String or fling?

Greg Vos

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Guys can I get your comments on stringing the blades after assembly to the hub bar? I will be stripping a xenon rotor to inspect it after the recent advisory from Trendak, the question I have is will it be better to string them or fling them ?
We have space to rev the rotor and let it spin down, so what will be the better technique?
 

Dmorris

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I currently own an IFR Carbon Cub FX3. This is my 3rd Carbon Cub. Owned a Xenon, 2 TAF 2000's.
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Thousands and adding every week!
Having owned a Xenon and a couple RAF 2000's I've done both multiple times. Personally, I had better luck stringing my Xenon blades and flinging my RAF blades. Seems the RAF blades found their happy spot better flinging. Not so much with my Xenon. My Xenon flew incredibly smooth.
 

bryancobb

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Stringing gets them to a good initial ballpark starting point as far as "sweep" and "180 degrees" apart goes. This "static" arrangement may or may not result in the smoothest arrangement in flight. The second step would be to track the blades, on the ground, while they are spinning. This gets you an initial track where the rotor is supporting as much of the aircraft flight weight as possible. Sandbag your aircraft to the ground and tie it down so it won't take off at high RPM (as close to flight RPM as possible). Readings of spanwise out of balance can be taken during these spinups and weight can be added to correct it.

All of this is done on the ground to get things smooth enough for flights for Track and balance. This is still not good enough and you are not finished. You will probably have to make 10 to 12 short flights at cruise speed to get the rotor dialed in. An electronic vibration measuring device will be needed. If a gyro has the same max vibration level as a heli does, you need to get in down to BELOW 0.15 IPS to avoid excessive wear of parts.

Don't be shy about deliberately throwing some of these adjustments out of adjustment if the electronic device indicates it. Many times the smoothest rotor occurs when one or more things are not set perfectly.
 
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XXavier

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On the art of rotor stringing, and on the diverse approaches to identify and reduce vibrations, I've found an interesting paper:


Among the references, there's Mike Goodrich. And also a paper by V. Vainilavicius that is, in my opinion, quite valuable, if it's also true that the author mentioned the importance (???) of the direction of the wind on rotor vibration...
 

bryancobb

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On the art of rotor stringing, and on the diverse approaches to identify and reduce vibrations, I've found an interesting paper:


Among the references, there's Mike Goodrich. And also a paper by V. Vainilavicius that is, in my opinion, quite valuable, if it's also true that the author mentioned the importance (???) of the direction of the wind on rotor vibration...
That is a very accurate and easy-to-understand writing. There's one facet of gyro balancing that was omitted, is the most difficult one to correct for. Unlike helicopters (which don't have the issue I'm about to discuss) gyrocopters or gyroplanes or autogyros have rotors that are self-regulating and the RPM is not constant. Their rotors are constantly "on-the-hunt" for the optimum RPM for what's going on at a specific moment in time. By the time physics exerts an acceleration or deceleration force on rotor RPM, that moment has passed and the chase goes on forever.

Now I'll the talk about the unaddressed issue I mentioned. I'll use the the ice skater that is often used to understand rotors. The skater hold a 10kg weight in her right hand and spins at 75 RPM. Everyone can see the "rotor" is out of balance and if you could measure it, it would be statically out of balance too.

Now put a 20kg weight on the other arm at the skater's half-span point (elbow). Now it would balance statically but would create many forces that would make the left arm (rotor blade) a totally different creature. It has a lot more mass than the right. It has a mass at it's mid point that must be lifted by a flimsy rotor blade so flexing or "coning" is different than the right. The right blade has its weight at ts tip and that creates more tension in the blade at all RPM's and makes that blade flex less.

Also since the heavy left blade has more mass, it has different accel/decel characteristics than the right one. The perfect rotor would have opposing blades with exactly identical masses. Their spanwise centers of mass would be located at exactly the same distances from the center of rotation. Both would have identical airfoils, twist, stiffness, smoothness or surface profile, and would make exact amounts of lift at all airspeeds, takeoff weights, and rotor RPM.

Because so many things cause vibrations and we cannot tend to all of them at once, a smooth rotor in flight is a compromise and most times, adjustments that deliberately vacate "the perfect setting" and search for the smoothest rotor for your machine, your blades, and your flight regime in which you spend the most time.
 

hismiths

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Additional question. After assembly, can I have one of the several helicopter tour company mechanics @ PHKO/KOA true and balance my blades? If I purchase a certificated gyro, can my regular A&P do the hub/blade portion of a 200 hr/annual, or do I need a rotor mechanic?
 

