90E tail vibes - gone

[rchelijc]

It seems this has been done to death.

Mine never really had much vibes anyway - in-flight the vbar read 400 at 2000rpm headspeed. But there was always this "sound" that I wanted to get rid of.

And it was off and on - some days it was louder.

As of now, it's completely gone. This is what I did - hopefully it might help someone.

Pre-work: I was installing a new ESC, so I thought i'd loosen all frame bolts and nuts, and retightened them by one after the other in increasing torque strengths, but not too tight.

Then I took off the tail blades, and then removed the tail head form the heli. The sequence of what I did after that as below.

Note: those that know how to dynamic balance the tail, then probably not helpful to you. The only additional thing I did was ensure no end-play on the grips, and dynamically balanced the grips and hub first without blades, then with blades.

1. Tightened the tail grips to the hub (tighten feathering lock nuts)
Not tight, but just until the there's almost no end play when I try to pull the grip away. I did both sides. Then...

2. Checked distance from blade grip hole to centre hub hole using a calliper. I didn't care what the distance was, as long as they matched 'physically' - that is lock the calliper when measuring one side, and then made sure that the other side was the exact same, which it was.

3. Machined the bolts and nuts that hold the tail blades so that each bolt and nut is within 0.001g from the other. Easily and quickly done with a dremel and a microgram scale. I matched the bolt and nut separately since I wanted them to be independent of the blade grips I'll be using them with.

4. Stuck it on the dubro tru-spin - the tail head assembly is perfectly balanced.

Side note: I found this quite remarkable - that the distances and weights matched. Kasama's machining tolerances must be really low.

5. Removed tape from the tail blades. I had tape on the CG on one side cos they were out of balance out of the box (by about 0.1g). However, I marked the CG on the light blade just in case I needed to replace the tape.

6. Mounted blades and tightened them in the grips, ensuring they were totally straight outwards.

7. Back on the dubro, and sure enough the heavy side consistently dropped below centre. I added a long piece of tape to the CG on the light blade, and kept trimming off until completely balanced.

8. Mounted on the tail shaft. The method of tightening the grub screws I used is the same when I tightened the frame bolts, or when tightening nuts on a car wheel. I used red loctite on both, then screwed one in until it just touched the shaft, then I did the other side exactly the same. Then I tightened one side a little more, then the other, then over and over again, all the while jostling the head assembly roughly on the shaft to make sure the grubs sit nicely in the holes on the shaft.

Done.


Took it outside, and spun it up at my different head speeds (without mains). I nearly fell off the chair by how silent my tail has become. Even my smaller helis aren't as silent. touching the boom, there is almost no 'buzz', even at 2050 headspeed.

[Tom Kreuzberger]

Great Mate!

Thats how it should be.

Tom

[seemore74]

Question here guys, being new to the vbar I dont know but when I check my event log after a flight I keep getting "raised vibration" warning throughout the entire flight, is that something to worry about? Other than that the heli seems to fly great and do what it should do when I say.

[rchelijc]

Originally Posted by seemore74

Question here guys, being new to the vbar I dont know but when I check my event log after a flight I keep getting "raised vibration" warning throughout the entire flight, is that something to worry about? Other than that the heli seems to fly great and do what it should do when I say.

I get that too, it's normal. Whenever the blades flutter (blade fart) or during hard maneuvers you'll get that. There's a threshold, not sure exactly but over 1000 I think.

Try just one flight of hovering only. You should not see any raised vibes on the event log for this.

[Mercuriell]

That's a great and methodical run through - worth being a sticky !

[rchelijc]

Originally Posted by Mercuriell

That's a great and methodical run through - worth being a sticky !

Unfortunately, not without photos... I feel sticky-worthy threads should always be photo-ed to the max... I might edit them someday.

But for those who have battled tail vibes on the 90E/N then know exactly what I'm doing when and to which part I guess...

[rchelijc]

Well, while I was at it, I realised decided to balance the main rotor head as well. But I was too lazy to take it off, so I did these 2 things. As far as I'm concerned, this is good enough, but if you really want perfection, then you need to take the head off with the shaft, and stick it on a dubro.

Note: I'm assuming the shaft is sitting true in the 3 bearing blocks, and that the whole drive system is built securely with no unintended play.

1. Tighten your blade grips as you normally would, but back of a little so that it's very snug but not too tight.

2. Take the heli outside, and spool it up at the max head speed you run, then bring your collective to zero pitch, and hit throttle hold. Important because you don't want to load your blades while spooling.

3. Turn your heli off! Then carefully tighten up those blades without moving them - your blades should now be straight out now.

4. Put your heli on the table, and drop it to one side, supported by books so that the rotor disk is approx perpendicular to the ground.

5. Balance your head by adding tape to CG of the lighter blade until perfect.

You'll notice that the anti-rotation bearings work one way, while in the other direction, it's free to rotate, for autos for example.

6. Flip your heli on the other side and do the same, reason is as above.

7. Find a flat piece of wood / pencil, about 1cm wide, and about 0.5cm high. Put this flat piece directly below the main mast, and let the skids balance on it. If your CG is right, it should balance there perfectly.

