Lessons learned by a "superior" rotor balancer

Mike G

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My experience is only with Euro type gyros so this may not be applicable to single place and Bensen type gyros.

After writing about tightening or « pinching »the roll and pitch pivots and having received a few private requests for help/advice I decided to try to write down what I’ve learned during the development of the Smart Avionics PB3 and PB4 Dynamic propeller and rotor balancers.

This is a very basic intro into the terminology I use, a list of rotor design features that I find important or useful and some tips for dynamic balancing. Some of this is a repeat of what I’ve already written but at least it’s all in one place.

Static or Dynamic balancing???
My experience with the major European rotor manufacturers is that a rotor supplied as a matched pair of blades balanced by the manufacturer is pretty good out of the box and shouldn’t need static balancing. Here my opinion is based on my experience with Magni, ELA, AutoGyro, Trendak and Averso, I’ve balanced a number of others but don’t have enough experience to include them in my list, that doesn’t mean I think they’re no good, I just don’t have enough time with them to comment.

Since, even those “good” rotors that arrive at your site have been dismantled and re assembled it’s worth a dynamic balance if you have access to a balancer.

If you’re dealing with a rotor that was repaired, is of unknown history, built by an unknown or little known builder then a static balance is probably the best starting point before you consider a dynamic balance. I found these videos and this document to be very good at explaining the process. There are surely others out there.

Skywheels track and balance .pdf
http://www.rotaryforum.com/forum/showthread.php?t=35423
https://www.youtube.com/watch?v=DAq3u2Fwq_c
https://www.youtube.com/watch?v=th-ftIIV4CI&t=78s

Blade alignment or “stringing”
This is aligning the blades using a string that runs between identical points at each blade tip and ensuring that the string runs over the centre of the rotor or bearing axis (one of the videos shows this quite well).
If you assemble or reassemble a rotor this is the first thing you do before tracking and balance.

Tracking
There are two basic methods for tracking:
1) The traditional (what I call) “optical” tracking that can be divided into 2 types.

a) During prerotation at lower than flight Rrpm

This includes the traditional “flag” method where you put different coloured markers on each blade tip and during prerotation a second person approaches the spinning rotor with a cardboard “flag” that is marked with the different colours as it touches. This tells you which blade is flying high and which is flying low.


This can also be done using small reflectors and a mirror or a powerful lamp, strobe lamp or tip lights.

b) During flight.

The methods using a strobe, mirrors or tip lights can also be done in flight, the “flag” method obviously cannot.

The advantage of b) over a) is pretty obvious in that the blades are at flying speed and coned hence it’s the more accurate method of the two. It’s also much safer.

In my opinion tips lights are safer because the pilot doesn’t have to mess about with mirrors or lamps while flying I've heard somebody say that they find tip lights distracting when flying so that's to be considered.

The downside of these methods is that they are theoretical. They are based on the theoretical assumption that a perfectly tracked rotor will have the tip of each blade flying along the same tip path. Helicopter mechanics will tell you that the best tracked rotors do not always have identical tip paths.

2) Dynamic tracking

Helicopter tracking is often done using a second accelerometer in the cockpit. The aim being to achieve the tracking that produces the lowest cockpit vibration possible. With a suitable dynamic balancer and knowhow it is possible to carry out the same procedure for gyros.

If you don’t have access to a balancer then the reflector method or tip lights are your best solution. Tip lights have the advantage of being a one man operation but they are heavier than the reflectors with the potential for losing one on flight or unbalance if they are not perfectly matched.

Adjustments during balancing

There are 3 basic adjustments that can be made to a rotor to improve balance and tracking, 4 if you include “pinching” the roll a pitch pivots.

1) Spanwise balance

This is the simple adding or subtracting weight from one blade to make it have the same mass and CofG as the other. In reality you are making the mass times CofG radius of each blade the same.

