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Aerial Photography and Video > Main-blade-induced vibration ....?
 
 
Hogster
Key Veteran
Location: Surrey, UK

Hi all,

I seem to be having problems tracking down a vibration problem on my Logo 10. The blades are both exactly balanced (weight and CofG point), the tracking is dead on, and yet the whole airframe is still vibrating quite noticably. The main shaft wasn't bent the last time I checked (no crashes/hard landings since then), and the only thing I've done since the last time it was flying perfectly is install 550 blades (were 500s) and a longer boom ....

What other things can I check?

I tried spooling it up without the blades, and although there's a strong buzz in the airframe, it's at the frequency of the tail rotor (I imagine).

Could it be to do with the physical balance of the helicopter? Having added my AP2000i I had to add 90% forward subtrim to keep it relatively motionless in hover ..... could that affect vibration?

I would really like to track this problem down as I like my ships to run as smooth as silk (and I've had that before with this platform)

Many thanks for any advice,


David
04-30-2007 Over year old.
HOMEPAGE  
 
 
AceBird
Elite Veteran
Location: Utica, NY USA

Anything that rotates can cause vibration. Is this shaft drive or a belt drive tail?

Ace
What could be more fun?
04-30-2007 Over year old.
 
 
Hogster
Key Veteran
Location: Surrey, UK

Belt driven (and it's a new belt as I installed a longer boom).

The thing that's making me point the finger of suspicion at the main blades is that I can see the airframe oscillating at the frequency of the main blades ..... if it was something to do with the belt or tail rotor then I doubt the vibration would show itself so vigorously in the frame? Unless it's somehow oscillating at a harmonic of the tail rotor speed? .... hmmmmm


David
04-30-2007 Over year old.
HOMEPAGE  
 
 
airboss
Elite Veteran
Location: OC ,california

main

shaft bearings might be NFG...

..OC Synergy ..@ OCMA
04-30-2007 Over year old.
 
 
Hogster
Key Veteran
Location: Surrey, UK

How does one check the bearings? Would slop be the only problem?

Another thing I should have mentioned is that when stationary, the rotors noticably slant downward at the tips .... and when they're rotating at full speed and pitch is applied the tips slant upwards slightly ....

Could that point to anything being worn out/loose?

Thanks,


David
04-30-2007 Over year old.
HOMEPAGE  
 
 
AirFoilAviation
Veteran
Location: Quincy, IL.

Couple things we have found over the years. If its a wobble then I think the head is correct. Check the dampeners first and probably replace them, then check the ball links for any movement then to the blade grip bearings. If they have slop this can cause a flutter which will induce a wobble.
The last thing I would look at is the head speed, if its to slow this lets the lead blade move back which will throw the head out of balance on the lead lag effect.

I would put my money on worn ball links.

Hope some of this helps you.

John

Team AirFoil
04-30-2007 Over year old.
 
 
airboss
Elite Veteran
Location: OC ,california

remove

the bearings to check them,they are very inexpensive,so if you remove them than replace them.also replace dampners and check grip bearings..

..OC Synergy ..@ OCMA
04-30-2007 Over year old.
 
 
hovernut
Senior Heliman
Location: Fraser Mi

"How does one check the bearings?"

One way is to hold the bearing between your thumb and index finger and spin it with your other hand, if you feel any rough or notchy areas anything other than perfectly smooth, replace it.
04-30-2007 Over year old.
 
 
AceBird
Elite Veteran
Location: Utica, NY USA

There wasn't a problem until you stretched the boom and increased the blades, am I right?

Doesn't a gear on the main shaft drive the belt? Does the belt vary in tension as the pulleys rotate? New pulleys? so many things it could be.

Ace
What could be more fun?
04-30-2007 Over year old.
 
 
Disciple4123
Veteran
Location: Waynesboro, VA USA

Centre of Gravity off?

Quote 
Could it be to do with the physical balance of the helicopter? Having added my AP2000i I had to add 90% forward subtrim to keep it relatively motionless in hover ..... could that affect vibration?


Hogster I feel your very rear heavy helicopter with cyclic compensation to keep it level may likely induce cyclic vibration. On a heli with a good C of G the main blades are primarily lifting with little or no cyclic involvement. Now when you trim it fore 90% like you said not only do you run into travel range limitations, but the rear of the rotor disk supports more load than the front, requiring the cyclic movements, leading to vibration. I also feel that excessive constant cyclic loading will likely wear out the rotorhead faster than a balaced setup that only uses the cyclic control to change the angle of the rotor disk and runs a relatively neutral mode when right stick commands are not put in.

I second the comments that loose head dampening and a headspeed too low can cause this, but I'd move weight around so when you string hang the heli from the rotorhead centre it doesn't tilt excessively either way.

Finally I will ask, is the 90% trim you indicate needed while the AP2000 is engaged, or while flying under manual control that you need the trim as well?

Eric
04-30-2007 Over year old.
HOMEPAGE  
 
 
airboss
Elite Veteran
Location: OC ,california

bearings

they get damaged easily from crashing most are pretty low quality and easily flat spot on impact.check your CG.hgster hit a high note.

..OC Synergy ..@ OCMA
05-01-2007 Over year old.
 
