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Check those servo arm screws!

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Old 07-23-2013, 07:02 AM
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G4guy
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Default Check those servo arm screws!

I recently lost my scratched built Tsunami at Kentucky Jets. It was was flying so well, especially with the new Jet Central Cheetah.
On the final flight, as I rotated the aircraft rolled 90 degrees left, I corrected and it did it again at least 4 or 5 times before it rolled over and went in to the corn field. All this happened in a few seconds. The plane was pretty much destroyed, but all the control surfaces were still attached with the servos still functioning.
The good news for me, the turbine survived with no damage. The 3 Sullivan fuel tanks did not rupture, whew.
So when I returned home, I laid the wings out on the work bench. I thought I was going to find a flap issue, but to my surprise, they worked and moved correctly. When I hooked up the ailerons, the servos they both worked, but one aileron did not. I thought maybe stripped gears, but no, the arm was off the spline, even though the retaining bolt was still in the shaft. I had the lock washer installed. I'm thinking the arm came off and maybe put the aileron in an offset position sometime after I started or during the takeoff. I happened so fast, I didn't have a chance.

What I learned:
Always Loctite the bolts, I'm going to check my other aircraft.
Do a full control surface check before taxiing out, especially at a busy jet meet.
Go over the airplane at least once a year, I hadn't looked at the servo mount since I built it, 3 years!
Thanks to everyone for the compliments on the design, number 3 is on the building table now.
Regards,
Ron
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Old 07-23-2013, 07:20 AM
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Default RE: Check those servo arm screws!

Ron, yes, I saw your jet at KY Jets, and she was beautiful! Sad to see it destroyed.

One question: is it ok to use Loctite on servo bolts? I thought I saw sowewhere that Loctite can attack plastic arms. If the arm is metal, I suppose Loctite is acceptable. Seems I remember something about using a drop of CA instead of Loctite.

Anyway, sorry for your loss. I did not realize your Tsunami was a one-of-a-kind scratch build. Impressive! If you ever make a kit to sell, I'm in

Randy
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Old 07-23-2013, 07:25 AM
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Default RE: Check those servo arm screws!

I haved problem wit Dubro arms ,the strip over 200oz and leve the airpline with no control.
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Old 07-23-2013, 07:39 AM
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Default RE: Check those servo arm screws!

I also read that Loctite attacks plastic.
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Old 07-23-2013, 07:52 AM
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Default RE: Check those servo arm screws!

This is a good reminder. Thanks for posting. Like Jim says above, too much loctite can damage components as well, but a little drop won't hurt metal to metal (screw to servo gear) connections. Even aluminum.

On my checklist (I'm the only one that has one that I've ever seen) that I keep tethered to my waist, I have an Acronym: F.E.A.R.

This stands for:

F: Flaps

E: Elevator

A: Ailerons

R: Rudder

I pull on all the surfaces and check their movement, position and condition.

Even this can't catch everything.

Ali was doing the good graces of putting my Ultra Bandit through some paces (he was spanking it) and he was saying that it continually kept getting out of trim. I found that the mounting bolts on the ailerons had become a little loose and with hard movements, the aileron would move in the servo box.
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Old 07-23-2013, 07:53 AM
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Default RE: Check those servo arm screws!


Quote:
ORIGINAL: VF84sluggo

Ron, yes, I saw your jet at KY Jets, and she was beautiful! Sad to see it destroyed.

One question: is it ok to use Loctite on servo bolts? I thought I saw sowewhere that Loctite can attack plastic arms. If the arm is metal, I suppose Loctite is acceptable. Seems I remember something about using a drop of CA instead of Loctite.

Anyway, sorry for your loss. I did not realize your Tsunami was a one-of-a-kind scratch build. Impressive! If you ever make a kit to sell, I'm in

Randy
The spline is metal, and the bolt is metal, so maybe a small amount just on these two parts. Keep it off of the plastic arm if one is being used. Or check the bolt regularly!
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Old 07-23-2013, 08:00 AM
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Default RE: Check those servo arm screws!

I've had issues with the dubro servo arms in the past. The plastic becomes brittle, it also shrinks. When that happens the servo arm bolt loosens and can start to back off. Even with loctite, once the arm shrinks a bit, it starts to become sloppy on the output shaft and eventually strips. I had this happen, along with another servo arm splitting lengthwise through the clevis holes on my kingcat. Fortunately I caught it on the ground. Best thing you can do with dubro servo arms is leave them at the hobby store or throw them away.
Sorry to see your jet destroyed, it's a very nice looking design.
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Old 07-23-2013, 08:07 AM
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Default RE: Check those servo arm screws!


Quote:
ORIGINAL: VF84sluggo
One question: is it ok to use Loctite on servo bolts? I thought I saw sowewhere that Loctite can attack plastic arms.
This advice came from hitec, not to use threadlock with their karbonite servos. The output shaft is karbonite which is just plastic not metal, and hitec advises not to use threadlock in that.

As you only need a little drop on the bolt into metal output shafts the threadlock should not be near the servo arm so it is not an issue with plastic servo arms.

see http://hitecrcd.com/faqs/servos/general-servos
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Old 07-23-2013, 08:25 AM
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Default RE: Check those servo arm screws!

Thanks for the info Harry!
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Old 07-23-2013, 08:31 AM
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Default RE: Check those servo arm screws!

Ron I would suggest using lock washers. I'm pretty sure most metal gear servos come with these standard.
If not I would suggest these:

http://microfasteners.com/lwt04-4-lo...th-plated.html


Really sorry about your jet. Are you headed to North Jackson next month?
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Old 07-23-2013, 08:33 AM
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Default RE: Check those servo arm screws!

