Troubleshooting a tail fin resonance

[Anthony Oxlade]

Hi Guys,

I had a hard landing on the maiden of my TT conversion and managed to strip the 54T gear and crack my tail blades. Upon rebuilding i have observed a tail fin resonance at 2300 rpm headspeed. This is with the heli spun up on the bench with main/tail blades removed. It is appears as a severe side to side vibration of the lower tip of the tail fin. The vbar analyser shows a large spike at the frequency of the tail, with no other significant peaks.

At 1900-2000 rpm the tail seems to spin smoothly, with peak vibration levels around 100 from the vbar.

By trial and error i have found that the fin resonance only occurs with the tail blade grips and bearings in place. If i remove these but leave the tail hub attached then the spectrum is smooth and there is no tail resonance. The only issue i could find with these parts is that the inner radial bearings were slightly notchy. Has anyone had this cause a tail resonance before?

I'm not sure what parts to replace and tempted to just order parts for a completely new tail assembly!


The only

[GAR]

Is the tt shaft bent?

I have broke off both tail blades to the hub and not bent the boom or striped any gears BUT BENT the shaft!

[Anthony Oxlade]

It looks ok by eye but it is hard to tell. I assumed if the TT was bent there would be high vibration levels even with the tail assembly off? Also the frequency of the vibration peak is exactly that of the tail.

I'll order another shaft just incase but they take a week to get to the uk There are none in stock here.

[GAR]

Maybe the shaft is not bent.

I have not seen a perfect straight (new) e6/7 shaft yet.

In the days of using Easton arrow shaft for tt drives...they were straight, as an arrow.

[Tommy Wagner]

I had a similar problem on my old N5  and it was the tail shaft or the tail hub that was slightly bent, bye it looked perfect, I even took the motor out and dial indicated it to make sure the crank was not bent when I replaced the tail shaft and tai hub the vibe was gone.


Tommy

Sent from my iPhone using Tapatalk

[Anthony Oxlade]

I will try one or two things tonight whilst i wait for my batch of new parts. First I'll put the suspect tail assembly back on and replace the TT with the belt. If the resonance disappears then i assume it must be related to the TT...

[Anthony Oxlade]

Hi Guys,

Thanks for the suggestions. The source of the vibration was a bent tail hub. I had a minor blade strike that didn't bend the tail shaft but the tail hub somehow was slightly damaged.

My bench test is now silky smooth across the whole rpm range. Peak vibration level on the vbar at 2300 rpm is just 40

Can't wait to get flying again!!