Violent Oscillation During Spool-Up/Down

[flyingw]

At a certain rotor speed during spool-up and then during spool down, my newly-outfitted Protos 500 heli suffers a violent period of oscillation. When the desired head speeds are reached the machine runs smoothly and without vibration. Otherwise the heli flies well.

I checked the blades with a jeweler's scale and they are exactly the same weight.

Any ideas on what could be causing this and how to stop it from happening?

I was thinking of adding some tape to each blade (each side) to change the resonance of the system.

Appreciate any ideas. Thanks,

Paul

[Russell Harding]

Blades tighter or looser might help. How are they? Usually I've found tighter is better as long as they are nearly aligned at initial spool.

[flyingw]

Code3Medic,

They are snug, but not tight. After taking them out of the foam blade holder I swing them into position and place them pretty straight before spooling up. I could tighten them up a bit.

Do you think this could have something to do with it: The flat surface on top aft of the servos is not so flat, and I mounted the FBL unit with a stepped arrangement of mounting tape. It is not as rigid as it would be if mounted with one big piece on a flat surface. Please see the photo below.

Thanks,

Paul

[mike d1]

I too have fought this on both spool up ad spool down. Its happened on spool down badly enough for the v bar to cause a tip over more then once. I traced it to worn head dampeners. I also changed the spool up rate on the yge esc to heli middle from heli slow and thats all but eliminated it on spool up.

On Spool down after hitting throttle hold it will still shake every now and then, difference is I am always watching for it now. When it does shake I give it a little positive collective, not enough to get airborne again just enough to get it light on the skids, seems to settle it down till the blades stop. I think what is causing it now are the rubbery/flexy skids. They allow the heli to vibrate pretty badly on the ground and the fbl unit starts to overcorrect making the situation exponentially worse, that's my theory anyway.

Mike

[flyingw]

Mike,

Very interesting. I also thought of using collective to stop the affect after seeing a video on youtube of a full scale heli taking off to stop the resonance. I tried it once on a takeoff. She took off way before reaching full headspeed but without drama.

I forgot all about it until after landing and while she spooled down I failed to react in time. I'll have to practice catching the resonance before it gets bad.

Got new head parts on order and will try that as well..

Thanks,

Paul

[qntt]

Same here... The problem was a bad swashplate with lots of play out of the box. Changed that and the problem went away. Quality control at MSH is not what it used to be.

[kayakboy69]

couple thing come to minde one i have run the helicomand and ikon on the boom block with zero issues helicomand is on 3m grey outdoor and ikon was on squshie zeel tape. i found that a tick on the tight side

you can also test it if the are snug by spooling it up hover it set down and come to a compleat stop at zero pitch and then spool back up.

I have found that if your throttle hold is set max pos max negative and you land and hit the TH the blades will slip and cause a wobble on the next spinn up.


and dont forget to make sure you have the same gryo gain in throttle hold as normal mode !!! took me for ever to figure out why it would spin at soon as i hit TH and landed in a mini AUTO

[Jumna]

Aggravating, I have that on a different heli, there's a point of combined rpm & ground contact that really gives it the shakes, but I've gotten good about avoiding it on spool up and spool down. Taking off on thick grass helps.

[flyingw]

Replaced the grips, dampeners and bearings. The resonance got a lot better but was still slightly evident during spool-up/down - I was able to control it using MikeD's collective technique described above.

Today tried a new set of SAB 465s purchased from Bill at MSH-Direct. The SABs are a bit lighter than the RJXs and the resonance is almost completely gone now.

[01ws666]

I started seeing this issue when I stretched mine.
Running rjx 470mm's balanced, red msh dampners.

Mine always does a little shimmy on spool up it's not too bad (I take off and land on asphalt btw)
But on spool down if I'm not careful it'll shimmy pretty hard and then snap to a stop.
I've actually broke a blade grip on spool down when it snapped one time!

Besides this issue she's as smooth as glass in the air... Kinda weird,
My fix for this that's worked for the last few hundred flights is I now bleed off all the headspeed the last couple inches before touchdown so I'm at full collective just before I land.

[flyingw]

Helicopters are finicky complex machines. My heart goes out the engineers, test pilots and their families who developed the real ones.

[qntt]

During spool up/down the FBL system tries to stabilize the heli and if there is even a little play anywhere in the head the system will sense that play as a dead band where the heli doesn't respond to the corrections it sends to the servos. In conclusion it freaks out and over corrects one way then freaks out again in the opposite direction and so on until the centrifugal forces tighten up the play or the RPM gets high enough so that the frequency of the oscillations is considered by the system just noise.

Tighten up everything you can and it will go away. Especially check the swashplate for play. Ideally there shouldn't be any.