Head Shake on spool up HELP !!!!

[Hovering]

I just got my HDX-500se ready for the first hover. Balanced the blades and re checked balance put weight on the CG of the light blade. These are the stock blades.
On spool up I noticed a nasty shake that went away as the RPM's came up, she hovered just fine very happy with the way it hovered. But when I set it down and the RPM's started droping it realy started to shake violently. It got to the point that the hole heli was doing a dance and I thought it was going to tip over.
I removed the blades and checked for off balence of the head at below hover RPM's and at Hovering RPM's
I did see just a very little out of balance at low R's but nothing like with the blades on. Fly bar is even on both sides of the flybar cage also.
I hope somebody can help me with what to check next, I'm at a loss where the problem is coming from.

[lotus7]

This sounds familiar!!!
I had a similar vibration , and when checking the tracking I lost my head - Check out my earlier post
I would recommend you find the cause before the samehappens to you - it can be dangerous!

[netmeister329]

I've seen similar problems and think its due to hamonic resonance. This is assuming you have the blades balanced and tracking is properly setup. On spin up you hit a hamonic frequency that causes your bird to resonate. Similar to how a vibrating truck or train engine might make a window pane vibrate. Or if you drive an old car you might hear one of the muffler baffles rattle at certain engine rpm's

Actually I have a ceiling fan suspended on a 5' shaft. At low speed there is a profound wobble but if I increase the speed there is no noticeable wobble.

The most likely suspect is a small runout in your main rotor shaft. ( it is slightly out of true ) . The rotor head is traveling in a very small elipitical orbit that is apparent at low resonate rpms. But goes away as you spin up.

If you put the shaft in a lathe put a surface gage on it you might see it out a couple of thousands. You might be able to see run out if you put the rotor shaft in a drill and turn it on.

If the bird only does it once and seems fine until the next time then the problem is due to the blades have not "unfolded" after storage/transport.

[Hovering]

netmeister329
Thanks for the info, the hole bird is brand new but your right the shaft could be out of round even though it's new. I did not get a chance to check the tracking. How could it cause this problem if it is off ? I thought that the tracking just effected the flight performance.

[lotus7]

Just read on another forum that this could be caused by the main blades are too tight in their grips , causing a leading or dragging blade. As sson as you have enough headspeed , centrifugal action overcomes the tightness and pulls the blades straight.
Just checked my blades and one was stiffer than the other!

[netmeister329]

The grips not being tightened equally could cause wobble but once you get head speed up they should "unfold" and you should not see any wobble until the next time, assuming you folded the blades up for transport. As far as tracking goes you are right tracking should not really cause any wobble.

[Hovering]

Thanks guys,
But it also happen after I landed when the head speed was decreasing. So the blades would of been straight from the centrifugal action.
I'm thinking I either got a bad main shaft or bad main bearings in the kit or both. Just ordered both, I'll see and let ya all know.

[3secondsToImpact]

Once spooled up are the blades tracking perfectly?

[CompuFoil]

This happened on my TRex as well. At low RPM's it would quake like crazy, but get smooth at around 2500 RPM. I had everything balanced perfectly. I even replaced the o rings in the head and it didn't go away. I found the problem when I was checking my blade angle. Once side of my flybar was bent down so one paddle was about 1/4" low. It must have happened while I was working on it, and had it laid head down on my lap.
Eric

[Hovering]

I got the new bearing and main shaft the other day and am going to put them in today. I'll check the flybar again and tracking after. I also have some new main blades coming, MAH 425 should be here next week.
Thank's Eric and 3secondsToImpact. I'll let ya know how it goes.

[Hovering]

Hi all,
I am happy to tell you all the head shake is gone !! I replaced the main bearings with new ones, I also checked the fly bar paddles they were only .18mm different so I left them alone.
But besides the main bearings there where two things that stuck out. One of them was one of the main blades I could not put the screw through the blade holder and it with out screwing it through the bottom. But the screw would go through fine out of the blade holder. Plus after I got the screw through it even with out a nut on it if I kept turning it it would tighten up on the blade. I thing that the Brass bushing in the Blade is not in the Blade square from top to bottom. Witch probably caused that blade to be to tight. So I reamed out the brass bushing with a drill.
The seconded thing that stood out was the main Blades were 1.5 Deg. off from each other, So I fixed that. I think I was excited when I was finished with the build and over looked checking the other blade while setting the Bird up

Thanks to all of you you pointed me in the right direction and solved my head shake problem !!!!!!!!!

Thanks again
John