Building Mariah Gale

Vertical compliance shouldn’t be necessary so long as the rotor is properly tracked.

Mast flexibility along with the greatest possible lead lag stiffness of the rotor should keep inplane resonance of the rotor/mast assembly above the excitation frequency. A slider does about the same thing as mast compliance except that it only works in a fore/aft direction.

I don’t have precise answers but a friend of mine had a gyro with a “redundant” mast; two pieces of 1” x 2” aluminum tube structurally bonded together. That thing shook no matter what blades were being used.

My gyro with 2-½ inch round 2024 x 0.120-wall tube was dead smooth. The same blades on Bob’s gyro rode like a jackhammer; we even swapped rotorheads to no avail.
 
I don’t understand why there is no vertical shake.

I don’t understand why there is no vertical shake.

Thank you Chuck,

I don’t understand why there is not a vertical component to the two per rev shake.

I will trust in your experience.

That makes the design of the mast structure and controls much simpler.

I would like to make the mast removable because at this point in the design she is just over ten feet tall. Without the mast she is just over 6 feet tall. Most of my friend’s shops have a 7 or 8 foot door. My garage has a 7 foot door.

Thank you, Vance
 
Is that a bad thing?

Is that a bad thing?

My Sport Copter hub bar appears to have built in lead lag compliance.

Do you feel that is a bad thing?

Do you feel there would be value in fabricating a more rigid hug bar?

The Sport Copter blades seem to shake less than the RAF blades did.

Mariah Gale’s gross weight will probably be over 1,400 pounds so I feel that rotor shake could become an issue.

Thank you, Vance
 
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A seesaw rotor must be as stiff as possible in a lead-lag direction Vance.

Arthur Young’s precursor of the Bell-47, the Bell model 30, had a violent 2/rev shake above 30 mph until he jury-rigged external bracing. He was able to provide adequate internal stiffening on later models.

The rotor in conjunction with its supporting structure is a resonant assembly like the resonant bars on a glockenspiel. If it happens to be resonant near 1/rev, vibrations are magnified and show up in the airframe as 2/rev because the rotor’s rotation doubles the frequency.
Swedish yoke.jpg
That’s Larry Bell in the RH seat and test pilot Floyd Carlson in the LH seat.
 
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I will take that as a yes to make the hub bar stiff.

I will take that as a yes to make the hub bar stiff.

Thank you Chuck.

I will take that as a yes to my question about do I need a more rigid hub bar.

That will be interesting to experiment with.

I felt that with the low frequency of a two per rev shake that we could not raise the frequency high enough to change the response of the airframe to the perturbation. I suspected that was why Sport Copter built a compliant hub bar to lower the resonant frequency.

The wooden blades must have had a very different resonant frequency that my aluminum blades.

Arthur Young’s helicopter had very soft engine/transmission mounts to manage the airframes response to this two per rev shake. I wonder if that allowed him to raise the frequency of the hub bar above the soft mounted system's natural frequency.

We are increasing the section of the Pratt Truss to raise the resonate frequency of the airframe.

I have never understood resonance and been a cut and try sort of person. I would love to have more understanding and less cut and try.

It seems to me that a heavier, longer aircraft would have a lower resonant frequency.

I know that Mike Schallmann had some success with braces on his hub bar.

I would be grateful for any effort to help me become confused on a higher level about mitigating shake from the teeter rotor system.

Thank you, Vance
 
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I’ve gone through this several times before but let’s try one more time:

Grasp a stick of welding rod between thumb and forefinger about ¼ of its length from one end while pretending it’s a rotorblade vibrating in the lead lag direction. Thump it in the center or at one end and it vibrates at its highest fundamental resonant frequency. You’re holding it at a resonant node.

Now imagine you have an old fashioned crimp on lead fishing sinker and you crimp it to the center of your welding rod. What happens when you thump it? It still vibrates but a lower frequency.

The same thing happens if you apply spring restraint at the center; the stiffness of the mast also lowers the resonant frequency.

I’ve suspended a set of DW rotors from the ceiling by cords attached to their nodal points while driving them from center by a variable speed saber saw. They were resonant at ~6 Hz, -360 rpm. That’s static; centrifugal stiffening will raise the frequency but there isn’t a wide margin.

A rotor that is resonant at its rotational speed gets 2 hits/rev from the airstream as well as from mass forces above and below the teeter bolt. It’s like pushing a child on a swing; if the pusher was fast enough, he could push the child from the back, run around and give him a push form the front; 2 hits/cycle like the rotor. Instead of the pusher rotating, the child is rotating in effect. (A pendulum, the swing, has its plane of swing fixed in space like a gyroscope and can’t rotate but the rotor can.)

A rotor that is vibrating at its rotational speed gives the airframe two whacks/revolution. If it’s being excited slightly above or below its resonance, there is a phase shift; the stick rather than moving fore/aft, may move diagonally.

