Model Aircraft Design

Many years ago I read a series of articles in Radio Control Modeler Magazine by Chuck Cunningham. These inspired me along with brother into a life long, very rewarding hobby of building and designing model airplanes. We both couldn’t believe that with just a few simple formulas that we could design and build our own models. I’ve kept that series of articles through the years and still follow those simple principles in designing model airplanes today.

Lets talk about a basic design for a model airplane. When one first approaches a conceptual design you need to think about a few questions about how you want your model to fly. Will the model be a trainer, sport, intermediate, or any number of combinations? How large of a model do you want to design and build? What kind of power will you be using? Will the model be gas or electric? I’ll use very standard assumptions to arrive at a model that will, if built and balanced properly, fly with almost no trim changes on the first flight - nothing but basic design.

Lets start with a basic gas powered model with a 60” wingspan and a 12” cord, powered by a .60 cu. in. 2-stroke engine. The numbers will be almost the same for an electric powered plane or one with a smaller or larger engine and wingspan. So, dig out your calculator and follow along.

Wing Area and Aspect Ratio

Wing area is nothing more than the length of the wing X the cord. This will be a constant cord wing 60” long by 12” wide or 60”X12”= 720 sq. in. Next, the Aspect Ratio or the wingspan squared, divided by area of the wing (AR= B²/S) will give a basic idea of the flying characteristics of the model. Higher or lower will determine it the model is a floater or a brick. It also helps determine the power required in order for the model to fly. Using the formula and the values so far:

B²/S=AR or 3600/720=5 or an aspect ratio or 5 to 1 (5:1)

Most sport models have an aspect ratio between 4:1 and 7:1. Below 4:1 and you become a NASA test pilot and above 7:1 results in a glider type model. Using the chart in figure 1 an aspect ratio of 5:1 results in a model with good overall handling and glide ratio. So, with the above assumptions, a wingspan of 60” with a cord of 12” our total wing area is 720 sq. in. and the aspect ratio is 5:1. With these in mind we’ll continue to the basic fuselage and use the assumptions to arrive at the basic overall fuselage dimensions.

 Figure1

Basic Fuselage Design

With our assumptions from above we’re ready to layout the basic fuselage design. To keep this as simple as possible in designing a simple sport model airplane, we have established the wingspan and the cord of the model. We will assume that the fuselage will be 75% of the wingspan of the model and our formula will be 75% of 60” or .75 X 60=45, thus our overall fuselage length will be 45”. If we look at the side view of our model we know that the fuselage is basically two different components, the nose and the tail with the wing somewhere in the middle. In our example we’ll use a nose length of 20%, 11” or the distance from the back edge of the prop to the leading edge of the wing. At this point we’re not going to worry about the C/G, as we will discuss this later in the design of the model. For the tail moment we will double the nose moment or 40%, 18”. This length is the distance from the wing trailing edge to the leading edge of the horizontal stab. Yes, I know that to be pure in design the length should be from the back of the prop to the C/G of the wing and the tail moment should be from the Wing C/G to the Tail C/G. This would involve more Calculations to arrive at the desired results. I’m just trying to Keep It Simple.

Horizontal Stabilizer

Over the many years I’ve assumed that the Horizontal Stabilizer to be in a range from 20% to 30% of the area of the wing. I generally use 22 to 23% in my designs. Please note that Deltas and flying wings are different designs and require different considerations. With our assumptions from above we’ll use 22% of the wing area. So, 22% of 720 equate to roughly 158 sq. in., and we will assume it to be a span of 3 times the cord or 3C. We’ll round these numbers off and use just a little math, thus our cord will be 158/3=52 and the square root of 52 equals our cord of 7”. The span of the stabilize will be 3C or 7” X C = 21”. Our Stabilizer now has the diminutions of 21” X 7”.

The Vertical Fin

Again over the year I’ve assumed the Vertical fin to be in a range of about 1/3 the the area of the horizontal Stab. I generally assume this to be from the top of the horizontal stab to the top of the vertical fin. So, again with just a little math we can arrive at some basic designs. The horizontal stab has an area of 158 sq. in. So, 1/3 of 158 equate to roughly an area of 52 sq. In. Using the square root or 52 we arrive at a Vertical Fin height of 7”and a cord of roughly 7.25”. Kind of an ugly looking airplane, so just adjust the height and the cord to arrive at a set of figures that will keep approximately the same area for the vertical fin. One could add a dorsal fin to increase the area and lower the height and width of the fin profile.

In the next article we’ll look at the rest of the design dimensions, as there are still other assumptions that need to be considered. So far we have our basic design of a wingspan of 60” and a cord of 12”. Overall the fuselage length is 45” and the nose moment is 11” while the tail moment is18”. The horizontal stabilizer is 21” X 7” and the vertical stabilizer is 7” high. We still need to consider the area of the elevators, ailerons, and rudder. Also, the thrust lines along with the incidence and of course the Center of Gravity. Later I’ll discuss how to lay out the design in CAD and design a good flying simple model airplane.