Mike G

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Greg

The old timers in the gyro world managed to get a reasonably smooth rotor using “static balance”, “stringing”, “slinging” and “tracking using the flag” and then flying and judging changes in vibration by the seat of the pants. Like any skill it must have taken them a lot of time (probably years) to develop the experience and feel necessary to achieve their results. I take my hat off to them.

If you are asking the question, I assume that you, like me, don’t have those old timer’s skills.

Today we have electronic dynamic balancers that allow us new comers to track and balance a rotor with much more repeatability and accuracy than before. With a good balancer, and knowing what you’re doing, a track and balance shouldn’t take more than a day.

It’s a pity you didn’t attend my training session in SA last November.

On another note I see that you are asking a few questions about a Xenon that you are rebuilding for a customer and that JM (who’s a lawyer) is giving you technical advice. I’m an engineer and I wouldn’t give any technical advice based on the limited information available. But I’ll give you a bit of legal advice.
If your name is going on the log book of that gyro then you shouldn’t be considering anything but replacing every damaged or worn part with the factory recommended parts and setting up the rotor (slinging or stringing) as per the manual. Selecting a different bearing or considering drilling new holes in a mast that has cracks etc is asking for trouble. If that gyro ever crashes for whatever reason, his widow will not thank you for having saved him a few Rands by “fixing” his gyro cheaply.

Brian You probably have a lot more experience than me balancing helicopter rotors, but I suggest you get some experience balancing gyro rotors before you give any advice about it.

Xavier, I had nothing to do with writing that paper, the author seems to have used some old stuff I put on the forum a few years ago, as did another paper that was put on a thread about balancing a year or so ago.

Mike G
 

bryancobb

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Brian You probably have a lot more experience than me balancing helicopter rotors, but I suggest you get some experience balancing gyro rotors before you give any advice about it.

Mike G
Mike,
I consider you a friend even though we have never met. What incorrect things did I say that ruffled your feathers? I understand that I didn't buy your balancing setup and choose to keep searching for a DSS Micro, but I never said a critical word about you or your balancer.
 

Mike G

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Greg sorry to hijack your thread I don't like it when people do it to me.

Brian

Sorry if I hurt your feelings it wasn’t my intention.

First, I get nothing out of any PB4 balancer sold. In fact developing the PB3 & 4 has certainly cost me a lot of money, not to mention time, effort and grey hair. I helped develop it because I saw it as a good and reasonably priced solution for gyro owners struggling with the old timer’s methods of track and balance and vibration problems generally, plus it kept me busy in retirement.

Since you’re a helicopter guy your decision to get a DSS is probably a wise one because it is a helicopter balancer whereas the PB4 was developed primarily as an autogyro rotor balancer, and the track and balance processes are similar but different.

So I don’t hold anything against you for not buying a PB4, quite the contrary.

So why my comment to you?

For a number of years I’ve been trying to convince gyro guys to form groups and get into dynamic track and balance with a PB3 and now PB4. There is surprisingly little interest and the most common response I get is that it is too complicated and they don’t think they’ll understand.

In fact it’s a very simple, if somewhat laborious, process that requires the user to follow the steps one at a time and work and fly with the maximum of precision. The biggest hurdle seems to be getting used to the Polar Chart and Frequency Spectrum graphic formats that are unusual for most.

As I explain during my training sessions (in which there is virtually no math), once you’ve got used to the polar chart and established your move lines you can forget the rest, it’s just like a video game......

“How do I get these points on the chart to move to the centre when they are only allowed to move up and down the move lines?”

That’s it, to track and balance a gyro rotor with any modern dynamic balancer (and good training) you don’t need to know anything about skaters with weights on their arms, gyroscopic precession, Coriolis and all the rest of the mumbo jumbo and folklore out there.

For me your post is just going to convince more people that it’s too complicated for them.

I shouldn’t really care anymore, I’ve probably trained just about everybody who is interested in balancing and am concentrating now on developing my latest toy that I hope will help avoid some of the accidents we see.