There are a number of ways and methods to do this:

a) A simple threaded hole in the blade tip that allows you to fit a screw and washers as weights.
b) Using two or three “turns” of duct tape stuck to the blade tip.
c) Adding weights to the inner end of the blade.
d) For extruded blades simply taking the inner end blade cover off and putting washers in so that they are centrifuged out during rotation or removing the outer end cover (not possible with AutoGyro rotors) and popping the washer(s) in and replacing the cover.

I prefer a 6mm (1/4”) threaded hole, preferably not directly into the fibreglass if its a “plastic” rotor. Magni supply a 5mm taped hole in the fibreglass tip that does the job but I fear could be easily damaged as you keep screwing the screw in and out to add/subtract washers and I am reluctant to fly with the Smart Avionics tip LEDs using this little thread due to their weight. The other down side is that when you’ve finished the balancing you have to weigh the washers and make a lead bar that goes down the 5mm threaded hole.

ELA use a big 10mm steel plug that is good and bad. They have the annoying habit of installing it with loctite and then filling the hex (Allen key) slot with mastic that you have to dig out to get the plug out. On a recent brand new ELA I couldn’t get the plug out at all it was stuck so well. I usually replace these plugs with a 10 mm threaded bar that I drill and tap to 6 mm. That way I am manipulating a steel screw into a steel plug with no risk of damaging the fibreglass blade. Also when I’ve finished, the actual washers I used for balancing are simply put behind the 10mm plug and the 6mm screw (with button head) is screwed back with a light loctite.

AutoGyro use the washers down the extruded blade technique that at first sight is appealing. However I find it a pain because you have to get up a ladder to take the blade cover off the inner end of the blade (no need for ladders with washers on the blade tip) and getting washers out again can be difficult requiring one person up the ladder and another shaking and rattling the other end of the rotor to try to get the washer to fall back out. When they do fall out the chances are you won’t catch them and they fall on the floor or worse in the engine bay (been there done that).

From memory Averso have a metal end cover on the blade held with 2 screws. This is ok but each time you remove the cover to put washers/weights behind you are at risk of damaging the threads in the blade. I’d prefer to put a threaded hole in the end cover, that way if you mess it up you only have to replace the cover.

I can’t remember what Trendak has, I think it’s a plate screwed on the end like the Averso can someone correct me please?.

Some don’t give any access to the blade tip for balancing and put large weights (usually self sticking lead weights for car wheels) on the inner end of the blade. I find this a bit permanent because you can’t take them off and put them back more than once or twice before the glue stops working.

If there are no other ways of adding weights, a couple of turns of duct tape is another solution. The inconvenience of this method for me is that it’s not very precise, it’s a pain trying to clean the sticky glue that’s left behind and you then have to weigh the tape you ended up using and find some way of adding that weight to the rotor blade tip or wherever you put the tape. If you use this method then rub the tape with your hand after it’s in place to warm up the glue, especially if it’s a cold day. I didn’t do this when I started balancing and the tape started to come off in flight, very noisy and frightening when you don’t know what it is.

2) Chordwise balance

This is adjusting the rotor chordwise or perpendicular to the span to put the CofG on the bearing centre axis. There are 3 ways to do this:

a) Moving or “Shifting” the rotor along the teeter bolt.
b) Having an extended teeter bolt and adding weights.
c) By misaligning or “sweeping” the blades so that they are no longer “strung” correctly.

I have recently come across 2 rotors in production (and another homemade one) that had no facility to “shift” the rotor. It appeared the designers thought that if they built it with tight enough tolerances it would work or they relied on misaligning or sweeping the blades.

The major European manufacturers all have some method of shifting the rotor chordwise. Magni, ELA and Averso use a screwed “barrel” and I think they all use a 1 mm thread pitch.



This gives a pretty fine adjustment. 1 mm is about 40 thou so 10° rotation gives a shift of about 1 thou. The only criticism I have of all of them is that they don’t put any graduated scale on the rotor head to allow you to accurately shift the rotor and it’s a pity to have such a fine adjustment possible but not be able to make the adjustments with precision. 1 thou of an inch shift is detectable by all of the balancers so why not use it.

