Often, RC modelers find that their servo output arms are coming loose during 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 your giant scale airplane and that’s Vibration. The majority of vibration comes from the engine and it affects 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!
Good article with some nice tips. What about hard mounting the engine vs mounting the engine with vibration isolation mounts? I have seen both ways recommended and would like your opinion plus the pros and cons of both ways.
Another good thing to do is get away from the plastic/nylon servo horns. They can fail at anytime even without being stressed.
Thanks Gerry, all good tips. Another one I use to reduce engine vibration for conventional mounts is to use the mount as a template and cut the outline from an old inner tube to fit between the mount and the firewall. The hard rubber absorbs the vibration without letting the mount wobble about.
Comments are closed.