Vibrations vibrations

[Grumpy]

The problem with the Trex 600 Pro/Dominator is that in standard trim the motor mount is fixed, so you cant change pinions. This means that you need a motor that is exactly or very close to the stock Kv (530Kv). 560 Kv may be a bit high. The 4035 designation usually refers to the size of the stator. A 4035 has a 40mm diameter x 35mm long stator so a 4025 is a significantly smaller and likely to have less torque and lower ultimate power rating.

Personally i'd not even be considering changing the motor at this point as there really isn't any evidence at all that the motor has any problem. Trouble shooting a heli problem by random chang-out of major components can get very expensive very quickly! A 'wobble' in flight wouldn't usually be down to vibration anyway. All the stuff that spins on a heli does so at high speed, so vibrations will be high frequency. A 'wobble' is usually down to a head problem, such as worn or too hard dampers, head gain too high.. or maybe notchey thrust bearing

[Rich23]

Although having a press is handy it is not required to change the bearings. You will need a main shaft to knock out the old bearings and a socket which fits the OD of the new bearing to seat it.

PM me if you need help

Rich

[Grumpy]

The arbour press isnt so much to remove the bearings, it's the press the shaft out. The larger Align motors have a 'flange' machined into the shaft rather than the more common circlip in a groove. This means that to get the can off the stater you have to press the shaft out of the can. You could maybe use a large vice, or even a hammer instead of a press, but the shafts really do take some shifting.

I've got an arbour press so if anyone needs an align motor worked on I'm happy to help.

[cjm180]

Thanks for the replies everyone.

Grumpy, yes i agree. I am just trying to cover the bases. How much running would be considered time to change motor bearings? particulaly align motors?

Its all re-assembled now, so i will try and get a test flight tonight to see if its any better. I see the wear on that thrust bearing was bad. Also the same (outer) race on the other grip looked to be fine. In fact the other race in that pic is from the other side. So it shows that the blade / fethering shaft assembly has been harder on one side than the other. This i think is a telling sign, as for me it shows either side of the head are not balanced or not concentric and its maybe allowing more loading on one side as the spindle / blade assembly moves axially to the spindle more over on one side.

I can also say they are the original head dampers in it also. Maybe one has seen more wear than the other side, and that in its self has helped any excentricity either side of the head block.

See how the flight goes.

On the motors, and i know nothing about motors, i see the 750mx is a 700 size heli motor. i see the specs for this motor are alot higher than those of '600' size motors such as the scorpion and xnova's. Would the wattage recorded in the ESC be a measure of motor size required electrically?

[p1andy]

Your thrust bearings are in a really poor state. I have previously had wobbles like your having and found it to be the thrust bearings but i have never had a set look like yours, normally i end up with individual ball impressions. When was the last time you had them out to grease/maintain?

In fact just last night i changed my protos ones out and they were damaged, new thrust bearings and dampers and she should fly nicely again.

[cjm180]

er.... last time I had them out? err, 2 1/2 years ago just before they went in for the first time



Had a quick flight on it tonight and again it seems ok. I will get it down the field next week and put some decent flights in on it and see how it goes. I can confirm it took 200+ flights to get em like that. So a bit more than the 20 flight check recommended in the manual.....

[p1andy]

That could well explain why they are so bad then. I dont check after the manual recommended flights as that would only be a few days flying, but i do haul them apart to ram more grease in now and then.

Glad it looks like your sorted.

[scallybert]

Personally i'd not even be considering changing the motor at this point as there really isn't any evidence at all that the motor has any problem. Trouble shooting a heli problem by random chang-out of major components can get very expensive very quickly! A 'wobble' in flight wouldn't usually be down to vibration anyway. All the stuff that spins on a heli does so at high speed, so vibrations will be high frequency. A 'wobble' is usually down to a head problem, such as worn or too hard dampers, head gain too high.. or maybe notchey thrust bearing

Exactly.

The 'buzzy' vibration that blinds sensors and provokes uncontrollable rolls, etc, is a much higher frequency than the kind of vibration that hits resonances and provokes oscillations.

[cjm180]

Hey

Got 6 flights in today on the 600l, and while it sounds better the problem still exists.

Got home and pulled the motor out. The gear is worn quite bad, which was a supprise to me. There was a lot of black dust around the bottom or frame mounted motor bearing. The small washer had some wear on it also.

The big find tho was that the motor can has been rubbing on one of the servos.



So i immediately thought motor bearings again, but they dont seem too bad. There is a little bit of play, but not as much as deep as the grooves are. So maybe that servo has moved a little over the 200+ flights?

It certainly was not built like that as the motor was free to turn. Anyways, i have decided to trade the motor in for new as there is damage on the can. Its 50% off a new one if u trade in your old.

So thanks grumpy for the offer. I will def bear that in mind for the future.

So new motor, i will replace the fram mounted bearing in the lower motor support and see how it goes then. I will also replace the pinion gear. Will i need to change the main gear or will it 'bed in'?

