Trex 450 Wobbel

Kent P. · Posted: Sat Feb 25, 2012 10:17 pm Post subject: Trex 450 Wobbel

Hello all,
I recently bought a used trex sport 450 and have been slowly building it up to see what it and I can do. I am using a Spectrum D6xi and a ar 6100e with hitec65's as cyclic and a ds420 for the tail. Has the 780 align gyro. It lifts off fine and starts to hover very nicely but as it settles in to the hover the whole thing starts to wobble pretty much till it is out of control. I think I need to relevel the swash plate. I just wanted some input so I don't waste a lot of time where I don't need to. I have checked rotor balance and shaft straightness etc and all that is in order. I do not have a swash leveler yet but it is in the mail. So while I wait for that I thought I'd ask this here. Thanks for all that take time to read this.
Cheers,
Kent P.

tombo242 · Posted: Sun Feb 26, 2012 12:11 am Post subject:

Hi Kent,
That sounds like a good set-up. Just what form does this wobble take? e.g. Is it always fore/aft or side/side or circular?

A good description may help us to give a few possibilities.

Tom.

Kent P. · Posted: Sun Feb 26, 2012 2:17 am Post subject:

Hey tombo242,
The motion is a circular one. It doesn't seem to be the same frequency as the rotor speed is why I'm pretty sure it isn't a balance problem. Am I thinking in the right direction?
Thanks,
Kent P.

tombo242 · Posted: Sun Feb 26, 2012 5:34 am Post subject:

Hi Kent,

If the swash was off the heli would just drift in the direction of the tilt. It would not go in circles.

Two things occur to me:

1. Could be a harmonic of the rotor speed caused by out of balance bades or maybe tracking. Setting up by eye can cause some problems. Are you sure that you do not have a twisted blade?

2. Tail belt running slack or tight - not likely but worth a check.

Just initial thoughts.

Tom.

sbobby · Posted: Sun Feb 26, 2012 9:48 am Post subject:

+1 for

tombo242 · Posted: Sun Feb 26, 2012 4:12 pm Post subject:

Good thought there sbobby, that could be it!

Tom.

Kent P. · Posted: Sun Feb 26, 2012 10:20 pm Post subject:

Thanks sbobby and tombo242,
I have balanced the blades and I don't think they are to tight as I read somewhere that you are supposed to tighten them and turn the chopper sideways and shake it and the blades are supposed to move. So I did that and it does that, but here is something to consider. At the rotor main housing where the blade grips attach the spacing between the head and grips is not even. Would that cause this wobble and would I correct it by replacing the rubber dampers in the housing?
Thanks again,
Kent P.

tombo242 · Posted: Mon Feb 27, 2012 7:55 am Post subject:

Hi Kent,

I think that this unequal spacing could be the cause of the problem. If the blades are not exactly the same distance from the mainshaft centerline, then they will not be in dynamic balance. The effect will be the same as if the blade further away was heavier than the other.

I think that stripping the head and ensuring that everything on both sides of the feathering shaft is the same is an excellent idea. Those blades must match exactly.

Tom.

Kent P. · Posted: Sat Mar 24, 2012 11:15 pm Post subject:

Hello all,
Well here is where I am at. I rebuilt the head and balanced and matched everything I think. I installed a new main shaft and feathering shaft and got new damper bushings. Changed out some rough bearings in the blade grips and set the swash plate up with a tool set the blade pitch with a tool etc. I still have a wobble after the chopper is up. So I made a fixture to clamp the chopper to a vise and powered it up and the motor surges at about half throttle or more. Do I have a speed control problem or something else electronic? Thanks in advance for any more advise on this as I am confused at this point.
Cheers,
Kent P.

tombo242 · Posted: Sun Mar 25, 2012 6:40 am Post subject:

Hi Kent,

This is the sort of problem where we really need to have the heli in our hands to make a really quantifiable decision.

Looks like an ESC problem, but there again may not be. Even more info may be required here!

Tom

solentlife · Posted: Sun Mar 25, 2012 12:51 pm Post subject:

Various things come to mind
(Forgive if some are already sorted / covered)

a) Having exact same measurements both sides of head - that is feathering shaft, flybar, flybar weights, paddles etc.
b) Having weights checked and same - that is rotors same weight and same CoG and balanced, flybar weights, flybar paddles etc.
c) tail rotors at 90 degrees to main rotors / frame
d) main shaft correct way up and no play in main bearings at frame blocks
e) dampers not worn
f) Blade holder bearings not worn
g) feathering shaft threads cleaned out and bolts fully tight and home
h) Main gears not warped
i) steady rpm / throttle
j) smooth curves without sudden jumos

I have to say that I've had wobble in past - and usually due to my useless flying ! WEhere I've got into a toilet bowl effect by overcontrolling on lift off. Initially seems ok then it starts ... till I have to put the heli down to stop it. I don't say its you .. but we all do it sometimes.
If not overcontrol - then it's more likely to be a dynamic inbalance where weight is greater one side and revolving round the model. But usually that would have some relation with rotor speed, and increasing rotor speed would create a really bad violent shake in extreme.

Does the heli vibrate when at speed ?

Kent P. · Posted: Sun Mar 25, 2012 7:32 pm Post subject:

Good morning tombo242,
Hoeabout if I try toputup a videoof the chopper doing what it does?This might take me awhile but I'lltryand if You can see it maybe it will give You some more ideas. In themeantime I have a spare esc and I'll sub it in to see if that makes any differance.
Thanks for your time and ideas,
Kent P.

tombo242 · Posted: Sun Mar 25, 2012 11:24 pm Post subject:

Hi Kent,

No guarantees but any extra info is always welcome.

Tom.

Kent P. · Posted: Mon Mar 26, 2012 4:06 am Post subject:

OK,
So I can't figure this out but I is happy ending. I mentioned yesterday that I had a spare esc and so I threw it in there today and I can now finally hover my Trex. I had also mentioned that when I put my chopper into a holding fixture that I fabricated that the motor seemed to surge or pulse. Can anyone explain how that would make the bird wobble like that? I also thank you all for the replies so far. With respect to over flying, after I saw that the Trex would fly I perpously moved the stick around a lot more than I should and it does sort of do the same wobble but I can settle it down by steadying the stick where as before even if You didn't move the stick it would just start the wobble. Well I need to go burn up another battery so have a great rest of the day.Cheers,Kent P.

chopper54 · Posted: Mon Mar 26, 2012 4:11 am Post subject: post subject

Have you checked your blade tracking when airbourne. Out of track blades can cause serious wobbles. I had this problem with my blade 400 because of a very slightly warped blade.