With a sufficient number of toothpicks in place (at left) to define the outline of the firewall, the motor assembly is ready to be removed. With enough toothpicks (above), it is possible to capture the outline of very intricate curves, even those of air vents. However, this would also mean blocking the airflow from the vents, so this area will remain open.
than actually completing it. As shown in the pictures, I attach the toothpicks on the aft side of the undersized firewall. In turn, this means the infinitely thin cross-section being defined will be the backside of the actual firewall, and the forward edges of the actual firewall will have a slightly smaller diame- ter to match the fuselage taper. After carefully removing the tape, spin- ner, and sliding the motor/mount/toothpick firewall out of the plane, it is a simple task to trace the firewall outline onto a piece of poster board for a more durable template. For the firewall itself, a variety of materials can be used; 1⁄8-inch 5-ply birch plywood, 1⁄8- inch carbon fiber end grain balsa laminate, or 1⁄8 or ¼-inch fiberglass end grain balsa laminate. The laminate materials yield the best ratio of strength to weight, but require the addition of hardwood dowels at the mounting points to avoid compression of the laminate.
I prefer to use the fiberglass end grain balsa laminate as it is less expensive, easier
to work with, cheaper (than carbon fiber), and sacrifices negligible strength in this ap- plication. Either 1⁄8 inch or ¼ inch thickness can be used. I prefer ¼ inch as it provides a little more gluing area and can be just as light with a greater number of lightening holes.
When working with laminate materials,
care must be taken to avoid delamination during cutting/drilling. Fine tooth saws, bits, and sharp drills (especially Forstner style bits) used with light pressure will min- imize potential for delamination. Drilling from both sides is another technique that will minimize delamination. With 3⁄8-inch dowels installed, CyA’ed in place, and drilled for the motor mount, the entire as- sembly is trial fit in the fuselage. To evalu- ate the fit, a couple of tools can be very help- ful. Using a standard flashlight or flexible wand flashlight aimed at the joint between the firewall and fuselage side can show high/low spots. A small adjustable dental type mirror can give a better sight path to
see mismatched areas. Scrap music wire with a 90-degree bend (or thereabout) can be used as a feeler gauge to find void spaces. While not ideal, void spaces of up to 1⁄8 inch can fairly easily be addressed when gluing the firewall (more on this later). With a good fit confirmed, 2-ounce cloth is laminated to each side of the firewall to lock in the dow- els. The glue used is not critical; any decent laminating resin or medium viscosity CyA will work.
The tail ring mounting structure can be quite minimal; small triangular tabs pro- truding from each side of the fuselage to- ward the mounting holes on the tail ring can suffice. My preference is to use a half former across the top of the fuselage. The half for- mer adds very little additional weight, but adds significant strength and is much less likely to get knocked loose during mainte- nance of the motor. 1⁄8-inch fiberglass end grain is plenty strong for this structure, and constructed using the same toothpick process.
Forstner style drill bits are used to cleanly cut holes (above left) in laminate materials. Drilling from each side reduces the likelihood of delamination. Various lights and mirrors (above right) make it easier to assess the accuracy of the fit of formers/firewalls and bent scraps of music wire can be used as feeler gauges to find gaps/voids within the structure of the model. Evaluating the fit of the
firewall is much easier with the use of backlighting (below left) from a small flexible pen light. Hardwood dowels (below center) inserted at mounting points of the firewall prevent crushing of the fiberglass end grain balsa laminate. The fitted firewall (below right) is laminated with 2-ounce cloth to capture the hardwood dowels.
FLYING MODELS
27
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