spoilers in balsa sheeted foam wings

[Weird Harold]

I am working on my first balsa sheeted foam core 2M wing. I would like to add spoilers with two individual servos, but I need help with the linkage, control horns, and methods of hinging. Does anyone know of any articles, diagrams, or personal words of wisdom.

Thanks for the info!

Rich

Ollie, are you reading this??

[Ollie]

An important consideration is to locate the spoilers where they will do the least harm to the flow over the wing when they are closed. Back at about 60% of the chord from the leading edge will be more tolerant of bumps and slight misalignments.

There are two ways to go. You could buy a commercial unit (made in Germany) that pops up or you could build a barn door unit that hinges at the front edge. If the wings are not sheeted yet, you could put a patch of thin plastic under the area where the spoilers will be located to act as a release film. After sheeting, the skin can be cut away for the spoiler and easily removed because it is not stuck to the core. i9f the skin is strong enough for the size of the spoiler you can use it. The spoiler blade can be hinged with clear plastic tape, like packing tape. A 1/16 ply control horn can be added to the bottom of the spoiler and the foam core cut away to clear the horn. the spoiler servo should be a micro mineature servo that fits within the thickness of the wing. A short pushrod connects the servo to the horn. It is a good idea to do a scale drawing of the crossection that shows the servo output arm in both open and closed positions in relation to the hinge and horn in both open and closed positions so that the linkage geometry can be designed to operate properly.

[KevinSheen]

Ollie's got a good idea, I'm going to expand on it just a bit.

Make sure that you plan out your spoiler blade and servo locations well in advance of sheeting the wing. Also plan on some reinforcements just under the upper skin of the wing. This will provide a little added support (instead of just the sheeting) when you remove the core to locate the servos in the wing. Also plan out your servo wire tunnels back to the center of the wing.

Here is one way to tunnel the wire for the servos (I'm sure there are numerous ways). I normally will route a 1/8" deep slot in the bottom of the foam that will closely fit a 1/8" x 1/4" balsa stick that will act as the tunnel cap. Inside of the first slot in the foam, route another that is about 1/8" deeper and narrower than the first slot. Something like this:

-------- -------------- foam bottom surface
!_ _! tunnel for balsa cap
!____! tunnel for wire

Lay the wire in the tunnel and glue the balsa cap in place. Try to get a really good flush fit between the cap and the surface of the foam. A dremel tool with a router bit and a straight edge makes this task a piece of cake. Put the end of the cap right in the area where the servo will be.

For the upper surface reinforcement, you can use a 1/32 inlay of ply or some uni directional carbon fiber or anything that will help keep the upper sheeting stiff when you remove the foam for the servo bay.

Sheet the wing.

Once the sheeting is finished, open up the servo bay with the dremel and router bit. Careful with the depth initially until you get the wire out. Once the wire is located, you can have at it until you get the depth you need to keep the servo inside the wing.

On the top surface, I like to layout the blade area in pencil and then go after it with the dremel again. Finish it by carefully squaring up the corners and then line the spoiler bay with 1/16" balsa on the edges. After the glue dries, you can sand the inset balsa smooth with the top skin.

I made my last spoiler from balsa that was capped with 1/64 ply. The 1/64 ply really keeps it from warping in the sun.x

hope this helps some,

Kevin

[Weird Harold]

Ollie and Kevin, thank you both for some very good ideas.

I plan to do some scale drawings of the installation, but I'm still not too sure about what the geometry of the linkage should look like( i.e. the angles, shapes, and sizes of the servo and blade horns).

I also will use the Dremel router, but wanted to ask which bit works the best. Do you use a cylindrical bit with a flat bottom?

The use of a plywood doubler or layer of fiberglass on the bottom side of a balsa blade to allow flush sanding with the sheeting on the top sounds like an excellent idea.

I appreciate your help!

Rich

[KevinSheen]

Yes, the cylindrical router bit is the one I use, here is a link:

http://www.dremel.com/html/products/accessories/routing.html

I use 652 and 654.


I actually used RDS connectors on my last RES project. It is a center spoiler (only 1). If you want some pictures of it, I can post them but it may not help you.

No pictures but this might help you with the linkage:

http://www.djaerotech.com/dj_askjd/dj_questions/controlhorns.html

One last item, you can always mock-up your servo, linkage and blade to see what works best. Some scrap balsa and tape can usually help to cobble something together that is reasonably close without too much effort. A blade control horn can be as simple as a small plywood triangle with a hole for the pushrod on the lower end.



good luck,

Kevin

[Weird Harold]

thanks Kevin.

Rich