Wobble

[scallybert]

Wobble seems like a good word for it.

I've had this happen on a couple of helis, and I don't know what causes it.

It's a kind of oscillation in both the pitch and roll axes simultaneously, such that the ends of the main shaft make circles, and the main shaft makes a cone.

On a GAUI EP100 that was otherwise well-behaved, it could be provoked by banging a skid on the ground, or violent manoeuvres.

On a G5 with training gear, it wobbles at 2450 RPM head speed; not so much at 2400; gone at 2350.

So, the question is, what might cause this ?

[scallybert]

On a G5 with training gear, it wobbles at 2450 RPM head speed; not so much at 2400; gone at 2350.

Thinking about it, I'm not sure of these head speeds - though I am sure of their relationship to each other.

The ESC governor is expecting a 6 pole motor (which the stock Outrage motor is), but this is an OT-BL460, and I'm not sure this is a 6 pole.

I've got a suspicion I may be running slower than I thought.

[acwman]

Could it be that the blades are nipped a tad too tight???

I get this on the 500 when Practising autos, I generally nip the blade bolts tighter just incase I botch it and the blades fold back in the blade grips and take a chunk out of it.

[Steve]

All over wobble is usually a good indication of a head speed that is too low.

[scallybert]

Could it be that the blades are nipped a tad too tight???

Thanks - definitely interesting. I may well have some issues around how the blades sit in the grips.

All over wobble is usually a good indication of a head speed that is too low.

This is what I wanted to hear.

It surprised me that a lower headpseed made it go away, as I thought I'd heard this somewhere, and the EP100 experience seems to support it.

Plan now is to put in a regular motor (known to be 6 pole) so I can at a known HS like my happy G5; and tear the head & blades down.

[scallybert]

The motor was an Outrage OT-BL460 (shipped by mistake - seems nice), rather than the regular KV1700.

Counting the poles, the OT-BL460 looks like an 8 pole motor compared to the 6 pole 'regular'. Hence running it on the same governor settings will make it run at 3/4 the revs. (3/4*2450 = 1840 RPM).

I swapped the motor for the 'regular one'. It would have been easier to change the governor settings to 8 pole - but I wanted to be sure by going back to what I know...

I also zeroed the head mechanically. (ie links back to equal std lengths, then set tracking mechanically.) Then fitted a set of the g/f blades I got from EtheAv8r, which seem to be more accurate than some of the Outrage g/f ones.

Test flights in the backyard appeared free of wobble & vibes - which is good, as that was the problem in question.

Definitely higher head speed. Possibly better balance & tracking. These blades may be a tighter fit in the grips - possibly at odds with what was expected to help.

[Biggles]

If you are using training gear then it sounds like a sympathy wobble to me !!!

[scallybert]

If you are using training gear then it sounds like a sympathy wobble to me !!!

Strangely, the only reference I could find to 'sympathy wobble' was a posting from you here about a year ago...

Personally, I think people tend to take training gear off too early. I can fly without it, but it can make orientation much easier, calm things down a bit, and give some protection against bad landings.

That aside when I say 'wobble', I don't mean just the TG vibrating around. This is a much lower frequency effect, like a slowly wobbling top.

TG can certainly play a part in this, as it's going to change the MoI of the heli, hence the natural frequencies in various modes, etc. However, the other G5s didn't display this wobble (though one of them has another vibe issue, hence has massively cut-down TG.)

I think the reason for the wobble here was I'd messed up wrt the head-speed. Increasing it to what it should have been (and is on the other G5s) seems to have eliminated the wobble, and the TG seems pretty vibe-free too.

[darryl]

i've had this with training undercarraige when the head speed was to high

hth

[EtheAv8r]

This is interesting, because my G5 with Outrage wood and Outrage fibreglass plus training UC wobbled horribly! With the Outrage FG blades (carefully balabced) it wobbled very violently and the hover ended in a crash. Without the Training UC and with the alternative FG blades you now have - it was smooth and wobble free!

[tourerjim]

Battery if your using same one for them or the only thing is head speed otherwise you are looking at a machanical fault.

[scallybert]

i've had this with training undercarraige when the head speed was to high

This time round, I was running at 3/4 the headspeed I meant to be.

I was going for 2450, and was 1838.

Doing the maths on my earlier post, it wobbled less at 1800, and not at all at 1763.

These are all 'too low' in that the recommended headspeed is 2200-2450.

However, dropping from 1838 helped. I suspect that going above 1838 might also help.

I think there are two important properties of these wobbles: how stable they are (ie whether they naturally die down or build up); and whether they are stimulated by an imbalance or tracking errors in the rotorhead.

The latter is intuitively related to headspeed (just like you have to push a swing at the right time & frequency).

The former isn't so obvious. My GAUI EP100 could be provoked into a worsening wobble by banging a skid on the floor. Other than that it was generally well-behaved. My guess is that the instability of the wobble was down to too low a head speed - ie with a higher head-speed any wobble would naturally die down.

With wooden blades it wobbled horrendously from take-off. I guess they weren't well balanced, and stimulated the wobble even more. Or maybe they tracked really badly.

[scallybert]

This is interesting, because my G5 with Outrage wood and Outrage fibreglass plus training UC wobbled horribly! With the Outrage FG blades (carefully balabced) it wobbled very violently and the hover ended in a crash. Without the Training UC and with the alternative FG blades you now have - it was smooth and wobble free!

Do you know what headspeed you were running ?

This (rather boring) video [ame="http://www.youtube.com/watch?v=b-KIgYa2Vg4"]YouTube - Outrage G5 with Training Gear on 20090502[/ame] is my first G5 with TG, Outrage FG blades, and a lower than recommended headspeed. No sign of wobble, very well-behaved. (And not very well set-up...)

That same G5 (having been crashed) is now at 2450 RPM, with TG, another set of Outrage FG blades. A bit vibey, but no wobble and well-behaved. (And still not very well set-up...)

[scallybert]

BTW, I posted last night to say I'd fixed the problem.

Hopefully it will still be fixed next time it flies...

[kevinh]

is that you flying in that video mate,


if so loose the under carriage as you seemed to master the hover both side on a partial side on, you seemd to be able to hover long enough in one place.....

its a big step i know but you will notice the difference without them!!