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servo installation advice

Old 12-03-2011, 04:01 AM
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jcoop65
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Default servo installation advice

Things were going great, third flight in on my mx2, then it happened. As my plane lifted just after take off silence! I was taking off in the direction of the lake
that is just north of our runway. Needless to to say there was no time for anthing except a water landing. This would have never happened if i would of taken more time on my throttle servo instalation. The servo broke fee from its mounts and throttle slammed shut. I would like to hear your ideas and ways that you all install servos and make sure they are not going to break free of there mounts. I did not think that there was a lot of force on the throttle servo. What an eye opener.
Old 12-03-2011, 04:30 AM
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Default RE: servo installation advice


ORIGINAL: jcoop65

.......third flight in on my mx2

I did not think that there was a lot of force on the throttle servo.
Same force on the servo during the first two flights.

It is not the force, it is the vibration what destroys things.

Sorry to read about the mishap.
Old 12-03-2011, 05:01 AM
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jcoop65
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Default RE: servo installation advice

I do agree about the vibration. I made an aluminum plate to mount the servo.
Old 12-03-2011, 05:31 AM
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Default RE: servo installation advice


ORIGINAL: jcoop65

The servo broke fee from its mounts and throttle slammed shut.
As noted, the vibration is a real issue with gassers. I noted the above comment, did you remove the throttle return spring on the carb?

Some guys leave it on and I've seen several incidents similar to what you describe when they leave that return spring on the carb, it just puts more stress on the servo and especially in situations where the servo/mount is just glued to the side of the fuselage, the thing just falls off when the glue joints (usually) fail.

Don't remove that spring completely as it keeps the shaft/butterfly centered in the carb bore, but I cut the little tang off the spring.
Old 12-03-2011, 06:37 AM
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Default RE: servo installation advice

If the whimpy little throttle spring pulls the throttle off its mount, it wasn't installed properly. I leave my springs as-is from mfg and I never have had this happen. It's not the root cause here. Even the OP admitted it was his throttle servo installation.
Old 12-03-2011, 07:29 AM
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exeter_acres
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Default RE: servo installation advice

I have made my own boxes.. similar to these...
but these are very nice and the Jtec guys are awesome

http://www.jtecrc.com/servoboxes.htm
Old 12-03-2011, 08:07 AM
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K-Bob
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Default RE: servo installation advice


ORIGINAL: exeter_acres

I have made my own boxes.. similar to these...
but these are very nice and the Jtec guys are awesome

http://www.jtecrc.com/servoboxes.htm

Man, that is one tough looking servo box.

These will work too.

shop.aztechaeromodels.com/Secraft-Side-Servo-Mount-AA-SSIDE.htm
Old 12-03-2011, 04:37 PM
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aussiesteve
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Default RE: servo installation advice


ORIGINAL: JoeAirPort

If the whimpy little throttle spring pulls the throttle off its mount, it wasn't installed properly. I leave my springs as-is from mfg and I never have had this happen. It's not the root cause here. Even the OP admitted it was his throttle servo installation.
+1
Old 12-03-2011, 08:46 PM
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phantomdriver
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Default RE: servo installation advice

Jtec servo box looks like fort knox
Old 12-04-2011, 06:21 AM
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Default RE: servo installation advice


ORIGINAL: jcoop65

I do agree about the vibration. I made an aluminum plate to mount the servo.
The right type of wood will absorb more viabration than aluminum..............
Old 12-04-2011, 07:22 AM
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Default RE: servo installation advice

Maybe a picture of the alum. mount to clarify. The first question is how the mounting screws screw into the alum mount.
Old 12-04-2011, 07:47 AM
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Default RE: servo installation advice

ORIGINAL: jcoop65

I do agree about the vibration. I made an aluminum plate to mount the servo.
Some types of aluminum can be very brittle if not annealed (heating followed by rapid cooling).

From http://en.wikipedia.org/wiki/Aluminu..._applications:

"One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life."

The failure of brittle material under the stress of vibration is called fatigue:

http://en.wikipedia.org/wiki/Fatigue_%28material%29
Old 12-04-2011, 01:45 PM
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jcoop65
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Default RE: servo installation advice

here's the bracket, the base plywood is very thin and it split. I felt aluminum was the best bet. Its aircraft quality about 1/8 thick.
Old 12-04-2011, 01:52 PM
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jcoop65
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Default RE: servo installation advice

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Old 12-04-2011, 01:56 PM
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jcoop65
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Old 12-04-2011, 03:14 PM
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Default RE: servo installation advice

It looks like the wood is too thin to hold the servo screws firmly. The total thickness of the wood where the servo screws hold onto should be around 1/4", maybe less if the wood is aircraft grade.

You could use 2.5mm - 3mm ply wood to replace the alum. plate to save some weight. Two extra small pieces of ply wood can be glued on top of of it for the servo to sit on it and the mounting screws to screw into.

My concern with alumn plate is to secure the wood pieces to it and the extra weight.
Old 12-04-2011, 05:41 PM
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jcoop65
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Default RE: servo installation advice

In this case i was not too worried aout the weight, I wanted to make sure the servo stayed put no matter what. I did consider your wood idea but i felt the plate would be my best bet.
Old 12-04-2011, 06:01 PM
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Default RE: servo installation advice

Is the aluminum piece in a "L" shape with a bolt through the end? If so...that is very strong! Capt,n
Old 12-05-2011, 04:38 AM
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jcoop65
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Default RE: servo installation advice

Yes it is. The bolt on the bent end goes through the main bulkhead, and then two more bolts on oposite end. I also glued the bracket in place, and the servo is
held in place with bolts and nylon capped nuts.
Old 12-05-2011, 05:02 AM
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Default RE: servo installation advice

Just a note to ensure that the linkage geometry is setup correctly as well. If not, then instead of it breaking loose you'll just burn up the servo.
Old 12-05-2011, 06:41 AM
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Default RE: servo installation advice

How the original set up was?

What failed?
Old 12-05-2011, 06:55 PM
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jcoop65
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Default RE: servo installation advice

The original setup had the servo mounted on a plywood plate glued in place. the servo scews went through both pieces. I think what happened is that after the first flight I could not get the idle to go complety down, to kill the engine. I proceded to adjust the trim, however the throtttle was all the way back on the idle set screw. this in turn put too much pressure on the servo were it mounts, and started the failure process. Id like to say that this is my first big plane and im learning things the hard way, I do have help from an experienced flyer but im slow to take his advice. Experience is everything, but you have to get there somehow.

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Old 12-06-2011, 05:24 AM
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Default RE: servo installation advice

ORIGINAL: jcoop65

however the throtttle was all the way back on the idle set screw.
You can remove that screw, it's not needed and can restrict the ability to completely close the throttle butterfly to kill the engine as you found out.

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