Model Airplane Design Made Easy

Many years ago I read a series of articles in Radioview of our model we know that the fuselage is
Control Modeler Magazine by Chuck Cunningham.basically two different components, the nose and the
These inspired me along with brother into a life long,tail with the wing somewhere in the middle. In our
very rewarding hobby of building and designing modelexample we'll use a nose length of 20%, 11" or the
airplanes. We both couldn't believe that with just a fewdistance from the back edge of the prop to the
simple formulas that we could design and build ourleading edge of the wing. At this point we're not going
own models. I've kept that series of articles throughto worry about the C/G, as we will discuss this later in
the years and still follow those simple principles inthe design of the model. For the tail moment we will
designing model airplanes today.double the nose moment or 40%, 18". This length is the
Lets talk about a basic design for a model airplane.distance from the wing trailing edge to the leading
When one first approaches a conceptual design youedge of the horizontal stab. Yes, I know that to be
need to think about a few questions about how youpure in design the length should be from the back of
want your model to fly. Will the model be a trainer,the prop to the C/G of the wing and the tail moment
sport, intermediate, or any number of combinations?should be from the Wing C/G to the Tail C/G. This
How large of a model do you want to design andwould involve more Calculations to arrive at the
build? What kind of power will you be using? Will thedesired results. I'm just trying to Keep It Simple.
model be gas or electric? I'll use very standardHorizontal Stabilizer
assumptions to arrive at a model that will, if built andOver the many years I've assumed that the Horizontal
balanced properly, fly with almost no trim changes onStabilizer to be in a range from 20% to 30% of the
the first flight - nothing but basic design.area of the wing. I generally use 22 to 23% in my
Lets start with a basic gas powered model with a 60"designs. Please note that Deltas and flying wings are
wingspan and a 12" cord, powered by a .60 cu. in.different designs and require different considerations.
2-stroke engine. The numbers will be almost the sameWith our assumptions from above we'll use 22% of
for an electric powered plane or one with a smaller orthe wing area. So, 22% of 720 equate to roughly 158
larger engine and wingspan. So, dig out your calculatorsq. in., and we will assume it to be a span of 3 times
and follow along.the cord or 3C. We'll round these numbers off and use
Wing Area and Aspect Ratiojust a little math, thus our cord will be 158/3=52 and the
Wing area is nothing more than the length of the wingsquare root of 52 equals our cord of 7". The span of
X the cord. This will be a constant cord wing 60" longthe stabilize will be 3C or 7" X C = 21". Our Stabilizer
by 12" wide or 60"X12"= 720 sq. in. Next, the Aspectnow has the diminutions of 21" X 7".
Ratio or the wingspan squared, divided by area of theThe Vertical Fin
wing (AR= B2/S) will give a basic idea of the flyingAgain over the year I've assumed the Vertical fin to
characteristics of the model. Higher or lower willbe in a range of about 1/3 the the area of the
determine it the model is a floater or a brick. It alsohorizontal Stab. I generally assume this to be from the
helps determine the power required in order for thetop of the horizontal stab to the top of the vertical fin.
model to fly. Using the formula and the values so far:So, again with just a little math we can arrive at some
B2/S=AR or 3600/720=5 or an aspect ratio or 5 to 1basic designs. The horizontal stab has an area of 158
(5:1)sq. in. So, 1/3 of 158 equate to roughly an area of 52
Most sport models have an aspect ratio between 4:1sq. In. Using the square root or 52 we arrive at a
and 7:1. Below 4:1 and you become a NASA test pilotVertical Fin height of 7"and a cord of roughly 7.25".
and above 7:1 results in a glider type model. Using theKind of an ugly looking airplane, so just adjust the
chart in figure 1 an aspect ratio of 5:1 results in a modelheight and the cord to arrive at a set of figures that
with good overall handling and glide ratio. So, with thewill keep approximately the same area for the vertical
above assumptions, a wingspan of 60" with a cord offin. One could add a dorsal fin to increase the area
12" our total wing area is 720 sq. in. and the aspectand lower the height and width of the fin profile.
ratio is 5:1. With these in mind we'll continue to the basicIn the next article we'll look at the rest of the design
fuselage and use the assumptions to arrive at thedimensions, as there are still other assumptions that
basic overall fuselage dimensions.need to be considered. So far we have our basic
Basic Fuselage Designdesign of a wingspan of 60" and a cord of 12". Overall
With our assumptions from above we're ready tothe fuselage length is 45" and the nose moment is 11"
layout the basic fuselage design. To keep this aswhile the tail moment is18". The horizontal stabilizer is
simple as possible in designing a simple sport model21" X 7" and the vertical stabilizer is 7" high. We still
airplane, we have established the wingspan and theneed to consider the area of the elevators, ailerons,
cord of the model. We will assume that the fuselageand rudder. Also, the thrust lines along with the
will be 75% of the wingspan of the model and ourincidence and of course the Center of Gravity. Later I'll
formula will be 75% of 60" or .75 X 60=45, thus ourdiscuss how to lay out the design in CAD and design a
overall fuselage length will be 45". If we look at the sidegood flying simple model airplane.