Designing big birds

Recently, a few flying bud- dies and I were sitting in front of a wood-burning stove talking about the weather and, of course, model airplanes. Colder outside temperatures signal the traditional Northeast modelers' migration back into our workshops, so we discussed ways to design a first-time, giant-size model. Ideas on airfoils, engine sizes and power-toweight ratios were thrown around, but the most interesting idea we came up with was a basic guide that almost anyone could use to start the design process and to get the creative juices flowing. Sure, you could simply enlarge a proven model design until you have a giant version, but if you want to start with a blank sheet of paper, it will take a little more effort.

What better way to welcome the building season than with some design notes and illustrations to help you get started?

DESIGN 101

First, I didn't just pull these figures out of the air. Refer to Figure 1, and you'll see the basic dimensions for a well-balanced, constant-chord, high-wing monoplane. Hey! It's intended as a first-time endeavor, so let's keep it simple. I researched several basic model aerodynamic-design resources and settled on the information supplied by several model designers. Remember that these are just starting points and not forged-in-steel absolutes.

The diagram shows the basic wingspan, chord and tail-- moment information needed to establish the foundation of your design. Engine size and airfoil choices can be determined later. The fun part is sketching in the shapes and details that you like after you've made a few simple calculations. You must draw two basic sketches before you can develop your own construction plan-the wing-plan view and the fuselage side view.

THE WING

A rectangular, constant-chord wing is the easiest to design and build because the ribs are identical and the airfoil shape is constant from root to tip. The mean aerodynamic chord (MAC) needed to determine the position of the balance point is also constant and requires no further calculation; the wing chord and the MAC are the same.

The aspect ratio (AR) is the relationship between the wingspan and the wing chord. The AR affects the model's induced drag, or the amount of drag caused by the wing when it develops lift. For a given wing area, the more you increase the AR, the lower its induced drag becomes. This is why gliders have such long and slender wings. To determine the AR, all you have to do is square the wingspan and divide that number by the wing area.

Because longer, slimmer wings are subjected to higher twisting and bending forces, you can't just make your wing panels as long as you want. Figure 1 shows an AR range of between 5 and 7. This gives a reasonably wide choice of wing planforms. Given a 90-inch wingspan as your starting point, an AR of 5 would give you an 18-inch chord and 1,620 square inches of area. An AR of 7, however, produces a 12.75-inch chord and an area of 1,170 square inches. Do you want a model that looks like a clipped-wing Cub (low AR) or one that looks like a Storch or a Pilatus Porter (high AR)?

Ailerons are shown with dimensions that are percentages of the semi-span area and wing chord. A strip aileron is shown the full span of the wing panel and with a width that's 15 percent of the wing chord. For a barn-door aileron, use 40 percent of the semi-- span for the length and 25 percent of the wing chord for its width. Pretty easy!

When it comes to airfoils, we could start an entirely new column on the subject. But for the sake of argument, there are three basic choices: a flat-bottom airfoil for a trainer, a semisymmetrical one for sport flyers and sport-scale airplanes and a fully symmetri- cal airfoil for aerobats (see Figure 2). For a first giant flyer, let's stick with a semisymmetrical one.

FUSELAGE AND TAILPLANE

Once you have established the chord length of your wing, you can determine the overall length of the fuselage and its various moments. Draw your airfoil on the plan, and mark its 1/4 MAC location, or balance point. The nose moment, or the distance of the spinner's backplate from the 1/4 MAC, should be 1.5 to 2 times the chord. With a wing chord of 18 inches, the distance should be between 27 and 36 inches. The tail moment is the distance between the wing's 1/4 MAC and the tail plane's 1/4-MAC location; it should be between 2.5 and 3 times the wing chord. Again, with an 18-inch wing chord, the tail moment would be between 45 and 54 inches.

In the top view, the tailplane (horizontal stabilizer and elevator) is shown like a smaller wing with parallel leading and trailing edges. This is to simplify our diagram. Tapered surfaces can also be used, but you will have to calculate its MAC and its 25-percent location. The tailplane area should be between 18 and 22 percent of the wing's area, and its AR should be between 3 and 5. The elevator area should be approximately 35 percent of the total tailplane area.

The rest of the fuselage details are then developed around the thrust reference line location and the placement of the landing gear. A good place to start is to draw the reference line about 1/3 of the wing chord below the wing. Place your center of gravity (CG) on the reference line directly below the 1/4 MAC. For simplicity, I show a tail-dragger, so the main gear axle should be at roughly 20 degrees to the CG and at a sufficient distance below the reference line to provide adequate propeller and ground clearance. The gear stance (distance between the main wheels) should be about 1/4 of the total wingspan.