Servo Mounting Technique
by Len Buffinton
You’ve waited patently for the box to arrive. It seems like it took forever, and in this case it probably did. But the wait is over and it’s finally here. You carefully open the box and unpack the shiny painted fuse, long sleek wings and crystal clear canopy.
NOW WHAT?
Stop stalling and drooling, it’s time to assemble these pieces into the beautiful masterpiece you’ve seen in videos or maybe on the flight line. But where do you start?
Everyone has a different approach to “building” their models. The approach you choose is irrelevant as long as you arrive at the end with a nice model and a good, solid radio installation.
This article will discus different techniques for mounting servos in two common types of sailplane wings.
1) Foam wing sheeted with wood
2) Full glass, hollow molded
Both of these wing types are commonly found on many of the European designed sailplanes. The foam/sheeted wing is the most economical, whereas the fully molded glass wings are the most costly.
Starting with the servo selection, you need to choose a servo that can do the job required. Most of these wings are pretty thin, so the choices of servos will be somewhat limited. Each of the major radio manufacturers have added some sort of thin wing servo to their lineup. These servos are usually side mounting type and have torque values in the 40-80 oz range. Hitec, JR and Airtronics for example, have servos designed to lay on their side, complete with mounting lugs. Choose your servos based on what you need. Strength and size are the main considerations.
The first wing type we will talk about is a foam/sheeted wing. Lay the wing on your bench, bottom side up. You will see the cutouts made for the servos, although sometimes the covering is still over the holes. There are usually one or two for the aileron, and one for the flap, depending how big the wings are. If the plane is fitted with spoilers, there will be an opening for one also. Starting with the cutout closest to the tip, place the servo into the opening; be sure to keep the servo arms going the same direction in each wing panel. 
It would be wise to find the servo covers and make certain they will fit properly now, and the servo arms are corresponding with the way the covers are made. Dry fitting all the parts will save hours of aggravation later. There is nothing worst than getting all the servos mounted and hooked up; only to discover the servo covers wont align with the arms. Arrrrggh, that’s frustrating. (not that it’s ever happened)
Check the depth of the cut out also, you want the servo to be as close to the skin as possible, but not stick out. If you need to remove a little foam to make the hole deeper, it’s OK to do so. If you find the depth of foam between the bottom of the cut out and the top skin of your wing is very little, ( less than an 1/8 inch) it would be a good idea to scrape it right down to the sheeting and apply a little fiberglass cloth and resin to the exposed skin bottom. Doing this will stiffen the skin and keep it from flexing when the servo is operated.
Once the resin and cloth has cured, you will probably need to shim the servo back up to the correct height. Depending on the situation, you may need just a little thickness, or perhaps you need to make up a good bit of thickness. Aircraft ply or light ply can be used as a mounting base. 
Determine the correct thickness needed and cut the proper pieces to fit the hole.
Now is the time to choose the method you want to use for mounting your servos into the wing.
There are a few different ways to do this;
1) Simply glue the servos into the wing. This approach is common with sailplanes because the wings are so thin; there’s no room for mounting brackets.
2) Mounting with tabs already on the servos
3) Purchasing some commercially available mounting plates. These mounting plates are designed to be glued into the wing. Once the glue has cured, the servo is secured into the plate with small screws. The only down side to this method is the cost. These mounting plates are not cheap, but of course neither is the plane.
4) Make our own mounting bracket. This is accomplished with some plywood, some hardwood and a few small screws.
Gluing the servo directly into the wing
The best method for gluing directly into a wing is to get some shrink tubing at your local electronics store or any of the battery pack manufacturers online. Bring along the servo to make sure the tubing will fit well.
Cut the shrink tubing to size, place it over the case of the servo.
Then shrink it on nice and tight.
Plug in the servo to your receiver and check the centering of the arm. You wont be able to unscrew the arm and re-adjust it once its glued in, so get it set up now. Make sure the servo is going the correct way when the stick is moved. In some cases, when you reverse a servo, the center will change. Don’t forget to put the screw back in, use blue locktite too.
