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Old Oct 31, 2005, 09:14 PM
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SoarScale
United States, WI, Wind Lake
Joined Nov 2004
814 Posts
P61C Black Widow series (#3)

The picture here shows the top surface sheeting added. Since I only have a picture of the end result, it is probably worth explaining how I got to this stage.

Again, the 3” wide balsa sheets were pre-joined on a flat surface with wood glue and sanded for finish. The outline was then cut for the flaps and nacelle fitting at the TE. The center join curve was pre-cut prior to gluing to ensure an excellent fit between the two top surface sheeting halves. When happy with the fit, the sheeting was glued. Now, a point here about the glue and the method of gluing.

Firstly, you’ll remember that not only do I have the foam bed for the bottom surface (the surface the wing was constructed on), but I also have a foam bed that follows the top surface airfoil contour almost exactly. This is where the benefit, in my mind, of the foam bed technique really comes in handy.

The glue used to attach the top sheeting was Polyurethane. One version is guerilla glue, another is Elmers Probond Polyurethane Ultimate Glue. I used the later. The glue was applied to the tops of the ribs and spars of both center section halves. The sheeting was misted with water (the curing agent) and the top sheeting placed and taped in position. Then, a few sheets of wax paper were laid over the sheeting (just in case of glue seepage) and the foam cores laid on top of that and weighted down evenly. The structure was then left overnight to cure.

In my personal opinion this has a few advantages.

1). The sheeting can be applied in one piece giving an excellent final finish
2). The foam provides even pressure over the complete top surface and to some extent, allows the sheeting to conform directly to the smooth, accurate, airfoil contoured surface of the foam core. The polyurethane glue will fill any voids between sheeting and ribs also.
3). Since the sheeting is applied over slow curing glue, once the sheeting is applied, foam cores added over the top and weighted, the whole structure then cures to the exact dihedral and geometry you need (no warps, twists etc).

Now, this result can only be achieved by having accurate, no warp, no twist cores so core template design and cutting as well as core cutting itself needs to be accurate.

The only items left to do on the center section after sheeting was to cut out the small flaps, wrap some 1/16 balsa around the leading edges, check out the hinging mechanism I chose, hook up the control rods to the servo’s and flaps. And create the servo covers. All pretty simple stuff.
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