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Old Nov 08, 2012, 06:28 AM
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Australia, VIC, Melbourne
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Interesting issue......

The final Su-35 repair/upgrade/maintenance was to go over all aspects of the plane to be sure it was all 100% fine. Which then reminded me of seeing the TV Rudder servo stalling now and then... fairly often really. The servo itself had worn the screw holes due to the pressure against it!

When I undid the TV nozzles, the right hand one was 'medium tight' and the left hand one was 'quite TIGHT' when moving through various position combinations. I found that if it was DOWN and the rudder tried to move it, that it was under high duress and unable to move it correctly - thus the servo mount issue.

After a lot of moving it around, I figured out a 'peak' in the friction/resistance that suggested the 'ball' had some high spots in it. It is not perfectly round!
The right hand one was the same, but just not as tight to matter as much - but it was unacceptable in one combination angle/position.
But the left hand one was highly unacceptable!!!

After a LOT of 'total manouevering around' through all angles, for 10 minutes or more - to get the friction to wear it down a bit (???) - I decided it was fruitless. The right hand one did improve a bit, but it wasn't that bad to start with.

But all the moving made rub marks as clues to where the issues were.
Then out came the 'somewhat fine' wet and dry paper.... and I sanded away merrily. Testing... sanding..... testing... sanding... yawn.... testing.... sanding.....
For over one hour in total across the two of them!!
End result... NICE 100% free moving, but no slop, TV "balls"!!

I had never thought about that before..... that they could be 'sub-standard', like most part of any PNF are, so I should have checked that from the start too really!!

Seeing the servo screw holes in the plywood plate were wrecked, I decided to put an MG servo in there, because it drive two TV's, whereas the other 'vertical travel' servos only drive one each. The MG servo being 5mm longer got me past the worn screw holes.

So now all is free, smooth and perfect!!! Yay!! But it took quite a while in total !! (2.5hrs or so!)

Inspection completed and passed......
I will take it out one morning this week, or at worst on the weekend.
I actually didn't "fill and finish" the repaired areas, as I want to see it FLY again FIRST.... plus the damages aren't that bad and they don't stand out much at all after 1metre or so. But I will do it all fully next week - if it is still alive after these next flights!!

...
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Old Nov 08, 2012, 06:48 AM
Life begins at transition
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wow, hell of a lot more patience then I! I would have just stuck a 7kg servo in it
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Old Nov 08, 2012, 06:55 AM
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LOL.... well 3x 7Kg then... seeing the area that hits a 'bind' then affects vertical and lateral moves to and from there.
Of course it is nicer to be totally free and smooth over all possible travels.
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Old Nov 08, 2012, 07:31 AM
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I have have had more problems with the TV's. They are very rough probably due to dust in the gimbals. I have had both L+R servos go dead in both locked up and down positions creating interesting landings. Also had the center servo lock up full right. So replaced them all with Hitecs. Its a pity you cannot remove and service them. For my big epoxy glass Euro that I'm building I will make my own which will be on axis bearings like a giro.
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Old Nov 08, 2012, 07:33 AM
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Nice, how did you dismounted the nozzles?

Or did you sand them right in the plane?
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Old Nov 08, 2012, 07:59 AM
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Sanded in-plane. Just move them to each 90deg position at full angle.... and sometimes to the 45deg position if required. Using a strip of wet and dry, about 155mm to 20mm wide, by 60mm long so it can be controlled.
It was probably 240 grit (I think) or at least 320. And then lots of rubbing/sanding, moving/testing, again and again.
It took a lot of rubbing, so it would be hard to go too far!

Yeah Ace... for sure the servo(s) could stall, possibly at very bad times!! LOL
Some position combinations just add up to a notable 'lock'.... very high friction, verging on jamming.
And my rudder one would have died for sure soon enough! I haven't tested it yet, but by the first motions I saw I thought it had a chipped tooth.

The stock servos are probably ok, IF the TV balls were totally free for ALL positions they can ever get to.

This was an 'accident' just waiting to happen!! I can just see..... "Hmm, crashed again. This time it was........"
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Old Nov 08, 2012, 08:44 AM
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Well lets see.........What next!
With only 40 flights on it I have had this stuff happen. Just a note. I have thirty years experience flying all types of models and full size from fighter to civilian types. So I tend to be anal on inspections and looking after stuff. I'm also the club test pilot and senior instructor so I think I'm pretty decent at flying these things.

