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Old Jan 25, 2010, 10:22 PM
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Help!
Why is my helicopter wobbling at lower RPMs?

I just finished building my TREX 450 and for some reason the head seems fine at higher rpms, but when near the lower speeds, the head begins to oscillate hard. The video explains all.

Do you guys think its something with the shaft, even though its a new shaft?

In the video, notice how it gets really bad when I throttle down.

http://www.youtube.com/watch?v=rxISBmRpL44
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Old Jan 25, 2010, 10:46 PM
crashs come easy patience dont
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check to see if the main blade`s are to tight first.
if you can move them by hand farely freely not where they`ll blow in the wind but not tight enough you have to force them to move.
then loosen the jesus bolt in the head hub and check to see if the hub is seated on the shaft correctly.
then check the main shaft.
then check the swashplate center bearing, if you crashed hard this could have gotton damaged.
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Old Jan 25, 2010, 10:51 PM
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Quote:
Originally Posted by bad400 View Post
check to see if the main blade`s are to tight first.
if you can move them by hand farely freely not where they`ll blow in the wind but not tight enough you have to force them to move.
then loosen the jesus bolt in the head hub and check to see if the hub is seated on the shaft correctly.
then check the main shaft.
then check the swashplate center bearing, if you crashed hard this could have gotton damaged.
That is what I was kind of thinking too. Blades might be too tight.

It would be nice to see a little more.
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Old Jan 25, 2010, 10:58 PM
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Ok, i am going to try with a spare align shaft I had. The shaft is new and perfectly straight. Will post results in a moment.
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Old Jan 25, 2010, 11:07 PM
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Assuming nothing bent or poorly set up, this can be pretty typical on spin up and spin down if the blades are either too loose or too tight. Sometimes what you are seeing are the blades spinning out with inertia to their proper lead/lag position and then they smooth out. Once the RPM comes down they go out again because the inertia is reduced. This will be especially apparent if the spool up is not slow and smooth because the rotor "kicks" on startup and moves the blades out of their straight out position momentarily.

I had this fairly bad on a 500 of mine and my blades were too loose. I snugged them down just a bit so they would not fold or move when the rotor started moving but they were still moveable with force from my hand. I would pull them out straight before spoolup and the problem went away.
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Last edited by Xrayted; Jan 25, 2010 at 11:14 PM.
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Old Jan 25, 2010, 11:16 PM
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Here is a video up close with a new shaft and without the main blades. The wobble is not a huge amount but still noticeable. I don't understand what could be possibly going wrong.

http://www.youtube.com/watch?v=QF90Ex--1xk

i don't think this is bad enough to be a problem, but I am fairly new to helicopters. It just seems that everything is set up perfectly, and I am worried that anything might be going wrong.
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Old Jan 25, 2010, 11:21 PM
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Originally Posted by scolen3 View Post
Here is a video up close with a new shaft and without the main blades. The wobble is not a huge amount but still noticeable. I don't understand what could be possibly going wrong.

rotor wobble 2nd test (0 min 56 sec)

i don't think this is bad enough to be a problem, but I am fairly new to helicopters. It just seems that everything is set up perfectly, and I am worried that anything might be going wrong.
Maybe pull the entire head while still on the main shaft and balance the entire unit on the shaft with flybars and blades. Its the long way home but Ive had to do it before to get rid of any unbalance or vibes. Make sure the flybar is set directly center with same amount of bar on each side and the paddles are not off in weight from each other.
Your vid doesnt look that bad to me really and I dont think i would have noticed it. You cant always rely on looking at the head button movement as they may not have drilled it perfectly center. Its not a precision part so it may not always appear to spin true.
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Old Jan 25, 2010, 11:22 PM
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Yes, I replaced the shaft, then balanced the blades perfectly by screwing them together then balancing them. A few pieces of tape later, and the thing is perfectly smooth. thanks for the answers guys.
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Old Jan 25, 2010, 11:26 PM
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Take the head apart. There is a known machining defect where the the dampers seat on this head.
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Old Jan 25, 2010, 11:29 PM
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Take the head apart. There is a known machining defect where the the dampers seat on this head.
Can you explain in further detail?
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Old Jan 25, 2010, 11:39 PM
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Originally Posted by scolen3 View Post
Can you explain in further detail?
Yes, it's not machined square. Nothing you can do about but verify that is the problem.

I will try and find the tread again but it was some time ago showing pictures of the EXI head stripped down. Clearly a machining defect that causes the feathing shaft to be cocked off from 90 degrees to the main shaft at a neutrual position.

It might have been at Xheli's forum, which has been redone but not archived.

It sould be pretty obvious without even measuring it. The one I saw was very obviously not machined square.
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Old Jan 25, 2010, 11:53 PM
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I will check it, but I would not be surprised if they revised it. I don't have the stock metal head, with the plastic arms, but the full metal upgrade. Although they might be using the same parts though, I don't know.
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Old Jan 26, 2010, 12:11 AM
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This is where the problem has been.



For whatever reason from time to time they get this hole machined off center which causes all kinds of problems.

I am sure they use the same head block for the full metal upgrade.

This one is okay.
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Old Jan 26, 2010, 12:25 AM
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Check to see if your main gear isn't out-of-round and also if your mesh is correct.
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Old Jan 26, 2010, 12:29 AM
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I just checked, and from what I can see, fortunately, it looks like my block was drilled straight.
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