Workbench Secrets — Custom Servo Installation

Mar 12, 2014 4 Comments by

This neat workbench trick is the work of Ken Park who worked out his servo installation issues while working on an EDF jet with relatively thin wing panels. Here’s what Ken has to say.

Canada Install Servos

Well, it’s been a lot of fun getting into RC Jetsbut what I have noticed is that the really impressive ones come mainly as an empty shell with little or no instructions on how to install your hardware, an example are servos.  Since everyone uses different types and brands, things like servos vary in height, weight and size.  So you can see why the companies simply instruct you to fill this space with your servo!

IMG_2160

(Above) Here are some of the tools needed for the job.

 

My Viper Jet EDF kit is beautifully built but it has a nice hole in each wing for both the aileron and flap servos.  The wings top surface has the airfoil shape and they instruct you to mount you servo so it is level and flush just under the bottom surface of the wing so the hold down plate can be attached.  So you can see you got to fit a square shape into an area that is far from square.

Canada Install Servos

(Above) Out of the box, the Viper Jet’s wing panels are assembled and covered, but servo installation methods are up to you.

To proceed the first finding to is to figure out the exact contours of the space my servo is going to occupy.  The light went off in my head and I used an old school Carpentry wood workers tool known as a  wire profile gauge.  I placed tape over the wing where I wanted to use the device to prevent it scratching the wings finish.  I then pushed down on the wires to get an exact depth gauge and profile.  I then transferred the profile from the gauge onto a sheet of paper.  I did a left and right side profile and found both to be nearly exact in profile front to back.

Canada Install Servos

(Above) This is a Carpenter’s “Wire Profiler”, it’s great for duplicating shapes during a modeling project.

So working from my side profile I can now lay my servo on its side in its simulated location that I want to fill so it fits flush under the 1/16-inch ply hold down plate.  In this case I am using Hitec 225MG servos.  From this you can now see the area and profile contour shape left that needed to be filled.

Canada Install Servos

Using a pair of scissors I cut out this bottom area profile and simply use its shape to make 3 little ribs that I wanted making a shim plate ramp.  I notched the ends so they would self-align with a top plate of the ramp.  The entire shim plate ramp was all made from 1/16th ply.  Each ramp for each wing used 3 ribs.  Each rib had the proper contour of the top of the wing and was found to be an excellent fit when inserted into the model.  I finished of each shim plate ramp at the front of the wing with some scrap 1/16-inch to complete the ramp shapes.

Canada Install Servos

(Above) Here’s the full size profile traced onto paper to form my cutting template.

Canada Install Servos

Canada Install Servos

(Above) I used 1/16 inch ply to make a ramp that matched the contour to match the interior wing shape, using the paper template.

So with each ramp made it was slipped into place and each servo was then checked to see if it would fit flush with the bottom hold down plates.  Sure enough it worked great and all is left to do is permanently gluing each ramp into its location and then mounting the servo’s in a fashion so you can gain access if need be.  Tam Jets have some nice L-brackets that let you screw in each servo on its side and would work very well in this application.

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(Above) Ready to test fit in the wing.

Canada Install Servos

(Above) Hitec aileron and flap servos attached.

You can follow the rest of the photos to see how I went about fine tuning this building task and I am sure, the technique can be applied to other similar models. Like they say, building a model is really building a bunch of smaller projects and then when you run out of the small stuff, your model is ready to fly!

Canada Install Servos

(Above) Servos and Ramp epoxied into place so the arms line up with the control horns.

Canada Install Servos

(Above) use some tape to workout the size and position of the servo hatch slots. You don’t want them to interfere with the servo arms or the clevises.

New Servos

Completed servo installation with linkages for the aileron and flap installed. For a neater, smoother hatch cover, I use clear tape to secure it in place instead of screws. Remember, the servos are attached to the wing structure not the hatch cover.

 

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4 Responses to “Workbench Secrets — Custom Servo Installation”

  1. Martin says:

    Nice, but what if the servo/gear/arm need to be exchaged?

  2. Norman Staples says:

    Very clever. Ken Park shows what this hobby can inspire. A difficult problem simple solution. Simple for some impossible for others. They should teach this in school.

  3. Ken Park says:

    Hi Martin

    If you must glue a servo into place first add a layer of green masking tape between the servo and the platform – In this case the Flap-servo was lightly glued into place with epoxy – To get the servo’s out you can unscrew the Aileron servo move it out of the way then too remove the Flap servo you use the flat of a large screwdriver to pop it up and loose

  4. SFT2 says:

    I’ve glued more than a couple of servos into airframes and have never had one either come loose on it’s own or fail to let go when I needed it out. Wrap the servo with strapping tape, the stuff with fiberglass tow in it, so that the ends of the strip of tape overlap on the surface being glued. Wipe it with alcohol, glue it in (epoxy, aquarium silicone, or whatever you prefer), and let it cure. When you need it out just cut the tape on the nearest convenient side of the servo, peel the tape off the surfaces of the servo you can reach, and pull it out. I’ve found it best to completely cover the side of the servo being glued with the tape. That way there is no glue left on the servo at all.

    Ken,
    Excellent work. The only thing I would change is replacing the ribs with foam, most likely urethane (Great Stuff or marine grade 2 part “pour foam”) with the edges rounded, to distribute the load across the upper wing skin. Not the load from the servos, that’s fairly minimal, I’m talking about the aerodynamic loads on the outside of the wing skin, specifically at negative angles of attack when there’s positive pressure on the skin. However, I’m somewhat prone to overthinking things and the servos are far enough behind the center of pressure that it’s very likely irrelevant. I’ll defend myself with the skin of a sailplane that was molded out of a layer of .5 oz glass over two layers of .75 oz. glass and bonded to a traditional wood rib and spar structure. That skin felt like well shrunk Monokote, but gave a bunch more strength and didn’t dip between the ribs. And I will NEVER DO THAT AGAIN!!! Geez, that wing likely paid for a therapist’s new golf clubs and took up most of my senior year in high school.

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