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Old Jun 17, 2011, 04:08 PM
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USA, CA, San Diego
Joined Oct 2007
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V120D02 tail vibration fix Xtreme Boca bearings upgrade

I've decided to dedicate a blog section to the tail vibration problem for obvious reasons.

Getting these tails vibration free seems to be one of those things that many of us have problems with, including yours truely. It also seems to be an inherent problem with these small Heli's in general. Part of the reason is that the tail blades are spinning 3.4 times faster than the main roter. So if the mainblades are going 4000 RPM's the tail blades are at 13,600 RPM's.

You know how the tail fin buzzes back and forth and it's difficult to hold onto without it buzzing your fingers off? If your lucky you don't have this problem "very unlikely" or it's not very bad "mild buzzing" that is about where I'm at now. Dynamic balancing helps but doesn't get rid of the tail buzzing completely.

This was recently referred too as the holy grail of small heli fixes. You can tune out part of the problem by dynamically balancing the blades using clear tape but I think there is more to the problem than this.

I believe a major part of the problem is that the bearings do not fit snug enough into the plastic bearing retainer's which tend to stretch out over time. Using higher quality precision bearings should also help.

Ideally the bearing retainer should be C/Aed into position and held firm so it can't vibrate or move at all.

After looking closely at what happens to the outside part of the bearing while running the heli I realized that the bearing retainer "outside part" spins or wiggles around while the tail is spinning. The amount of vibration seems to depend on how loose the bearing fits into the hole. You have to look very close while it's spinning. The outside part of the bearing tends to vibrate and slowly spin around in circles. This condition alone is enough to cause the tail to vibrate. No amount of blade balancing will matter as long as those bearings aren't properly secured.

The outside bearing holder "right side" is the one that seems to wear out fastest because it's closest to the tail blades. Mine was so worn that the bearing was wiggling around and falling out.

I believe I have found a suitable replacement for the stock radial flange bearings from Boca bearings. These high quality close tolerence high precision and sealed 440c stainless steel bearings combined with the Xtreme aluminum tail box should give me a chance at having a vibration free tail. These bearings are $12.95 each and have free shipping in the US. They are much higher quality than the standard bearings that the Xtreme tail comes with.

Before assembling the tail I prepped the new bearings and tail piece by lightly scuffing with 600 paper and cleaning everything with isopropyl alcohol. This gives the C/A more grip and ensures that the bearings won't come out or spin around in the tail box which is a common problem.

This is the link to Boca high quality metric radial flange bearings 1.5x4x2 to fit the Xtreme tail box or stock plastic tail box.
http://www.bocabearings.com/bearing-...F681XZZ-15x4x2

I'll post pictures here and update this blog to let you all know how it works out.

The first picture below shows a few important things to look out for.
Yellow arrow = Clear Scotch packing tape is to balance blade.
Blue arrow = -002 oring which gets rid of play on tail pitch link and also keeps the link from popping off to the inside which is a common problem.
Red arrow = Source of vibration problem, this bearing retainer tends to spin around and simply does not fit the bearing retainer tight enough to hold it securely. This bearing also has quite a bit of play in it.

Update:
I'am happy to report that this new tail assembly is completely vibration free. No more strange noises or buzzing tail fin, it's smooth as can be.

I believe that using the Boca 440C high speed sealed bearings made a big difference. The real trick is that you have to C/A them into place so they can't spin around or vibrate. I also needed to sand appox. .001" off the back of the drive shaft gear for better gear mesh.

I also made a new custom tail fin from 1/16" carbon fiber plate and new tail blades from 1/32" carbon fiber plate which you can see in the pictures.

I'm using the Xtreme tail box with the rear reinforcement bar left off to reduce weight. The Xtreme tail box uses a clamping type system to hold the tail box onto the tail boom which is much better than trying to secure it using set screws.

A small problem is that the tail bar that comes with the Xtreme tail box is precisely .0052" too short so when you tighten down the screws it squeezes the entire tail assembly inward causing the bearings to bind up. It's really overkill on strength anyway so it's an optional part in my opinion.

Ideally the tail parts should be spinning freely with minimal resistance. You need to check for free spinning of the tail assembly with the drive shaft installed, tail assembled with the front gear removed. It should spin freely with minimal resistance, about five or ten spins per flick.

The machine screws for the tail fin are also too short to work correctly, mine just slipped out of the hole when I tried to tighten them. I had to use longer machine screws from my Walkera parts to make the fin mount stronger.

The Xtreme tailbox is a really nice piece that is worth the extra effort to get it working right. The clamp type tail box securing system alone makes the Xtreme tailbox worth having. The extra weight of this tail box might be a problem for certain set ups. It's about a gram heavier than the stock plastic version so the extra weight can be a problem unless you rebalance the heli by moving the battery.

Two things would make the Xtreme tail box better. Xtreme needs to make a slightly wider rear bar and include longer screws for the tail fin. It would also be nice if Xtreme had an upgraded Boca bearing option for this kit.

Dynamic balancing:
You will most likely need to dynamically balance your tail blades for optimum smoothness. In order to be completely vibration free one of my blades required a single piece of clear tape 3/16" wide about 3/4 the way towards the tip wrapped around the leading edge of the lighter blade.

The procedure is basically the same as balancing the main blades. Use idle up for max RPM's and check for vibrations. Move the 1/8"-1/4" wide piece of tape from blade to blade, idle up the heli after moving the tape checking for vibrations, going by feel until you find the light blade. You will notice the vibrations drop when you add tape to the lighter blade.

After you find the lighter blade adjust the width and the placement of the tape until the vibrations subside. Going by feel, idling up the heli to check for vibrations each time you adjust the tape. This is a good place to take your time and get it right.

The c/g is not always the best place for the tape. Moving the tape out past the c/g seems to work best.

This ten minute dynamic balancing procedure made a huge difference. My heli is incredibly smooth now. I can turn my gyro's up to 3 oclock with no servo arms bumping around and there is zero vibration on any of the tail parts. No more of that insane buzzing when you grab the tailfin. Very smooth and precise tail movements with zero wagging and no jumping around, tail hold is much better also.

If your tail is still vibrating after performing these procedures, or if it takes a lot of tape to smooth it out and assuming that your tail blades are balanced then most likely your tail shaft is bent.
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Last edited by chevell; Aug 15, 2011 at 07:17 PM. Reason: Update
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