What do you think of my stick shake?

gyroblazer

Newbie
Joined
Jul 17, 2022
Messages
16
Location
Outback NT, Australia
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Subaru ej18 powered (peter green homebuilt) 28ft patroney rotors
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55

These are 28ft fiberglass rotors

They used to be on my old open seater machine. I didnt even know string-lining was a thing when i assembled them on my old machine so it always had a slow sort of shake to it.

They were about 10mm off center before :unsure:



I string-lined them in the hayshed the other day and they are straight now.



But now I am getting this fast vibration. It seems to be vibrating in an anticlockwise circle really fast (see video)

I made some shims out of a beer can this morning and experemented flying around. I placed the shims in between the teeter bushing outer lip and the fiber washer. I tried with two shims on one side, landed, then tried the other side. No difference, still get the vibration each time.

Notice how in the video i pull some hard G's when i turn left and the stick goes smooth. It also seems to settle down in vertical descents with engine at idle sometimes.

I talked to a mate and he reckons that either I didn't stringline it straight, or the blades need balancing. Another mate suggested that the blades might be too slow and coned up more than they should be for the teeter height. I can pull the blades off and check my string-lining job but after that it seems to be getting too technical for me.

What do you reckon from watching the video?

Thanks
Gyroblazer
 
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WaspAir

Supreme Allied Gyro CFI
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Balancing is always a worthwhile effort. To install a new blade on my Bell 47 last month, we strung the blades and did a static balance first, and then flew it with tiny adjustments to sweep and weight to get it really smooth. These can be surprisingly small adjustments - for the drag link sweep adjustment, we changed about 1/8 of a flat of turn on the nut (under 8 degrees of rotation of the wrench). On a 37 foot diameter rotor with total rotor head and blade weight of over 250 pounds, we were changing tip weight at about 2 grams per adjustment. The easiest way was to wrap some masking tape around the tip, fly it, add or remove tape, and repeat until the flight was smooth. Then we peeled off the tape, weighed it, and put an equivalent weight of small washers in the tip.

blade string.jpg
 
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GyroChuck

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From an article by Greg Gremminger

Rotor head normally 9 degrees tilted back from keel

Keel angle normally 9-12 degrees nose down, 11 degrees preferred.

170lb. Pilot use 9 degrees down, for a heavy 220lb. Pilot use 12 degrees down

Author: Greg Gremminger ([email protected])

Subject: Rotor Shake components

You are perceptive to ask this question. Stick shake pattern is a very interesting subject, although a bit complicated when you get into it. It can be a good tool to help minimize rotor shake! I'll try to explain some of the things stick shake can indicate. But first, you need to appreciate that if there is ANY play in your stick controls, the stick shake may not accurately represent what the ROTOR SHAKE is doing.

For a better representation of ROTOR SHAKE, I have mounted a small laser pen rigidly to the head (on the cross bar or something rigid to the head - using shaped nylon block clamps), and pointed the dot to land on the dash or my knee or somewhere where I can observe it in flight. With the laser pen, there is no slop that might otherwise be in the stick itself! If the stick/controls are very tight (little friction as well), the stick shake can be used like the laser dot can. If the head roll and pitch pivots have a lot of friction, the rotor shake will be transmitted into the airframe, and neither stick shake or laser dot patterns will accurately indicate the sources of the shake.

1/per rev circle or oval pattern indicates a balance problem - some combination of span and/or chord balance, and/or a tracking problem:

- Span balance: blade-to-blade dynamic balance - not necessarily the same as static balance if the blades are not mass and geometrically identical.

- Chord balance: Adjustable by blade "string" or by chord balance adjustment screw (on some hub designs). This is not a simple matter of "stringing" the blades! Stringing is a GEOMETRIC balancing. Even if geometrically "strung" most blades will "search" for their MASS center by moving in the vertical blade attachment bolts - essentially "re-stringing" themselves for their mass center! Whether balance chord-wise geometrically or by mass, the aerodynamic center may still not be over the rotor axis. There is no substitute for quality consistent blades! A quality blade has consistent geometry, mass distribution and aerodynamic accuracy. Some blades that "just will not balance" are probably blades where the geometric, mass and aerodynamic centers are not in the same place!

- Tracking: Tracking is not solely the tips visually tracking the same point. If the aerodynamic properties of the two blades are not identical, minimal tracking "bounce" may not occur when the blades track visibly. On the laser dot, tracking appears as a 1 per rev oval or circle which is probably not in phase with the other balance 1-per rev circle or oval. The combination of balance and tracking 1 per rev shake will likely cause oval or even linear shake patterns with the long axis in any direction. This may be a source of side-to-side shake, or shake in any direction.

