Rotor balancing issues

nomie

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I need some help with my rotor balancing!

I did read some topics regarding this on this forum but I must admit some of the technical discussion went a bit over my head. If the experts could perhaps steer me in the right direction given my unique situation I would really appreciate it.

My story:
After landing my 2013 Magni M16 I discovered 1 few coin-sized chips on the rotor. My AP repaired these and repainted the rotors. On replacing the rotors they had terrible vibrations. The AP then statically balanced the rotors, and using a DSS personal microbalancer got the rotor vibrations reduced by a lot.
However, he is running out of things to adjust to get the last bit of vibrations dialed out.
The curious thing is - when loading the rotors (Like doing turns), they are silky smooth. But as soon as you go back to flying straight and level the vibrations return and some cabin hop also starts.
He is currently looking at the control rod stiffness as suggested by the Magni agent but I am not sure if this is maybe more guessing than anything else.
Is there perhaps anything else that can be tried? Thanks!
 

GyroChuck

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From previous posts

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.

discussion:

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
 

GyroChuck

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Chuck Beaty Rotor Shake

If a rotor is smooth during a vertical descent, you’ve eliminated track, balance, rotorhead running off-center, out of pattern, etc.

When the shake occurs only during forward flight, it is due to the 2/rev aerodynamic input. Then you have to look for incorrect undersling, rotor in-plane resonances, etc.

In-plane resonance is caused by either the mast being too stiff or the rotor being too soft in-plane.

Date: July 09, 2001 08:40 PM
Author: CA BEATY ([email protected])
Subject: Blade track

The thrust vector of a rotor, John, is normal (right angle) to the hub bar. With out of track blades, the thrust vector rotates in a conical path. Stirs the rotorhead and the stick in a circle. Also produces a 1/rev lope in the airframe; the high blade has more aerodynamic pitch than the low blade and as it enters the advancing side of its orbit produces more lift than when the low blade is the advancing blade. Some of the shake -the stirring motion- can be canceled by slightly overbalancing the high blade.

Scott Malone is right, it's very unusual to find a rotor is smooth when out of track.

Date: July 12, 2001 03:39 PM
Author: CA BEATY ([email protected])
Subject: Rotor shake

Reply to John Landry's post of 7-10-01

There are 2 types of shake in the rotors we usually fly on gyros:

(1) One/rev: This is both the easiest to understand and to correct.
(A) Out of pattern and out of balance.
In both cases the CG of the rotor is off-center and stirs the rotorhead and stick around in a 1/rev circle.
To correct, balance the rotor or align the blades with a string.

(B) Out of track: One blade flies higher than the other because of differing aerodynamic pitch. The rotor thrust line is perpendicular to the rotor and rotates in a conical path, stirring rotorhead and stick in a 1/rev circle. Also, the high blade, as it enters the advancing side of the rotor disc produces more lift than than the low blade when it becomes the advancing blade and causes a 1/rev vertical lope of the airframe. The stirring motion can be reduced by slightly overbalancing the high blade, confusing people using electronic balancing machines who don't understand the underlying principles. Electronic equipment is not a substitute for brains. Blades having "equal lift" will fly at the same height.

(2) Two/rev: This is the most difficult to understand and the most vexing to rectify.
(A) Insufficient undersling: The CG of the coned rotor is directly above the teeter bolt and in forward flight with cyclic flapping, rotates in a circular path at 2/rev. When the blades are oriented at 90°-270° to the flight path, the rotor CG lies on the rotorhead axis. When oriented at 0°-180° to the flight path, the rotor CG is behind the rotorhead axis as a result of cyclic flapping. It moves in a circular path that is difficult to visualize without the aid of a model.

(B) Each time the rotor is broadside to the relative wind in forward flight, the drag is higher than when endwise, producing a 2/rev fore and aft shake. The only way to cure this is by the use of a limber mast.

There is, of course, some overlap between the undersling generated shake and aerodynamic drag shake.
Both forms of 2/rev shake will generally disappear at zero airspeed.

Other sources of 2/rev shake result from blades not having identical characteristics; i.e., different pitching moment coefficients or different chordwise CG locations. Pitching moments can be equalized by bending trailing edge tabs.


Date: July 12, 2001 06:34 PM
Author: CA BEATY ([email protected])
Subject: Galloping blades

Ken, I'm sure you've seen a see-saw. It balances with equal weight kids.

See-saw rotors behave the same way.

When out of track, the circular stirring part of the shake can be eliminated by adding weight to the high blade and the electronic balancing equipment may not know the difference. The person operating the equipment should.

The vertical lope can only be removed by getting the blades in track.

PS: It is sometimes possible for a rotor to be smoother out of track than in track if the airfoils of the 2 blades aren't the same. But this isn't a case of equal lift; it is a situation where the shake from operating the blades out of track balances out a shake from blade pitching moments.

But this is a very rare occurrence. As I mentioned in a previous post, out of 1,000 sets of blades, Ernie only had to scrap one set that may have come unlatched in the bonding fixture.


