher level also.

Your example, in my opinion is a nice, high level mistake in pursuit of a worthy objective.

Thank you, Vance

[Victor Duarte]

Vance,
although i am certainly not the most skilled to answer.. i try to share the little knowledge i may have with simple words..

the advantage of a 2 bladed tetering rotor is that you can change the rotor pitch by tilting the axis of rotation.
in a 3 bladed rotor, you can hardly change the plane of rotation manually, the efforts due to gyroscopic inertia would be too high. ( but i have seen a 3 bladed rotor without swashplate on a gimbal head).
so, to be controlable, the 3 blades rotor has to have a swashplate... the plane of rotation is no longer defined by the hub but by each blade having their own pitch (cyclically).
the need for lead/lag hinges is another mess..
i joined a drawing, please correct me if i'm wrong, i will change it.
Thanks
Chuck, i really like your rotor and i think it could be a good candidate for a heavy gyro.

[Vance]

Thank you Victor.

I would think that if you put the hinges close to the mast and the blades are not too heavy, the control forces using an offset gimbal would be low. As the hinges move out the forces trying to pull the head back in line would be quite high.

I believe Miss Champion has a tilting head and She seems to fly well. Mr. Pitcarin is not a particularly strong fellow and there was a sequence in vertical something that I just watched at the I Max at Wright Patterson with Miss Champion flying along a river. It was wonderful with the big rotors turning slowly. It apeared very gracefull, almost like a gull, flying gracefully with little effort.

Back to the point, if an offset gimbal adds stability and a swash plate feels less stable, it seems to me that making an offset gimble three or four blade rotor with lead lag and flap hinges would work fine. Am I missing something?
Wouldn't tilting the hub still provide a cyclic input that flys the rotor to it's new angle?? Wouldn't the offset gimbal offer the same benifits for stability?

At the risk of getting off track, I have always been facinated with Kaman's servo flap system of rotor control. He claims that it adds a lot to stability. Any thoughts on this??

To be clear, I don't see anything wrong with a two blade teeter rotor system with an offsett gimbal, I just want to better understand the design compromises. I feel in the dark about rotors and understanding the different schemes often sheds a little light for me.

Thank you, Vance

[Victor Duarte]

Vance,
i just can report that gyros with 3 or 4 blades with an offset gimbal flew very well. I'll ask my instructor precisely how his 3 bladed prototype was in flight. I just know from him that it was harder to prerotate, harder to handle, very dissimetrical and harder to land because of its finesse, but it was very efficient in flight.

[C. Beaty]

Here are a couple of 3-blade gyros, Victor, that were easy to prerotate, easy to handle, symmetrical in flight and easy to land.

The first, photographed in 1974 at Williston Florida, used 3 Hughes OH-6 blades, had a fairly stiff rotor to airframe coupling; allowing a wheel to be lifted with the rotor at zero lift pitch if near flying RPM while on the ground.

The second, photographed in 1989 at Clewiston Florida, had 3 homegrown blades, each consisting of a laminated birch spar, balsa afterbody with fiberglass cover. There was an internal lead bar near the leading edge for ¼ chord balance. Airfoil was Boeing VR-7.

Neither had flap or drag hinges. The ’74 picture is a frame from an 8 mm movie camera; the ’89 picture is a frame from a videotape.

Ernie, of Dragonwings fame, flew a 3-blade gimbel head gyro that was not difficult to handle. The flap hinges were on the center of rotation and the 3 blades were linked together by a sliding linkage that held them off the droop stops to simplify ground handling.

His first attempt at a 3-blade rotor had offset flap hinges and was without the linkage to hold the blades off the flap stops and went about as your instructor described.

[KenSandyEggo]

Chuck, I thought a gyro would shake apart, get struck by lightning or something if you tried to fly one with heli blades. Didn't you once post that they don't work on a gyro, or are OH-6 blades different than 269/300 blades?

[C. Beaty]

Never said anything of the sort, KJ. I flew Hughes-269, 300 and OH-6 blades for years. In fact, I once bought a truckload of runout TH-55 (military 269 trainer) blades from Ft. Rucker.

They’re a little draggier than some rotors available today but at the time, were better than contemporary gyro metal blades.

At $15 for a set of rotorblades, a crash was of no consequence.

[Victor Duarte]

Chuck,
I'm very sorry, i didn't see your post before :(
that's eye candy and brain candy to me !
you are sayin the exact contrary of what has been told to me...
I suspect he dropped the 3 bladed because..it was not his machine and he didn't feel familiar with it..

your description makes me want to i nvestigate again the 3 bladed thing..
your solution avoids resonnance, and appreantly is pretty smooth and efficient, that's what i want... I don't know what are your hangar challenges but 2 or 3 blades, i will have to put them off to tuck the gyro to my garage..

Thank you very much for your imput.

[C. Beaty]

Victor, if you’ll send me your postal address, I’ll mail you a CD with clips of both those machines in flight as well as the first flight of a helicopter using the rotorhead from the first machine. There are also some still photos on the CD showing close-up views of the rotorhead/control system of the second gyro.

Don’t expect a Hollywood style production with fancy graphics and the latest music, just some fuzzy, shaky stuff taken from copies of original videotape.

My E-mail is: cabeaty@att.net

[KenSandyEggo]

Darn, I bet they're up to at least $18 a set now. Where can I order some? :D Is that something I heard that's B.S. that gyro pilots flipped the Hughes blades upside down to fly them?

[Victor Duarte]

Wow,
thank you so much for your generosity, it will be something i will delightfully watch and re-watch while i will draft ...

I don't care about the music, my eyes will be busy enough :D

Thank you very very much Chuck, Sir Beaty should i say.

[C. Beaty]

Darn, I bet they're up to at least $18 a set now. Where can I order some? :D Is that something I heard that's B.S. that gyro pilots flipped the Hughes blades upside down to fly them?
Ken, Hughes blades use NACA-0015 airfoils, meaning they’re 15% thick and symmetrical. Being symmetrical, they fly as well rightside up as upside down.

However, they’re also twisted 8º (tips twisted down) to improve helicopter hovering performance and to delay retreating blade stall.

Autorotation requires opposite twist; root ends twisted down. Hughes blades flipped upside down and spun in the opposite direction have appropriate twist for a gyro.

Until I came along, the Army got scrap prices for runout rotorblades, about the same price per pound as runout beer cans. When I said $15/set, I meant to say $15/blade.

The blades on your J-2 are identical to 269 blades except they’re untwisted.

The A&S 18-A also uses symmetrical blades with NACA-0012 airfoil section.