Stick Shake and control friction

Mike G

Junior Member
Joined
Jun 16, 2005
Messages
1,917
Location
Lillebonne France
Aircraft
Owned Magni M16 now ELA 07
Total Flight Time
550FW + 500 gyro
I have recently tracked and balanced three gyro rotors and all of the owners expressed disappointment in the lack of impact of tracking and balancing on “stick shake”. Added to that, TexasAutogyro’s recent post implies that you can cure stick shake by using a dynamic balancer.

Experience makes me believe that stick shake is primarily a problem of control system damping and that unless you have a major 1/rev vibration due to unbalance or tracking (perhaps TexasAutogyro’s initial situation), then the usual stick shake vibration is 2/rev and cannot be reduced with a balancer.

To demonstrate this I took a couple of videos of the Smart Avionics test bed gyro and flew two flights, one with the roll pivot bushing and roll control tube friction clamp “squeezed” tight and another with these loose. I also recorded stick shake with a PB3 but dropped my tablet and no longer have the data. However I think the videos speak for themselves.

The thing to notice is the frequency at which the stick is shaking; this clearly is not “ketchup bottle” 1/rev frequency. This is 2/rev and the frequency spectrum (now lost) clearly showed this.
https://youtu.be/Vwd5g8o9yCw

In this video there’s virtually no shake, but the difference is nothing to do with tracking or balancing.
https://youtu.be/eAzHjj-W-bI

The “improvement” was exclusively due to increasing the friction damping of the control system by “pinching” the roll pivot and roll control tube friction clamp. I tried to tighten the pitch pivot but due to the design I couldn’t get it any tighter.

Perhaps using a hydraulic damper would have the same effect as friction damping.

The one thing that isn’t obvious in the video (probably due to the cameras inbuilt stabilizer) is that with the friction increased the stick shook less but the cockpit vibrated more.

There’s no such thing as a free lunch.

Mike G
 
Pure Dumb Luck. I have been flying Peg exclusively for a few months. I never thought much about stick shake in her and finally realized why. Here is a short clip from today's flight. I have a ding in one of my blades and I am embarrassed to say when i cleaned them last, and some of the apparently smoothness comes from slop but still a very pleasant flying experience.
https://youtu.be/BLervdUZH6w
 
Mike On page 2. of the posts I have pictures of my motorcycle shimmy dampers mounted on my RAF,they work very well in reducing stick shake.

The amount of resistance is controlled by adjusters on the dampers,
 
Back in my RAF2000 days I relentlessly pursued reduction of stick shake. I eventually got mine as good as I think it could get. At a point I was trading stick shake for cabin hop. My Xenon was the smoothest flying Gyro I've ever flown. Here's a video clip.

https://www.youtube.com/watch?v=NJa2aI9YhEI

David Morris
 
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Tail heavy rotor blades can cause 2/rev stick shake.

The advancing blade receives a noseup aerodynamic impulse that reaches a maximum at 90/270 degrees on each revolution. This occurs 2x per revolution.

Joe Pires'’ DW rotor blades are properly balanced about their aerodynamic center.
 
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I have sportcoptor blades,have never been able to get them as smooth as my RAF blades,however I

like the sportcoptor blades performance over the RAF ones.
 
Chuck
Joe's stick shake is 1/rev by the looks of it (definitely ketchup bottle shakin frequency), so I'd suspect hanger rash giving a slight unbalance or tracking error. Would the tail heavy blades give a vertical or horizontal 2/rev? That's another cause to add to the list of 2/rev causes.

Joe I've added a new "never to do" rule. never work on the gyro with part of the rotor blade sticking out the hanger door. Either push the gyro completely in or out. Why?? it just happened to me, I was working on the gyro and one of the fw guys in the hanger opened the hanger door into the rotor quote "sorry I never thought to look up". I now have to re do the track and balance.

