Balancing the G770 Tail Blades with SK

[A VIKING]

I have talked before about setting up my G770 tail blades for good balance but I would like to bring it up again because it works so well for me with the big 290mm tail rotor.

I had removed the blades from my Goblin to set up autorotation bail out with the SK and on reinstalling the blades and on my first run I had a tail vibe of 16+.

This is a totally stock 770 tail rotor with legendary sloppiness in the design.

So I went through a process I now have adopted as the only really accurate and simple way to set up the tail blades to eliminate the vibes.
The tail blades were found matched in weight from the factory so that's not a factor.
We know that rotor blades have there own CG, and I am not talking just span wise CG but also cord wise CG. If you were to draw a line at the cord wise CG location in the span wise direction and then draw a line at the span wise CG location and draw that line in a cord wise direction the two lines would intersect somewhere on the blade.
The trick is to align these two points with each other so if you could draw a line from one blades intersecting point to the other, the line would pass through the center of the output shaft for the rotor and this would have a positive effect on reducing vibes.

The easiest way for me to find these points on the blade is to use gravity to get me close.
So I will hang one blade in its grip with the hub in a vertical position and let the blade hang down and find its own balancing point. I then carefully tighten the blade bolt to pinch the blade in the grip to prevent it's movement.
I do the same procedure for its opposite blade.
Now I'll take it out for a short run at max rotor speed and look at the logs to see how much vibration is present on the SK log.
If it's still a little high I will pick out a blade and mark the hub with a felt tip pen.
I will loosen the blade grip slightly to allow an adjustment of the blade ever so slightly in the leading direction, then I'll tighten the blade bolt back down and try another run.
It is a bit of hit and miss which will usually burn up one battery to get it done but you will be rewarded with very low tail vibes.
If after the adjustment it gets worse, I will move that blade back and try the adjustment on its opposite.

So what do I see as a result of going through this?
I started with a tail vibe of 16+ and after the blade alignment my vibes are now a maximum of .7 with a low of .5. Now matter what RPM I am running and no matter what 3D move the ship is going through in flight the vibe stays constant between .5 and .7. That's pretty good for a sloppy 290mm diameter tail assembly at around 10,000RPM.
I am very happy with it.

Looking at the photo you can see how much out of alignment the blades have using the square as a visual guide. Looking at them you wouldn't think they couldn't possibly be in balance but they are.
Letting the blades swing free with looser blade bolts to find their own happy spot when running only works for me in hover. Once I start changing pitch on the tail the vibes come back.

My main rotor runs the same vibration level with sport flying but fly 3D and it will pick up a spike of 7 at a frequency 2X the main rotor, and that's common with this head I believe, something you just can't eliminate.

Anyway my G770 fly's really smooth, my Skookum told me so.


To hot outside to fly so I thought I would write something down, hope you find it interesting and maybe useful.

[TBur]

Good stuff ! Is their any correlation to vibe direction shown in data analyzer and this sort of thing ? Ie something identifiable that points us to blades as opposed to other mechanical tail issues ?

[A VIKING]

Originally Posted by TBur

Good stuff ! Is their any correlation to vibe direction shown in data analyzer and this sort of thing ? Ie something identifiable that points us to blades as opposed to other mechanical tail issues ?

No sir.
There is nothing in the log that will tell you this type of alignment of the blades is needed on the tail, only that its out of balance.
You will only be looking at the vibe spike at the frequency that your tail rotor turns on the SK log chart to know it is out of balance. Now you need to figure out why.
The key is if the blades are matched in weight and if you check their individual span wise CG and find them very close and they still vibrate then the alignment process of the intersecting points of span wise CG and cord wise CG between the two blades, that I mention above, need to be aligned through the center of rotation on the hub.
We can't easily check cord wise CG on the blades so we have no way of knowing if the blades are matched that direction.
So the only way of getting it right is with running and checking the SK vibe logs at the frequency of your tail rotor.

I am an advocate of doing this because it has always corrected my imbalance on my stock 770 tail rotor without doing anything else. You can methodically change tail blades many times, which many do, and eventually you will find a set of blades that will be matched and run relatively vibration free.
But if you start with a set of blades that weight the same but still vibrate dynamically they just need to be tweaked into better alignment in the hub prevent the vibes.

