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07-24-2006 12:43 AM  14 years ago
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fastsc92

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NH

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Violent Vibration
After crashing my hawk pro a few weeks ago I got the parts needed and fixed it up sometime last week. I wasn't able to fly it until last thursday but when I started flying it again there was a violent vibration that would start off ever so slight and then nearly shake the bird to pieces until I set it down.

I'm still running the training gear but I can hover rock solid until it starts shaking and the gyro goes nuts. I'm not sure if the training gear is causing the vibration to magnify or not, but you can see it start in the balls of the gear, then it starts to get violent.

When it crashed I just needed a crash kit and replaced the main shaft (didn't look bent but I swapped it out anyways, feathering shaft, flybar, and boom. After this first vibration problem I went back home and re-balanced the blades, they were off a tiny bit, brought it back and everything flew perfectly. Today however, didn't change anything over the weekend, started it up and the vibration was back. The tracking is good, pitch is the same for both blades, everything is tight, shafts spin free. The vibration comes and goes as I hover it, and seems to go away under more load or positive pitch as I climb slowly. If i need to bring down the heli and lower the stick, the vibration gets realy strong and continues until I give it some pitch/throttle.

Upon bringing it home, I noticed that one blade was sagging lower than the other...took the grips off and sure enough the feathering shaft was bent. How it got bent..i have no clue. There was never a hard landing, drop, crash ect since I fixed it and there was only two tanks through it since it was apart.


I've scratched my head on this one and still can't find the source. At this point I'm not sure if the feathering shaft was the cause, or if the vibration somehow caused it to bend under the oscillation. I hate casing down problems and spending money where I don't need to. Can someone give some input as to what might have been/still is the cause?
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07-24-2006 01:00 AM  14 years ago
rf_reggie

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Arlington, TX-America, F-ya

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Got kids?
I think very possible to bend the feather with severe vibration. But sounds like someone dropped it. Or could be engine related, maybe a new glow plug or since your 1st flight after repair the temperature/humidity changed and the engine is out of tune so bad it shakes. Is your fuel in tank "boiling" (rich) or "frothing" (lean) ?
Oops...
Ain't nothin the charge card cant fix
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07-24-2006 01:11 AM  14 years ago
fastsc92

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the engine runs like a dream and doesn't miss a beat. Its def. coming from a rotational source. As far as a mis-handling issue...I live alone and it sat on my kitchen table until today, so it couldn't have been dropped.

I really hope that this is the problem. I checked the bearings in the grips and they seem smooth enough, no binding, plus they dont pivot more than 15 degrees anyways. I don't know, I'm still stumped, but I don't know where else to look. Its posible that the feathering shaft somehow was bent upon installing it (i dont know how that can possibly happen...), and that was the vibration to begin with, but I dont want to replace it, and have the vibration remain and ruin another shaft.
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07-24-2006 01:31 AM  14 years ago
SteveH

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Texas

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fastsc92,

A bent feathering shaft will definately cause a vibration, and with training gear on, it will seem worse than it is as they have a tendency to amplify any rotor head vibration. The feathering shaft on a Hawk, being only 5 mm, is realtively easy to bend. If you had a hard landing even without a boom strike, it can be enough to bend the shaft.
The government cannot give you anything without first taking it from someone else.
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07-24-2006 01:44 AM  14 years ago
fastsc92

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NH

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Its just strange how it could have been bent since it was present from the first day of its assembly, before the model even landed. I'm wondering if something else is causing the vibration and that in turn bent the shaft do to oscillation.
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07-24-2006 01:49 AM  14 years ago
SteveH

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Texas

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I'm not there, but I would think not.The government cannot give you anything without first taking it from someone else.
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07-24-2006 01:57 AM  14 years ago
fastsc92

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NH

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I guess the only way to find out is to get another shaft and test it. If the problem comes back I'll try to post a video. Now I just need to get a shaft....sucks that it'll cost me nearly $14 for shipping alone, and I'll have to wait at least a week for it.....
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07-24-2006 02:06 AM  14 years ago
SteveH

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Texas

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If they are not bent too bad, I straighten them when I'm in a bind with a dial indicator and a drill press.The government cannot give you anything without first taking it from someone else.
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07-24-2006 02:21 AM  14 years ago
rf_reggie

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Arlington, TX-America, F-ya

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maybe:
blade grips, too tight or loose? If not the only thing left is the head block. Maybe it has a crack that can only be seen by flexing it.
Oops...
Ain't nothin the charge card cant fix
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07-24-2006 02:48 AM  14 years ago
perfesser zero

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Dublin, VA., USA

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Blade tracking...
maybe it's out or the blades are too loose in the grips. I've had a boom strike once when the blades were to loose in the grips and they just folded back and "whack", got the boom. I also agree with SteveH that the spindle (5mm) will bend at the drop of a hat (which it's supposed to do). Also make sure that the flybar is centered and not bent too. Are the paddles on backwards?

Later...