Here are the scales I made for my particular rotor.



Here is my rotor when I was testing the effectiveness of an extended teeter bolt.



Autogyro (and I think Trendak) use shims. Rotors are installed initially with 3.5 mm of shims each side of the rotor cube or block to separate the rotor from the teeter towers. AutoGyro supply shims in 0.1 mm intervals from 3.0 to 4.0 mm. This method has the advantage of allowing you to shift the rotor in 0.1 mm (about 4 thou) intervals and is very repeatable in that if you go back to a previous setting you know you have exactly the same as before. I find with all of the screw barrel arrangements that often if you go back to a previous setting the balancer doesn’t give you the same reading as before. I’ve never worked out why, perhaps because of the tolerances and backlash in the threads or the difficulty in being precise because there are no graduations marked on the rotor. It’s debatable if AutoGyro should provide 0.05mm shims, I’m sure they’ll say it’s unnecessary.

The down side of this shim method is that you have to take the teeter bolt out each time to change the shims. I found this time consuming and a struggle but I watched an AutoGyro guy do it and with practice he obviously had a technique and made it look simple. The other aspect I don’t like about doing this is the risk of damage to the teeter bushings on the AutoGyro, from memory they’re PTFE coated and are probably pretty easy to damage. I would recommend using a hoist to lift the rotor each time unless you have the AutoGyro technique.

3) Dynamic Tracking

This is tracking the rotor using a dynamic balancer. The classic method comes from the helicopter industry and they seem to use an accelerometer mounted vertically in the cabin and one mounted horizontally at the top of mast. I know of two of the major European manufacturers who use this technique. I find it a very effective method of reducing tracking vibration but it requires two accelerometer inputs to the balancer. It also means that you are trying to get your head around two polar charts and multiple move lines at the same time and they often disagree, as you improve on one chart you get worse on the other. This isn’t rocket science but it isn’t for balancing newbies, get some time in doing simple dynamic balancing first.

The classic Bensen design allows the pitch to be changed by shimming between the blade carrier and the teeter block. This introduces a chordwise shift and hence an unbalance because the C of G of the coned rotor is at the height of the teeter bolt (thanks JC for educating me on that). Since the typical blade carrier width is about 70 mm (2.5”) and the typical undersling is about the same, we can say that the chordwise shift due to a tracking pitch change is about the same as the shim used. This is one of the reasons that the old hands will tell you to get your tracking done first. There’s no point in balancing chordwise before you track because you’ll have to do it again.



The thicknesses of shims used for tracking are usually pretty small, 0.05 mm (2 thou) to 0.15 mm (6 thou) is about par for the course and this amount of chordwise shift is not very important but you can detect it with a dynamic balancer. It’s worth noting that 0.05 mm (2 thou) is only about 3’ of angle, that means one blade has increased by 3’ and the other has decreased by 3’ so the total difference is 6’. It still means we’re working with some pretty small angles.

Note that lighter rotors with a greater undersling will tend to have even more chordwise shift than the thickness of the pitch shim.

Another design is the half moon between the block and carrier. This has the advantage of allowing you to change the pitch by simply tightening the nuts on one side and loosening them on the other so you don’t have to undo the bolts to fit shims and the pre rotating and braking torques are transmitted via the half moon and not the bolts in shear.

If the bolts holding the block to the carrier have a 1 mm pitch thread (about 24 TPI) then turning the nut one flat (60°) is the equivalent of a 0.16 mm (6 thou shim).


However I personally don’t like this system primarily for dynamic balancing because you don’t have the measurable and repeatable pitch change you get from the shim method so it’s difficult to calculate the required adjustment from the polar chart. I also find it difficult to get the pitch setting I want and obtain the specified bolt torque at the same time. For the guys doing this regularly I image there’s a feel that comes with experience but for the occasional user (like me) I didn’t like it. Added to that it must be more expensive to produce.