Think of it this way, you have two blades pulling on the hub spindle and the output shaft of the tail rotor. That line of pull or centrifugal force needs to be in a straight line through the center of the output shaft. If that line of centrifugal force passes outside of the center of the output shaft the rotor will vibrate because of this.
Draw a straight line on a piece of paper and put three dots spaced equally on the line.
The center dot will be the output shaft of the tail rotor.
Now take one of the dots and displace it away from the line.
Now redraw the line connecting the outermost dots.
You can see the line no longer passes through the center dot which represents the output shaft. A vibration will occur.
That is exactly what we are doing in this case...aligning those pulling forces.

Sorry to be long winded but I enjoy thinking and talking about this stuff.

Just flew my Goblin in a stiff wind and downloaded the vibe log and I continue to see .3 vibes on the tail rotor.
Life is good.

[Luvmyhelis]

In a way the logs do tell us something more than just looking at fore aft, up down or lateral vibes. It is how the vibes are mixing and moving. With some training and insight, you can tell what you are seeing. Georgi is the master at this! He told me at one point he spends roughly 20 to 30 minutes with his head literally IN the vibes, becoming one with the heli. Scrolling back and forth, separating them. Zooming in and out. Spreading them. I have witnessed him not only identifying specific issues as described above, but adding in one of the bearing race's are chipped, then tossing in something about frame harmonics due to a specific heli model, while letting guys know their lower motor bearing has issues. Then discussing how the logs show the blade profiles are slightly off. True story! I am about half that good I think? Others here that have come and gone have also seen astounding things with their talents at doing so, along with the amazing guys helping out everywhere now such as Omerco and Xtrayted.

This is much why I appreciate and honor those who hop in and help diagnose vibes for others here. Guys don't realize sometimes how much time guys like georgi do commit to them out of his own personal time just looking at one log. The effort and personal sacrifice of his life in doing so? In the old days there were dozens of them at times screaming for help. Much why I spent tons of time trying to learn myself, and still have a long long way to go.

[georgi UK]

=] @ Martin, I always look forwards to your excellent contributions here, it's always been like this, many quiet viewers, you give them plenty to think about so keep it a coming! thanks.

@ LMH Randy, you are part of that core group that can analogize logs, without question!

[SRJay]

Good post, I'm wondering though while transporting your heli do you then leave the tail blades extended? If I understand you correctly you are locking the blades down within the grip so they can't swing on their own at all?

[A VIKING]

Georgi, don't encourage me or I won't shut up.
So for the new guy and the quiet reader.

TBur asked about correlation to vibe direction to sort things out.
This can be useful in some situations but if you are new to accelerometers and the vibrations they sense you need to understand how those accelerometers "see".

Accelerometers are "directional". They only see vibrations in the direction they are pointed. That is why we have 3 axis in which the accelerometers are sensing. The vertical, lateral and fore and aft accelerometers. Its important to know also that the accelerometers "see" all frequencies of whatever they are attached to and in this case our helicopter.

Because there are three accelerometers sensing vibrations, you will see on the log for a given "spike" the reports from all three accelerometers.

Because the accelerometers are directional in sensing, one of the accelerometers will "see" a given vibration better than the others and that will be the vibration spike you see on the chart at its maximum amplitude or movement. You can still see on the log what the other accelerometers saw at that frequency but because they were not in the perfect position to "see" this particular vibration, they will report the vibration with less than the actual vibration amplitude.

For a given installation of the accelerometers, you would expect a certain trend of reporting from them.
This "trend" is based upon the integrity of the mounting system to which the rotating component is attached.
If you normally saw a lateral vibration being produced in a new ship at any of the positions of rotating components that we are interested in (engine, main rotor, tail rotor or TT shaft) and you suddenly see a phase shift or that normally "lateral" vibration shifting over to the fore and aft position for example, you should investigate why.
What is happening is the support structure or the rotating component itself has changed. You may have loose hardware, a crack or possible a failing bearing in the system.
This is important for you to understand, if you see a sudden phase shift with the maximum vibration spike changing from one accelerometer to the other, something is broke or needs maintenance.