The solution, as Arthur Young learned in 1946, is to make the rotor sufficiently stiff and the mast sufficiently soft that the rotor/mast natural frequency is above rotational speed.

Lead-lag hinges, to be effective, must be close enough to the center of rotation to keep the resonance well below rotational speed. Then ground resonance rears its head and drag dampers as well as landing gear suspension dampers are required. I’ve done it and it works but I didn’t have landing gear dampers and it would always go through ground resonance during the takeoff roll as the blades were being accelerated. The gyro would dance from wheel to wheel but would normally pass through. Occasionally it would get so bad I’d have to stop and start over again.

So there you have it: rotor stiff inplane mounted on a soft mast.

It’s not so much a case of isolation as one of tuning.
 

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Yyyiiipppeee!!!

Yyyiiipppeee!!!

Thank you Chuck,

I remember being confused by this in the past.

I am sorry to have asked you to repeat it.

I didn’t understand that they were on edge.

I feel confused on a higher level now.

I feel that with my wider chord Sport Copter blades would have a different resonant frequency that the Dragon Wings.

Ahah, as I typed this I realized that larger is stiffer and raises the frequency and heavier lowers the frequency and longer lowers the frequency. I feel confused on a higher level now because I am working with the variables.

I can see where it will be easy to experiment with. This is very exciting.

It is rare for me to jump my confusion two levels in such a short period of time.

I am grateful for your patience.

Thank you, Vance
 
Vance, take an actual stick of welding rod and perform the experiment. You will learn more about resonance than just by reading this. You’ll learn the nodes aren’t precisely 25% from each end on solitary sticks of welding rod; off the top of my head, I think they’re ~23%. A node is a point where there’s no translation; only pivoting.

Anything that lowers the frequency will shift the nodes toward center.
 
I didn’t understand the application.

I didn’t understand the application.

I have done the experiment Chuck; I just didn’t understand the application.

I just did it again anyway.

I have been down this road with a Harley Davidson frame with a Stroker engine in it.

It shook the handlebars so bad you couldn’t hold on.

I jammed a piece of wood between the front cylinder head and the frame.

The handle bars became dead smooth and the tail light broke off the back fender because it was shaking so badly as we headed to the bar in town 10 miles away.

I did not think about the blade flexing in that direction because of a lack of imagination.

I am aquiver with excitement.

Thank you for your patience, Vance
 
Chuck,

To be sure I understand correctly...

You suspended the rotors from the ceiling in their normal orientation, not on edge?
Then you excited them with the sabre-saw in the horizontal plane?
 
Yes, Mike. The saber saw was clamped to the hub center and run by a variable voltage transformer with a magnetic pickup (a Plymouth/Dodge ignition sensor) sensing plunger motion. The output signal was sufficient to drive an electronic frequency counter directly.

That was static resonance. With the rotor at normal rpm, the frequency of resonance increases considerably.

Earlier, when we were first exploring reasons for 2/rev shake, we made up an aluminum plate in the general shape of a SkyWheels hub to stiffen the rotor inplane. It did indeed eliminate 2/rev shake.
 
Thanks Chuck.
I recalled you posting about this before, but for some reason I had pictured the blades suspended on edge with your holding the sabresaw vertically to excite the blades. I don't know why I got stuck on this, but I did. I can now join Vance on his journey to higher levels of confusion. :)

My 23' Dragon Wings should have been turning ~340 RRPM on my Bensen according to your spreadsheet (I don't have a rotor tach). I did run a set of 23' DW's on an Air Command with the same AUW as mine and it did turn right at 340 RRPM.

The AC mast was longer and more limber than my doubled up 1x2 mast so it had little shake. I ended up building a slider for mine (thanks for the help), and also machined out some taller towers after experimenting with riser blocks. I now run a set of 22' DW's, and they are mounted in the top hole of the teeter block. The 22 footers should be turning ~360 RRPM.

I still have a slight high frequency shake in the stick, feels like a 2/rev. I also have the Bensen pump handle with the U-joint pivot, so there is no slack in my controls and I feel every thing in the stick.

I may try to make a wider hub bar like you described, or perhaps wider steel straps to use a couple of turnbuckle type arms running outside of the teeter blocks to see if that removes the last bit of shake. I really should finish the projects I have now before taking on any more, but where's the fun in that?

The smoothest set I've flown were a set of Bensens on a KB-2. It was scary smooth. I had to look up at the rotors occasionally to make sure they were still there. :)
 
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The greatest amount of inplane flexing occurs in the steel attachment straps, Mike.

This Averso hub looks to be a good way of stiffening things up.
 

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More progress today!

More progress today!