Once that’s complete, you can clean the tubing with acetone or lacquer thinner. This will remove any oils or grease which may have gotten onto the surface.
Its important to dry fit everything as stated above. Make sure the servo fits nicely. Use a straight edge to align the servo in the opening. Typically, the output shaft is toward the T/E of the wing. This allows the servo cover to fit and still give plenty of room for the servo arm to move. Take some time now and tape around the opening. I use the blue painters tape, 2” wide. This tape will keep any extra glue from getting onto the surface of my wing, and provide a place to make pencil marks.
Mark the corners of the servo, as well as the center of the output shaft. Check the height again; allow a little room for error. Its better to have the servo a little bit low than to have it stick out.
Next, you need to make the wire connection.
You’ll always want to have access to the connectors. I would recommend you dig out an area of the foam so the servo connector will fit. Be sure you can get to it if you ever have to replace a servo. If you are happy with the way it all fits, it’s time to glue it in. Mix up some 15-minute epoxy. You will need to add some filler to the epoxy, the best for this application in Micro Fibers, also known as Flox. This is available from West Systems or any good composite supply store. Mix the combination to the consistency of mayonnaise. Not too dry and not runny. You need the mix to be stiff enough not to run all over the place, yet loose enough to allow it to soak into the wood and foam.
Apply the mix to the servo being as neat as possible and assuring good coverage, but not so much it pushes out and creates a mess around the servo. Keep the glue clear of the servo arm and wiring. You can use some rubbing alcohol on your finger to smooth the epoxy that oozes out around the edges. I like to get the mix to come up the sides a little, just to form a little pocket. I feel this helps hold the servo in place.
Once you are happy with the fit and alignment, check it again! Be certain the height is good.
Now, leave it alone and don’t move anything until it has completely cured.
This method allows you to remover or repair a servo in the future. You would slice the shrink tube and remove the servo. Pull out the old shrink tubing, being careful not to damage the wing, prep a new servo and glue it right to the existing area with epoxy.
Some, including myself, have also just glue the servo directly into the opening without adding the shrink tubing. It works fine, but the few times I’ve has to remove a servo, I tend to be nervous, not wanting to damage the wing surface its mounted too. If it foam, you are going to rip out a bunch of it getting out the servo. On a glass wing, I use a pair of wide pliers and give it a sharp twist. I never had a damaged wing, but I do this shrink method now.
Mounting with screws
If the servo type you choose have mounting tabs on the sides, which allow the servos to be laid flat, they can be attached directly into the wing pocket without the need for a “specific mounting plate” however, you still need something to attach it to.
Keep in mind the thickness if the wing, you want to secure the servo but you don’t want the screws coming through the other side. Because these wings are thin, it is a possibility.
Its best to cut a piece of aircraft ply and fit it into the servo cutout. Check the height of the servo with the ply in place. If you are happy with it, epoxy the ply into the hole. Once the glue has cured, align the servos as described above. Mark the corners, pilot drill the screw holes very carefully. DO NOT GO DEEP WITH THE DRILL BIT. Uses very short mounting screws to secure the servo in place. This method allows you to remove the servo for wiring, centering, and servicing.
Commercially available mounting plates
These mounting plates are available from a few different vendors. They are either aluminum or molded plastic. They are specific to the brand and model of servo, so be certain which ones you order.
The plates have little holes in them to help the glue “grab” when pushed into the epoxy. Everything about mounting these plates is identical to the method described above. Just be certain the height is good. Shim the opening with wood to get the correct height.
Plastic servo holders and servo covers all in one
Another choice for easy setup is pre-molded servo mounts and covers. These mounts are available to fit the round cutout found on may smaller foam core/wood sheeted models. The servo mounts right to into the round base that gets glued into the hole.
Make your own mounting brackets
If none of the above methods appeal to you, its certainly possible to make your own version of a mounting bracket. You will need to mount a plywood base in the hole as described above. Then, create another plate with mounting lugs on it to secure the servo. This mount then screws to the plate you previously mounted in the servo bay.
Whichever method works for you, take the time it requires to get a good installation.
REMEMBER
Dry fit the parts and double-check everything before gluing!
Happy flying.
