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Max thrust trainer issue


David Payne 4
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I have a max thrust trainer that on it's maiden flight suffered with roll axis osilation 

There is no gyro fitted

The servos are new with no slop

The receiver is new

The motor was at a constant speed 

The conditions were windy

Anybody have any ideas what to check?

The aileron hinges are firm and good

My wot 4 performed as normal on the same day

 

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Flimsy foam ailerons springs to mind.

Alternatively are the servos strong enough for the surface size / speed?

 

We did have an issue like this recently at our field which was traced to a poor servo extension causing one aileron servo to "wave" after returning to centre. Only happened when full deflection was used and stick was allowed to spring back to the centre, other servo never did it.

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If we assume the balsa structure is rigid enough then it could be some form of aerodynamic disturbance caused by a part of the aileron at small deflection creating a projection that has a disproportionate effect on the aileron function. At larger aileron angles the airflow will be turbulent and thus a small projection have little or no effect.

Where possible for accurate control at small control deflections it pays to fully "shroud" the control surface gap to maintain a smooth as possible airflow over the surface.

Just a thought.

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2 hours ago, Andy Gates said:

Flimsy foam ailerons springs to mind.

Alternatively are the servos strong enough for the surface size / speed?

 

We did have an issue like this recently at our field which was traced to a poor servo extension causing one aileron servo to "wave" after returning to centre. Only happened when full deflection was used and stick was allowed to spring back to the centre, other servo never did it.

Agree. Bad connection at an extension lead is the most likely cause of servo 'waving' at centre. Easily checked before digging deeper.

Edited by Cuban8
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I have often noticed the phenomenon of servo bounce when checking one out on the bench when either the battery I'm using is almost flat or when there's a bad connection. This limits the current the servo can draw and causes an oscillation between pulling the supply voltage down which slows its operation there by it draws less current and allows the voltage to recover speeding it up again leading to an overshoot. All this confuses the electronics in the servo, leading to spurious movements.

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I've suffered this a few times. That is I couldn't find what was causing my flight instability through static inspection.  3 times I've been able to isolate the issue to the iron on film graphics of the the sunburst. That is the film would lift up spoiling the air flow over the wing.  

 

When on the ground the film laid down fine. But as the angle of attack got higher the film in spots lifted up. Try to clean the surface real well and seal all the film edges with a going over with an iron and/or seal the edges with a clear varnish.

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If you have checked all the hinges, control horns, pushrods and clevises then it could be a servo. The foam RIOT has quite a coupled rudder so worth checking  the rudder servo and the aileron ones. From experience not just check them for a few seconds, but for minutes. 

 

I have a model with brand new servos and everything checked out on the ground and it flies well, but would occasionally would do something rather odd. In the end it was sitting on the bench (with the prop off) and I just kept stirring the sticks and after a couple of minutes (nothing before that) a servo would make a un-commanded movement and then recover, then return to its usual duties for an other 30 seconds to a couple of minutes and then do it again. Just one faulty servo but it took a lot of finding.

 

My son had a model that flow many time suddenly developed un-commanded aileron servo movement, that turned out to be a servo plug to Y lead connection. As the lead flexed the servo moved with no command input change, just unpluged it and plugged it back in and it was ok until we sold the model a few years later.

 

Again with the the model restrained or the prop off run the motor and watch to see if either the aileron or rudder servos move when no commanded.

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Hi all

thanks for the replies 

I've had the plane out again today and it's performed perfectly 

After checking everything suggested and found nothing that looks incorrect,  I fitted one of the capacitors into a spare channel on the receiver and the only other difference was the wind was a lot less, 

I'm going to run a separate power feed to the receiver and not use the bec feed off the esc

 

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IMO I would just replace the ESC as if part of it is playing up/faulty then it might pack up completely. OK it will only give up with a dead stick to deal with, but from experience it never happens a convenient time.

 

Although the RIOT is not weight critical (have flown some porkers due to repair glue!), what's the point of making it none standard and over complicated. There are hundred of RIOT's and thousands of models out there that use the ESC BEC to power the RX and they don't have a problem and a lot of them have the retracting UC off the same BEC.

 

Unless its your RX....but that's an easy swap to test.

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It is the max trainer not the riot so it has an overlander 60A esc, and recommended motor to the build instructions 

The esc was used in my wot 4 without any issues,  however the wot 4 has the standard ripmax mini servos and the max trainer has standard size digital servos with a higher power rating 

Dropping in a 2s 1000mah lipo won't add too much weight to the balsa model and will cost nothing as I already have them

 

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I missed the digital servo bit earlier.

How many servos are there and what is the BEC current rated at?

It might be the BEC is trying to supply more current than it can so the voltage is dropping out.

Analogue mini servos to standard digital servos is quite a jump in power required

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1 hour ago, David Payne 4 said:

Dropping in a 2s 1000mah lipo won't add too much weight to the balsa model and will cost nothing as I already have them

You'll need to use a regulator if you want to use a 2 cell LiPo or you'll kill the servo's and possibly even the RX, use either a regulator or a 2 Cell LiFe or NiMh's to get the correct acceptable voltage.

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David,


Should you be looking for a regulator to drive the receiver from your 2S Lipo this one maybe well worth a consideration. It seems to be aimed primarily at the drone industry but I'm sure it will be eminently suitable for a fixed wing setup as well. It's a switching device and I've recently obtained two to play with by paralleling them together just to see what happens. This is for another thread and so far they been as good as gold just idling together for a couple of days; but that's only as far as I've managed to get. They're very small, lightweight and compact, at a cursory glance so far look to be very well-made, a good list of protection features, reverse polarity input, thermal cut out, output short-circuit and over current. I'm wondering if the output short-circuit just might be a current fold back procedure, I intend to try this when I can eventually get there… 2 to 8S input, max 36V together with 3 voltage outputs, 5, 6 and 7.4V - all at 5A; and this is all for less than £8 for cash, can't be bad.. Being so small and light the one area I have slight reservations about is temperature rise, particularly at high output rates so I shall be keeping a close check on that, contact thermometers at the ready…. The output plug is standard receiver style….


Good luck


PB

Edited by Peter Beeney
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