Metal to metal then locktite is fine.

Locktite can be very aggressive to certain plastics, others it won't bother at all. If it attacks it, it will make it brittle and weaken it.
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Old 07-23-2013, 08:53 AM
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Default RE: Check those servo arm screws!


Quote:
ORIGINAL: Bob_B

Ron I would suggest using lock washers. I'm pretty sure most metal gear servos come with these standard.
If not I would suggest these:

http://microfasteners.com/lwt04-4-lo...th-plated.html


Really sorry about your jet. Are you headed to North Jackson next month?
The lock washer was installed, I think i need to check the airframe more often!
No, I will be in Europe next week, South America this week!
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Old 07-23-2013, 09:19 AM
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Default RE: Check those servo arm screws!

Then you need a bigger hammer!
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Old 07-23-2013, 09:36 AM
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Default RE: Check those servo arm screws!


Quote:
ORIGINAL: LGM Graphix

I've had issues with the dubro servo arms in the past. The plastic becomes brittle, it also shrinks. When that happens the servo arm bolt loosens and can start to back off. Even with loctite, once the arm shrinks a bit, it starts to become sloppy on the output shaft and eventually strips. I had this happen, along with another servo arm splitting lengthwise through the clevis holes on my kingcat. Fortunately I caught it on the ground. Best thing you can do with dubro servo arms is leave them at the hobby store or throw them away.
Sorry to see your jet destroyed, it's a very nice looking design.

+1

We just removed an entire set of Dubro arms from a model due to them having so much slop on the output shafts, that with power applied to the servo (8711) you could physically strip the dubro horn around the output shaft. This wasn't an isolated case either, I had three brand new packs on the wall, and they were all this way as well. And before someone asks, YES they were for JR servo's. After four brand new sets of these arms all being the same, I'll never use them again.
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Old 07-23-2013, 09:51 AM
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Default RE: Check those servo arm screws!

Harry, iirc Spektrum say exactly the same thing.
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Old 07-23-2013, 09:52 AM
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Default RE: Check those servo arm screws!

Ron,

As other have stated LocTite and plastic do not play well together, I would not recommend this if any plastic is involved.

p.s. My legs have recovered from retrieving your model from the field...
I don't know what was in that tall grass in KY but it sure did burn my legs!
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Old 07-23-2013, 10:42 AM
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Default RE: Check those servo arm screws!

+5 or 8 ( I think ) on the loctite.

I was goin to fly my P-47 last weekend but the low ceilings prevented me from going up, last night I was checking the ailerons and when I pulled on the right one it came off half way out [:-] [X(]

Rafy I too have a check list or two, acctualy I have three check lists since ever but you are the only one I have seen carring one.

Yes, checking the control sufeces before a flight is my thing too
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Old 07-23-2013, 12:42 PM
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Default RE: Check those servo arm screws!

In the past I used a dab of silicone sealant on the top of the screw/servo arm, I now use a small amount of Loctite combined with a socket head cap screw of the correct size, not had any issues with either method.

Peter
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Old 07-23-2013, 02:49 PM
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Default RE: Check those servo arm screws!

Wow, it just happened to me. It was one of the elevator servo which lost the screw, good luck to me it was not stuck but flapping.
This is not a jet, it is the 72" AJ Slick. Since its a 3D plane it has big surfaces so I was able to land her with the other half w/o damage.
It taught me a lesson, check all those screws periodically
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Old 07-23-2013, 02:52 PM
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Default RE: Check those servo arm screws!


Quote:
ORIGINAL: RCISFUN

Ron,

As other have stated LocTite and plastic do not play well together, I would not recommend this if any plastic is involved.

p.s. My legs have recovered from retrieving your model from the field...
I don't know what was in that tall grass in KY but it sure did burn my legs!
Thanks Rich for your help, I also had a case of poison ivy on my leg, arm, chest. Not bad, but a little Clorox took care of it when I got home.
At least I believe I found the culprit, that's always a good thing....

Ron
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Old 07-23-2013, 03:00 PM
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Default RE: Check those servo arm screws!

silicone rtv makes an excellent threadlocker and its safe for plastic
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Old 07-23-2013, 03:07 PM
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Default RE: Check those servo arm screws!

I recall Maj Woody recommending CA instead of Locktite if plastic arms are being used.

I found a reference here: http://www.rcuniverse.com/forum/fb.asp?m=9600907

Sluggo
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Old 07-23-2013, 05:22 PM
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Default RE: Check those servo arm screws!

I always use loctite on metal output shaft servos. I place a dot of loctite on a paper towel and roll the threads of the screw in the loctite, then install. I almost learned the hard way about globs of loctite and plastic servo arms. It softens the plastic and allows the splines to slip, or at least it did with mine. Now on everything 20cc and larger / faster I use aluminum arms & loctite. Sorry about your plane.
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Old 07-23-2013, 06:22 PM
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Default RE: Check those servo arm screws!

I use RC56 canopy glue on my metal gear servo screw threads.... and then a little drop on top of the screw. Dampens any vibration. Locks the screw in and allows for easy removal should you need to.
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Old 07-23-2013, 07:10 PM
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Default RE: Check those servo arm screws!

Zap a Dap a goo, use a toothpick to run a small bead from screw to servo arm. Has never failed, does not allow screw to rotate out. If you need to remove screw just use your fingernail to scrape off before using Phillips screwdriver
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