Head geometry optimization

[curmudgeon]

I built my 806 according to specs using the stock servo arms (20mm holes), stock swash balls, and outer hole on the blade grips.  I ended up with a low number on the 6° geometry step on my Spirit Pro.  This would not be a big deal except that I am still experiencing side to side vibrations after 84 flights.  I have to lower the electronic cyclic gain on the Spirit to improve this side to side head/skids vibration.  Another pilot suggested I use a shorter servo arm hole.  I guess by lowering the mechanical gain I can increase the electronic gain.  I agree a shorter servo arm hole would increase the 6° geometry number and put it in the ideal range for the Spirit, but I am concerned about any negative tendencies a shorter servo arm would cause.  I am not sure if there is any ball adjustment(s) I can make to either the swash or the blade grips to improve the 6° geometry number.

Please share your thoughts on head geometry optimization for this helicopter.

[Mike Spano]

If it's a constant shake like you describe, it's likely mechanical and not a geometry issue.

If it's not constant, here are my thoughts (with the limited info I have) 
On the vbar they call for 8 degrees cyclic, but most of us end up running between 10-11 to get it to perform properly. Just putting the number where it needs to be and letting the degrees land where they land.

Knowing the servo type would help along with the head speed.

Heads peed can cause this as well, too low...

Can you elaborate more on your setup?

[curmudgeon]

I posted on the "Side to side vibration" thread back in July 2016 about this issue after doing only 6 flight.  I was told to let the head dampers break in; about 25 to 30 flights.  Now I am on flight 84, and the side to side shake continues.

Rail 806/116 blades
14S
12T pinion
25T tail gear for 5.1:1 tail gear ratio
KDE 700XF-455kv-G3 motor
Kontronik 200HV, BEC set to 7.9v, using Kontronik governor
BK BLS 8002HV cyclic and 8005HV tail MKS HBL599 X6 cyclic servos and MKS HBL880 X8 tail servo
Spirit Pro FBL
Jeti REX7 receiver
Stock Head Dampers:  80D Orange inside, and 90D black outside

At 80% Kontronik governor throttle (about 1,750rpm) I get pretty bad side to side shakes during hover.  Decreasing the cyclic gain all the way down to 20% (minimum cyclic gain for Spirit Pro) does not improve the side to side shake.  Decreasing the head speed down to about 1600rpm makes the side to side shake go away almost completely.  No side to side shakes at 1500rpm.

[Rodney Kirstine]

Have you tried running 4 soft dampers yet?

[curmudgeon]

Have you tried running 4 soft dampers yet?

No, I have not.  Would that be the next step?

[Rodney Kirstine]

If you're not doing really hard 3D flying then I would suggest giving it a try, yes.  I'm running them on my 766 and it cleared up similar symptoms that mine was having.

[curmudgeon]

Thank you.  I'll give the softer dampers a try and report back.

[Mike Spano]

There is something else going on in think. Mine doesn't shake. I'm flying the same servos. Just to be sure, check the blades, they are packaged by people, so there is a chance maybe they got miss matched. Check the motor shaft for true and motor bearings.  After that, I would get a different, better, fbl. Maybe a vbar. All fbl are not created equal.

[alan]

Replacing a flybarless unit due to an aileron bobble would be silly. Lots of folks running the spirit. May not be a vbar but it is highly unlikely it spirit itself is the issue.
Agree with Mike that you need to rule out mechanical and balance issues.
I have found in the past specifically with the Bavarian demons that with harder dampening one needs the run lower overall head gains. For grins you may also look at that. Not familiar with Spirit settings but if you can lower just the aileron component you may want to try that also. Softer dampeners may also be the ticket.

Cheers
Alan



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[Mike Spano]

Alan, the issue he is describing is a constant wobble, not one on just stops.  That's why I think it could be a fbl issue. Even when I have had horrible wobbles on stops, it hasn't wobbled just sitting there. Wouldn't hurt to just check to be sure the tape is good and the gyro mounting plate is secure and not vibrating. I have had that happen on an n7 before....forgot lock tight and plate got loose and caused all kinds of issues. 766 being electric I doubt it, but it's worth checking.

Servos are new, so not an issue with centering.

I honestly think it's mechanical.

With 806 blades, me personally, I wouldn't go very soft on dampening, I would worry about those big blades and that boom.

If all mechanical things seem to be ok, and everything is balanced, you may simply have to fly at a higher head speed than you are set at now. The slower the hs, the higher the likelihood of this happening.

[curmudgeon]

If all mechanical things seem to be ok, and everything is balanced, you may simply have to fly at a higher head speed than you are set at now. The slower the hs, the higher the likelihood of this happening.

I am not having any issues at 1500rpm.  The side to side vibration is noticeable at 1600rpm and is worse at 1750rpm.  I don't notice any issues during flight, just during hover at those head speeds. 

[alan]

Gains can be the cause of this behavior. What I dealt with on the Bavarian Demon was exactly what you are now experiencing. It does sound like it could dampening or head gain issue assuming all other causes are ruled out.


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[curmudgeon]

Not familiar with Spirit settings but if you can lower just the aileron component you may want to try that also.

Spirit does not have separate aileron and elevator gains.  Just cyclic gain.   

[Mike Spano]

90% sure this is mechanical of the problem gets worse as hs goes up . Typically it's the other way....the lower the hs the worse the bobble....the faster things spin the worse the wobble means something is out of balance, missaglined, or bent. Maybe a bearing going bad.

[Rodney Kirstine]

Mine is with a CGY-750 and I only tried what I am suggesting after exhausting every other avenue and it absolutely took care of the issue but YMMV.