 
Hogster
Key Veteran
Location: Surrey, UK

Wow guys thanks so much!

I will look at the following: bearings, dampeners, spindle, getting the CofG fixed, trying a slightly higher headspeed ....

Something that was suggested over on APLanding.com was to try removing the twin boom braces, as they can sometimes cause the heli to vibrate at a certain frequency if they're inducing stress into the airframe. And from what I can remember when I last reinstalled them (after installing the new boom), I needed to give them quite a bit of upward force to get them to locate on the support-balls under the horizontal fin ....

Eric - that 90% subtrim is without the AP2000i in use (I haven't had a chance to test it in an open area yet). I dialled in the 90% of subtrim to leave my normal trim free for in-flight adjustments. That subtrim simply tilted the swash forward a bit.

Thanks once again guys


David

Edit: AceBird - yup that's correct .... as far as I can remember the vibration wasn't nearly this bad (if at all present) before I changed the blades/boom.
05-01-2007 Over year old.
HOMEPAGE  
 
 
Hogster
Key Veteran
Location: Surrey, UK

Hi guys,

Ok I dismantled the rotorhead this morning and found that one of the thrust bearings had worn CONSIDERABLY more than the other one, that the spindle had pretty bad scrape/engraved marks on the surface and that one or two of the radial bearings were damaged .... although that could have been me trying to get them out!



Also there didn't seem to be a 6th O-ring .... I could have lost it as I was dismantling it .... I'm not sure ....

Anyhow, I've ordered a new set of radial bearings, new thrust bearings, new O-rings, and a new spindle which should arrive tomorrow ..... am looking forward to testing it!

By the way, I tried running the tail rotor up to flight-speed without the rotor head installed (just the main shaft sticking up) and there's no vibration whatsoever ... so obviously the tail rotor and belt are fine Also the main shaft isn't bent at all.

Will continue to update you all on my progress!


David
05-01-2007 Over year old.
HOMEPAGE  
 
 
AceBird
Elite Veteran
Location: Utica, NY USA

David,

Now that I see Eric’s post I am wondering if you balanced the heli for the added weight in the tail. I assumed you did and I agree with Eric if you didn’t

Ace
What could be more fun?
05-01-2007 Over year old.
 
 
hovernut
Senior Heliman
Location: Fraser Mi

David, when you reassemble the head, put some greese on those thrust bearings and a little on the O rings it will make a differance.
and remember on the thrust bearings the large center hole goes inside towards the head block and the smaller center hole towards the blades.
You might have already known this but I guess it doesn't hurt to put it here just incase.


Tony
05-01-2007 Over year old.
 
 
Hogster
Key Veteran
Location: Surrey, UK

Hi Tony,

Thanks for that, I wasn't aware of that


David
05-02-2007 Over year old.
HOMEPAGE  
 
 
Hogster
Key Veteran
Location: Surrey, UK

Btw, when lifting a heli by the flybar (as close to the mast as possible of course) to check the CofG, what should hang parallel with the ground? The skids or the tailboom? Can one assume that either of them are at 90° to the mast?

Cheers,


David
05-02-2007 Over year old.
HOMEPAGE  
 
 
FCM
Elite Veteran
Location: Back in Blighty!

Looking at the pitting on the thrust race, I would guess that this bearing has been installed the wrong way around i.e. opposite to hovernut's advice. Not sure if this would make the heli vibrate on it's own but it may be reponsible for marking up the feathering spindle.

It will be a good thing to get your C or G somewhere close to over the mainshaft. A little bit forward is okay but the way you have it now will make for some 'interesting' flying characteristics when descending from altitude!

Anyway, good thing you found this before any of it became a real problem

Boom or skids should be okay.

Paul.
05-02-2007 Over year old.
 
 
hovernut
Senior Heliman
Location: Fraser Mi

David, when you lift it up to check CG just hold it up infront of your face and look at the main shaft, you can usually tell if it is leaning or straight up and down, I have also used something as a referance such as a piece of molding or a corner of a wall but make sure your referance is plumb and straingt by holding a long bubble level against it. then just make sure the shaft is parallel to the referance.

hope this makes sense.

Tony
05-02-2007 Over year old.
 
 
Hogster
Key Veteran
Location: Surrey, UK

VIBRATION PROBLEM SOLVED!!!!

Hi guys!!

Just installed all the parts that arrived in the post this morning (basically a new set of everything you can see in that photo above, minus the two screws), greased the thrust bearings and o-rings, re-centred the flybar and reset the paddle pitch .... and it's as smooth as glass now!

Also I've found that the balance is more or less ok, and only 50% subtrim is needed to get it to stay level in a hover .... When I pick the heli up by the blade grips it stays nice and level - perhaps it wasn't as badly off as I thought it was ....

Now I just need to track down the tail-drift issue and the slightly funky performance of the ESC (see my other thread on here at the moment) and it'll be perfect!

Thanks so much for all your advice guys, I couldn't have done without your help


(a very satisfied) David

PS. I also took the liberty of ordering one of those small discs to go above the rotorhead .... no more circular scratches on my hand from slowing the rotors down now!
05-02-2007 Over year old.
HOMEPAGE  
 
 
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Aerial Photography and Video > Main-blade-induced vibration ....?
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