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Other › low frequency wobble when cold out
11-27-2006 02:28 AM  11 years agoPost 1
demosa

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Tigard, Oregon, U.S.A.

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I've been flying my SE with Medusa 3400 and 11t all summer long with no problems (although several crashes here and there).

The last rebuild I did I changed the rubber dampeners. I flew it for about 10 flights in 65 degree weather with no problems.
Yesterday, I got out in 45F and flew around a few batteries. But I noticed that in a hover I now have a low frequency wobble.

My theory is that when it's cold out the rubber dampeners stiffen up and induce this low frequency wobble. Is this a reasonable theory?
I'm thinking of using a 12t which will increase the head speed anyway but should I be putting silicon-based lubricant on the dampeners instead ?

Any other theories that apply here ?
Thanks

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11-27-2006 05:24 AM  11 years agoPost 2
marc8090

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Long Island, N.Y.

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Sounds like a reasonable theory. I wouldn't lube them, just give them a few flights to see if they break in. If not, then raise your headspeed a little.

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11-27-2006 07:39 PM  11 years agoPost 3
flyride

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Salt Lake City UT

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I have had the same problem ,I just installed the red dampers and it was flying great then one day it started shacking like the heli was out of ballance and also a lot in the tail and I havent figured it out yet so any info would be helpful.Also I did lube my dampers and I run about 3000 on the head but the faster it is the worse it gets.I am in the prosess of replacing the whole tail blade grip assembaly,shaft and tail bearings but I dont know it it is the problem.

and the blonde said "RECTUM, Hell damn near killed em!!"

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12-03-2006 02:21 AM  11 years agoPost 4
demosa

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Tigard, Oregon, U.S.A.

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Well I went to the 12t pinion because I prefer slightly higher head speed (~2800) and this wobble went away.... silky smooth now.

Demos

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12-04-2006 03:07 AM  11 years agoPost 5
jhamlinjr

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Milledgeville, GA, United States

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Im having same problems.Seems to be focused around main gear. Changed tail rotor shaft. Everything else is new. I do have a wobble when I watch main gear at low head speed. I was thinking maybe the main gear isnt true. Thinking of going to cnc main gear. Mabe the shaft coming from motor is bent or front tail drive shaft. Something is bent. Guess Ill try front tail drive shaft next. Cant really see how motor shaft could be bent. Anyone heard of this?

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12-05-2006 04:23 AM  11 years agoPost 6
demosa

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Tigard, Oregon, U.S.A.

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Low frequency wobbles are usually NOT because of shaft problems or main gear etc...

Demos

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12-05-2006 04:47 AM  11 years agoPost 7
jhamlinjr

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Milledgeville, GA, United States

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Perhaps you can enlighten us on what the problem might be.

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12-05-2006 05:14 AM  11 years agoPost 8
bexusflexus

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Tulsa, OK

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One or both main gear bearings may be shot. Have had this happen several times causes a low speed wobble, most notably when spooling up.

even not doing anything would have been better than nothing...

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12-05-2006 06:27 AM  11 years agoPost 9
Stork

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Pawnee Rock, Ks USA

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Cold dampners
Demosa,

I think you hit the nail on the head about your dampners. They are cold therefore harder. I don't think lubing the dampners will help wobble in this case all-though lubing the dampners so the feathering shaft can move and center is a good and common practise.

Are there softer dampners available? Maybe a thinner shim is in order?

Clay

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12-05-2006 11:51 AM  11 years agoPost 10
SSN Pru

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Taxachusetts

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3D flyers use very stiff dampeners in thier heads so i doubt the cold weather would make it wobble. it could, however, be a combination of low headspeed and stiffer dampeners. a "wobble" is most definitely a symptom of low headspeed on the trex. do a search about it. its even been video taped before.

pru

Stupidity can be cured. Ignorance is for life!

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12-05-2006 06:40 PM  11 years agoPost 11
Stork

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Pawnee Rock, Ks USA

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WOBBLE
Pru,

You opinion is most valid, however cold weather will indeed cause this problem. I've seen it happen on the larger helis, usually with those that are using hard dampners. More than likely the helis headspeed is/was borderline using the dampners and the cold made the wobble show up. However we do not know what the head speed is. If the head speed is aound 3K then the dampners are too tight. If the head speed is around 2500 then I say jack the headspeed up to 3K and see if that gets rid of the wobble.

Clay

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12-05-2006 08:34 PM  11 years agoPost 12
keyz

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, UK

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im prolly gona sound real stupid now
what kind of wobble is it ?
side to side or for aft ?
i may have missed you state this fact

If you need to get high , go and fly !

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12-05-2006 09:33 PM  11 years agoPost 13
Stork

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Pawnee Rock, Ks USA

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The whole helicopter has a low frequency shake. The movement is almost circular.

Clay

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12-05-2006 09:53 PM  11 years agoPost 14
demosa

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Tigard, Oregon, U.S.A.

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I can only state the fact from my experience :

I've always been using the same type dampeners (the stock Align ones).
During the summer with ~2800 HS no problem. I was flying the 430L motor with 13t pinion (I don't have a tach so someone else measured it for me).

Then I switched motors and pinions. I went to Medusa with an 11t. My head speed went down although I don't know by how much (no tach). But I could tell from the way it sounded and the cyclic response was slower. But it was flying fine in 65F weather.

One day I got out to fly in 45F and I saw this low frequency circular wobble (about 2 cycles per second) mostly noticable at the bottom of the skids. It would only be noticable at a hover. Coming in from up-high flying and settling into a hover was fine, then a split second later it would start wobbling. Pumping the collective would cure it for a bit then it would come back. I flew like this for a few days in the same temperatures and it was consistent.

Then I went to 12t pinion. This certainly raised the head speed. And the wobble is gone even in 42F. If it comes back I'll look elsewhere for a problem.

Usually problems with rotating shafts show up as buzzing at the fins and I have no buzzing at all anywhere. So I can only suspect the dampeners and my exprience so far verifies this ...

Thanks,
Demos

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12-05-2006 10:24 PM  11 years agoPost 15
Stork

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Pawnee Rock, Ks USA

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Exactly
Demos,

I believe you are exactly right, and with the head speed you are running you probably won't have any more problems.

Clay

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12-05-2006 11:05 PM  11 years agoPost 16
ktm520

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newburgh, in

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this may or may not be relevant, but my rex recently developed a "tail bob" (tail oscilated up and down at approx 2 Hz, not a wobble, only oscilation about the pitch axis). the pro wood blades i put on were alittle bit tighter in the grips than the last set and i wasn't able to get the blade as loose as i usually run them. i went ahead and flew it thinking it wouldn't matter. well, it did. i first tried replacing the dampers with no success, before i thought about the blades. finally, i filed down the blade roots and got the grip tension correct and the bob went away. i'm running low head speed, 430L, 11t, 65% throttle at hover (don't have a tach yet, probably 2100-2300).

on second thought, this has nothing to do with temperature, so i pretty much just wasted your time . . . lol.

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12-05-2006 11:38 PM  11 years agoPost 17
keyz

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, UK

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must admit , i was kinda tending towards the blades been to tight or out of balance somewhere as there was a mention of a "bump"

If you need to get high , go and fly !

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