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shop talk

Steve Condon

Installing servos in wings (without using glue)

Nowadays, almost all models using ailerons and flaps have the servos installed in the wings. The installation of these can be a pain or it can be a breeze. So can the un-installation. Many folks use methods that involve gluing the servo in place. This works fine, but when it comes to the need for removing, repairing and reinstalling a servo, the super-tight wedge method is what I prefer. Here are a few tips to make it easier on you. This method assumes you are working with a bagged wing skinned in carbon/glass or wood. A similar method to be used on hollow molded wings is discussed at the end of this article.

Making a template

First, if you know youíre going to be installing more than one wing servo of the same size, make yourself a template so you get it right every time. It doesnít have to be anything fancy, a piece of cardstock (perhaps the backing off one of those memo pads real estate agents leave on your doorstep) or a scrap of 1/16" ply thatís about 3" x 3" will work. To create your template, trace the exact size of the servo "footprint" and cut it out. Remember to create a "well" for the servo arm to reach its max deflection, but ensure that the shape of your hole is not a square. There must be a "step" so that all four sides of the servo make contact (see photo. one).

Test the accuracy of the template by slipping it over the servo. You DONíT want it to fit perfectly tight. In fact, there should be a space of about 1/32-1/16" on the side opposite the servo wire as shown in the photo. The reason for this is that it is difficult to push a servo straight down into its hole because of the wire. With the hole a little wider, you can lower the wire end of the servo into place and have just enough clearance to slip the rest of the servo into the hole without smashing the wing skin. The taller the servo, the more clearance youíll need.

Once you have the template to a satisfactory size, you are ready to begin.

Tools for the job

Things you will need to make this easy (this may be a good time to update your Christmas list):

  • Dremel tool with router base and 1/4" coarse flat-bottom cutter

  • Scribe

  • Wrench to tighten chuck on Dremel tool

  • Scraps of 1/32" and 1/64" aircraft ply

  • Scissors

  • Dustbuster or other small vacuum

Bagged Wings

Mark

This method assumes a carbon or wood skinned wing. Locate your template in the position for your servo. While holding the template firmly in place, use the scribe to scratch the outline into the surface. Do this for all servo locations.

Cut

There are two approaches to this procedure. One is quicker but requires a very steady hand. With this method we use the router, at full speed, to cut along the scribed line on the wing surface just barely "eating" the line. Once you go all the way around the perimeter, stop.

With the second method, which you may want to use until you become comfortable with your hand/router coordination, we cut the wing skin with an X-acto knife. An X-acto knife with a broken point #11 blade is perfect The shorter, stronger blade will provide the needed leverage we will need when cutting through the carbon fiber .

With the composite layer removed, you now have a template in the wing to run your router around to cleanly remove the necessary foam.

Set the depth of the router bit so it will make a cut that leaves the surface of the servo exactly flush with the wing skin when the servo is in place. Once you have it right, tighten the bit with a wrench. DONíT FINGER TIGHTEN ROUTER BITS. Take the safe route! (Pun intended)

Proceed to route out the rest of the foam. Use the vacuum to suck-up all the foam bits. Clean-up the corners using an X-Acto or a small file.

Test Fit

If you made your template the correct size, the servo should fit just as described--it should be a snug fit on the way in, and have a little clearance once in place. Now would be the time to do your wiring. When the soldering is all done, be sure to hog-out a little cavity for the small amount of excess wire you may have.

Wedge in place

Once your wiring is all done, plug the radio in and turn it on. Check the program and ensure that your wing servos are centered where you want them, that the polarity is correct and the servo arms where they belong. When you are satisfied with everything, use your scissors to cut pieces of ply the same height and width of the servo sides youíre going to wedge. Donít forget that you canít go full width where the servo wire is located. Either cut the piece short enough, or put a notch in it to clear the wire.

You want the wedges to come-up flush with the skin. If the gap is big use one or two pieces of 1/32" and then a finishing piece of 1/64" to make it really tight. When putting the wedges in, it may help to sand a slight bevel on the part that leads the way. You may only have to use wedges on one side of each axis. I prefer to use wedges on the side opposite the wire, the servoís bottom, and a skinny wedge on the small surface on the servoís top.

Cover with Tape

When you are pleased with how tight the servo is wedged-in, use an adhesive-backed vinyl servo cover or your favorite hinge-like tape to tape across the servo. Youíre done.

To remove

If you ever have to remove the servo, take the tape off, user the tip of a small screwdriver to pry the end of one of the 1/64" wedges up. Once you can grab it with your fingers or some needle-nosed pliers, pull it all the way out. After the first one, all the others should come easily.

Molded Wings

A similar method to be used on hollow molded wings follows much of the same technique, except you really donít need the router. Mark the holes in the same way leaving a little extra room to maneuver the servo in and out. Cut the wing skin away with an X-Acto, small saw, razor blade or even your Dremel router.

The critical issue is to set the depth so the servo is flush with the wing skin. In the case of a molded wing, a triangular wedge of balsa or rohacell works perfect. It will take some trial and error, but fashion a wedge to be glued to the inside of the top skin that will hold the servo perfectly flush with the bottom skin. When done, make two or three "walls" out of 1/8" or 1/4" thick balsa to go around the opening between the top and bottom skins. Carefully fit the walls and then glue them in place with 5-min epoxy.

Once the walls are in place, wedge and tape the servo in place as described above. I have found that this method works extremely well for securing servos under the most strenuous launch loads and yet, the servo is still removable with relative ease in case a gear needs replacing or some other problem arises.


 
 
 
     
 
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