8. From above, carefully spin by head by hand AT THE HUB (ie, not pushing the blades from one side). If reach is a problem, you might have to figure out how to do this another way. What you're trying to do is spin the blades by hand at a reasonable speed but not 'disturb' the heli with sideways forces.

9. Your heli should sit perfectly balanced spinning away happily.


If it is rocking back and forth, then it's not balanced. The momentum of the heavier side is throwing the heli forward and back. Greater accuracy can be achieved by using a narrower piece to balance the heli on, but then again, the practicality of this depends on how 'efficiently' you can hand spin the heli without disturbing it.

I any case, once this is done, I reckon it's good enough to fly.

[Stallion]

@rchelijc: It seems you know a thing or two about the Srimok Tail! I have Tail vibe issues at the moment! Changed Tail shafts. Balanced main and tail blades(including hub and grips) but still the tail fin is vibrating about 5 to 10 mm! Then i landet the heli and watched the track of the tail blades while it was spooling down! they where out of track by 5 to 10 mm at the tip! I adjusted the Tailblade grips as you discribed with no endplay! The hub was fresh out of the box. My guess is that this is causing the vibes! i was not amused that a brand new tail hub is already bend or not running true! At the last order i ordered two tail hubs (they are realy sensible) threw that on and the same shit all over again!! i did change the bearings in the grips! it has to be the hub! Any ideas what else it could be or does Kasama has a quality issue here!!!????
What i will try tomorrow is to loosen the grips a tad to have about 0,1 to 0,2mm endplay and may be the grips will centre better!

My method of checking the hub is to install it and thighten it put the blades in straight put it on the bench and hold the grips down with two fingers and turn the hub with a finger then watch the tip of the blade! If it wobbles or moves then it is bend or untrue! And having two fresh ones in a row bent out of the box makes me not happy!!!!!!!! :-(

[Mercuriell]

There were a batch of tail hubs that were out of true about a year ago and it is possible some may be still around in dealers - mount it up on a spare tail shaft in a drill and check it runs square and true. If they are faulty your supplying dealer should be able to help you or mail them back to Kasama. There are very stringent QC checks on them now.

[Stallion]

@John: What i have noticed on the two hubs i have lately ordered is that the machining quality seems to bee better and there are more threads on them which is a good thing except that i can not fold the tail blades completely for transport any more. I hope i get that sorted out quickly i realy would like to fly again!!!!! But i am a bit disapointed about the matter! Even if i send them back to Kasama it realy takes time to ship it back and forth.

[rchelijc]

If the tail hub is not running true, you'll get vibes for sure.

Also, I'm assuming you're installing new hubs because of a mishap? If so, check the ball links - I'm not a big fan of the hex bolt through the hollow ball link design, but if the bolts are even slightly off on each, then you'll get slight off pitch differences that'll be more pronounced at higher speeds.

I'm curious as to why you mentioned on spool down only - have you tried testing (without main blades) at all headspeeds? Out of track tail should not become back in track again at different headspeeds, and then off track again in other headspeeds.

However, vibration can cause resonance, where at certain speeds, the tail will look out of track.

[rchelijc]

After a long time of problem free operation, I came across the same issue with tail vibes when rebuilding after a crash.

It was a major struggle with vibes, and it took a while before I recalled writing something here, A quick visit back to this thread, and realised that I should have read this at the start of rebuild - would have saved me heaps of time!

Anyway, the rebuild thread is here.

To summarise, about the tail, the Kasama tail is designed to be ultra-accurate to a fault. It is not very friendly to inaccurate assembly. Compared with my test machine the 2007 MA Razor 600E (which has unbalanced tail blades and some wear and tear on one of the tips), the Razor is much more forgiving to lazy builders.

Basically, you need a digital caliper with POINTY tips to setup the tail. You need to ensure the blade bolt holes on the tail grips are EXACTLY the same distance from the hub - measuring is not good enough. You need to just fit the pointy tips into the hole, tighten the caliper, and then ensure the opposite end is the same.

That is, PHYSICALLY verified rather than by measurement. I would guestimate that such physical measurements are probably accurate to 0.0001mm, taking into account elasticity of the metal in the calipers.

Then, balance the tail blades precisely with a microgram scale to about 0.002g accuracy. You can improve on accuracy of cheap microgram scales by taking 10 or so, alternate measurements, and averaging them out. You can find your own balance between sample size and free time.

Finally, mount the tail, spin up and VERIFY with vbar vibe analysis. Take a tiny piece of tape and move it along one blade and test, and then the other blade, until you find that vibes drop to its lowest.

End result:
1. Every effort made to assembly the tail with precision
2. Final verification through actual running of the tail, and tuning with tape.

Previously, many have recommended the 95mm tails, as being less likely to vibrate, compared to those above 100mm.

If you do all of these above, you can run any tail, up to 115mm or more if you so feel like.

Good luck with the tail!