Model Airplane Design


Initial steps on preparing an original model airplane design


Top view of the Robin four channel RC model airplane design

Top view of the Robin four channel RC model airplane design

Creating an original model airplane design is a challenging and rewarding aspect of the RC flying hobby. Drafting a model aircraft plan is really not difficult. The task employs a series of steps and skills that can be acquired by anyone.

Following are some initial thoughts and considerations on planning a model design. These are by no means final statements. Rather, they are starting points from which you can evolve and mature your particular model airplane design.

Robin prototype (blue fuselage) and final version

Robin prototype (blue fuselage) and final version

The first step in laying out a model airplane design is to determine what type of airplane you wish to build. Will it be a trainer, a sport flyer or some sort of scale project? Will your subject be a high, mid or low wing aircraft?

Model size

Once you know what type of plane you wish to design the next step is figuring out the size of the model. I usually start with determining the wingspan of the model airplane design, as the wingspan will be used as the basis for further model dimensions.

The wingspan can also give you an initial idea of the size of the motor for your finished project. For gas powered model airplane designs, the size of the engine is important, as you will have to design and build in the necessary fuselage support configuration. There are less stringent requirements on electric powered model motor structures due to the smoother operation of electric power as compared to a gas engine.

Once you have determined the wingspan, there are several rules of thumb for fuselage and tail surface model airplane design dimensions.

You next need to determine the wing chord, or distance between the wing leading and trailing edge. For normal sport flying aircraft, the aspect ratio (wingspan squared divided by the wing area) should be no less than 5:1.

The aspect ratio for a constant chord wing is simply the wingspan divided by the chord. I always subscribe to the theory of keeping things simple when starting out on new projects of this nature.

Thus for your initial model aircraft design projects use a constant chord wing. A constant chord wing, using a single rib size, is easier to design and build as compared to a tapered wing.

Robin fuselage CAD plan

Robin fuselage CAD plan

You will have to choose an airfoil. There are a wide variety of airfoils you can select from and they vary with the type of aircraft being contemplated.

An aerobatic model would use a symmetrical airfoil shape while an antique airplane plan would use a higher camber rib shape to account for the slow flight speeds characteristic of these aircraft.

Clark Y airfoil

Clark Y airfoil

My airfoil preference is a standard Clark Y, designed in the 1920s for full scale aircraft. The Clark Y works well with everyday sport RC model airplanes. I have used the Clark Y on all my RC aircraft design plans.

Fuselage length

You will next determine the length of the fuselage. A good rule of thumb is for the fuselage length to be around 75% of the wingspan. The nose length (distance from wing leading edge to the prop) should be around 20% of the fuselage length.

The distance from the wing trailing edge to the leading edge of the horizontal stabilizer should be around 40% of the fuselage length. The horizontal tail surface area should be around 25-30% of the wing area, and the vertical tail around 35% of the stabilizer area.

Yard Ace prototype top view

Yard Ace prototype – wing span was too short

Note that all these model airplane design planning parameters are just starting points for your final model airplane plan. I am a firm believer in the benefits of the TLAR (That Looks About Right) approach to model aircraft design and layout.

If in doubt as to your initial aircraft dimensions, consider adding a bit to the nose or tail length, or increasing the wing span. You can quickly get into trouble on your first flight if the tail or wing surfaces are too small.

Final version of Yard Ace RC plane

Final version of Yard Ace with 4 inches added to the wing span

You will almost always be alright if these surface areas are a bit too large or the moments too long. For example, my first version of the Yard Ace had a wing span that was too short. The model flew much faster than anticipated to maintain flight. On the updated version of the Yard Ace, I simply lengthened the wing span – very easy to do when drawing a model plan with TurboCAD – and the airplane flew much better with the increase in wing area.

The center of gravity should be located a third of the wing chord back from the wing’s leading edge. I always add a bit of positive wing incidence to my model airplane designs.

As a final note, make every effort to design your model airplane as light as possible. One of the huge advantages of electric model airplane flight is the ability to keep motor mount and overall airframe structure to an absolute minimum, due to the lack of vibration as compared with a conventional gas engine.

Model aircraft that are built at a minimum weight always fly better than the same model airplane design completed with a heavier weight.

Nose detail of the Blackburn RC model plane

Nose detail of the Blackburn RC model plane

Author: Gordon McKay