For tracking without a dynamic balancer, which is more suck it and see, I imagine it’s pretty convenient to squeeze the nuts on one side and relax the others, go and fly and try again, but dynamic balancing is a more about measuring what you’ve done and calculating the next move.
AutoGyro,ELA, Averso, Trendak and I noticed TAG use the shim method, Magni don’t have any pitch adjustment and they seem to be no worse than the rest so there might be an argument that we’re wasting our time with tracking.




4) “Pinching” the role and pitch pivots.


This isn’t balancing or tracking it’s the introduction of friction in the system. It is only possible on machines that have sufficient flexibility in the design around the roll and pitch pivots to cause a “pinching” of the bushings as you tighten the pivots.

The following photo shows an ELA (AutoGyro very similar) setup and you can see that the supports (end plates) for the pivots are pretty thin (about 5mm) so as you tighten the nuts on the pivots they flex enough to “pinch” the bushings and produce friction.



In the second photo you can see the very solid sections of material used.



Thicknesses like these would be difficult to flex and pinching would probably not be effective (in fact on this prototype it wasn’t).

Should I invest in a Dynamic balancer???

I’ve said this before but here it goes again. Unless you’re out in the sticks (à la Birdy), a professional in the gyro business or fascinated by vibration don’t buy a balancer for yourself. If you are part of a group of gyro enthusiasts (or PRA chapter) buy a balancer between you and find the most enthusiastic user and get him to balance (or at least be there for the balance of ) each of the groups gyros. That way you’ll climb the learning curve faster and establish a data base.

5) Can I make money balancing gyro rotors??

If you’re honest probably “no”. In my opinion, balancing a rotor properly (assuming it needed balancing) takes a long time, at least a day (6 to 8 flights just to establish the move lines) and usually uncovers some weird vibration effect (I’ve picked up, prop, engine, cockpit, tail and undercarriage resonance plus a suspected tail flutter) that takes even more time to understand.
If a gyro vendor is always balancing the same one or two models then, with experience, he/she can reduce balance time considerably because they don’t have to create “move lines” each time.
If somebody balances your rotor in 2 or 3 flights it either didn’t need balancing, the unbalance was so small that you probably couldn’t detect the improvement, they’re taking you for a ride or they’re much better than me.

Even the best tracked and balanced Euro rotor (I got my 1/rev below 0.1 IPS in all three axis at the mast) still vibrates horribly because of the 2/rev so your “customer” will always be disappointed because you may have reduced or eliminated the 1/rev but the 2/rev is still there and he won’t understand that he’s paying for a balance and it still vibrates.

Remember we aim to reduce the 1/rev vibration to below 0.1 IPS by tracking and balance, yet the average Euro gyro will have between 2.5 and 5 IPS at 2/rev.

You can probably help pay for your balancer by balancing props for all the fixed wing guys. Balancing props is less complicated and doesn’t require any flights. Don’t try balancing props on windy days and always check for equal pitch before you start.

If you’re now asking “why bother with balancing”, studies by NASA https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760005640.pdf
show that the most disagreeable vibration frequency for the human body is 5 hz, that’s 300 rpm or very close to 1/rev for most rotors.



The NASA study didn’t go below 2 hz because then you’re getting into the sea sickness range. So yes it’s worth reducing the 1/rev by tracking and balancing. It’s also worth measuring the 2/rev and complaining to your manufacturer that it’s unacceptable.

6) Which balancer do I buy?

While I haven’t been able to compare all of them I’m pretty confident that they are all have pretty much the same accuracy. I did compare the cheapest (PB3) to the most expensive (Vibrex) and the results were very, very similar. See the article in PSF
http://www.rotaryforum.com/forum/showthread.php?t=43921&highlight=powered+Sport+flyer&page=2

I would recommend that if you buy one get one with the frequency analyser, you may not understand what it is at the beginning but once you get used to it is an invaluable tool for detecting other vibration problems such as prop unbalance, carb unbalance (Rotax 9XX series), component resonance etc.

