Bad Vibes — Loose Servo Output Arms

Oct 15, 2012 10 Comments by

Often RC Modelers find that their servo output arms are coming looseduring flight and for various reasons, this is not a good thing. Actually servo arms coming loose is a symptom of something else that’s troubling their giant scale airplane and that’s Vibration. The majority of vibration comes from the engine and it can affect other parts of your airframe.

Here are a few tips to minimize the affects of engine vibration.

First, make sure your engine is properly bolted in place and that the engine mounts (and its standoffs if used,) are secure.  Always use quality hardware and install wide flat washers under the nuts to spread out the load and help prevent the nuts from crushing into the firewall.

 

Second, always run a properly balanced propeller. Whether you use a composite or wood prop, throw it on a balancer and make it doesn’t have a heavy blade. Use a quality balancer like the one from Du-Bro that has precision bearings and an adjustable base.

 

Third, make sure your tail surfaces are properly installed and hinged and the linkage is free of slop. Sometimes a cheap CA style hinge will break and go unnoticed, so check these before each flight to be safe.

I like to install jam nuts on the threaded ends of the pushrods to lock the clevises securely in place. It doesn’t take much to eliminate play in your linkages.

 

And, fourth of course, make sure your servos are properly installed.

  • Always use the rubber mounting grommets and be sure to install the brass inserts from the underside. This prevents the servo mounting screws from crushing the grommets.
  • With most of my big planes, I like to install a removable servo tray made from a sheet of lite-ply. This makes inspection and maintenance a lot easier.
  • I also glue additional layers of wood under the tray where the screws are inserted to stiffen the tray and to increase the amount of material the screws can thread into.

  • Also, before you screw your servos into place, be sure to “harden” the threaded screw holes with a drop or two of thin CA. This will greatly reduce the chances of the screws stripping out of the wood.

These issues usually affect your throttle, rudder and elevator servos. Engine vibration is most concentrated in the fuselage and it seldom affects the aileron and flap servos out in the wings.

Another good tech tip for preventing the screws from backing out is to apply a very light smear of clear silicone sealant or Goop adhesive to the servo arm and the screw head. Just a little bit is all that’s needed. You don’t have to cover the entire screw.

 

So, whenever you encounter a recurring problem, (especially after a rough landing or a crash,) look at your airplane as a whole and do everything you can to minimize the effects of engine vibration! Fly safe!

 

Gerry Yarrish

About the author

Senior Technical Editor About Me: I have a lifelong passion for all things scale, and I love to design, build and fly scale RC airplanes. With 20 plus years as part of the Air Age family of magazines, I love producing Model Airplane News and Electric Flight.

10 Responses to “Bad Vibes — Loose Servo Output Arms”

  1. Mike Merryman says:

    If you cover the screw head with sealant, how do you remove the screw later if you need to? Thanks for the excellent tips!

    • Gerry Yarrish says:

      you dont cover the screwhead entirely. just a small dab on one side of the screwhead and the arm. it comes off cleanly with a sharp hobby knife

      • John Sohm says:

        Jerry, just curious why you use some RTV sealant instead of just using a tiny drop of blue LocTite to keep the screw from loosening? I mean the sealant probably works well but why not use something designed just for that purpose.

  2. Rick Davey says:

    I do almost everything you suggested and will do the rest…the RTV on the servo screw is a must and it comes off easily…Highly recommend that and the jam nuts…nice article.

  3. Rick Apitz says:

    I’ve also used a drop of RC-56 canopy glue on the screw threads on metal gear servos…. helps dampen vibration and keeps them from backing out and are easily removed.

  4. FHH says:

    Paranoia regarding screws vibrating loose is a good thing…

    I agree with Rick Apitz

    A dab of RC-56 (canopy glue) on the screw heads of servo arm retaining screws will prevent them from vibrating out . Its easy to peel the glue off the screw and arm when you need to get the arm off.

    RC-56, being water based, will not attack any plastic and I’ve never had it cause corrosion of the black coated servo arm retaining screws.

    It does require an hour or so to set up properly. (blue or red lock-tite recommend 24 hours for full cure and will damage many plastics)

  5. Anonymous says:

    If you use Goop, it’s very easy to pull off in one piece with needlenose pliers after it cures. Similar to the canopy glue. Does not attack plastic.

  6. Terry Bolin says:

    good article,, Several good tips.. Gary I am having a aileron that pulses just a little when the B25 os46la motors are on WOT..I have balanced the props and motor is mounted tightly to the black fiberglass motor mounts supplied by manufacture..the servo holds steady untill WOT, would you suspect a vibration still causing this servo to hunt ? (maybe a 1/16″ up or down) The linkages are very firm with no slop…Thanks for your imput!

  7. Tom Pomeroy says:

    2 motors need to be synchronized to run smoothly.
    Tp

Copyright © 2014 Air Age Media. All rights reserved.