Your thoughts and input on the Velocity 90.

[DominicD]

bruno are you sure its from the tail ? The V90 has a strange sound to it because of the main gear mod, you can usually hear it in flight vids.

[bruno95]

Yep, comes from the tail or start somewhere in the heli and end up with tail resonance (as i mentionned ended up with tail blade broken in flight but on the second heli the same day got also tail haft bend after too much resonnance).

I know the sound of the main gear mod 1 i've it during all flight like almost any heli running mod 1.

I made also trial just staying stationnary and controling RPM limitation via the radio (potentiometer)..increasing from 1850 rpm to around 1950 rpm makes the resonance sound appear and become incredible loud from the tail.

I'll make a video next time i go to the field.

I think something is entering into resonance and as all the tail control, tail blade holders, tail transmission gears are very tight (no play thanks to the good design), then nothing is absorbing this resonance (which is not the case for T700N where you've too much play everywhere). Again, as mentioned before, it appeared after removing the last remaining play in the tail control by changing fro DS650 to better tail servo.

Forgot to mention as well that i tried to reduce the tail gain down untill it didn't hold but problem did not dissapear.

[DominicD]

Do you have some kind of strap or bridge on the boom supports or are they all on their own ? I find even with the 50 that i need 1 or 2 supports on the booms supports otherwise they resonate.

[backburner1955]

The spread on those boom supports look like you can use the support brace for the T-Rex 700. It is only $7.99 and looks real nice.

[bruno95]

Thanks for your hint/proposal (any other are welcome )

Yes i've the K&S version of the support brace.

Bruno

[royroy11]

with the trex 700 torque tube, where are you guys placing the two bearings? evenly spaced, or not? if they're not spaced evenly, does it matter which side is closer to the center?

[Costas]

Ideally you don't want them equally spaced.

If they are equally spaced there is a much higher chance that you could experience some harmonic related vibrations along the TT. Spacing the bearings un-equally helps reduce this issue.

It will not matter which bearing is closest to the centre - the main thing is that both should not be equi-distant from centre.

[backburner1955]

What about adding a third bearing?

[DominicD]

That just increases the frequency of the harmonic.

[backburner1955]

What if they were all spaced differently? Would that not cancel each out?

[DominicD]

no, thats what makes them harmonics

[Costas]

Originally Posted by backburner1955

What if they were all spaced differently? Would that not cancel each out?

It will not cancel them out as such but it will prevent them from occurring.

The relationship of the number of bearings Vs the resonant frequency is just simple maths.

For instance - If you use two bearings equally spaced then there is a relatively high chance of experiencing vibrations at 2x, 4x etc of the fundamental frequency. The fundamental frequency is related to the RPM the TT is rotating at.

So if it was for example rotating at 1,000 times per sec then you may experience vibrations at 2,000 or 4,000 or even 6,000 and 8,000 Hz. This is for a twin bearing equal spacing setup.

If you now configure the TT with three equally spaced bearings then the mathematical relationship between the vibrations and RPM now become multiples of 3 eg 3,000, 6,000, 9,000Hz etc.

The idea is to mount them at odd distances from each other so the spacing does not favour harmonic vibration generation.

[DominicD]

I don't believe the TT RPM has anything to do with the resonate harmonics. And I don't know about you but most helis don't run a 60,000 RPM tail

[Costas]

It would be interesting to see whether any vibes induced in the TT are simply related to its operating RPM or whether say lower [or higher eg engine] frequency vibes can set it off.

I think many of us have witnessed engine related or even tail related vibes in tail boom supports etc.