If you’re not particularly technical and will need a lot of hands-on assistance consider after sales tech support. Having a sales tech nearby who can come and debug things for you can be very important in the early days. Companies like Dynavibe seem to have support people and their website is very informative and worth a visit no matter which balancer you buy.
The PB3/4 is cheap and the support you’ll get is Mark Burton at Smart Avionics, me or other users. Having said that we did manage to balance a gyro in the middle-east with me in France, Mark in the UK and the manufacturer on the other side of the world and all by 4 way emails and phone calls. It was long but we got there.

Lessons learned the hard way.

This is something I wrote for Brent some time ago when we discussed his PB3.

1) Always note, each time, what adjustments you’ve made, you think you’ll remember but you won’t. Always make your notes clear and be systematic to write it in the same format.
2) Mark “Master” and “Slave” on the appropriate blades, when you’re up the ladder and want to verify which is the master, it helps.
3) Always, always, always park the gyro with the master blade in front and work systematically from the same side of the gyro, especially if your chordwise (teeter shift mechanism) is a screw thread.
4) Always work alone, don’t trust anybody else to make the adjustments for you. You’ll be amazed how confused some people get, even intelligent people, in fact they’re the worse, they do what they think is right, not what you tell them. Every time I work with someone we make a mistake.
5) Try to fly as smoothly and precisely as possible preferably with no turbulence. Try to really nail the airspeed each time, it’s the best way to keep a constant Rrpm during this exercise.
6) Don’t let other people talk to you, especially when you’re making an adjustment. There’s always someone who arrives when you’re up the ladder and asks “Hey what ya doin” and when you’ve finished answering his questions you’ve forgotten whether you’ve made the adjustment or not especially as the day goes on.
7) Try to keep the aircraft weight constant, so add a gallon when you think you’ve used one. As the weight goes down so does the rotor speed and unbalance force (hence vibration) is proportional to the square of Rrpm so you’ll get lower readings that aren’t true.
8) Be as precise as possible with adjustments, especially if you use a threaded mechanism to do the rotor chordwise shift.

I've tried to make this a generic as possible. Any questions???

Mike G
 
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Joe Pires

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Yes just one. Does this mean you are or are not coming to Florida soon?
 

eddie

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Mike,great article explains a lot of issues when balancing rotors.




Best regards,
 

Smack

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great detail

great detail

Mike,
Wow, that write-up took a lot of your time. Thank you.
Also, thank you for adding A LOT of detail.
So few here add sufficient technical detail (way too much "I think..." without support).
Keep it up !!
Brian
 

Resasi

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Mike what a great piece of work. That is a keeper.

Thank you.
 

ckurz7000

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This is a great, practical write up, Mike. Thanks for taking the time to do it and share your experience with us.

-- Chris.
 

wolfy

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Thanks Mike for your effort good topic mate.
Could I ask you, I also have a PB3 but have never been able to get the move lines to record and so the algorithm feature cant work. I have just been plotting my move lines manually on a polar chart.
If you have the time could you explain how to record move lines and get the algorithm working?
wolfy
 

bryancobb

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Mike,

Do I understand correctly? A gyro pilot just has to be content with cruise 2-per-rev at between 2.5 and 5 IPS ? That seems to me to to be so high that parts will be wearing out and breaking way too often to be safe.

Am I not understanding.
 

Rambler

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Mike,
Here is a pdf of the Skywheel rotor blades captured from the web of the
website that used to exist. It describes the balancing procedure for these types
of rotor blades. Also here is a link from this forum on the subject.
http://www.rotaryforum.com/forum/showthread.php?t=35423
Hope this helps everyone. I own 2 sets and thank you for this important post. Excellent work. Thank You. :yo:
 

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GyroCFI

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at Farrington's

at Farrington's

I remember back in 1989 after I purchased my Super J-2 and trailered it down to Farrington's for an annual. We had to align the rotors after transport. We did the flag method with the grease pen on each of the rotor tips. Made a big Q-tip looking device out of a 10' pole and T-shirts and masking tape. The first time I touched it to the rotor that was spinning at about 400RPM I was a little over zealous and it threw that pole out of my hands and about 40' away... I was a little more cautious the next time and only one of the pitch links on the rotor system had to be changed to get them all perfect.