Mike G
 

Greg Vos

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R44/22 H269/300 MD 500 Magni (all); Xenon RST; DTA; ELA; MTO
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Greg

The old timers in the gyro world managed to get a reasonably smooth rotor using “static balance”, “stringing”, “slinging” and “tracking using the flag” and then flying and judging changes in vibration by the seat of the pants. Like any skill it must have taken them a lot of time (probably years) to develop the experience and feel necessary to achieve their results. I take my hat off to them.

If you are asking the question, I assume that you, like me, don’t have those old timer’s skills.

Today we have electronic dynamic balancers that allow us new comers to track and balance a rotor with much more repeatability and accuracy than before. With a good balancer, and knowing what you’re doing, a track and balance shouldn’t take more than a day.

It’s a pity you didn’t attend my training session in SA last November.

On another note I see that you are asking a few questions about a Xenon that you are rebuilding for a customer and that JM (who’s a lawyer) is giving you technical advice. I’m an engineer and I wouldn’t give any technical advice based on the limited information available. But I’ll give you a bit of legal advice.
If your name is going on the log book of that gyro then you shouldn’t be considering anything but replacing every damaged or worn part with the factory recommended parts and setting up the rotor (slinging or stringing) as per the manual. Selecting a different bearing or considering drilling new holes in a mast that has cracks etc is asking for trouble. If that gyro ever crashes for whatever reason, his widow will not thank you for having saved him a few Rands by “fixing” his gyro cheaply.

Brian You probably have a lot more experience than me balancing helicopter rotors, but I suggest you get some experience balancing gyro rotors before you give any advice about it.

Xavier, I had nothing to do with writing that paper, the author seems to have used some old stuff I put on the forum a few years ago, as did another paper that was put on a thread about balancing a year or so ago.

Mike G
Thanks Mike
a few years ago in Cape Town we connected your equipment to my then Magni M16 we buggered around for close on two hours with absolutely no idea of what the rotor was doing or if the shake was improvable 😔 you eventually said ok time is a issue and we should do it another time? You came to FAFK with Dave Lehr.

so some guys saying it’s complicated and they won’t understand it? resonates well with me, since then I have been to some interesting countries and have done some very in depth technical as well as flight training with only modest tools and no formal vibration assessment training, that said I seem to have got the machines to fly well

I have in the past built a xenon from the ground up for a client and then student, that was 2017, I was always chasing perfection (my workmanship with respect will put most AP’s locally to shame) I have seen some work from local AP’s that is bordereing on criminal and I could name a number of them who I would not let lose on my lawnmower!

In this (xenon rebuild) I commissioned a well known gyro operation who has vibration assessment equipment and at cost had them to Cape Town to asses the rotor vibration I was trying to rid, after 6 hours in the hot South African sun the gent said there is nothing wrong, the vibrations are below the normally accepted standards and not harmful to the rest of the gyro, he presented me with printouts and charts that would bore an actuary and confuse a judge. ( I have in flight video of a open glass of water on the centre console and I wanted no movement in the glass) he went on to say that I got it right and better than any Magni would ever have with its heavy composite blades, I did this with old fashion self learnt techniques, I balance the propellers using the same methods.

Based on that and my years of working in a high tech environment as an technician and working with gyro copter for some 13 years add my exposure to and flying helicopters for circa 25 years I have a bit of a feel for what is a dangerous vibration, ..with helicopters and having spent time with certified aircraft engineers who balance the blades with Chadwick it seems a sure way to get good results, but it’s a different cost and a different, more complex piece of flying machine.

i did not attend your course for two reasons, it was a lot of money and without purchasing equipment to continue balancing for clients it would add no value and I don’t work on gyroplanes to make a living, secondly I was not in SA at the time.

i asked the question fling or string because having built Large RC helicopters for the film business some 20 years ago long before drones and digital media when we used to build RC choppers to carry a ARRi 3c with a 400ft film magazine on the nose ( these were big RC helicopters) I used to fling them during set up and we needed a clean vibration free platform for shooting film in those days there was no such thing as gyro stabilization, it was all up to the artisan and his humble tools.

