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My design process begins with a 4 x 6" photograph which shows a very good side view, looking right down the wing. I enlarge the photograph in steps until it reaches the size that I'm planning to build, in the case of the Critical Mass, about four feet. I needed an extra step here as my best photograph pictured the airplane pointing to the right. Since I habitually draw plans with the nose pointing to the left, I had the final enlargement reversed so that it pointed to the left. The enlarged photo was a bit blurry, but not as bad as you might think. It was easy to copy onto drawing paper with acceptable accuracy. I also compare the proposed design to other airplanes I have experience with. Since the real Critical Mass is a modified Sea Fury, it was logical to consider modifying my NATs and Walker winning Sea Fury. My initial Critical Mass drawing copied many moments and areas from Sea Fury drawings. I wanted an airplane of similar size, considering the weight, wing loading and performance of the Sea Fury.
The detail design process begins with a calculated tail volume and a general layout showing the location and shape of the major components. My two Sea Fury's had removable wings and a bunch of noseweight. I could save some weight by eliminating the removable wing and using a heavier, and more powerful, modern engine to replace the Sea Fury's noseweight. Aerodynamics were reconsidered in the light of my having more experience in designing. I decided to use similar, but not exact, moments and areas. The Critical Mass has a longer fuselage than my Sea Furys, but the extra length is mostly visual, not structural, overhang. This overhang is the result of moving the vertical fin and rudder back. Moving the vertical tail surfaces back by slightly stretching the scale fuselage outlines dramatically improves the realism and general appearance of the semi-scale design. It also considerably eases the installation of a movable rudder with a decently long pushrod linkage. The location of the stab and its relationship to the wing is basically unchanged. Cross sections of the fuselages are about the same size.
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Next, I made a small cardboard version of my proposed semi-scale Critical Mass. This helped me to visualize how appropriately the wing and tail surfaces fit the appearance of the real airplane. It also allowed adjustment of moments and areas necessary for a competition stunt ship. I tried two different wing configurations in an attempt to convey the appearance of a "clipped" wing racer. "Clipped" wings looked awful compared to the much more elegant appearance of the original Sea Fury wing. In this case, "elegance" won out over "realism". I also tried several different stab/elevator combinations to see how the appearance varied with different combinations of aspect ratio, taper and span. While determining the appearance of the emphanage (stab/elevators), tail volume was maintained at the original calculated value. In the photo, you may be able to see how I enlarged the vertical fin and rudder. I tried 5%, 10% and 15% enlargements of the photographic image and decided on 15% as being the most attractive. I can't honestly say that I have ever experienced negative effects of side area, so this wasn't a large consideration. At this point, it looked like my semi-scale version would build into a competitive airplane.
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Next, I had to consider how to build it with attractive lines and a competitive weight. To finalize the design, I built a mockup, "half fuselage", on a 3/4" MDF base. The mockup was mostly pinned and taped together with a few small glue joints to facilitate easy disassembly. Disassembled parts were modified or used as templates. This step was to refine the shapes of the bulkheads, and visualize the wing/fuselage intersection, size of the radiator "cuffs", the unusual "jet style, low drag" shape of the wing fillet. It also clarified obvious structural problems and their solutions before beginning construction. The final prebuild steps were to make canopy and fuselage molds as necessary. There are no commercial canopies having the exact shape required, and by making my own, the quality and appearance of the final canopy was assured.
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By now it is apparent that my semi-scale airplanes don't simply begin by drawing pictures on a clean sheet of paper. The design process isn't difficult, but it is time consuming. It sorta depends on whether you think the result will justify the effort. This post also makes the point that there are no easy shortcuts to the construction of my designs. They are pretty and fly well but are altogether too hard to build. Video sets are necessary to duplicate these construction techniques. These video sets and laser cut parts exist for most of my airplanes. My plans are inadequate for building without the videos. They lack templates, rib profiles and construction details. For this reason, they are only available with video sets.
Al