Never felt any stick shake on my J-2, was always smooth as glass.
 

Mike G

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Thanks for the comments.

I just realised that I left out the text about tracking adjustments under "Dynamic tracking" so I've edited the text above.

Joe
I've promised myself for the last couple of years to go to Bensen Days or Mentone to do a forum about balancing and each year something has cropped up, this year is no exception but I'm waiting for a confirmation of a meeting, if it falls thro I might just come over, I owe you and Laura a dinner from when I was last there and I really want to meet CB.
Now I'm retired money's a bit tight so you might have to let me sleep in your hanger:eek:
I would really like to balance your Aviomania, the French one is still grounded due to engine problems.

Wolfy OK perhaps we'd be better doing by email or Skype or something so as not to clutter up this thread.

Brian
You understood perfectly.

Rambler thanks for the link I'll edit it into the text.

Mike G
 

Mike G

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Wolfy
I've just found part of my PB3 training program for a Eurogyro manufacturers mechanics, it shows in detail how to install move lines. PM me an email address.
Mike G
 

500e

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a lot more now
Mike,

Do I understand correctly? A gyro pilot just has to be content with cruise 2-per-rev at between 2.5 and 5 IPS ? That seems to me to to be so high that parts will be wearing out and breaking way too often to be safe.

Am I not understanding.

Me to Bryan.
Would ground our hellis with that sort of vibs I think we must be at cross purposes with the gyro persons:peace:
I have an interest in gyros but no experience
 

All_In

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Mike... Buddy you earn my U-ROCK! award for making the world a better place.

Also along with your article we already have online I will add this a a new page in the how to section of PRA site. Thank you!!!

We agree 100% that you need a trained person not just the equipment.

I was talking to Jake on the phone a few days ago about getting a PRA balancing team at all PRA chapter fly-in's.

This post prompted me to email the board with the idea...


PM'ing you with more!
 

Jean Claude

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Mike, thanks for your great post. I did not understand why 1/rev vibrations seemed harsh while 2/rev stronger seemed soft.
Fig. 4 of the NASA studie makes to me now understand that this is due to the mechanical behavior of the passenger himself.

 

bryancobb

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Your Thoughts JC

Your Thoughts JC

Mike, thanks for your great post.

JC,

I would like to hear your thoughts on the idea that helicopters can be "dialed-in" to a much smoother condition in all vibration modes than a gyro can ever hope to be, because of the constant (or near constant) RPM.

It seems virtually impossible to come up with an effective way to counteract vibrations on a machine that is constantly re-writing the rules of engagement.

I`d like to say from my experience, the main reason rotor vibrations need to be minimized is to avoid premature wear or damage or failure of aircraft components. The vibrations or what I call "shakes or oscillations" that the pilot can feel and that make flight unpleasant are NOT the ones that cause components to fail.

In the category I`m speaking of that breaks things...the pilot CANNOT feel the difference between a 3.0 IPS vibration (unairworthy by FAA standards) and a 0.5 IPS vibration (well within acceptable standards of airworthiness).

Teach me sir :)
 
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kolibri282

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Extremely well written, clear, extensive, simply great stuff! Filed it right away. Thanks for sharing!!
 
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Jean Claude

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Bryan,
I do not think that the difficulty is due to the inconstancy of the rpm.
With a two blades rotor, the vibratory forces from a rotor of Robinson R22 or Magni M 16 appears to me of the same magnitude.
But the slower rpm of the gyroplane gives it a larger IPS, despite the same acceleration. It's just the mathematical properties of sinusoide derivative.
IPS is not directly representive of mecanical stress, when the frequency is different.



Of course, the stick vibration is particular, due to the control mechanisms of cyclical pitch.
 
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