I usually don't pay a lot of attention to the axis of the vibration, only making a mental note of its direction for latter reference if it changes.

I try to keep it simple when looking at the vibe log.

Identify the big spike.
Check the frequency of that spike so you know what rotating component is at fault.
Look at the attaching parts of that rotating component to the ship to insure its solid and not suffering from worn bearings, loose hardware or possible cracks in the structure.
Balance the offender by whatever means possible.

So from what you know so far, you know that for a given rotating component it will be imbalanced through its total 360 degree rotation but somewhere in that rotation it will become visible to one of the accelerometers because of a natural weakness in that mounting system.

So what if I told you that when reviewing your vibe log you saw a big spike in the vertical direction at the rotating frequency of the main rotor?
What are the possibilities? Think about it.
The vibe is being reported as "up and down", not lateral or fore and aft.
So what did you come up with, what will you check out?
If you answered loose mast in the vertical direction, blade tracking, loose servo mount, really worn out control link ball ends, all of these could effect blade track and as such create a vertical bounce to the main rotor.

I'll stop here otherwise I'll keep rambling and may confuse you which I hope is not the case.
Remember keep it simple and logical, you have plenty of time latter to make it more complicated but for now just focus on the big spikes and in doing so, as they are reduced, you will see the smaller harmonic spikes come down also.

Have fun with this and when you see a full scale helicopter fly over just remember and be proud that you could solve a vibration issue on that ship also because it uses the same technology...accelerometers and frequency.
Pretty cool.

[A VIKING]

Originally Posted by SRJay

Good post, I'm wondering though while transporting your heli do you then leave the tail blades extended? If I understand you correctly you are locking the blades down within the grip so they can't swing on their own at all?

No sir, I keep them locked down at that sweet spot so it's not lost.
I just turn the rotor until the tail blades are in a good position for transport, then fold the main blades.

[TBur]

Viking - Great to see success stories using the tools available

[Scottyb410]

Originally Posted by TBur

Viking - Great to see success stories using the tools available

In my somewhat humble but often prejudiced option, the SK vib log has been the best addition to helicopter systems since the FBL.

I'm just saying.

[A VIKING]

Originally Posted by Scottyb410

In my somewhat humble but often prejudiced option, the SK vib log has been the best addition to helicopter systems since the FBL.

I'm just saying.

Yes sir!

[Luvmyhelis]

I sooooo agree! No more stinking guess work, points right at the problem and lets a guy know where and what he has to address. Good stuff, and the reason I equipped all my current helis with the 720s. It hurt to do it. But the payoff is huge.

[A VIKING]

Originally Posted by Luvmyhelis

I sooooo agree! No more stinking guess work, points right at the problem and lets a guy know where and what he has to address. Good stuff, and the reason I equipped all my current helis with the 720s. It hurt to do it. But the payoff is huge.

No more guessing or just accepting what you have.
ALL of my ships had a very noticeable tail skid vibration to them. If I played with the tail I could improve the vibration I saw but I could never eliminate it.

Now I just smile when pulling up to a hover and check the tail before and after hitting throttle up and seeing the tail skid sit motionless with NO vibration at all, like it wasn't even running.

Until installing and using the SK features, I never saw that before.
My bearings have to love it also.

[B0tt0mline]

I will use this guide for setting up my new 770 with the SK720

Hopefully it works for me too

[A VIKING]

Originally Posted by B0tt0mline

I will use this guide for setting up my new 770 with the SK720

Hopefully it works for me too

If you are using stock blades I believe it will.
I tried this method many times with the same repeated positive reduction in tail vibes.
At the present, looking at my SK vibration logs, the tail is below 1 in vibration with no spikes at any time.
I could not run them visually straight and I could not run them loose, in both cases I would run a tail vibe of 16+.
If the blades weight is the same and you mount them so that if you drew a line through both blades COG with that line passing directly through the center of the tail rotor output shaft they will spin quietly with very little vibration.
Never flew any other tail blades but the SAB's so I don't know how the others react to this.