TT2

My Feedback: (13)

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.


(Click on pic for full size)

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.


(Click on pic for full size)

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.


(Click on pic for full size)

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!


(Click on pic for full size)

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

MHawker

My Feedback: (16)

Tom,

Very nice! I just got done complaining about the servos I bought being too big for the holes in my Extra and saw this. I don't have the ambition to do what you did, but I'll live vicariously through your work!

Good job.

Mike

MinnFlyer

My Feedback: (4)

Beautiful job!

SilverEagle2

My Feedback: (9)

Could you also take a premade plate and heat it up to conform to the airfoil?

That would be a little easier than making my own. I have never worked with composites before.

Cheers

TT2

My Feedback: (13)

Thanks guys! Yes, it's quite a bit of work, but it goes fast once you've got the tools made and the process down to a science. You can save the tools for reuse on other planes.

I forgot to add one other bit of info...In step 7 when you glue in the 1/32" ply servo well walls, it is difficult to keep them seated against the foam while the glue is setting up. I used a piece of scrap EPP foam cut to size and jammed it into the well. The EPP (expanded polypropylene) foam has some 'springyness' to it and it held the ply pieces perfectly in place.

It depends upon the type of plate. I primarily use West Systems' 105/205 resin and it has a pretty low heat deflection temperature, so I suppose it could be heated and bent...I've never tried it, but I'm going to right now. Contact me via email if you would like for me to make the plates for you.

-Tom

WreckRman2

My Feedback: (5)

Very nice...

WESCO

My Feedback: (11)

TT2 what servos are you using in your 31% and where are you getting your carbon fiber Wesco

TT2

My Feedback: (13)

WESCO: I'm using (2) Hitec HS-5925 for elevators and (2) HS-5925 for rudder. I'm using (4) Futaba S9402 for ailerons...unless I decide to go with more Hitec digitals. I'll probably go with a HS-225 for the throttle.

I ordered a BUNCH of carbon fiber from carbcom.com for an auto racing project that I've put on hold. Thus, I've got way more than I'll use in years for RC stuff, so if you need some, shoot me an email and we can work something out.

-Tom

AV8TOR

My Feedback: (2)

For those who do not have vacuum bagging I learned this from Dave Platt, put a scrap piece of monokote over the wing (you can just lay down wax-paper) and lay-up the carbon or fiberglass cloth over the paper and wing. After the last layer is on you can put on the Mylar on top to gloss up the top layer. Not as good as vacuum bagging but will give you a nice gloss finish. Finish just as TT2 describes. Dave Platt uses this technique to make landing gear doors before cutting the wells out.

coloradoz

My Feedback: (38)

Hello Tom, in your step 11 you state that you "Rough up the CF hatch cover in the areas where the mounts will attach." Why is this? Also, you glue the mounts in place with thick CA or epoxy, clamp and allow to fully cure. Is this the servo mount or hatch cover mounts. For the servo mounts, I take it that the fit has to be absolutely precise so that the servo/servo-mounts will 'just fit' inside the well. Do you leave the servo mounted for alignment purposes too while this is glued into place?

...ajs

TT2

My Feedback: (13)

This is to ensure a solid bond to the carbon fiber. It is possible that a residue on the surface would inhibit proper bonding.

These are the plywood servo mounds that are glued to the carbon fiber hatch cover.

This is easily done by cutting the sheeting (in step 4) slightly larger than the cover. The resulting hole should be the width of the cover plus the thickness of the interior 'wall' pieces plus 1/32" (slight gap).

When gluing the 'walls' in place you could use the carbon fiber covers as a guide to ensure a correct fit. It would be a good idea to put two layers of masking tape around the carbon fiber cover (to provide the slight gap) and then positioning it while the 'walls' are curing.

-Tom

coloradoz

My Feedback: (38)

Thanks Tom that explaination helps a lot! Without defocussing this thread - do you approach retract wells with the same technique (or is that already covered in a different thread)? I'm more concerned with degrading the integrity of the wing with all of these cutouts. Ive got a Bridi UFO that I want to refinish and refitting with retracts is part of the plan - any tips on reinforcing the wing area and fitting the retract wells?


Thanks!
...AJS