Eddie I've seen your post and there's a video on youtube, I intend to continue my testing to see if I can reduce the vibration that goes into the cockpit before I invest in dampers.

Mike G
 
Dmorris;n1122033 said:
Back in my RAF2000 days I relentlessly pursued reduction of stick shake. I eventually got mine as good as I think it could get. At a point I was trading stick shake for cabin hop. My Xenon was the smoothest flying Gyro I've ever flown. Here's a video clip.

https://www.youtube.com/watch?v=NJa2aI9YhEI

David Morris
David let me fly his Zenon and it was smooth and it was also pitch stable when I put full throttle and pull the power back all the way several times during that test flight for me.
Hi brother David. I miss you so much!!!!
 
Mike G., stick shake resulting from tail heaviness is primarily horizontal; the input from the blade being mostly torsional.

And as you well know, a shake free rotor also requires a soft mast as well as a stiff in-plane rotor.

Gary Yansen, an old time gyro ace, used to fly my sort of Bensen with round tube mast and and Hughes OH-6 rotor blades and complain that it scared him; it was so shake free that he felt sure the rotor had departed.

That’s Gary doing the vertical spin in my gyro following the 3-blade sequence after the see-saw rotor had been reinstalled.

https://www.youtube.com/watch?v=YnLXL3gmESM
 
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Would one (or more...) vibration dampers like this one be of some use...? They could be fitted easily to the control rods. Those dampers are used in power lines, to damp wind-induced vibration...
 

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XXavier;n1122853 said:
Would one (or more...) vibration dampers like this one be of some use...? They could be fitted easily to the control rods. Those dampers are used in power lines, to damp wind-induced vibration...

I think the snag with using something like this is those dampers are tuned to a particular frequency, we have a rotor that is varying in RPM (frequency) depending on load.
 
helipaddy;n1122861 said:
I think the snag with using something like this is those dampers are tuned to a particular frequency, we have a rotor that is varying in RPM (frequency) depending on load.

Probably, but power lines have a also varying vibration frequency, since the tension of the wire is a function of a number of variables, as wind speed and temperature... And those simple 'Stockbridge dampers' apparently work quite well...
 
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In my opinion (education welcome) a power line will have a natural frequency that is dictated primarily by its length and mass per unit length with variations due to temperature. The exciting frequency (wind etc) has to be at (or near) that frequency to get it to resonate so it will resonate at certain wind speeds but not at others. Like a guitar string it only has one frequency for a given length and tension.
The Stockbridge dampers are designed to have the same natural frequency and resonate out of phase hence dampening the resonance.
The same technique is used in "tuned mass dampes" in buildings to dampend out vibrations due to earthquakes. Google "tuned mass damper" in youtube for some visual examples.
My understanding of these (again education welcome) tuned mass dampers is that they damp out a single impulse or short term type excitation (like a guitar string being plucked or an earth tremor shaking a building) but I'm not sure they work with a continuous "forced excitation" like we have with a rotor out of balance or a 2/rev vibration that is always there. The energy of that continuous excitation has to go somewhere and my feeling is that it has to go into creating heat by friction or viscous (hydraulic) damping or some other means of absorbing energy.
My two cents worth.

Mike G
 
Maybe electrically adjusted dampers like on high prrformance motorcycles like the Honda cbr1000 which have the damping rates controlled by the computer sensing the speed of the bike could be modified where strain gauges on the stick would reduce the damping effect when they sense pilot input
 
Have you though about the magnitude of the exciting force when wind blows past an electric cable and when a gyro rotor spins?

-- Chris.
 
I’m not sure that I’m correctly interpreting Xavier’s picture of a power line damper but it looks like a resonant absorber.

If so, that’s a commonly used technique in helicopters.

Slide 28 in the attachment shows several such schemes. The lower LH corner illustrates the tuned vibration absorbers used in the Bell 222 helicopter transmission mounts which also support the rotor pylon.

http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2004_10_28_Hubschrauber.pdf
 
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