Foam and Balsa Wood RC Airplane

We built this RC Airplane as our Senior
Project in our Engineering 4 Class in High School. Outlined here is our Project Overview:

What is the project?

In a simple statement, our project is to build a radio-controlled airplane. In more detail, our project is to build the body of a RC Airplane out of different but appropriate materials and then install the necessary electronics to turn it into a flyable aircraft. We will use different materials and processes when building the plane and learn about electronics while installing the ‘guts’ of the plane.

Why are we doing this?

We have several reasons behind building an RC Airplane. One, we are interested in flight and airplanes and how they work. Two, our goal is to learn new processes such as using composites, setting up servo motors, and how to create an airfoil that lifts an object into the air. Three, our goal is to make an aircraft that flies. Four, we hope to have fun throughout the engineering process. Of all our goals, this is the one we are most confident in, because we love engineering.

What do you intend to learn from this experience?

We hope to learn about avionics and the physics of flight. All of this knowledge will be implemented throughout our project. We hope to learn some new manufacturing processes, such as using carbon fiber and/or plastic spray-on coating. We hope to further develop our CAD skills (specifically SOLIDWORKS), our CNC routing skills, and our other skills we have learned in past Engineering classes.

These are some basic aviation terms that are
helpful to know when constructing your airplane and conducting research:

• Aileron - The hinged piece used to rotate the plane in respect to the y-axis. Side-to-side "Roll."
• Flaps - The hinged pieces on the edges of the wings used to slow the plane when landing and to kick the nose of the plane up. They also adjust the amount of lift a wing creates.
• Elevator - The hinged piece on the rear of the plane below the rudder, used to control rotations in respect to the x-axis. Up-and-down "Pitch."
• Rudder - The hinged piece on the rear of the plane attached vertically which controls rotation in respect to the z-axis. Left-to-right "Yaw."
• Push Rods - The metal rods used to connect the control surfaces to the servos. These are run through the fuselage and are control the elevator and rudder.
• Chord - The width of the wing from front to back.

Materials

To begin building your RC Airplane, you are going to need to start gathering and purchasing materials. For our plane, we needed:

• Foam
• Balsa Wood (1/4 inch thick sheets, square and semicircular rods)
• Monokote Shrink Wrap
• Servo Motors, Brushless Motor (Propeller Motor)
• Battery (and Battery Charger)
• Transmitter (Remote Control) and Receiver
• ESC (Electronic speed controller)
• Propeller
• Hinges
• Extra Wires and Connectors
• Wheels
• Push Rods and their Connectors (called Control Horns)
• Carbon Fiber
• Glue (Gorilla Glue, Super Glue, and Epoxy)
• Miscellaneous Fasteners.
• Other items that you see fit as you customize your design and encounter problems in construction.

Tools

Another huge part of building the plane is using the right tools. Here is a list of the tools we used to build our plane:

• CNC Router
• Band Saw
• Iron (for forming shrink-wrap)
• Screwdrivers and Micro-drivers
• Laser Cutter
• 3D Printer
• Hack Saw
• Hand Drill
• Chop Saw
• Oven (for curing carbon fiber)
• Heat Gun (or very hot hair drier)
• Dremel
• Sander and Sand Paper
• Patience
• Creativity

The first step in building an RC airplane is to decide on your criteria and restraints. Choosing a wingspan and model type (glider, trainer, aerobatic, or racer) that is right for your plane is essential to the design of your plane. We chose to create a “trainer” with a 5-foot-wingspan. Our plane is fairly large compared to others and being a trainer means it flies at a slower speed. The wingspan is a personal preference, but it determines the width and shape of the wing. The necessary size of the wings can be calculated using an online website. We used this site for our calculation: http://www.flyrc.com/wing-load-calculator/. The type of plane is part of the determining factor in how much your plane can weigh, and how powerful the motor is. The trainer category is a good place to start, which is a weight of around three pounds.

After doing the modeling phase, we went back to CAD and began creating the final iteration of our design in SOLIDWORKS. We created the fuselage in two parts so that we could easily install the electronics, we created the wings using the airfoil shape that we had decided on already (see Wings section), and then we designed the horizontal and vertical stabilizer. We did not include the flaps in our model, as we knew they would just replace a chunk of the wings/horizontal stabilizer/vertical stabilizer. We also did not include the electronics, because they were not needed for the model to be used to make physical parts. We also translated some of the parts (like the two stabilizers) into Rhino so we could laser cut them later.

General Dimensions

The fuselage is 42.25 inches long, the wings are 10in x 60in, and the vertical stabilizer is 24in wide. This is a pretty big plane, but the ratios would remain about the same for a smaller plane.

One important part of creating an RC airplane is modeling. We both made a 3D model on the computer and a cardboard model in real life to get a sense of the scale of the project. When creating a cardboard model, only worry about basic shape, detail, and dimensions. Getting every little detail on your model is a waste of time, but making each aspect dimensionally accurate is important.

