HELICOPTER VIBRATION IN ANCHORED CONDITION

[ashimaero]

Hi,
A helicopter is anchored , and its lateral vibration is less then 0.1 ips at 92% RPM, whereas at 100 % RPM its reaching 1.5 ips.
Track is well within the limits and tried many mass balance but still its not reaching low level of vibrations.
Could anybody suggest what should be look upon?
Its a hinge less rotor with precone....

[Kiwieng]

Is this Main rotor or Tail rotor?
What manufacturer?

[Flying spanner]

Can I ask why you have it anchored? I can't see much joy coming from track and balancing ground lats with a tethered helicopter. Sometimes the difference between ground lats being serviceable, or not is landing on grass and not a hard surface.

Does the clock angle change between 92% and 100%?and does the track remain flat from 92% to 100%?

[ashimaero]

Is this Main rotor or Tail rotor?
What manufacturer?

Its a main rotor...Its a conventional helicopter with one main rotor and tail rotor...both rotors having 4 blades...

[ashimaero]

Can I ask why you have it anchored? I can't see much joy coming from track and balancing ground lats with a tethered helicopter. Sometimes the difference between ground lats being serviceable, or not is landing on grass and not a hard surface.

Does the clock angle change between 92% and 100%?and does the track remain flat from 92% to 100%?

Its a ground test vehicle, needs to be anchored to prove the integrity before flying..

The track remain flat from 92 to 100%...whereas clock angle changed in the same quadrant by 20 to 30 deg

[ashimaero]

What all should be suspected for high lateral vibrations in helicopters ?

[Flying spanner]

Since the clock angle is steady and no matter what moves you do the clock is in the same quadrant, it needs weight change. Coming either from hub centering, phasing, weight etc.

Not knowing how your head setup and assuming multi blade my ideal process for balancing would be static balance of the head, dynamic balance of the head on the machine, put the blades on and go from there.

As it seems your experimenting with something, the easiest way to do mass balancing would be to have a circular balance plate on the head to allow numerous weight locations.

As said earlier, weight is only one option. Being that it is a hingeless system I don't think you could change the phasing, but if your phasing is out it may take a lot of weight to correct for this. Also checking things like your hub centering, grip spacing and things like that.

All that aside, with regards to the dynamic balancing. This may be sucking eggs but A "move line" has to be established, because the logical place to add weight may not be where it is needed. It may take some large weights to get it out of the clock angle it's stuck at, but you need to throw some weight around and establish a move line.

[ashimaero]

Since the clock angle is steady and no matter what moves you do the clock is in the same quadrant, it needs weight change. Coming either from hub centering, phasing, weight etc.

Not knowing how your head setup and assuming multi blade my ideal process for balancing would be static balance of the head, dynamic balance of the head on the machine, put the blades on and go from there.

As it seems your experimenting with something, the easiest way to do mass balancing would be to have a circular balance plate on the head to allow numerous weight locations.

As said earlier, weight is only one option. Being that it is a hingeless system I don't think you could change the phasing, but if your phasing is out it may take a lot of weight to correct for this. Also checking things like your hub centering, grip spacing and things like that.

All that aside, with regards to the dynamic balancing. This may be sucking eggs but A "move line" has to be established, because the logical place to add weight may not be where it is needed. It may take some large weights to get it out of the clock angle it's stuck at, but you need to throw some weight around and establish a move line.

Thanks for the quick reply.
Head has been checked in both static and dynamic condition vibrations were very less. After putting the blades only situation is like this. We have mass chambers at 0.7R not at root. Tried differnt mass corrections and the system has shown senstivty to even 4 grams.
Clock angle is changing with mass correction.
Its a two segment blade.
Still no success to bring down the lateral vibration.
However at flat pitch (nill thrust condition) everytime a new value of lateral vibration is coming without chnaging anything. On the other hand at negative pitch and positive pitch values are repeating.
Is something to do with the flat pitch ?

[Flying spanner]

With regard to flat pitch. If you have a symmetrical blade profile, at flat pitch you will have zero control load and this could be causing the blades to flutter.

Have you looked at the vertical vibes? Just because the blades are in track doesn't mean that that they are in balance. They are in balance when the lift they produce is equal across the head. Vertical vibes can show up in laterals because they cause a rolling moment (some vibe gears get lateral measurements from subtracting two vertical velocimeters)

Are you able to get the clock angle to move across the clock?

