Wing tip master model process.

The Robin has a number of compound contour fiberglass parts. All of these parts are available for purchase, and will save a great deal of time when they are, however, in keeping with the spirit of die hard homebuilding, the Robin plans include all of the contour templates to build the parts. I have used this master model technique for many years. This process involves the use of 1lb/cu ft green urethane floral foam. This foam is readily available at Hobby Lobby and Michael’s craft stores. I always watch for sales at these places and load up when ever the price is right. I have actually built a full size fuselage master model using hundreds of the foam blocks. I will never forget the night I bought out the complete inventory of the Michael’s store in Wichita Kansas. I showed up at the checkout stand with 6 shopping carts full of foam. The lady at the register eyed me suspiciously; I knew what she was thinking, so I confirmed her doubt. I bent my wrist, struck a pose and in my best lisp I told her, “I have a HUGE!!! Wedding” The all knowing nod of her head said it all!!!

The basic principal of this process is exactly the same as the process amateur telescope builders use to make their optical glass lenses; they use the glass to abrade the glass. Since both materials abrade each other as the same rate, a spherical interface is formed. In my molding process, the foam is precut to the approximate shape and sections of the blocks are hot glued together. Care must be taken when doing this to ensure that the glue line will be beneath the eventual final contour. Its also important not to mix blocks of a different manufacturer for a given mold. All foams are not created equal and when the final shaping occurs the foam sanding block will not abrade the mold foam equally.

The following is a series of photos that illustrate the process used to make the wing tip master model. At the end of this process, I complete the master model by filling and fairing using automotive finishing processes. For the one time builder, the final step would be to seal the foam surface. I use spray paint of any type, sand able primer is acceptable. This surface is waxed with up to 5 coats of carnauba wax (NEVER, NEVER, NEVER, use a silicon based wax!!!!)  The outer surface of the foam is then laid up with a layer of fiber glass. The resin is allowed to cure and the surface is filled with either lightweight automotive Bondo or a slurry mixture of epoxy and micro balloon . The skin is then released. Building a part in this manner is slightly heavier than molding a part, but structurally they are the same. 

The process starts by building full size templates of the contour sections and the tip rib. There are two ways to do this, full size laser cut cardboard templates can be ordered, or the plans can be scaled up on a copier. The section stations drawing, have grid lines that are used to check for the exact scale factor when copied. Glue all of the paper templates to a piece of sheet aluminum or thin steel sheet. A word about adhesives here is important, do not use a water based adhesive, the preferred adhesive is 3M 777 spray. If you use a water based adhesive, the paper will swell and the contour lines will be distorted.  The reason I use metal templates is to facilitate cutting the foam on a band saw. I use a 24 tooth metal blade with minimal kerf. I guide the back side of the blade on the template when I cut the foam. All of the templates will have at least 3 #30 holes drilled into them so a round toothpick can he used to hold the foam to the template. Experiment with the correct drill size prior to drilling the templates to get a tight fit to the particular brand of your tooth picks.



using the tip rib as a sanding guide.

 All of the foam blocks are precut on the band saw to exactly 3 inches of depth. In some cases there will not be enough height of the block for a particular template. In these instances, the blocks must be spliced. Use a minimum of hot glue to do this and try to visualize where the bond line will be in the final contour. It’s very important that the foam splice glue not be near the final contour.

The base rib in the case of the wing tip master is a Styrofoam (extruded foam only) rib. Since this is the point where the contour begins the transition to a constant section, there is no need to shape this surface. It is used as a guide to sand the much softer floral foam.



using the base rib as a sanding guide

 once the plan view contour is sanded into the foam, the corners of the foam blocks are cut off with a sharp butchers knife of a hand held hack saw blade. Cut the blocks back to within a 1/2 inch of the final contour. Do not over cut. Repairs are difficult.






at this pont in the process, the magic happens!!! blocks of scrap foam are used to sand the foam. 

a block of scrap foam is used to sand the foam

Since the contour varies on the part, choose different blocks for the different areas of contour. Just a few swipes is all that is needed to bring the contour into shape.







this shows how the sanding block is starting to take on
 the contour of the mold.

Larger blocks are used for the less radical contours; smaller blocks are used near the trailing edge where the contour becomes more severe












final shape of the master

Dont worry about the gaps between the blocks, they will be bridged in the next step, which is to drape the foam with a layer of fiberglass cloth. In the case of the wing tip, the final lay up is only 1 ply of 8 oz BID cloth. There is no need for a heavier laminate because the tip cap bonds to a full rib and the contour of the tip cap gives it stiffness. remember, this plane is only flying at 63 MPH!!!

This is  the final finish for the master because I was making a full mold for this part. As an alternate to making a tool, the one time builder would be finished at this point.

This is what the final tip looks like after it is molded

3.8 OZ!!!!!