- Precession effects in the spinning rotor, can complicate analysis considering the direction of shake. If the rotor is essentially flexing the roll and pitch pivots, precession complicates the analysis, because actual movement of the head can lag the actual force applied! So, it can be very difficult to think through all this! 2/per rev shake can come from several components:

- Improper teeter height (for the rotor load). This produces a 2/per rev essentially fore-aft shake. Teeter height criticality can be reduced by designing the blades for lower coning angles (higher weight blades, stiffer blades, and higher RPMs) A shallower coning angle, makes changes in coning angle allow a smaller vertical offset of the rotor CG from the teeter bolt on changing "g" loads.

- Cyclic drag changes on each blade (in the side-to-side position of the rotor) as the gyro moves forward through the air. This produces a 2 per rev essentially fore-aft shake. Not readily reducible except by good efficient blade design. You cannot adjust this away, but, there are schemes such as the Dominator "slider" or the RAF flexible mast to reduce the amount of fore-aft shake that transmits to the airframe. Worse problem on long blades and heavy ships.

- Conservation of momentum of the rigid 2-blade rotor: This produces a 2 per rev essentially fore-aft shake. This comes from one blade teetering up while the other teeters down, essentially like a twirling skater drawing their CG toward the axis of rotation. If a skater could cyclically extend one arm and draw in the other around the spinning circle, they would shake at a 2 per rev rate! This is not reducible, but the slider or flex mast helps minimize transmission to the airframe. Worse problem on long blades and heavy ships.

- Teeter friction. This also produces a 2 per rev shake essentially fore-aft. This can be reduced by keeping the teeter pivot lubricated or otherwise minimal friction.

- Slop (side-to-side, along teeter bolt) in the teeter pivot. Minimize this slop without adding friction to the teeter. Original Bensen guidelines say .010 slop is OK, but, this movement can cause some very intense cyclic shocks and "hard knocks" when the rotor hits the slop stop. It does this twice per rev! On the laser pattern, very sharp and hard hits are usually apparent as a "knot" or sharp turn in an otherwise smooth pattern - the stick can feel like it's hitting hard, maybe even without a lot of movement.

- Teeter tower sway. This is almost the same as "slop" above, but it is a bit less jarring. This will be more of a factor on tall teeter towers. Some people brace the towers with cross bars. Magni type rot hubs and teeter "blocks" eliminate this sway.

So, the laser dot shake pattern may be very complicated, not always intuitive - a combination of 1 and 2 per rev shakes in various directions. If you can identify only 1 per rev or 2 per rev shakes, you are lucky, the problem is easier to identify. If the shake pattern is cleanly circular, oval or linear (in any axis direction), the problem is probably 1 per rev combinations. Start adjusting track, chord and span balance and watch the results for improvements. Experienced rotor people can sometimes differentiate between 1 or 2 per rev shakes - but this is not very easy!

If the shake pattern has extra little loops or tails, the pattern is contaminated with 2 per rev shakes. First, make sure the teeter friction is minimal - clean and lubricate the friction bearings if possible. Also, minimize the teeter slop. Coimbinations of 1 and 2 per rev shake can cause very sharp or irregular shake patterns that feel like hard "knocks". A "hard knock" in the stick can also come from teeter pivot slop (above).

In my experience, the major source of 2 per rev shake is improper teeter height - especially on very flexible aluminum blades and heavy machines. Some combinations require as much as 6 inches of teeter height. CAUTION: extreme teeter heights require double bearing rotor heads to handle the overrunning loads imposed on such a long moment arm. Also, tall teeter heights increase the force or "feel" of the cyclic stick - this is sometimes a good thing though, to give the pilot some heavier stick feedback.

For both 1 per rev and 2 per rev shake, there is no substitute for quality blades. Besides the geometric, mass and aerodynamic center consistency, many of the 2 per rev shakes can be minimized with quality blades. Efficient blades minimize shake due to cyclic drag changes. Low coning blades, minimize teeter height sensitivity. Blades that can "hold" tracking and "string" adjustments may maintain minimized shake better because they don't dynamically misalign themselves.

One more point: only 1 per rev shakes may be analyzed effectively by accelerometers and polar plots. 2 per rev shake is not analyzable by polar plots, and any amount of 2 per rev shake severely confuses the results of trying to do a polar plot! Even if there is no 2 per rev shake, the combination of balance and tracking induced shakes, severely complicates "balance shots" based on polar plots.

