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Installing Servos |
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 you’re 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
doesn’t 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
that’s 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 DON’T 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 you’ll 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. DON’T
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 you’re going to wedge. Don’t forget that
you can’t 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 servo’s
bottom, and a skinny wedge on the small surface on the
servo’s 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. You’re 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 don’t 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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