Building a Prototype of Your Model Plane Design
Add structure to your prototype model plane design as you build
Introduction | Parameters | Initial Plan | Prototype | Test Flights | Final Plan | Buy Robin Plan
Now that I have the outline of the Robin RC model airplane design complete (see the initial plan section), it is time to start actually building the prototype. For this project, I am using the minimum amount of plan detail to construct the first version of the Robin. Depending on your building skills and the amount of complex structure in the finished model, this prototype build approach can work very well. If, however, you are attempting a more challenging model plane design, or you just like to have a bit more information on your construction drawings, you will need to add the appropriate building elements to your initial plan.
Completed wing with internal 1/8 inch balsa structure
An uncomplicated design
The Robin prototype is an uncomplicated design, using 1/8 inch square balsa for the primary aircraft structure. I will start with the wing. The first step is to make the wing outline with 1/8 inch square balsa. Construct the wing directly over the full size plan. Build the entire 20 inch wing at once – do not make two halves. The complete and covered wing will later be inserted and glued into the fuselage’s wing brace section. Be sure to cover the work area with plastic to avoid gluing the balsa to the paper plans.
Complete balsa components for the Robin
Pin 1/8 inch balsa for the wing’s leading edge and two sides. Add the 1/8 inch square balsa for the wing’s trailing edge as well as the leading and trailing edge of the ailerons. I added internal vertical and diagonal 1/8 inch balsa braces to the wing as I saw fit. I was seeking a balance between providing strength to the wing, yet keeping to a minimum weight.
I made an estimate as to where the two aileron servos would be placed, and added 1/8 inch soft balsa mounting plates. I also added some balsa sheet reinforcement to where I think the control horns would be installed.
Bevel control surface edges
When the glue is dry, remove the wing from your building board and cut the ailerons from the wing. Sand a 45 degree bevel to the wing trailing edge and the two aileron leading edges to allow for control surface movement. Don’t be too aggressive sanding this bevel, as you want to maintain the strength of the balsa as you later apply the covering. Repeat these same building actions to construct the stabilizer and elevators.
Detail of the Robin's electric motor mount and control electronics. Note carbon fiber rod in wing leading edge, which is commonly used with foam materials.
The Robin’s flat profile fuselage goes together in much the same manner as the wings and tail surfaces. Start by making the fuselage and vertical tail surfaces outline from 1/8 inch square balsa. Pin these parts to the full size plan and glue in place. Be aware that when we design a plane, the prototypes can come out tail heavy if we are not careful. To help keep the model balanced, select lightweight balsa wood for the aft fuselage and tail surfaces. Use a bit denser (heavier) balsa for areas that will support either flight loads or equipment such as the motor.
View of Robin's underside
Once the fuselage 1/8 inch square balsa outline is in place, add 1/8 inch sheet balsa for the wing and stabilizer support. For a light weight profile RC model, I decided to use a zero wing incidence. I added a 1.5 inch 1/8 inch balsa sheet for the motor mount, as well as a fuselage provision over the middle of the wing for securing the electronic speed control and servo brick.
Add internal balsa structure
Once these sheet balsa inserts are in place, add the 1/8 inch square balsa vertical and diagonal members to fill out and brace the fuselage and tail. Remember that you are “designing this prototype model plane” as you are building it, so always think ahead to what is required for the finished aircraft. For example, I need to add the vertical 1/8 inch square balsa members to fully support the wing and stabilizer mounts. Also, I will have to include a cutout in the rudder for the balsa joiner that connects the right and left elevator halves.
Electric motor mounting detail
When the fuselage is complete, remove from the building board and separate the rudder from the fin. Bevel the fin trailing edge and the leading edge of the rudder for control surface throw as was done for the ailerons and elevator.
I did not do much sanding on the Robin indoor aircraft prototype. I did take a look at the wing, and decided to add some carbon rod in the leading edge for a bit more strength at the wing’s center section. I will include this carbon rod in wing’s leading and trailing edge for the Robin’s final version.
Iron on covering
I applied lightweight iron-on covering to the entire model. I chose not to shrink the covering on this first build. I next glued the wing and stabilizer in place, cutting away the covering for a strong balsa to balsa glue joint. The wing looked fairly secure in the 1/8 inch balsa fuselage mount. For added safekeeping, I glued 1/8 inch balsa supports to the wing’s lower surface and fuselage joint. The ailerons, elevator and rudder are installed using clear hinge tape.
View showing the profile fuselage and two aileron servos
Motor and electronics
You can order the Ultra Micro 4-Site's UMX 8.5 mm coreless brushed motor, gear box, prop, spinner and AR6400/AR6400L electronic speed control and aileron servos individually from Horizon Hobby. An alternative is to purchase a complete 4-Site and strip out the motor and electronics. Shop around the internet for a good deal. Perhaps a fellow flyer has a 4-Site they are no longer flying that you can acquire.
The motor is fitted to the Robin next. To keep things simple, I decided to keep the plastic motor and gear box mount from the 4-Site, and adapt this to the Robin’s nose. This approach worked out fine. Note that I used some 1/16 inch plywood as added strength for the two motor mounting tabs on the fuselage. I glued the motor in place with about three degrees down and zero degrees side thrust.
Locate the center of gravity
I mentioned earlier that when designing a prototype plane, the tendency is to come out tail heavy. I made a trial fit of the electronics and determined that the center of gravity balance point came out a bit aft of the proper location. I decided to mount the ESC brick as far forward as possible and moved the two ailerons servos closer to the wing leading edge. On the final plans, I will add an inch or so to the nose to help with CG balance, as well as move the aileron servos further forward.
The final steps are to mount the 4-Site electronics using two sided foam sticky tape, glue in 1/16 inch plywood control horns on all control surfaces, and connect the servos to the controls with 0.025 inch music wire. Install the lipo flight battery with velcro and perform one last check for the center of gravity. We are now ready for the first test flight of the Robin prototype plane design!