Scott!
Tiger 50 - OS 50SX-H
Hawk Sport - OS 37SZ-H
Raptor 30 - OS 37SZ-H
JR 10X/R950S
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07-24-2006 03:02 AM  14 years ago
fastsc92

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NH

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flybar is dead centered, straight and the paddles are correct. It'll fly this way and at one instant (mostly under power) its smooth as glass and I can hold it within a 1 foot radius. Once i need to come down from 20 feet up or so, it starts to get the vibration. Its almost as if positive pitch or constant engine power smooths it out. Either way, its bent, not bad, but its certainly not straight. I still dont know how it got bent since it never landed hard...hell..it never even landed before I noticed this.

Steve, how do you go about straightening it? A new one will be on the way tomorrow, but I'd like to give this a shot and see what happens.
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07-24-2006 03:27 AM  14 years ago
Flying Foot Doc

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Plm Bch Grdns, Fl USA

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Similiar Vibration Problem
This may be a side bar question, but this is the closest I've seen to my problem. I had a blade strike and changed the blade, did a CG balance and still have the vibration. Is it correct to just change the one broken blade or should I always do the set? Thanks for letting me ask . Andy Levy
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07-24-2006 04:49 AM  14 years ago
SteveH

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Texas

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Is it correct to just change the one broken blade or should I always do the set?
Andy,

You should always change blades in sets. They are ballanced in span, cord, and overall. No two sets are alike, usually not even composite blades from the same manufacturer.
Steve, how do you go about straightening it?
I use a simple set of V blocks and turn the shaft while looking at it with a dial indicator, and use the drill press with just a rod in it and press it back straight. It's sort of labor intensive, but if you can get the shaft back within about +,- .001", it will get you back in the air.
The government cannot give you anything without first taking it from someone else.
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07-24-2006 04:50 AM  14 years ago
perfesser zero

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Dublin, VA., USA

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Flying Foot Doc...
Did you change the spindle shaft? Anytime I've had a rotor strike anything, the spindle had bent on the Hawk Sport. Depending on how bad the strike was the main shaft might also be bent. Check your frames to make sure they aren't broken around the area of the main shaft bearings.

Later...

Scott!
Tiger 50 - OS 50SX-H
Hawk Sport - OS 37SZ-H
Raptor 30 - OS 37SZ-H
JR 10X/R950S
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07-24-2006 11:54 AM  14 years ago
RICH.L

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gentry arkansas

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when i first started i had a kyosho nexus that would do the same thing,hover great with the training gear on and then gradually start shaking so bad that i would have to land and then take off again!
it was in fact the training gear that was causing resonance,once i was able to get the training gear off the shaking never came back.
rich
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07-25-2006 04:21 AM  14 years ago
dxtarl

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Trainer, PA

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my Hawk is doing the exact same thing. When i get to about 1/3 tank it leans out and then starts the vibration, but some time i can fly the entire tank. Seem that it only started since I got this last set of woodie.
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07-25-2006 04:57 AM  14 years ago
cassat

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Alberta,Canada

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Maybe check the C of G on the blades. I had an old set that would shake terrible even though they balanced and tracked ok.
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07-25-2006 08:41 AM  14 years ago
JohnHawk50

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Chesapeake,VA

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fastsc92
Look at the upper mainshaft bearing housing. Try this but
you have to look closeup. Hold on too the bird with one hand
and the other grab the mainshaft rock the mainshaft backward
and foreward than side to side. Look at the upper mainshaft
bearing, if you see that bearing move in the housing that is
your problem. You will have to replace your upper frame. I
ran into this problem in crash 4 of my hawhsport. Let me know
if this is the problem.
Enjoy the hobby.
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07-25-2006 11:40 PM  14 years ago
fastsc92

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NH

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Update guys

Took it out today and flew about a half a gallon(1.5 gallons total for my experience). I got a new feathering shaft, and it still vibrates. Pitch is the same for both blades, tracking is dead on, blades are balanced many times and engine is running smooth. It def. starts in the head cause you can see it start and then it gets real violent. Again, more throttle usally makes it go away. I got a short clip of it that I'll post later. My camera man had VERY shaky hands, so its hard to see, plus I was trying to get it to do it for the camera, but for the most part it was solid with a little shake. I'll post it later once I figure out how to resize it (100mb!!). I thought about the CG of the blades but how would I go about checking it?

Now/tonight I'm going to tear the frame apart and see if the bearing has some slop it in. I took it apart before and it seemed alright but I'll give it some more inspection now that I have some trouble areas to look at.

I also changed the boom and the torque tube in the 1st and only crash. Should this be an area to look at as well? Its strange since it doesn't do it all the time..it comes and goes since I can still fly it rock solid.
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07-25-2006 11:43 PM  14 years ago
fastsc92

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NH

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ok...i'm moving the main shaft and there is zero play in the upper bearing. I'm putting some good force in there and nothing is moving at all.

When I balanced the blades, I just added more tape to the far end of one blade. I never thought about the tape placement. Anyone have the correct proceedure to balance and check the CG of these woodies?
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