My List:
Fuse broke in half on landing?
Main wheel fell off
gear door hinge broke
L&R TV servo lockups
Yaw TV servo lockup
Front retract mount fell off
Lost both rudders when hinges failed in flight
Left wing spar coming loose inside wing
Right elevator stab bracket came off in flight
Developed stress cracks throughout entire fuse.
Exploding fan from FOD.. not aircraft fault.
TV gimbals failing. One very sloppy and both grind
So Just check everything out prior to flights. I do fly this in demo flights a lot so tend to be very high G most of the time. Still a pretty good aircraft.
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Old Nov 08, 2012, 09:36 AM
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Sanding the TV units never worrked for me.
The 360 units pivot on a single pin on the bottom and the rest is the smooth plastic inner ball surface against the smooth plastic surface of the outer clam shell/socket.
The moment I got too much sand or dust in between them and got any scratches on the surfaces, there would be too much friction under power which pushes the inner ball against the outer under pressure.

That's one major issue I find flying off of grass, worse I would suspect off sand etc. If you pick up dust into the duct there was always a good chance of it getting between the TV ball surfaces.
Once scratched up I had to replace them entirely.

It happened more on my Euro if I belly landed her.

Pete you may have sanded that high spot away entirely but I'd double check the nozzles under full thrust before you take to the sky. And not to give the wrong impression it was jjust my experience when I did try generous sanding it made it worse even getting down to 1500 wet sand grit. A high spot may be another thing all together.
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Old Nov 08, 2012, 09:55 AM
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ace,

Wow you've had some bad luck with yours.

Pete,

I had the similar problem with my tv units not getting full travel and having uneven movement do to servo and ball tightness. On occasions my tv unit would stay stuck in a slightly off position anytime I used yaw. All this was causing my plane to drop its wing when i would turn needing more than usual elevator input to stop the wing drop. It made for an uneasy flying and landing experience. I took Max advice and removed the nozzles and cleaned em and added some lithium grease, problems went away. Also need to adjust my servo travel as it was binding in full yaw.
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Old Nov 08, 2012, 11:13 AM
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Quote:
Originally Posted by v8truckin View Post
ace,

Wow you've had some bad luck with yours.

Pete,

I had the similar problem with my tv units not getting full travel and having uneven movement do to servo and ball tightness. On occasions my tv unit would stay stuck in a slightly off position anytime I used yaw. All this was causing my plane to drop its wing when i would turn needing more than usual elevator input to stop the wing drop. It made for an uneasy flying and landing experience. I took Max advice and removed the nozzles and cleaned em and added some lithium grease, problems went away. Also need to adjust my servo travel as it was binding in full yaw.
V8,

Thanks for your info. BTW, how do you remove the nozzles??

Thanks.
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Old Nov 08, 2012, 11:33 AM
Should've, Would've, Could've
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Quote:
Originally Posted by genghiskt View Post
V8,

Thanks for your info. BTW, how do you remove the nozzles??

Thanks.
if you just firmly pull on the nozzle it will pop off, least that's what mine did. Make sure not to pull the foam off the nozzle, but rather the portion under the foam including the foam.
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Old Nov 08, 2012, 11:35 AM
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All,

Thanks for all of this information!

These gives me more to inspect while waiting for replacement parts. I prefer the preventative maintenace over the post-crash repair anyday.
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Old Nov 08, 2012, 12:10 PM
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Mind you there was always expected to be slight binding in rudder.
We noticed this from the first 360 units on the F-35.
The horn on the rudder is well aft of the pivot center. The pitch servo is closer to dead center. So at certain angles there is a slight non linear path that peaks ina bind.
Thus the importance of keeping the ball smooth.

V8 did you take a pic when you pulled it. The units I have have a lip on the intake side that won't allow a straight pull. Replacing was always major construction; usually splitting the fuse seam at the unit.

It would be nice if they changed that.
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Old Nov 08, 2012, 12:21 PM
Should've, Would've, Could've
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No didn't take a pic, but I can do that for ya when I get off work later this afternoon/evening.
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Old Nov 08, 2012, 12:30 PM
You are a "go" for reentry
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Ya! That would be great even as a ref for all to know how they're secured. Haven't had a need to pull any lately so a visual would help.
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