I purchased a Lycoming IO-320 B1A today in Los Altos, Ca and delivered it to Jim in Greenwood so he will have something to mock up without having to dismantle the Predator. It is a nice engine but no fuel system and the bendix magnetos need to be rebuilt. It also has the heavy stock starter, no flywheel and no prop spacer. It appears to be identical to the engine in the Predator and has the prop governor which we are not going to use. That means we need to plug the crankshaft before we run it. I have an extra flywheel at the hanger and I will bring it and the first Catto propeller that I damaged. We will still have to add the weight of a few things to get the balance right. I am going to order my 6 inch propeller spacer next week and we have some ideas for the exit fan.

Jim concurs on the single round mast with a fairing and we are moving toward an electric clutch for the hydraulic pump for the pre-rotator with a belt driven off the flywheel.

Does anyone know why they do the controls the way they do with the bar across the back? It seems to me the logical way to do it is to run the push pull tube that pitches the rotor head to the middle in the back and the one that tilts it to the middle of the side. I would love to understand the why of how everyone does it.

I read Jim what Chuck wrote about rotor shake and he is struggling with it. If the main in plane flexing comes from the steel attachment straps why aren’t the straps made in one piece? It seems like they would still flex up and down but not in plane.

We decided to go with 1 inch .049 4130 tubing for the main truss and the truss to the tail. The Predator is 7/8 inch mild steel. Jim is going to order the tubing tomorrow. We hope to have the frame sides ready to mock up in around a month.

I will be working on the tail in the next month and hopefully I will have it ready in time to bring it up on my next visit. I picked up the elevators because they need to be shortened along with the horizontal stabilizer.

We are exploring aluminum aero tubing that they use on big boats for the suspension. This decision is still a ways out. This is one of the advantages of a boat guy.

I am off to bed.

I will probably not be back on line until Thursday night.

Thank you, Vance
 
Does anyone know why they do the controls the way they do with the bar across the back? It seems to me the logical way to do it is to run the push pull tube that pitches the rotor head to the middle in the back and the one that tilts it to the middle of the side.

Thank you, Vance

Vance,

Ron Herron ran his push-pull cables just the way you describe on his LW-4 autogyro. You can see a couple of pictures here:

http://www.littlewingautogyro.com/lw4pictures.html

If I dig up any more pictures, I'll post them.
 
Hello Vance,

also the Calidus is using those push-pull cables, one for pitch, one for roll-axis. However, some people who have worked with these cables in motor-boating say, if they break, there is no way of knowing in advance. They just give. Using these cables, however, makes it possible to design a very narrow and aerodynamic mast.

Kai.
 

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Does anyone know why they do the controls the way they do with the bar across the back? It seems to me the logical way to do it is to run the push pull tube that pitches the rotor head to the middle in the back and the one that tilts it to the middle of the side. I would love to understand the why of how everyone does it.
Monkey see, monkey do.

That’s the way Bensen did it and in the case of a universally jointed pumphandle stick, is about the simplest way possible.

Linkages that seem simple and straightforward to some people are things of great mystery to others. When I changed from a Mac to a VW on my old Bensen, I had to cut the pushrods and install idlers for clearance. My good friend Willie went ballistic; “Chuck, it won’t work.” Willie somehow had the notion that since the rotor head moves in 2 planes and the idler arms moved in a single plane, the right motion could not be transmitted to the rotorhead.

The Kellett KD-1 separated control motion into pitch and roll planes and ran the pushrods in tandem inside the mast tube fairing and then used bellcranks at the top to obtain the proper rotorhead motion.
 

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I read Jim what Chuck wrote about rotor shake and he is struggling with it. If the main in plane flexing comes from the steel attachment straps why aren’t the straps made in one piece? It seems like they would still flex up and down but not in plane.
It’s a question of width, Vance. Twice as wide; 8x as stiff.

Deflection of a strap flexing in width varies inversly as the cube power of width.
 
The Begining of the foundation of Mariah Gale.

The Begining of the foundation of Mariah Gale.

Thank you for the picture Mike, that is just what I had in mind only with a push pull tube. It seemed like the one could hide in the fairing behind the mast.

Hello Kai, we were going to use cables, four of them, when we were rubber mounting the mast. Now that Chuck has talked me out of that we are planning on using push pull tubes.

Thank you Chuck, I thought that might be the case.

Thank you again Chuck, I love it when you talk like that.

I can see where the coning angle might create a problem with one piece straps.

I wonder why Sport Copter made them soft in plane. They went to a lot of trouble to do it. I will private message Jon Dailey and see if he will share some information.

I got a commitment from Phil at Coastal Valley Aviation to get very involved with modifying the Mooney empennage. My wild fantasy is to have it ready to transport up to Jim’s June 25th after Terry gives me my Biennial flight review. That is when Ed and I will try out the frame sections that Jim should have done by then with the seats. This is the beginning of the foundation for Mariah Gale.

Thank you, Vance
 
Vance

Get with an A&P Mechaninc as well on the Engine as if you are not going to put a constant speed propeller on it (Which could give you a better climb rate and a faster cruise speed) you will need to plug the end of the crank shaft. If you don't you will very quickly blow all of your oil out onto the floor!!
 
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