Date: July 12, 2001 11:16 PM
Author: Randy Brooks ([email protected])
Subject: Tracking blades

Randy Brooks I have not seen any method posted for tracking blades. I flew helicopters for years before we had electronic balance machines and they were smooth. One of the best tracking devices that was used was a pair of cat eyes (reflectors) attached to blade tips..Take about a 1 inch diameter reflector and solder a no. 8 screw--put no. 8 rev-nut in blade tip. Paint a 1/4 inch black strip on one. Then used a six cell flashlight and in flight you can see how blades are tracking. As long as the black strip is in center blades are in track. For balance use masking tape. Put on turn on tip of one blade if it gets better add more turns until smooth. Take off tape and weigh then add weight. If it gets worse with one turn of tape removed and put tape on other blade. Takes a little time but you may have a smooth rotor for fifty cents worth of tape instead of thousands of dollars worth of testing equipment.

Date: July 13, 2001 10:51 AM
Author: Randy Brooks ([email protected])
Subject: Installation of reflectors

Obtain at least two no. 8 riv-nuts. Drill a hole to fit riv-nut in lower side of blade or tio at the 25% station chord wise. Solder no. 8 slot head screws on each reflector. Install the riv-nts in the holes. You will need a riv-nut tool similiar to a pop rivet tool except it has a threaded stem to fit riv-nut. Screw into riv-nut, insert into holes and pull. Screw reflectors into riv=nut and align facing the pilot. After rotor is spinning on the ground and in the flight direct flash lite on the reflectors. One reflector will have a 1/4 inch black stripe painted across the reflector. If blades are quite a ways out of track you will get two reflections. One clear and one with 1/4" black stripe in center. If track is close the black stripe will overlap reflection showing track of blades. If in track stripe will be in center. You can do this in flight at different airspeeds and it will show track of blades. Adjust blades until they stay in track at desired crusing speed. Some rotors will vary track at different airspeeds due to imprefections in construction of airfoil along blade length. To determine if vibration is due to balance or track observe the structure(instrument panel)or( top of door frame) if it moves laterally it is balance - if vertically it is track.

Randy Brooks, CFI (airplane and rotorcraft-helicopter)
 

querist

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I fly RC helicopters and RC gyros and blade balancing is a major concern there as well. I realize that full-scale blades are much longer and this may not be practical depending on your location and facilities, but one of the very first things I do is a simple hang test where I hang the blades together, facing the same direction, with a rod through the main hole used to attach the blade to the rotor head just to see if they hang straight relative to each other.

Granted, my rotor blades are only about .5 meters long each, but the same principle applies if you can find somewhere from which you can hang them.

The other steps used in RC helicopters would not work for full-scale because of the size difference. I hope this helps.
 

Abid

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You need to do a dynamic balance. If you do not know the IPS in different axis, you don't have the data to do the correct thing.
I suspect based on your saying that when the rotor is loaded (a few extra RRPM) it is smooth than when at normal 1G, that you have a tracking issue.
Unfortunately, Magni older rotorheads have no way to adjust tracking. Now though they have an eccentric bushing that does allow you to adjust tracking, but it has to be done with a dynamic balancer otherwise it is a waste of time. Adjusting tracking also has an effect on chordwise balance.

Look at threads here from Mike Goodrich about balancing
 

XXavier

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GyroChuck: Thanks for those valuable and informative posts...
 

Mike G

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I need some help with my rotor balancing!

I did read some topics regarding this on this forum but I must admit some of the technical discussion went a bit over my head. If the experts could perhaps steer me in the right direction given my unique situation I would really appreciate it.

My story:
After landing my 2013 Magni M16 I discovered 1 few coin-sized chips on the rotor. My AP repaired these and repainted the rotors. On replacing the rotors they had terrible vibrations. The AP then statically balanced the rotors, and using a DSS personal microbalancer got the rotor vibrations reduced by a lot.
However, he is running out of things to adjust to get the last bit of vibrations dialed out.
The curious thing is - when loading the rotors (Like doing turns), they are silky smooth. But as soon as you go back to flying straight and level the vibrations return and some cabin hop also starts.
He is currently looking at the control rod stiffness as suggested by the Magni agent but I am not sure if this is maybe more guessing than anything else.
Is there perhaps anything else that can be tried? Thanks!
nomie
If you want help you have to be more specific and detailed about what happened and exactly what you and your AP did.

It would also be interesting to know where you are, and who exactly you are dealing with as an AP and Magni rep. Perhaps someone could recommend someone else local to help. It sounds like your AP is lost, if he can't resolve the problem with a DSS it's surprising, the DSS is a pretty sophisticated and excellent bit of kit.

A video of your stick shake straight and level and in a turn might help.

