450 Please help with chasing a vibration.

[Bruno]

Trex 450 Pro V2 stock with Torque tube tail drive. Does have a Tarrot boom.

Had a crash and bent the tail boom. At least the 10th time I've crashed this heli.

Repaired her before and back up in the air without a problem.

I spool up the heli on the bench without the main rotor head and I get a strong vibration in the tail that resonates through the boom struts and vertical when at mid stick to the point that it will numb the finger tips to the touch.

I have chased this vibration to the point that I have literally changed every bearing (I have yet to rebuild the head but it is not on the heli) the in the heli from the main shaft (completely rebuilt the main shaft assembly), to the tail boom mount (bearings and gears) down to the bearings in the tail grips. I have tweaked the gear mesh while checking the gear mesh throughout it's rotation.

There is no evidence of the vibration running the following tests, motor only; motor and main gear only; motor, main gear and torque tube drive assembly only; motor, main gear and torque tube drive assembly and torque tube only. motor, main gear and torque tube drive assembly and torque tube and tail umbrella gear and tail drive shaft (without tail assembly). Once I install the tail assembly I get the vibration. I have completely rebuilt the tail rotor assembly and made sure it slides like silk. I am using a second new TT with new bearing and dust cover (off center).

I have tried it in HH mode and rate mode with no difference.

Could it be the motor getting weird? I only have maybe 20 hours of light flight time on the heli. No 3D

Can some one help before this heli takes a manual thrust flight out my second story window...lol

 

[Tony]

In all honesty, it only matters if you have a vibration when flying. I'm guessing your gear mesh could be just a little too tight, or you have a weird spot on the main gear. Your pinion could not be perfectly centered and so on. If you really want to check where this is, start with just the motor, no pinion, and spool it up. Then, add the main gear, auto gear and shaft, and spool it up. Then the tail block (no boom or TT) and spool it up. Then, add the boom, TT and tail block (no tail shaft). Then, add the tail shaft. Then add the tail grips. Then add the tail blades.

This is pretty much a process of elimination. Hope you find it.

 

[Lee]

I spent 5 months trying to track down a very bad vibration, that made my 450 unusable.
Turned out to be the tail boom itself.
I had a carbon tail boom on it. Change back to an original aluminium one and the vibe were gone.
Hope that helps you.

 

[Bruno]

I will continue to try to isolate the vibration.
What might be other indications that it's the motor? How long do the motors last?

 

[Lee]

I would doubt its the motor.
Not much to go wrong with them, unless you bent the shaft or damaged the bearings.
If you have checked all bearings, shafts, tolerances between gears, then the only other thing is the boom as i said.
Try adjusting the bolt tensions on the boom clamp holding the boom. Sometimes tightening or loosening them can alter the resonant frequency of the tail.
Obviously don't loosen them too mush that the boom would slip.

 

[Bruno]

I have run the motor without a load and it is quiet and smooth throughout speeds.
I will swap the tarrot boom for an align boom. This is the first time I installed a tarrot boom.

 

[Bruno]

So...I have basically followed everyone's recommendations from earlier posts and then some. I have completely rebuilt the tail including a new hub, grips, bearings, slider, tail shaft, and umbrella gear and loosened the boom by about 1 mm.

I have replaced the front boom drive umbrella gears and checked the bearings.

New TT and bearing and dust cover.

The entire main gear assembly is entirely brand new.

The main shaft bearings are brand new. New main shaft.

Pinion is flawless. Main gear/pinion mesh checked all round with about 0.5 mm movement.

Spooling up on bench in rate mode without head assembly and with tail blades.

Nothing has changed with regard to the vibration I can literally see in the tail boom supports at mid stick. Higher RPMs and the vibration resonance diminishes however I can feel the vibration with my finger tips in the boom supports and the boom.

I have never really paid this much attention to the tail.

Is what I am noticing this normal?

Should I install the head and see how she flies?

 

[Tony]

If this is during spool-up, then this may be normal. If you can, post a video of what it's doing. That will help us narrow it down more. But to me, this sounds normal. All helicopters have a resonance vibration when they are spooling up.

 

[Lee]

If its a small vibration, then it could be normal.
If the tail is a complete blur, and you cannot see the tail fin, then thats bad.
As Tony said, post up a video and let us see. Some things are really difficult to diagnose without seeing the heli in action.

 

[Admiral]

Where is the TT bearing located? I have been told if it in the exact center of the boom that it can create an harmonic vibration, it needs to be offset a bit.

 

[Tony]

Where is the TT bearing located? I have been told if it in the exact center of the boom that it can create an harmonic vibration, it needs to be offset a bit.

That is correct. I put mine a little forward of center. About a CM or so.

 

[pvolcko]

Check the boom supports too. Sometimes the ends come loose of the support rod and will cause some vibration and even tail wag in flight.

 

[Amir Tahami]

I had also a weird vibration on the tail.
Turn out to be a weight difference on the tail blades and. New blades and zero vibration.
But Tony's suggestion is good, a process of elimination.

 

[Lee]

Going on from what Amir just said.
I too, had bad vibes from a set of plastic tail blades.
Changed to Carbon blades and the vibes went away.

 

[Bruno]

Vibration is the same without blades.

- - - Updated - - -

How about I try a second tt bearing?
Where should it be placed on the tt?

 

[Tony]

How about I try a second tt bearing?
Where should it be placed on the tt?[/QUOTE]

See post 11 for the answer...

However, I say you just stick some blades on it and see if it messes with the gyro. Like I said before, ALL helicopters have a resonance vibration when they are spooling up.

 

[Bruno]

Tony, I plan to rebuild the head and see how she hovers. Waiting on new grips. One grip's ball link thread was stripped.
I am curious though as to where others have positioned a "second" tt bearing.

 

[Tony]

There is no need for a second bearing. It will just add unneeded stress to the TT gears.

 

[Bruno]

There is no need for a second bearing. It will just add unneeded stress to the TT gears.

Yea, I have read conflicting opinions regarding a second TT bearing and the majority (similar to your opinion) warn against it since it may only mask or intensify a problem.

 

[Bruno]

OK. Checked & set BX settings. All went well.

Took her out for a test flight 10 minutes ago.

Hovered very well. Just noticed a slight blur/resonance in the vertical fin.

No general vibration in the tail.

Did some loops, flips, rolls, and inverted. no noticeable problems.

I'm guessing if it ain't broke I won't fix it.

I will keep an eye on things.

Perhaps the amount of resonance that is remaining is typical.

All comments are welcome.

Thanks.