Hello everybody,

For a few weeks, I've got a problem with my T-rex 450 XL and have no explanation. about 100 flight and no crashes : (only hovering and FF)
I'm fling wood blades and saw random tracking. At what moment the REX during hovering began to have some intermittent shaking : very short one (1/2 seconde), at random moment (I can say between 2 and 5 secondes), during all the flight.

I did not notice this shaking during Forward flight.

So I have controlled all the heli and did not find something wrong.
I have changed the o-ring : I was thinking what It could setup this random tracking and shaking but NO WAY ????

So I have also changed :

-feathering shaft
-blades : with carbon blades the tracking is better but not perfect during all the flight but the shacking is always here.
-have a test with a friend's lipos
-change pinion (I fly a align 2800 Kv so I have 13T pinion and now a 12T,
align 25G controller and align servos)

-I have also changed the idle up curve of my controller : I have also used a pitch/gaz curve instead off the regulator fonction.

Nothing change anything ????

I have done a test with a 8cell GP1100 pack instead of a 3 cells lipos.
The heli is flying perfectly ????????

So with low rotor speed and more pitch the heli has a perfect hovering !!!!!! : no shaking !!!!!!

and with his usual rotor speed (about 2000-22000 rpm) it has this intermittent short shaking ???????

So I have no explanation because I always fly very smoothly and it appears one day like that without any explanation.

So does anyone can help me

thanks
regards

Olivier
__________________
Have fun with your T-rex
Olivier from France

I'm thinking main bearings and low head speed.

Hello cflight,

Thanks for your reply.
I was also thinking about main bearing, but I don't explain myself why there is no shaking at hovering with very low main rotor speed (under 2000 rpm) and more pitch .
And also I don't explain why (if main bearings are out) the shaking is intermittent.

Regards
Olivier

I would think, while under a heavier load with more pitch the rotor head and blades are held firmer, and while just spinning free with less load it is being allowed to wonder? At a low head speed dampening becomes an issue, with less centrifugal force the blades have a greater tendancy to flutter. Maybe softer dampeners?
Some reading to help, http://www.cybercom.net/~copters/helo_aero.html
Get back to us regarding your resolution, I'm sure alot of people would be interested to hear the outcome.
I plan on building a semiscale UH-1 that I intend to run a low head speed on and less agresive cyclic, so I am one who is interested in any resolve for low head speed issues. Good Luck

Thanks again Cflight for the answer.
I just have a look at the heli aero and I really don't understand (In my case) why tired main bearings will do intermittent shaking at high rotor speed, and no shaking at low rotor speed.

My wish is to fly my heli with high rotor speed with no intermittent shaking.

I will change the main bearings as soon as possible and tell you if it is the solution but really (may be I'm stupid) I don't understand why new main bearing will help me.

Regards
Olivier

Yes, the more I think about you're problem, it doesn't sound like bearings are going to cure it. If you spin a main shaft in the bearings and they don't feel gritty or real sloppy, i wouldn't change them just yet.
Can you describe the shaking in further detail, and possibly take a look at your oneway bearing and bearing shaft to be sure it is grabing properly? If the oneway bearing shaft has worn through the crome coating, replace this first.
With out blades on is there shaking present?

I had 2 fly yesterday evening and really I don't know how to manage this.
now the shake is still intermittent but not only at hovering, also in FF.
The heli is fling perfectly ans suddenly it shake for half to one seconde then again fly perfectly.
the shake is overall : main frame, boom ... shake.
I'will change main bearing and free wheel as soon as receive them.
I repeat, tracking is perfect and o-rings and feathering shaft are new. the shake is the same with wood or carbon blades.
belt tension is OK.

I really don't know where to look at to manage this problem ???

and yesterday, I changed my gaz curve (idle up) from 90% to 60% : rotor speed is poor and I have to fly with more pitch.
with this setup there is no shake. If I put again Ã* 90% idle up : It shake.
It is also the same if my controller works in regulator mode or in gaz/pitch mode !!!!!

regards
Olivier

This sounds like a dynamic balance issue.

CG and balance your main rotor blades first. Any weight you have to add to the lighter blade make sure you add it along that blades cg. Also if you are flying the stock length blades, make sure you are flying with a headspeed of at least 2000rpm and lower your pitch to 40%-50% at midstick.

I don't know if is the same issue but my 450Se was doing basically the same thing and it was the end points on channel 5 (Normal and HH mode switch) the signal for the gyro it was set at 100%. I change it to 32% and no more shaking.

Thanks,
-Doug.

Hi DougV,
I've got a GY240. my end points are set at 100%. I can try to reduced but the tail servo rotation rate will be slower !!!!
I will try this weekend. never mind if the tail is slower.

Regards
Olivier

Hi,

I have experienced a similar problem due to rudder control rod bind. Sometimes it would get caught in one (or both) of the plastic tail control guides (T-Rex 450V2). The problem was more frequent when the head was spinning fast. Slowing it down usually eliminates the problem. I have used 2 nylon cable ties for each control quide to restrict its movement (leaving about 5mm of free play area as the guide would interfere with the horizontal stabilizer) and a couple of drops of oil on the boom as well as the point through which the control rod moves. This seems to improve the situation greatly (and when I drop the gyro rate from 50 to 35% the problem goes away). I suspect minor balancing problems cause rapid vibration of the guides and force the rudder control rod to be stuck briefly (jerky rudder operation) causing the whole machine to shake. When the head spins faster the frequency of the vibration of the guides increases improving the chances of the rod getting caught.

Regards,

TVB

here's a question, on what axis is the heli shaking? if its in the yaw axis, its the gyro sensitivity. however, if its on the pitch or roll axis, the first thing i would check is the tightness of the blade grips. the main blades should not be free, but be able to be pushed back with a little force. the reason for this is so if they are not perfectly straight, centrifugal force will pull them out to the proper alignment. i have experienced this myself and as soon as the blades straightened, it was smooth flying. good luck and keep us posted.

Hello,
I have just find what was my problem.
2 swashplate servos are tired : the aileron servo and pitch servo.
there neutral position on load are not good.
the servo jerk at neutral position on rotor load and the neutral position is the pitch to hover.
this is why the heli has intermittent shaking at hovering because the neutral servo position has random jerk.

The overall heli shake is explain because if one servo change his position during hovering ( and the 2 other are rock solid) only one blade has a pitch change.
this mean that for a short moment the tracking is not good and the heli shake.

So I have tested the heli with very low and very high rotor speed to change the hover pitch rate. The heli is rock solid because the swashplate servos are working in a other position : a position that they don't jerk

and finally
I've tested the 3 swashplate servo at the workshop and I confirmed that the aileron and pitch servo have a worm potentiometer : the track is worm at neutral position.
Servos are Align 9X : about 100 flights.

So now know I have to update my swashplate servos.

Thanks for your help
best regards
Olivier

Nice detective work, Oliver. I think you had a couple forums scratching their heads!
May I suggest 65MGs? when you feel the time comes, they really take a beating.

Hello Cflight,
just went back from holiday so I fly my T-rex yesterday and it is now rock solid at hovering and fast forward flight : no random tracking, no intermittent shaking.
that was it : I confirmed that the old aileron and pitch servo have a worm potentiometer.
Now with 3 new swasplate servos : no problem.
I'am happy
regards
Olivier