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Design an RC Model Airplane – Parameters

Initial steps for creating an original RC model airplane design

Introduction | Parameters | Initial Plan | Prototype | Test Flights | Final Plan | Buy Robin Plan

 

Robin electric RC model airplane

Robin electric RC model airplane prototype


Creating your own model airplane design is not difficult at all. In this series of discussions, I will go from a clean sheet of paper to a prototype model airplane design. I will then build and fly this airplane to verify initial design parameters.

Lessons learned from these test flights lead to changes for the final version of the design. I will then prepare a final set of TurboCAD plans and build the finished aircraft.

You will see from start to finish how to conceive, design, draw a set of plans, build, test and fly a model airplane design of your own. So, let’s get started!

4-Site RC electronics and motor

4-Site motor and electronics

The first step in designing a model airplane is to determine what size and type of model you wish to have. This will be tied in to the type of flying you will be doing, combined with the radio gear and electric motor that will be employed.

Micro electronics package

For this model design series I aim to produce a plan that will use the electronics and motor from the E-flite Ultra Micro 4-Site acrobatic biplane.

I am impressed with the small size and light weight of the 4-Site’s electronics, as well as the geared engine’s power. These components should do well for any micro flyer.

E-Flite 4-Site biplane

E-flite Ultra Micro 4-Site will be a design starting point for the Robin

The next step is to settle on the aircraft parameters. You will have to determine such dimensions as the wingspan, fuselage length and tail surface areas.

The parameters of the 4-Site are a good starting point for a new design. In concept, I will make my model with a lightweight profile fuselage with a flat wing shape.

I plan on balsa for the entire aircraft construction with a lightweight iron-on covering. These design and construction techniques should yield the lightest possible model airplane weight.

 

Now that we have a basic understanding of our new model plane layout, the next step is to determine the aircraft parameters. The 4-Site biplane flies well with a 15 inch wingspan. For this new plane, which I am calling the Robin, I’d like slightly more relaxed slow flight characteristics. I will start with a wingspan of 20 inches, and to keep things simple, I will use a constant chord wing.

model airplane design parameters

Model airplane design parameters

Once the wingspan is chosen, the next step is to figure out the wing chord (width of the wing). A good rule of thumb is that the aspect ratio, which for a constant chord wing is the wingspan divided by the chord, should be at least 5:1. This means the chord should be no more than 4 inches for a 20 inch wingspan.

4-Site motor

The 4-Site's motor mount works well in the Robin

Wing aspect ratio

I used a 5 inch wing chord for the Robin, which is a 4:1 aspect ratio (20 divided by 5). For lightweight indoor flyers with a flat airfoil section, the wing’s surface area gains more importance for good flight handling.

Thus, for the Robin, it should be acceptable to have an aspect ratio less than 5:1. Were the Robin planned for a larger aircraft design, I would obtain the 5:1 aspect ratio by increasing the wing span to 25 inches.

The next design parameter is the fuselage length. A fuselage length of 75 percent of the wingspan is a good starting point. For the Robin, this will mean a fuselage 15 inches long.

I now need to determine the length of the nose, measured from the wing leading edge. A nose distance of 20 percent of the fuselage length, or for the Robin 3 inches, should work. For parameters such as fuselage length or nose moment, it is acceptable to make the dimensions slightly larger if needed for aesthetic or equipment placement purposes.

 

Robin wing and tail

Robin's wing (without ailerons) and stabilizer (without elevator)

I need to ensure the nose is big enough to fit the 4-Site’s electric motor. Thinking ahead, I will also need the engine far enough forward to allow for the flight battery placement. Finally, I need to ensure the Robin balances properly at the center of gravity (CG).

A tail heavy model is always a worry. The optimum way to counter a tail heavy model would be a bit longer nose to allow the motor a more effective CG balance arm.

Final parameters

The final dimensions that need to be determined include the wing trailing edge to stabilizer distance, followed by horizontal and vertical tail surface areas.

The stabilizer typically would be behind the wing trailing edge 40 percent of the fuselage length, or 6 inches. For the Robin I will start with 5 inches and counter with slightly larger tail surface areas.

The wing surface area is 100 square inches. I plan on a little more than 30 percent of the wing area for the stabilizer and elevator at 33 square inches, or 33 percent. More tail surface area usually translates to a more stable model airplane.

The same approach goes for the vertical tail area. I will include more than 35 percent of the stabilizer area for stability, and use around 18 square inches for the fin and rudder.

I now have a set of prototype design parameters for the Robin. I have an electronics and power package selected with a concept for construction. Hopefully, the final weight will come out at around two ounces. The only way to determine this is to build a prototype, and this will be discussed in the next section.

Author: Gordon McKay

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