I will not even go into the complex video down link and frame speed requirements as i had to build an electric motor to perfectly run selected frame speeds, and controllable in flight by the camera operator ... carrying the standard ARRi motor was not an option with its weight, so while I am not an AP under the CAA system I have a very good basic understanding of working with my hands....
David Morris ex xenon owner gave me the simple answer I was looking for 👍
 
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Mike G

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Greg, you have a better memory for names than I have, sorry I didn’t realise that we’d met. I well remember flying with you and trying to get some data with my PB3. It was back when I was still teaching myself how to use the PB3 and balance rotors. I can’t remember the details of what went wrong but I’m pretty sure that I now know what the problem would have been. About that time I realised that we had a problem with some Magni rotors that were proving impossible to balance correctly using the dual accelerometer setup I devised for the PB3. That problem is now resolved and with the PB4 track and balance is much, much easier, even if it is still a long winded affair requiring lots of short flights.

I respect your “old timers” experience based knowledge but assure you that with today’s equipment you can get very good results quite rapidly. Next time I’m in SA I’ll come and see you, you have experience that I don’t and I’m always trying to better understand gyro vibration problems. I’d really like to get a copy of that report with its print out and charts if you still have it.

Believe me it’s much simpler than it was and with your experience you’d pick it up in no time. It can’t be a coincidence that:

Arrowcopter wanted to replace their damaged Chadwick (Vibrex) with a PB3 after I gave them a demo, they couldn’t because of an insurance issue.

AutoGyro and Magni replaced their Chadwicks with a PB4 after a demo and training.

Wagtail want to replace their system (I think Chadwick) with a PB4 after a demo.

ELA, Gyro-Tech, Brako, Trendak and Silverlight chose the PB4 after a demo and training.

Argo chose the PB4 on recommendation in Poland, and SportCopter now use the PB4.





Back to the subject of this thread, your question got me thinking.

Based on your experience you probably know most of this but I’ve written it down for me (because I have a terrible memory) and for those who are new to this.

I’m not an old timer when it comes to rotor track and balancing (although I’m feeling pretty old) and I’ve never tried “slinging” a rotor. In fact I very rarely even bother to string one before balancing.

Some thoughts on setting up a rotor if you don’t have a dynamic balancer.

The simplest first move is probably to static balance the rotor as described in John Potter’s write up (post#4 above) and extremely well demonstrated by Jim Vanek in his video
. This will give you a pretty good “spanwise” balance.

The next step should be tracking because most of the tracking adjustment features on gyros either require you to shim between the hub bar and the hub/teeter block (as shown in John’s write up) or use the “half moon” set up that can be seen in Jim Vanek’s other video
at about 06:50. Both these methods cause a chordwise imbalance so there’s no point doing a chordwise balance before tracking because you’ll just have to do it again afterwards. Tracking can be done during pre rotation with the ‘flag” method or using tip lights or reflectors (again see Jim Vanek’s videos). Tip lights and reflectors can also be used in flight and is more precise than during pre rotation.

Once you’re happy with spanwise balance and tracking there remains chordwise balance or “shifting” the rotor along the teeter bolt. John’s write-up says there’s no way of knowing how far out the rotor is chordwise without a balancer, which is true, but your question about sling or string got me thinking. If the rotor was off centre chordwise and you strung the rotor it would remain off centre. However, in theory, if you sling the rotor it should misalign the rotor blades so that its Centre of Gravity is over the centre of rotation and you have effectively balanced the rotor chordwise. If you then put a string across the rotor you’d find it misaligned.

Note I said in theory. For this to happen there has to be enough clearance in the bolt holes for the blades to move to where they want to. There also has to be very little friction because the restoring force that should push the CofG towards the centre is very small. I did a little calculation. If you had a rotor with blades of 15kg (33lb) each and a CofG at 2 m (6’7”) and the chordwise eccentricity was 0.5 mm (0.020”), that means it is “shifted” 0.5 mm off-centre along the teeter bolt, which is a lot of imbalance by the way. If you pre rotate to 200 rpm to “sling” the blades the force created at the centre to align or sling the blades is only 6.5 N (1.45 lbf). I couldn’t believe this number at first because it suggests that slinging simply doesn’t work and yet there are “old timers” who say it does. I asked Jean Claude to check my numbers and he agrees.

So you have a choice. You can sling the rotors and if it works, my theory is that you’ve also balanced your rotor chordwise.

Or

You can believe Jean Claude’s and my calculations and decide slinging simply doesn’t work.

Re your question about bearings, SKF says the following:

“Bearings in the 52 and 53 series are no longer available and have been replaced with 32 A and 33 A series bearings, which are dimensionally interchangeable. Only size 3200 A is different, and has a width of 14 mm instead of 14,3 mm.”

Mike
 
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