I've seen several methods for mounting servos in sheeted foam wings / stabilizers. I'm hiding mine under flush-mount carbon fiber hatches for my current project (31% CA Models Extra 300L). The carbon fiber hatches are much stiffer and lighter than using plywood covers, the covers can be made to conform exactly to the airfoil, plus they're immune to any warpage problems in the future. I'm sure that there are faster ways to go about it (and I'd love to hear your ideas), but here's how I'm doing it:

1. Mark the position of the servo hatches on the wing (or in this case, stab) sheeting. I used a 2" x 2 1/4" rectangle pattern cut from posterboard since I had six servos to mount...4 aileron, 2 elevator.

2. Make the Carbon Fiber hatch covers. I used 5 layers of 5.7 oz. bidirectional carbon fiber plain weave cloth. The odd-numbered layers have the fibers aligned at 0 and 90 degrees, the even-numbered layers have the fibers aligned at 45 and 135 degrees. I alternated the layers to add stiffness to the final part. I used 5 mil acetate sheet (overhead projector film) on the outsides of the layup to give a glossy final surface top and bottom. You do not need to wax or spray PVA on the acetate...this will simply peel off after the epoxy is cured. I vacuum-bagged the layup for a few minutes to squeeze out the excess epoxy then allowed the layup to cure for roughly an hour. While the layup is still in a 'green' state, cut out the individual covers (2" x 2 1/4"), then lightly press them down at the hatch positions so that they conform perfectly to the airfoil, tape them into position, and allow them to fully cure.

3. Remove the CF hatch covers one at a time. Peel off the acetate from both sides of the cover, and mark the inside of the hatch with whatever the hatch is for (i.e. right aileron inboard servo) and the direction of the leading edge so that they won't get mixed up. Lightly sand the edges of the covers.

4. Using the covers as a guide, using a SHARP blade, cut through the sheeting and slightly into the foam. Remove the sheeting (a small screwdriver works good starting from the center of the sheeting and 'flicking' away the balsa to the cut line).

5. Remove the foam from the servo wells. I use a piece of 12 gauge solid copper wire (14 gauge would work even better) bent to the shape of the well with wheel collars in place to control the depth of the cut. This is chucked up in a soldering gun. The wheel collars should lightly drag the remaining sheeting surrounding the well while cutting the foam. You could alternately use a Dremel with a router attachment, but I've found that this takes quite a bit longer and gets foam dust all over the place.


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6. Cut the four interior servo well 'walls' from 1/32" plywood. I used this because it's light, strong, and can be easily cut with a pair of scissors. Test fit the pieces and trim as necessary...you may want to mark these pieces (LE, TE, left, right) so they don't get mixed up while trimming. Make sure you cut a hole for the servo lead to go through.

7. Glue the servo well walls in place. I use ProBond (or any other brand) polyurethane glue since it's light, easy to sand, and slightly foams up during the curing process to fill small voids.

8. Sand the servo well walls flush with the wing sheeting.


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9. Test fit the CF hatch covers and sand where necessary so that they fit perfectly into the top of the servo well.

10. Position the servo onto the CF hatch cover and mark where the slot for the servo arm will be. Cut the slot into the CF hatch cover (a cutting wheel works good along with small files for final cleanup).

11. Make servo mounts from 1/4" ply. Screw the mounts to the servo. Sand the mounts to conform to the shape of the CF hatch cover. Rough up the CF hatch cover in the areas where the mounts will attach. Glue the mounts in place with thick CA or epoxy, clamp and allow to fully cure.


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12. Make the CF hatch cover mounts from 3/8" spruce or basswood tri-stock. CA them in the corners leaving a gap at the top to allow for the thickness of the CF hatch covers.

13. Put the hatch covers / servo into place and remove any material from the inside of the servo well that interferes with the fit. Drill mounting holes in the corners through the hatch cover and into the tri-stock mounts. Sit back and admire your work!


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I plan on painting the hatch covers...you could could use Ultracote on the covers just as easily. I plan on using button-head #2 screws to mount the covers. Although I haven't done it yet, a length of inner nyrod (the yellow fluted plastic) inserted and CA'd into each of the servo mount and hatch mount holes provides excellent vibration-resistant bite for #2 screw.

Whew! Pretty long winded, but I hope that this has shed some light on the subject. If you have any comments or questions about any of the steps, please respond here.

-Tom