Bad vibrations

[tnarg.retrac]

Hi folks
I have been trolling all the posts I could find and so far, but still cannot find the source of my vibrations. I am hoping that you kind folk can point me in the right direction. I bought a Belt CP recently and am setting it up before its pilot flight. However, I get what seems to be a classic low rpm vibration which I cannot find.

The first thing that I did was to check the main shaft. It looked fine and just to be sure, I replaced it anyway.

Putting the head back on, I see that the swashplate has a vibration in it (it had this before I took the head off). The vibration is small (<0.5mm) but as the swashplate rotates, I can see this movement transferred into linkages connected to the servos. The head also has this vibration and it is most obvious by looking at the little button on the top of the head which wobbles around.

These vibrations are visible at slow speeds so I don't believe this is a balancing problem - more like an out-of-alignment problem.

I have also removed the feathering shaft and it too seem straight although I am not convinced that that could be the cause of the vibration. When putting back the blade grips, I made sure that they were still tight, but free to move. There is no lateral slop (pulling each blade grip in the axis of the feathering shaft, away from the main shaft).

Obviously the problem gets worse when the blades are attached and even though I have balanced them, I am sure this problem stems from the head somewhere. I have tightened all the screws and all the mobing bits can move as required.

Any suggestions on where else I should look? I don't really want to find this by buying a new head piece by piece or is that my only option?

Many thanks
Grant

[another_finn]

Did you roll the main shaft on glass to check it's straight? The low-RPM wobble is very, very common with the stock shafts... They may appear perfectly true, but there's usually a slight bend where the main shaft collar goes. Most of them arrive with this bend from the factory.

The only other obvious candidate is the main shaft bearings. Can you wobble the main shaft around by hand? Do the bearings feel rough, or do they move visibly in the frame if you try to move the head?

[Rotax800]

You talked alot about your main shaft but check your tail shaft. I just replaced mine last week. The vibrations were absolutely terrible.

[tnarg.retrac]

> Did you roll the main shaft on glass to check it's straight?

I rolled it on a flat surface but not glass - will do that tomorrow. The new and the old shaft appeared to be straight - certainly the movement I am seeing in the head is way more than what would have been a possible bend in the shaft - but I guess it gets amplified the further from the shaft it is.

> The low-RPM wobble is very, very common with the stock shafts...
> They may appear perfectly true, but there's usually a slight bend
> where the main shaft collar goes. Most of them arrive with this
> bend from the factory.

If the new ones are also bent, how does one solve this - bending the shaft by hand?

> The only other obvious candidate is the main shaft bearings.
> Can you wobble the main shaft around by hand?
> Do the bearings feel rough, or do they move visibly in the
> frame if you try to move the head?

No - that all seems tight. I'll take out the shaft and roll it on some glass and check again - perhaps the bend in it was so slight I missed it last time.

I guess the chances of the pastic pieces of the head being warped is pretty slim?

Lastly, to maintain the CG of the heli, I need to move the battery really far forward - the back of the battery slides in as far as the front vertical bit which connects to the skid (I am sure there is a proper name for it!). Is this normal and what is the best way to fix the battery so it doesn't come loose? Also, the canopy gets really difficult to put on as the battery sticks out too far. How do most people overcome this?

Thanks again for the help.

[helihathnofury]

...they get a trex canopy!!......

[Jonnyheli]

This is going to be confusing. There are now 2 Grants. Im goning to call you Grant2.

You don't bent the main shaft by hand. You buy new ones that are a better brand. The trex ones fit but you have to place it in upside down.

PS. Not all of the main shafts are bent but they have been on occasions.

-Jonny

[another_finn]

Lastly, to maintain the CG of the heli, I need to move the battery really far forward
Is it the 1800 mAh? I find the Belt CP balances pretty well with batteries in the 2100 - 2200 mAh range. As suggested, the stock battery really needs a T-Rex canopy to get far enough forward.

[tnarg.retrac]

Did you roll the main shaft on glass to check it's straight?

OK - I have rolled the original shaft and the new shaft on a glass mirror and cannot detect any bend in it. I will put the shafts on a lathe later today and see how true they spin. However, when spinning the motor up with no head or swashplate attached, I can see and feel no movement in the shaft. When I increase the throttle to about 40%, I can feel a slight vibration in the whole heli, even without the head. But the shaft seems to be running true. The collar just above the top bearing seems to rotate a little skew - that may be as a result of it being placed on the thin part fo the shaft. Speaking of which, I don't understand why the shaft needs that little indent in it - the collar doesn't need it...