I concur with Abid there is a lot of stuff about balancing already on the forum, it's your job to do the research and come back asking about the bits you don't understand, otherwise you're asking those who are trying to help to do your work.
This comment isn't directly only at you, it's a general trend for people to expect those of us who are prepared to help, to try to filter out the details from a vague request for help by asking you questions you could have been asking yourself if you did the research.

Sv.grainne's comment and advice is very valid, but it's been said many times before in other posts about balancing that you should have picked up on by doing your research. I accept that the search function here isn't too good.

We're here to help but "god helps those who help themselves".

Attached is something I wrote a long time ago but is still pretty valid.

Mike G
 

Attachments

  • Mike G Balancing tips rev 4.pdf
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Brian Jackson

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nomie
If you want help you have to be more specific and detailed about what happened and exactly what you and your AP did.

It would also be interesting to know where you are, and who exactly you are dealing with as an AP and Magni rep. Perhaps someone could recommend someone else local to help. It sounds like your AP is lost, if he can't resolve the problem with a DSS it's surprising, the DSS is a pretty sophisticated and excellent bit of kit.

A video of your stick shake straight and level and in a turn might help.

I concur with Abid there is a lot of stuff about balancing already on the forum, it's your job to do the research and come back asking about the bits you don't understand, otherwise you're asking those who are trying to help to do your work.
This comment isn't directly only at you, it's a general trend for people to expect those of us who are prepared to help, to try to filter out the details from a vague request for help by asking you questions you could have been asking yourself if you did the research.

Sv.grainne's comment and advice is very valid, but it's been said many times before in other posts about balancing that you should have picked up on by doing your research. I accept that the search function here isn't too good.

We're here to help but "god helps those who help themselves".

Attached is something I wrote a long time ago but is still pretty valid.

Mike G
Printed and studying. Thank you for posting this, Mike.
 

Abid

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I can also add that if you are using a dynamic balancer and have the measurements in IPS. After you get below 0.1 IPS, you won't really feel any better. In fact, if you get the IPS down below 0.05 it starts to feel worse because the 2/rev start to be perceived much more prominently. Below 0.1 IPS measured right at the rotor-head in multiple axis, is good enough to leave it alone
 

nomie

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I just want to say a big thank you to everyone for the help. This forum really is blessed with individuals with a wealth of knowledge!

Apologies if I gave the impression that I started this thread because I do not want to spend my own time and effort to resolve this issue. This thread was started out of frustration because we have already spend a very very long time trying to resolve it.

I am busy sifting through everything in this thread to see what we might have missed. I'll post some feedback if we manage to fix the issue in case someone else ever runs into the same scenario - rotors smooth under load after balancing with DSS, but still have vibrations during S&L. Just waiting for the weather to improve a bit - its thunderstorm season right now!

Thanks again.
 

nomie

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Melt: Any chance you have an electric motor up on your mast (just below your rotorhead) for a prerotator?

What kind of gyro do you have there? Near Johannesburg?

Hi. The Magni M16 uses a flexible drive cable as prerotator (See photo). In the rotor head area the AP installed a new prerotator drive shaft, removed the fairing around the mast, and installed the sensors for the microbalancer. The AP did put a set of rotors from a 'known good' gyro on at the beginning, but the gyro was happily hopping around on the ground even with that set of rotors. He then spent a lot of time getting the two blades balanced exactly, and then did the procedures as laid out in the posts above to get everything dynamically balanced.

Yes near Joburg :) Gyro is currently in a suburb of Johannesburg called Brakpan, and I live an hour or so drive away Pretoria.

1669355191178.jpg
 

Mike G

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nomie,

Melt


Sorry if I was hard on you, I've had a bad week.

You say that when you put a "good" rotor on your M16 it was "hopping around on the ground even with that set of rotors." so I take it that you never flew with that rotor?

If both the good rotor and your rotor caused excessive vibration I would suspect that some thing has changed in the rotor head or mast or even the gyro. Did you (or your AP) do any other maintenance while the rotors were being painted?

Did you re-paint the blades completely or only the patches? Did you strip any paint off prior to painting?

Did you match mark the blades, blade carrier (hub bar) and teeter block so that they went back in the same place as before?

You say "did the procedures as laid out in the posts above to get everything dynamically balanced.". I don't understand, none of the posts above gave any instructions about how to dynamically balance a rotor.

During the "dynamic balancing" of your rotor, did you actually fly or did he try to balance on the ground? Can you describe exactly what he did?

Can you post the polar charts and frequency spectrum? Your AP should be able to print them for you.

1.5 IPS at the top of the mast where I see he put an accelerometer isn't good, did he use one or two accelerometers?

The Magni distributor is near J,burg, what does he say? Can he balance your rotor?

As I said before, we can't help if you don't give us all the details. What may seem insignificant to you might well be an important clue.

Mike G
 

500e

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Hate it when there is patch paint, striping blade and re painting is the way to go.
We have found helicopter blades are not balanced end for end this makes a difference
Here is a jig we made to check it works on both multi blade and 2 blade rotors DSCF0021_zpsje4rujxh.JPG
 
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