I have found that flying with a laser pen dot pattern helps a lot in understanding what is going on. Patterns and amplitudes are readily recognizable, and any improvements are easily noted. The laser dot also helps in isolating the 1 per rev problems from the 2 per rev problems. A severely distorted pattern is a good hint to go after 2 per rev things first!

I do not represent the above mechanisms, patterns or shake directions as totally accurate. This is a very difficult issue to think through and analyze. I surmised the above (sometimes impressions) from long-term laser dot observations on the High Command and on a Dominator that had very severe rotor shake. I invite corrections and additions to the above, but, suggest that you simply install the laser and observe the results of blade adjustments, before you try to "argue" with the specifics above. In the end, it is usually a matter of making an adjustment to see what it does to the dot pattern. It is most helpful to simply be able to see when you make an improvement - which the laser dot helps make apparent!

- Greg Gremminger
 

gyroblazer

Newbie
Joined
Jul 17, 2022
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Location
Outback NT, Australia
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Subaru ej18 powered (peter green homebuilt) 28ft patroney rotors
Total Flight Time
55
FIXED!


Thankyou GyroChuck, Waspair and Mikeboyette for your insightful responses.

I tracked down the phone number of the fellow who built the blades. He's been retired for years now but he was really fantastic and helpful. He said to remove the prerotator and try it now. He is not a fan of heavy 4-5kg electric motors sitting at the top of masts and he always built his machines with the motor way lower down the mast near the motor and ran a shaft up to the head.

These blades have extra pitch in them compared to most other blades and he got technical with his explanation which you fellas would understand but not me, something about extra lift caused on the advancing side of the blade and how it rocks the head and transfers into vibration. He said that many people with electric starter motors on the top of the mast used to have problems with stick shake using his blades that were 27 or 28ft. But these type of blades are the cream of the crop in Australia.

I took the prerotator off and went for a fly and now they are smooth as butter!
 

Smack

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No one picked up the manufacturing (tooling, process) of the Patroney blades?
 

gyroblazer

Newbie
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Jul 17, 2022
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Location
Outback NT, Australia
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Subaru ej18 powered (peter green homebuilt) 28ft patroney rotors
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No one picked up the manufacturing (tooling, process) of the Patroney blades?

I think nobody has shown him a true interest in learning. He's a good bloke and really generous with his knowledge too.
 

Abid

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I think you don't have a stick. I think you have a T bar. Why?
 

Abid

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It lets me fly with one hand so I can use my left hand to shoot zombies with the .44 magnum
I always fly with one hand. What do you need the second hand for while flying. Does your throttle need constant attention?
 

gyroblazer

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Jul 17, 2022
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Location
Outback NT, Australia
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Subaru ej18 powered (peter green homebuilt) 28ft patroney rotors
Total Flight Time
55
What do you need the second hand for while flying.
Eating sandwiches, shooting, filming, dropping supplies, sending messages, whatever you want :)

Does your throttle need constant attention?

Not if im cruising but i'm always changing the throttle when im manouvering around like in that video. With the aircraft style throttle lever on the side like other gyros you wouldnt have a free hand.

I was always a fan of the aircraft style throttle lever until i tried this and now it feels really comfortable. I used to race motorcycles so it came very naturally
 

MikeBoyette

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It lets me fly with one hand so I can use my left hand to shoot zombies with the .44 magnum
My first gyro had a twist grip on the stick. It was nice to be able to fly with one hand. My second machine has a quadrant throttle on it. I tightened it so it would stay where you set it so you could fly one handed.
 

gyroblazer

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Joined
Jul 17, 2022
Messages
16
Location
Outback NT, Australia
Aircraft
Subaru ej18 powered (peter green homebuilt) 28ft patroney rotors
Total Flight Time
55
My first gyro had a twist grip on the stick. It was nice to be able to fly with one hand. My second machine has a quadrant throttle on it. I tightened it so it would stay where you set it so you could fly one handed.

Yeah my last machine was like that, but I always had to put my hand back on the throttle when the time came to dance around :)

I actually considered changing the throttle to the side lever on this machine when I was looking at buying it because its what I was used to but peter green (the builder) convinced me to leave it and said its great for flying in really cold weather because you can keep both of your hands out of the slipstream. He was right and I was perfectly comfortable on the cold morning that I took off from coober pedy when i bought it because I was behind that windscreen (y) and it feels way more natural since ive grown up on motorcycles and now i just feel like im riding a flying motorcycle.
 
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