[ashimaero]

With regard to flat pitch. If you have a symmetrical blade profile, at flat pitch you will have zero control load and this could be causing the blades to flutter.

Have you looked at the vertical vibes? Just because the blades are in track doesn't mean that that they are in balance. They are in balance when the lift they produce is equal across the head. Vertical vibes can show up in laterals because they cause a rolling moment (some vibe gears get lateral measurements from subtracting two vertical velocimeters)

Are you able to get the clock angle to move across the clock?

Yes we are able to get the clock angle to move across.
We checked vertical vibes too, its very low.
Its not a symmetrical airfoil.. Getting around 1000 N at flat pitch.

[Birdy]

Depending on your referance ( average chord or flat bottom), flat pitch on an assymmetrical blade will still produce lift.
An 8H12 profile for example, measured from the flat bottom, needs to be at about -4* to produce 0 lift.

[Flying spanner]

Really the only more help I've got is if you post your readings and adjustment, Pm me if you like. If you can get it to move across the clock thats a good thing. It's not uncommon for us to cut corners off weights to make 0.5gms adjustments, so getting movement from 4gms isn't anything to worry about.

[ashimaero]

Depending on your referance ( average chord or flat bottom), flat pitch on an assymmetrical blade will still produce lift.
An 8H12 profile for example, measured from the flat bottom, needs to be at about -4* to produce 0 lift.

Ya you are right.It depends on the reference.

[bladepitch]

Not thead stealing and just think its worth mentioning...no smart bum answers or anything but just Everyone offering help on every post.. Good work you lot, keep it up... hope you get it sorted..

[Birdy]

Funny, i was pondering this thread today while aimlessly smashing air particals, and the only thing i can think of is a harmonic gremlin.
Harmonic shakes and vides can be a pain to locate and isolate, and can show up along way from the cause.
The fact its not there, then a few rpm higher it is leads me to think harmonics.
Mind you, im no rocket scientist.

Hope you get it sorted, interested to hear the result.

[Birdy]

Ever watched the ips reading on a TR while slowly bringing the rpm up to 100%?
Up, down,up down, all while the clock never changes much at all.
Or am i reading the dynavide rong?

[LisaMTheiss]

I am new in flight training and getting flight lesson from Epicflightacademy. By the I am happy be in this thread and looking forward to your question and answers. Thanks

[Flying spanner]

Funny, i was pondering this thread today while aimlessly smashing air particals, and the only thing i can think of is a harmonic gremlin.
Harmonic shakes and vides can be a pain to locate and isolate, and can show up along way from the cause.
The fact its not there, then a few rpm higher it is leads me to think harmonics.
Mind you, im no rocket scientist.

Hope you get it sorted, interested to hear the result.

I had given this some thought also. As you say very hard thing to track down. This was my original concern about it being anchored. If you have vibe gear that has a spectrum mode you can check different frequencies. When you take normal vibe readings they are 1:1, if it were to be a harmonic it would have to be coming back to the velocimeter as a 1:1 and in phase it the original vibe. If it wasn't in phase you would have an unstable reading. All kind of pie in the sky stuff. I guess course of action forwards if it was a harmonic, would be to look at the spectrum and look at the multiples of the main rotor freq and try to get them lower or I guess just see if they go to custard when the other vibe sets in.

[Birdy]

I dont want to send this thread awol, but i have a dumb question.
A 2bl teetering rotor, balanced near perfect in mass and chord, hovers with a motionless stick.
Any speed above translation sends a slight 2 per shake to the stick.
Besides a miss set cone angle ( no flap hinge) and the inherant 2bl drag pulse, is there anything else i that would cause this vibe?
I know a 2bl will always shake some with forward speed ( inherant) but id like to know if there is anything else i need to check.
And please, keep it simple, so,s i can follow.

[Flying spanner]

Birdy, if you have hydraulically boosted flight controls I would check the sloppy links on the servos. The servos should stop control feedback from the head.

To put things simply 1:1 vibes are caused by an out of balance head, ie one blade flying high for one whole rotation. 2:1 are caused by the main rotors affect on the airframe, ie 2 blades passing over a loose transmission mount for each rotation.