After that i will inspect the tail. but i dont expect much there, but could be wrong. It just seems like a resonance vibration to me. Exactly like one. So if i can just shift the level of vibration i think it will be fine. Be nice if the gpro was like the demon, and tell u where the vibes are (motor, head or tail).

Its been perfect for the 200 odd flights its had.

[cjm180]

Ok, so i get the jist that the motor vibrations would be too high to cause this kind of oscillation. I cant think what else it could be tho. Its flown great the whole time so i am struggling to think its the FBL unit needs tuning sa little in the head.

The only other thing is the tail drive system. But i would expect poor tail performance and any oscillations to be in a forward / aft direction. This is certainly left to right.

Are you guys thinking that if there are vibes in the tail that that is messing with the FBL sensors causing the FBL unit to put a left right wobble in?

[scallybert]

I was thinking you had it sussed with the notchy thrust bearings in the grip(s).

The other thing I'd wonder about is a dodgy cyclic servo.

Sometimes the potentiometer tracks wear (or get damaged), so they get jittery in a very specific position.

[Grumpy]

Have you tried lowering the cyclic gain?.... I know there should be no obvious reason why a FBL setting that worked fine previously should now need reducing but helis are funny things sometimes. It's quick easy and costs nothing to reduce cyclic gain, so got to be worth trying.

As for tail or motor vibes causing it... nope, that doesnt sound likely IMHO. If vibes knock out the sensors you get loss of response and would usually end up picking up pieces.

[coolice]

Exactly.

The 'buzzy' vibration that blinds sensors and provokes uncontrollable rolls, etc, is a much higher frequency than the kind of vibration that hits resonances and provokes oscillations.

Have you tried lowering the cyclic gain?.... I know there should be no obvious reason why a FBL setting that worked fine previously should now need reducing but helis are funny things sometimes. It's quick easy and costs nothing to reduce cyclic gain, so got to be worth trying.

As for tail or motor vibes causing it... nope, that doesnt sound likely IMHO. If vibes knock out the sensors you get loss of response and would usually end up picking up pieces.

Hey Guys.

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.

It's worth checking the torque tube, I've seen it where the tube was too long and was jammed too tightly between the tail gear and boom block gear, which gave some funny issues. There needs to be a little float there.

Moving on to dampers, if it was these I'd expect to see it every flight, not sporadically as it seems to be(?) so far with a coming and going nature.
Is this vibration/wobble linked to a change in rotor rpm or rpm set point in an idle up?
Have the dampers been greased when assembling them into the head? If not this can see the feathering shaft grabbing and releasing as it moves slightly against the dampers. Even the Align POM ones need a smear of grease to allow the fine amount of free movement.

Another thought, have the main blades been checked?
Could the internal weight be loose and moving around in flight?
Could they be out of balance in the span wise C/G and thus lead/lag differently in flight? The latter would through off the rotor heads balance nicely.

Ian

[cjm180]

Hey up

Bert, I did think about a servo, now that I have seen one has been touching. Maybe this has put a bit of heat into it which may of effected the pot? How to I test them? just move them and put a bit of resistance on the arm and see if it feels ok?

Steve, yeah I can just drop the head gain for a test. It will help rule it out if nothing else.

Ian, I need to go thru the tail don't I. to rule it out. I suppose its worth a look anyways given the dodgy head bearing.

To summarise whats what:

What I am getting:

About 10 or so flights after freeing off the OWB, I started getting a very slight side to side wobble. It was noticeable in the skids. It was tiny. As soon as I put in input in to move it out of the hover it seemed to go away.

Over the next 10 or so flights it progressively got worse. It seems to get worse after a few flights. For example it will be a lot worse after the last flight of the day than the first.

It starts off very small after holding it in the hover after 5 or 10s. Then over the next 3 to 5 seconds gets worse. When u move it out the hover it stops. I can only see this in the hover. FFF etc all looks fine visually.

What I have done so far:

Replaced OWB complete for new.

Made sure main gear jesus bolt is not over tightened.

Rebuilt head with new bearings and dampers. All greased up. Found a worn thrust bearing.

Replaced main bearings, both the upper and lower.

removed motor - it has been contacting the servo. Small amount of play seen, not excessive tho. Also the outside of the can has rub marks on it. So elected to replace the motor (50% off if u trade in your old align motor).

Wear on pinion gear, will replace.

Ordered up a full set of tail bearings. Also ordered new motor lower bearing

I am away from my heli now for a few weeks. When I get home I will fit the new motor and pinion and take it for a fly. If its still there I will try and film it so you guys can see. I will then look to reduce the head gain a little and see what happens.

After that I will look to renew all tail beaings.

Hopefully with your help we can find the issue.

[Grumpy]

As you have ruled out the mechanical parts of the head then it sounds like head gain too high to me. I've had the exact same 'wobble' visible on the skids in a hover on a couple of my helis, the 7HV and the G630. Both could be improved by reducing head gain though the 7HV needed softer dampers to get the problem fixed totally. In my case it was more pronounced at moderate to low headspeeds.