An all-too-common error we see in RC planes is servos not being installed correctly.
The results of a small mistake can be devastating to the airframe, and your ego.
Text, photos and video by Tom Hintz
Posted – 6-2-2017
Installing servos in an airframe might not seem to be a skilled task but the reality is far too many planes (and other RC vehicles) are damaged or junked out because of a simple mistake during that installation. If my email is any indication, there are several questions that we can address for those new to RC flying and perhaps a veteran or two in this article.
While servo manufacturers go to great lengths to protect the electronics within a servo from vibration, how they are mounted to the airframe is also crucial. Manufacturers include the necessary pieces for mounting the servo but if not installed properly their ability to isolate the servo from vibration can be compromised. It should be noted that nearly all servos from all manufacturers use virtually identical mounting parts which speaks to how well they work.
Probably the most misunderstood among the hardware pieces are the tapered brass inserts. These pieces are almost always cone-shaped with a wide flat or rolled end and a smaller end with no flange.
I frequently see these pieces inserted into the rubber grommets but from the wrong side. The sleeve is designed to be inserted into the grommet from the bottom so the wide base will be on the wooden surface to which the servo is mounted. When the screw is tightened the wide base prevents the sleeve from cutting into the wood while the narrow top end of the insert contacts the head of the screw close to its shaft. That allows the screw to secure the servo firmly while allowing the rubber grommet to absorb vibration.
If the insert is installed from the top of the servo the narrow end cuts into the wood when the screw is tightened. That allows the wide flange and the head of the screw to compress the rubber grommet compromising its ability to dampen the vibrations the servo feels. That can lead to an early demise for an otherwise perfectly good servo.
My one exception to the included mounting hardware is the wood screws meant to secure the servos to the airframe. Nearly all manufacturers include Phillips-type screws. I replace these with hex-head screws from RTL Fasteners because they are so much easier to install and remove later without stripping the heads. Also, the threads on the RTL screws run all the way up to the head which gives them a more aggressive grip in more thicknesses of wood making them ideal for a bunch of other uses in an airframe in addition to servo mounting. The RTL screws are available in several lengths which further extends their utility in an RC airframe. (See Resources below)
The brass sleeve on the right is correct,
the one on the left is wrong but we see
this all the time.
Many planes come with pre-cut openings in which the servos are to be mounted. Bill Rutledge, from the FlyingRC.net Texas Bill Squad hit on a procedure that eliminates potential servo failure due to rattling against the side of those openings. Yes, he figured this out after having such a failure himself and finding the rub mark on the side of the failed servo. This makes perfect sense as the mounting hardware that comes with servos is meant to suspend them on the vibration-eliminating rubber grommets. Allowing the side of the servo contact the edge of the opening negates that dampening completely.
Wrap 3 or 4 layers of masking tape around one of the servos (assuming they are all the same size) just below the mounting flange on its body. Use that servo to check the size of all servo openings in the airframe. If an opening is too small expand it until the taped servo will drop into place without forcing it.
With the taped-up servo in the openings drill pilot holes in two diagonal mounting holes, install screws in those to lock the servo in place and then drill pilot holes in the remaining holes and run screws in those as well. Remove the screws and servo and put a drop of thin CA on each of the holes and allow to dry. Repeat the process on the remaining servo openings. Now when the servos are installed permanently (no tape) you can be sure there is no contact with the sides of the openings to transfer vibration to the servo.
This process might add half an hour to a build but can save that plane or a servo. We fly RC to have fun, not chase planes around or do unnecessary repairs. It’s your choice.
Most RC plane applications require the output shaft of the servo to be located towards the front or rear of the plane to fit the linkage pieces included with the kit. This orientation can also be important for clearance of the linkage or servo arms. The instructions should specify the orientation or have photos of the installation. Output shaft orientation can sometimes make a difference for routing the servo cables and keeping them out of harm’s way.
Heat shrink over servo extension plugs
makes them more secure and easier to
pull through the airframe.
Most servos come with cables around 12”-long. That means we need to add extensions to at least some servos in most applications. While most servo extensions look the same there are small differences in the plugs among manufacturers so we need to pay attention to this when buying them.
The plugs connecting the servo lead to the extension might seem secure but I always add a length of shrink tubing that covers the joint completely. This makes it even more secure plus introduces a level of protection from contamination should water or fuel get into that space. The shrunk down tubing also streamlines the joint making it easier to feed through the structure of a wing or tail section.
Some people would rather cut the servo cable and solder in a length of compatible wire to reach the length needed. There is nothing wrong with this approach if you use good wire and are confident in your soldering skills. Remember that we must isolate the individual splices with heat shrink tubing (I add another piece of heat shrink tubing covering all the splices) which can create an odd-shaped lump if the splices are not staggered. That “lump” can complicate stringing the cable through the structure in some airframes.
Chances are that more than one servo lead will have to be pulled through the structure of a wing or tail section on your new plane. There is a nearly equal chance that this task will frustrate you as it did me. Like so many things that frustration led to thought and a better way of doing things.
The good news here is that the plane manufacturers nearly always leave a string in place to pull the cables through. If you simply tie the string to the plug on the servo wire and start pulling your day is liable to get very long and that frustration will be back. The plug will simply fold over and make the cable seem twice as big as it catches on everything in the structure.
I do tie the string onto the cable, but use a simple slip loop that goes around the cable just below the end plug. Then I add a small piece of masking tape to hold the string straight off the end of the plug so it all stays in line as it comes through the structure. If the tape runs off the end of the plug a little and you work it down around the string it will skip over the edges of structure easier yet.
The final task is to make sure that the servo cables are secured so they cannot become tangled with things like servo arms that could pull them hard enough to disconnect them from the receiver. I have found servo cables wrapped around a servo arm that I previously thought were safely tucked out of harm’s way. The forces that act on the airframe also act on the cables and though lighter, they can still move in response to G-loads. Servo cables that move can rub against an edge in the structure and eventually wear the insulation away leading to a short. If it can go wrong, we want to find a way to prevent it.
Take the time to bundle servo and battery cables in logical groups. Secure those bundles to the airframe so they remain out of the way of anything that moves. This kind of work seldom takes much time but can be well worth it.
I get as anxious as anyone to get a new plane in the air but we must be smart about it. Taking the time to assemble I correctly will prevent future damage or the loss of the plane. This does not cost a bunch more or add excessive amounts of time to the build but can add years to how long your new plane lives.
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