So if we assume that it is not the head or the main shaft bearings, where do I go from here? Or is my assumption invalid?

Thanks again for all the suggestions.

[another_finn]

The thin part is just insurance that the set screw can't slip too badly on the main shaft. In negative G, that collar is basically supporting the weight of the helicopter on the upper main bearing.

If the vibration is just that - not a clear wobble in the head, and you only see it in a narrow RPM band, it could just be a resonant frequency. There's always some vibration in a helicopter system, and hitting the right harmonic can shake things quite a lot without there necessarily being anything wrong. If it goes away once you throttle up past the vibration band, it may not even be worth worrying about. Hard to tell without seeing how it behaves.

[tnarg.retrac]

If the vibration is just that - not a clear wobble in the head, and you only see it in a narrow RPM band, it could just be a resonant frequency.

The shaft itsenf doesn;t seem to have a wobble. Problem comes when I put the head on it I see there is a wobble. The swashplate moves around laterally and it seems like the top of the head is also moving. It is almost like the plastics are not in alignment.

Perhaps a short video of the problem would provide some insight?

Thanks

[another_finn]

Perhaps a short video of the problem would provide some insight?
That might help - it's often easier to see what's going on than rely on a written description. If the wobble only starts when you put the head on, it could be a worn or defective center hub that's out of alignment... But then you would see the wobble no matter what the RPM.

If the head is out of balance, the vibration would show up when you have enough RPM to make the forces big enough to shake things around. At even higher RPM, the speed could be enough to spin stabilize the head and mask the problem. Have you measured that your flybar is centered, and the weights are exactly the same distance from center?

[Jonnyheli]

Just play the song 'Good vibrations' by 'The beach boys' to your helicopter and then try to fly it. :thumbup:

-Jonny

[tnarg.retrac]

That might help - it's often easier to see what's going on than rely on a written description.

I have loaded one on YouTube here:

http://www.youtube.com/v/Su06bJ85_Hg

Apologies for the poor focus but being able to get in close enough is a hassle.

THings to note:
1) The wobble on the button at the top of the head.
2) The small but visible lateral movement in the swashplate.

Please shout if you need a better version or some different angles.

Thx

[another_finn]

I don't know - that seems like a lot of wobble at such low RPM, and it seems to start right at the shaft collar... The regular spot for the Esky bend. I would call that a clear-cut case of a bent main shaft, except you checked and it isn't.

It could be the frame rate, but it sort of looked like your flybar wasn't tracking properly... Did you check that the paddles are level? Do the links move freely?

One last straw... How tight is the jesus bolt in the center hub? Sometimes you can tighten it too much, twisting the head slightly out of true. If it feels like you could safely back off half a turn, it might be worth a shot.

[tnarg.retrac]

I don't know - that seems like a lot of wobble at such low RPM, and it seems to start right at the shaft collar... The regular spot for the Esky bend. I would call that a clear-cut case of a bent main shaft, except you checked and it isn't.

It could be the frame rate, but it sort of looked like your flybar wasn't tracking properly... Did you check that the paddles are level? Do the links move freely?

One last straw... How tight is the jesus bolt in the center hub? Sometimes you can tighten it too much, twisting the head slightly out of true. If it feels like you could safely back off half a turn, it might be worth a shot.

Thanks Mika - What would happen if I put the main shaft in upside down. If there was a bend there then it may show up some otherhow? If this is a bend then it is pretty constant with 3 different shafts that I have tried. Not out of the question but IMO seems unlikely.

As for the flybar tracking, I took another look and it was a bit out. Well spotted. It has not reduced the head wobble though.

Lastly, I back off the jesus bolt a bit but no real difference there.

I will see if I can source a T-rex main shaft and see if that will work for me. At least that should prove it one way or the other.

Thanks again for all the help.

Regards
Grant

[another_finn]

Depends on what batch they're from... There was a time when you couldn't get a straight one anywhere, but I hear they have improved since. Putting it in upside down would transfer the bend to between the bearings, so maybe you could get your main gear to shake too :D

Anyway, I'd call a T-Rex main shaft a pretty conclusive test. Never had the shakes again after I started using them.

