This tutorial is not the ONLY way to cut foam, it is how I do it to achieve the best cores possible with minimal risk of screwing up. I offer it for guidance only, hoping that you can a void some common pitfalls.

First of all, accurate cores demand that you have accurate templates. There are many different template styles available, including the template which is the full outline of the rib. I prefer the template style CompuFoil produces because it assures you that the root and tip template are parallel, (or at the washout that you told CompuFoil to give it). In contrast, the full rib type template must be VERY carefully aligned, root and tip or washout is introduced. Any downward slippage of the template, and you can kiss that core good bye. A full template with a base cannot slip down since it is resting on the work surface. Since the wire rides on the inside surface of the airfoil on the bottom template, you may be lead to believe that this will cause the airfoil to be too thin because of the path of foam that was lost due to the cutting action of the wire (called the kerf). This is why it is crucial that you ALWAYS cut the bottom surface first. And ALWAYS cut from leading edge to trailing edge as well. This will assure the best possible feathered trailing edge. The kerf does not appreciably affect the airfoil thickness because once the cut is complete, the top section of the foam will drop back into the bottom bed and close up this gap. The kerf does affect the accuracy towards the leading edge, however. The more vertical the cut becomes, the less the gap is filled in by the top portion dropping down, and hence the greater the amount of undercut at the leading edge you will experience. CompuFoil will adjust for this automatically on the printed templates with a mathematically corrected bottom template line. This correction compensates for the progressive undercut along the leading edge. The template that CompuFoil produces is actually two in one. Depending on whether you cut along the top or bottom solid lines will dictate which template you are cutting out.

My favorite choice for template material is a paper based phenolic board. This material is similar to Formica, but is much easier to work with. This material can be purchased through Tekoa and Aerospace Composite Technologies. My personal favorite is a 1/32 inch thick material from Aerospace. Produce two template images, either hand drawn or produced by your favorite software <G. Spray the back side with 3M spray 77 glue and glue them to the phenolic board. Cut the templates out using a jig saw and finish sand to the line, (I use a disk and drum sanders with 220 grit wet-or-dry for this). Finish off the airfoil edges with #0000 steel wool. This will put a nice polish on the edge and allow the wire to glide smoothly along its surface. Inspect this edge carefully for nicks. Any thing which can stop or slow the smooth running of the wire will produce a defect the length of the panel. The templates will be attached to the foam using rivets and tape. The rivets also assure proper alignment when you switch templates (top and bottom). Align the two templates on top of another, making sure that they are perfectly aligned. Then drill five 3/32" holes along the base, making sure the holes are far enough from the edges to let the skirt of the rivet head clear. (Rivets work well for this purpose because of the head). Prepare the rivets by sharpening the tips to a needle point as well. Print out a second set of templates and glue it to the back side of the phenolic. This will allow you to see the station lines when cutting both your left and right wing panels. Speaking of station lines, here is another thing to keep in mind, (something that all other airfoil software I've seen handles incorrectly). When you specify 9 stations on a 10" airfoil without sheeting, there is a line every inch from leading to trailing edge. When you define sheeting however, the CORE dimension, not the outside wing is divided up into ten sections. Why do this? Well, keep in mind that station lines are primarily for foam cutting purposes to help keep your bow speed at the root and tip in sync. On a sheeted wing, you're not interested in having the wire cross the trailing edge of the true airfoil. You want the wire to leave the CORE trailing edges at the same time. This is very evident if you have a fat section at the root and a thin section at the tip. The distance from the core t.e. to the airfoil t.e. is much greater for the thin airfoil than the thick, (because of the small angle that the top and bottom sheeting comes together at on the thin airfoil). If you followed airfoil spaced station lines instead of core spaced station lines, you would have cut past the trailing edge of the tip long before the root.

The first thing you need to do is cut the panels to the proper size and planform. Don't stop there however. Careful inspection of the panels will very often show that the surfaces are not flat. If used this way, you can very easily introduce warps into you wing. The way I handle this is to clap two pieces or wood vertically to a narrow bench. The end sticking up should have a notch in it for the bow wire. Hang the bow on these such that the wire is parallel to the bench surface. I adjust the wire to trim off about 1/8" of foam. Sprinkle a little micro balloon on the bench to make it easy to slide the foam blank. Slowly push the foam through the wire, surface planing off the thin piece of foam. Turn the blank over and do the same thing to the back side, (lower the wire another 1/8"). The blanks are now ready for use.

Wipe the hairs, (produced by the cutting of the foam), from the edges of the foam. These are tough for the wire to melt through and can build up on the wire - a major problem when you're trying to get the wire to travel smoothly. Use double sided sticky tape to adhere the foam blank to your surface. Position the bottom template on the edge of the foam blank and secure it with rivets. Now here is a very important tip. When you push the rivets into the foam spin them at the same time between your fingers. This will keep the foam from bunching up at the tip of the rivet and will give you a much tighter hold. When you cut your core, remember, always cut from the leading edge to trailing edge. After you have cut the bottom surface, VERY carefully wipe the hairs from both surfaces. At all times when wiping the hairs, don't let the hairs ball up. If you wipe a ball of this stuff across the surface, it will roll a trough into the surface. Now let me get into another aspect of foam cutting which will set the reason for the next step. When you cut cores, the wire is cooler at the panel edges than in the interior of the cut. You can see this in the core bed because the foam is right up at the same level as the template near the template, but about an inch away, it is actually a little lower than the template surface because the wire is hotter and has melted a little more foam (i.e. the kerf is wider, leading to overmelt). Take a long sanding bar and flatten the edges of both the core and the core bed. Be VERY careful when sanding the core! If this raised edge is not taken care of, when you go to cut the top surface with the top half pressed into the bed, you will be introducing a bend. Replace the bottom template with the top template. Notice how the rivets perfectly align the top template in relation to the replaced bottom template. Replace the top cut away foam into the bed and add weights to keep it positioned firmly in place. Cut the top surface from leading edge to trailing edge. Remove the hairs and flatten the edges in the same manner as the bottom surface. This completes the core!

What you want to do is pull down just enough vacuum to hold all of the layers together. Pull the edges of the bag to remove any wrinkles. Be careful not to squeeze any dings into your foam here. Transfer the bag to the lower bed (make sure the bed is on a flat surface!). Position it so that the leading edge of the core is snugged up to the leading edge of the bed. I like to use a couple of squares of double sided sticky tape on the bottom bed to keep the bag from sliding around after setting it. You don't need much, a couple of 1" squares will do it. Put the top bed in place, (also with sticky tape). For weight, I first lay some boards across the surface. I have several 7/8" x 10" x 3' long particle boards. I use particle boards, because they are very heavy, and flat as well. I then usually put a little extra weight on top of this, such as full gallon paint cans. You don't need a whole lot of weight! At this point release the vacuum. Why do this? You want the core to relax into the bed, just in case it had any twist or bend to it when you pulled the initial vacuum. Now give it the full vacuum and let the epoxy cure. You can take it out of the bag after a day if the wing was kept at room temperature. (At the next point, I consistently break my own rule). Let the epoxy cure for several days before removing the mylars if you are producing glass wings. The epoxy is still fairly soft, even after two days and your wing is very prone to finger dings. Like I said, I have the patience of a two year old when it comes to wanting to run my hand over my shiny, freshly bagged wing, but you'll be better off if you can wait longer than me.

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