Vibrations vibrations

[scallybert]

> How to I test them? [Servos] just move them and put a bit of resistance on the arm and see if it feels ok?

I'd suggest moving them (under power) slowly along their range, looking for any glitchy or jittery points, particularly around the positions corresponding to hovers.

Applying a gentle load may help.

Obviously, be careful...

[scallybert]

I get what you're saying about vibes, but say it starts at the back of a model, as it makes its way through the airframe it can change from a high frequency to a low one.
Nitro models were worse for high frequency vibration, as the cooling fans are running at engine RPM (15K+) and usualy in close proximity to the radio tray, so that was a good source of the high frequency vibes.
A tail rotor spinning at 10K is far less of a high frequency and thus can give different symptoms and say there is a tail based vibe, you may find that the boom and supports could dampen the vibration enough that by the time it hits the main frames and FBL platform, it manifests differently.
I've experienced problems by a tail rotor vibe and the model flew, but randomly twitched and felt horrible in flight.

Every airframe is different and so too is every makers FBL unit, some dealing with such vibrations differently and showing different signs on the airframe.
[...]

I think some of this is a bit muddled.

Our fundamental point is that vibes that upset sensors are a significantly higher frequency to the 'rythmic inputs' that provoke 'oscillatory wobbles'; and these are different effects.

For example, 12k RPM is 200Hz; a wobble is maybe 2Hz. I don't buy that vibrations change frequency through the airframe - at least not at a factor of 100 frequency, without massive attenuation.

Moreover, you refer above to vibes affecting sensors ["by the time it hits the main frames and FBL platform"; "a tail rotor vibe and the model flew, but randomly twitched and felt horrible in flight"].

Certainly, these effects are an important issue, but it's not the same as (eg) a badly tracking blade hitting on a resonant frequency of the airframe & stability system , and provoking a wobble.


> I've experienced problems by a tail rotor vibe and the model flew, but randomly twitched and felt horrible in flight.

Interesting. I've got a Goblin 500 that's a bit like this (and the Skookum says is reasonably vibey - frequency matches the tail RPM).

What was causing the tail rotor vibe ?

[cjm180]

Just an update

The new motor has arrived, and is now installed complete with a new pinion and lower bearing block. The old pinion had some wear on it, as did the lower bearing that sits below the pinion.

As you remember there was evidence the old motor had touched off quite badly on the servo. I assembled the motor on the motor plate and installed in the heli. There is quite alot of clearance between the motor and servo. So i am unsure how its touched off. There was a bit of play in the old motor, but not that much. I cant see how it has touched????

Anyway so to sumarize what been done:

1, New main shaft bearings - Felt a bit dry inside. Certainly didnt have that resistance to rotation a new grease packed bearing has
2, New radial and thrust bearings in both main blade grips - Found a worn thrust bearing
3, New head dampers - Couldnt see anything wrong here. Grips were tight. Replaced as had the head in bits
4, New motor - small bit of play, a little damage to outside of the can.
5, New lower motor bearing - Could feel wear, washer worn
6, New pinion - Old one showed wear

So it will be maidened again on Sunday. So fingers crossed. I will power it up tomorrow night without the blades in and feel the vibes. See if its any better than last time. Before i removed the main gear and ran it motor only and could feel a vibe that i felt was high.

If this does not work, i am going to try and lower the head gain a good 10 to 15 points, to an extreme to see if it goes away as grumpy has suggested.

Will report back as to its progress....

[cjm180]

New motor to servo clearance...

[cjm180]

Hey up

A little update:

Got 14 flights in on it today and it seems to of cured it. I think the problem was around the motor. It seems to of totally gone. I still cant understand how the old motor managed to contact that servo. There was not enough clearance in the bolt holes in the frame for it to move that far back. I tried to rock the servo but again it does not move that far. So dont know, but it appears to be fixed...

So thanks for all your suggestions and help, its much appreciated

[Mike Sanders]

Don't you just love it when a problem is solved.