[tnarg.retrac]

Depends on what batch they're from... There was a time when you couldn't get a straight one anywhere, but I hear they have improved since. Putting it in upside down would transfer the bend to between the bearings, so maybe you could get your main gear to shake too :D

Anyway, I'd call a T-Rex main shaft a pretty conclusive test. Never had the shakes again after I started using them.

I have noticed that the main gear seems a bit "warped" - when it rotates it is not completely flat. Could that be the cause?

[mnwizard]

Hi Grant - Just like Mika I had trouble with the Esky shafts being bent. I went thru 4 of them and couldn't track down the wobble.

Finally out of frustration I rolled the shafts accross a mirror. You can see even a very small bend this way, as the bend area won't touch it's reflection on the mirror.

All 4 Esky shafts were bent near the end.

I use the Trex shafts now and haven't had a problem. You just have to put them in upside down so the machined grove doesn't ride inside the main bearing. If you're putzy like me, you can file a small flat on the shaft for the set screw of the collar to tighten on, but most guys don't bother.

[tom dubya]

The only thing i would be really concerned about would be the swash movement. The button on top of the head always wobbled a bit when i was using the plastic. And i went though a few of the plastic heads and they all wobbled a bit on the top. Also the main gears don't always spin totally true. well in my case anyway and i have used the esky's,helimax and the blade 400 gears.

[DierWolf]

You need to check more than just the paddles being level, you need to make sure the Flybar Rod is the same length on both side and then make sure the paddles are screwed on or place on the same distance from the head or it will be an imbalance and cause all sorts of vibrations.

i really dont see a bend in the shaft all i see is the crappy esky swash moving slightly, you say this is a new bird, how loose are the 2 parts of the swash? maybe the center ball is very tight in its sleeve making for poor movement.

Also you have to take all these things with a grain of salt when it comes to stock plastic parts and have to remember the washout arms directly connected to the head is also connected to your swashplate and if all of that stuff is not set up right and balanced it will transfer down to the swashplate, couple that with the Crappy loose link setup its clearly seen by the swash moving about becuase of the play in the links.

So again check your setup on the flybar, check your swash plate itself (remove it and inspect how its moving it should be free and smooth) and check your blade balance.

[thunderhead]

Nice assessment, and perfect use of the "C" word. This looks fairly normal. Plastic parts are plastic parts. I use a Hi point balancer to balance my rotor heads. I find this tool and a good blade balancer indispensable. Once you get the blades on that will probably smooth right out.:thumbup:

[sutty]

Well I have to say I didn't think it looked that bad either. When I had a plastic head I don't remember it being much different to that and I didn't have any trouble with it. The button at the head of the shaft just isn't made accurately and each time I replaced it, I used to break the plastic one a lot whilst I was learning :(, it wobbled a different amount to the previous one. Once I think I had one that was actually centred. :) This was always the case whether the shaft was straight or not.

Can you not remove all head components, and just spin it up slowly, and see if you can see anything. Maybe you could gently hold a screwdriver against the shaft whilst it spins and see if it causes it to vibrate. (Never tried this, so have no idea what will happen, just a thought). If all is well with the shaft only then add components and see when things start to go wrong. Once you have found the culprit, make sure you balance and centre it correctly to minimise the impact. If it is the shaft, then you have your answer, inverted trex shaft, can't fault it.

EDIT: Just read the thread more carefully from the start and saw that you had already tried shaft only and couldn't detect a wobble. My mistake, sorry for going over old ground.

[bmorse]

Hi Guys,

I am new to this great sport and I enjoy every minute. I have a belt cp exhibiting the same kind of vibration. In fact I replaced the main shaft as well. I did recently have a small blade strike, but not enough to even break the blade. I had to replace the main gear because of stripped teeth. Now I am in the hunt for this vibration. I took the main blades off and spun it up thinking the blades were contributing to the vibration. As it turns out you can see the hole head moving along with small movements in the swash plate as described in the thread. Did anyone find the solution to this?

[copterboy]

Did you replace the main gear with stock e-sky gear?? They can come warped from new.

I replaced mine today with a Blade400 main gear (thanks Kianhon :thumbup:) and it's much better than e-sky gear. Would recommend you get one of these and replace the e-sky one - even if you not crashed it!!!

[helihathnofury]

The Microheli parts are much better, if more costly..........