View Full Version : Is your stick shake trying to tell you something?
01-19-2005, 07:47 PM
Okay here is a topic worth discussion....
I have had my share of stickshake with some of the gyros I have flown. On a few occasions I asked some of the "Old Timers" what was causing the shake and I was asked to describe what kind of shake it was.... 1 per rev, 2 per rev. Or was asked if the stick was let go did it shake back and forth or in a cirular motion.
How about it guys... what does all this mean?
If I let go of my stick and it kinda shook in a circle what does that mean?
01-19-2005, 11:51 PM
What is your opinion from your flight experiences ?
01-20-2005, 01:44 AM
If I let go of my stick and it kinda shook in a circle what does that mean?
It's just feed back Ron,the blades are go'n round in circles ,so the stick is just tell'n ya the the blades are still spin'n.
Sorry,that woz ment to be funny.
Circular stick shake I'v found to be mass inbalance.
Me old raf blades had the stick do'n circls,a few washers on one blade fixed that problem.
01-20-2005, 05:58 AM
Here's a posting from Greg on the "old" forum.
Author: Greg Gremminger (firstname.lastname@example.org)
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
01-20-2005, 07:12 AM
Wow, great information, thanks for digging that up Chuck.
07-17-2005, 12:55 AM
1/per rev circle or oval pattern indicates a balance problem - some combination of span and/or chord balance, and/or a tracking problem
Thanks for this info. I have 23' DW's with the stick doing little rough circles. I've reduced the 'roughness' and intensity of the shake by shimming the head block (.002" at a time).
So far its .018", I'd like to keep shimming until I either loose the shake or it gets worse again but do not wish to put too much stress where I shouldn't.
Your advice would be appreciated as too how much is too much with all the shims.
07-17-2005, 07:12 AM
Dave, you can’t hurt anything by going to more shims but 0.018” sounds a lot. Rarely do DWs require more than 1 or 2 shins.
The shims are for tracking and if the blade tips appear to be in track, look to other causes for stick shake. Deliberately adjusting the blades to be out of track sometimes produces a smoother rotor but it’s cover out of balance or out of pattern.
Track, balance and out of pattern always produce a 1/rev shake and stir the stick in a 1/rev circle. Severe out of track can be felt as a 1/rev vertical lope in the seat, reminiscent of a galloping horse.
2/rev shake is far more difficult to deal with. The first requirement is a soft mast. Braces between rotorhead and engine mounts nearly always make 2/rev impossible to solve except with a “slider” or equivalent. Redundant masts, a pair of 1 x 2s or mast tubes with extra internal walls nearly always produce more shake.
The reason soft masts, sliders, etc reduce shake has more to do with tuning than isolation. A rigid mast/rotor combination often is resonant within the operating rpm range of the rotor. Softening the mast tunes the combination to a lower frequency which is not excited by rotor forces.
07-17-2005, 09:22 AM
Being a tyro that has never had my hands on the stick.....(yet).... I ask a question may be a real "DUH" for you experienced gyronauts: Could blade shake (1/rev or 2/rev) cause any type of harmonic response in the airframe with one aggrivating the other?
07-18-2005, 09:39 AM
Redhorse: Yup, the resonance can be anything from completely benign to quite scary.
In the former category are the springs on my Gyrobee's instrument panel. They are quite soft and have a long travel. The panel tends to get bobbing up and down at one/rev, even though the gyro actually rides quite smoothly. The problem can be fixed with stiffer springs (which have a higher natural frequency) or with dampers.
Obviously, if the same thing happened in your mast or a flight surface such as a HS, it would not be funny. Fortunately, rotor vibes are at a low enough frequency that USUALLY this isn't a big problem with structural elements. Engine and prop vibes are higher-pitched and somewhat more likely to cause problems. Control pushrods will often go through a resonance during prerotation. They bow back and forth like guitar strings, making the gyro look rather rickety to spectators. This lasts only a moment, however, and stops as the blades speed up.
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