Model Airplane Design Part 2

       Let’s look at the rest of the dimensions for our new design, as there are still other assumptions that need to be considered. So far we have our basic design of a model with 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 is 18”. 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.

Adding Ailerons to your design

     We’ll start with the wing and add some ailerons to our design. There are basically two types of ailerons we’ll consider, barn door and strip ailerons. Both have advantages and disadvantages. Typical barn door ailerons just look Cool, while strip ailerons are very easy to install and hook up. Barn door ailerons require more setup using bell cranks and long control rods to actuate the control surface. While strip ailerons only require short control rods connected directly from the servo to the aileron. This is not to say that one could use separate servos to actuate either barn door or strip ailerons. The choice is really up to the designer builder. For simplicity my choice is usually strip ailerons.

The formula for aileron dimensions follows:

       Strip ailerons are 1/8 the width of the cord of the wing, or in our example 12”/8=1.5”. For a barn door aileron the formula is just a little different. Barn Door ailerons are ¼ the span and ¼ the cord so the length of our aileron would be 60”/4=15” and the width would be 12”/4=3” resulting in an aileron 15” X 3”. Not much to aileron dimensions and a very simple formula. Of course the designer can change any of these dimensions to obtain the desired performance of the model. So, both strip and barn door ailerons achieve the desired results with pros and cons between the two styles.

       Moving back to the Horizontal Stabilizer and elevator the formula is again very simple. We already know that the stabilizer is between 20 to 22 percent of the wing area. The formula for the elevator is 20% of the area of the stabilizer or in our design 147/5=29.4 sq. in. Or 21” long. Our simple formula is 29.4 /21”= 1.4”. I”ve rounded off to the next higher number just to keep is as simple as possible. The overall dimensions of elevator are 21” X 1.4”. Again you can adjust as needed to obtain the desired performance of the model.

Model Airplane Design Part 2

       The Vertical Stabilizer and rudder are a little different and require a new formula. The Vertical fin should be 1/3 the area of the horizontal stabilizer and the rudder 1/3 to 1/2 the area of the vertical fin.  Again, these are very simple formulas to achieve using a small calculator.

       So far all of the basic components of the design are finished and all you have to do now is consider the dihedral, wing and stab incidence, C/G, engine thrust lines, and landing gear placement. In our next article we’ll explain more on these details and conclude with a chart with all the formulas.