After studying, measuring, and scavenging the premade sets for parts and sizes, we designed our own fuselage on SOLIDWORKS to be carved out of foam. We decided to use a sturdier foam that can resist heat-deformation, and before we could put it on the router we had to cut it into smaller, rectangular shapes using a band saw. After that was done, we cut the fuselage out of two pieces of foam, each one in four cuts. (A roughing and finishing cut for both the top and bottom sides) Unfortunately, when we cut the bottom half, we had the foam slightly off center, leaving slight chunks in the leading edge, so we had to gently hand-sand these out later to match the other piece. We finished the outside of the fuselage by coating it in epoxy, which dried and hardened into a hard exterior shell.

We started the process of building the ailerons and flaps by building balsa wood cores to form the carbon fiber around. These cores were cut on the laser to the correct size and the balsa wood used were the leftovers from the wing ribbing. We then decided the pattern of layering we wanted to ensure the pieces are strong. We went with a pattern of the fibers going straight up and down, then at a 45 degree angle, and finally perpendicular to the original pattern. We took the uncured carbon fiber and traced and cut out the patterns for our pieces, which we then placed around the balsa wood cores to ensure correct sizing.

When we were satisfied with the sizes, we used the following layering to compress and heat the carbon fiber. We started with a bag on the outside which is sealed with tack on the edges to create a vacuum bag. Then we layered our pieces between two aluminum plates with a sheet of Teflon film between the carbon fiber and the plates. Before sealing the vacuum bag we wrapped a section of glass fibers around the entire contraption, to help the air escape from the center of the sandwich. Finally, we put two vacuum tube adapters on the top; one adapter for a pressure gauge to ensure we got a good seal, and the other for the actual vacuum line. Then it was a simple matter of connecting the vacuum line in the oven, and letting the carbon cure under the pressure from are vacuum bag and the heat from the oven.

When we pulled the pieces out of the oven the next day, we found that they had cured in a decent fashion. The edges of the pieces were not quite sealed, and if not for lack of time, we would have tried the process again but add another bag around the carbon fiber pieces, then put them within the other bag. This would have hopefully helped to make more precise sealed edges on the pieces, but in the end these gaps on the edges of our pieces were actually convenient. The hinges we used to connect the flaps to the balsa wood frame could be inserted through the gap. To make this work we had to make a special tool from a hacksaw blade that we then used to cut slits in the balsa wood rods holding the framing of the wings together. We then inserted on end of the hinge into the slit and the other into our flaps by cutting slits in the balsa wood core and sliding it in between the edges of the carbon fiber.We used this same procedure in attaching the rudder and elevator to the tail of the plane.

Attaching the Push-rods:
This is a critical step as it transfers the motion of the servos to the flaps, ailerons, rudder, and elevator. First, zero out your servos. This is done by turning connecting your motors to the battery and receiver and allowing them to go to their pre-programmed center. After this is done, measure out how long your push-rods need to be. Do this by connecting the push-rod to the motor (The end will have to be bent 90 degrees then 90 degrees again to create a ledge for the motor to rest on) then holding your control surface at its neutral position and marking where the rod should meet the linkage. Then bend this shape into the 90-90 ledge so it can secure to the control surface. Some push-rods have attachments that screw onto the ends and fit onto the linkages without bending, but for our ailerons this was not the case. Also note, some larger push-rods will not fit through the servo connector. To overcome this problem, we used a hand drill to widen the holes enough to fit the push-rods.

The most important and difficult part of designing the wings was creating the airfoil, the shape that give the wings their lift. To create our airfoil, we used the NACA website http://airfoiltools.com/airfoil/naca4digit which can create over 1,600 different types of NACA airfoils based off of the dimensions you enter. The dimensions it generally asks for are wing chord (front to back length), camber (symmetry of the airfoil), thickness (top to bottom height in % of chord), and the number of points you want plotted.

We suggest a NACA 2412 airfoil, which is the general airfoil shape.

After computing your airfoil's coordinates, copy and paste them into an Excel spreadsheet, from there you can put these points into a CAD software like Rhino 5 and make a physical airfoil. That process is explained in this video:

Note: You may need to use control-point-curve to connect the dots into a physical shape.

We decided on a balsa wood rib-and-rod design for the frame and then a cover made out of shrink wrap plastic. Using the Rhino designed airfoils, we laser cut the ribs out of 1/4 inch Balsa Wood.

We encountered a problem with the laser cutter: it burned the balsa wood. To solve the problem, we used trial and error to create our own setting. We ended up creating a preset that would cut at 14% power and move at 87% speed. This change allowed us to quickly produce airfoils and progress with the wings.Then all we had to do was space the ribs out on the rods and glue them in place. We didn't measure out specific distances between the ribs, but they averaged about 2" apart. For a better result we suggest placing the ribs closer together.

The final step involved coating the wings in Monokote, a special shrink-wrap made for model aircraft. Using an small iron, the Monokote adheres to the ribs of the wings and then a heat gun smooths the plastic out, creating a flat, uniform shape. Instructions for applying Monokote are included with the shrink wrap, but be careful when wrapping the edges of the wings or other places where the plastic only pulls on one side. The shrink wrap broke a few of our ribs during this process and could have been avoided by having a brace at the very back edge of the wing to hold the ribs in place. See pictures for the crooked and wrinkled areas where a rib broke.

As we learned, you want the thruster to have about 100 watts for every pound the plane weighs.

Choose motor, then battery, then ESC as the ESC is sensitive and could burnout with improper amps or voltage. For larger planes (like ours) use regular servos for the control surfaces (i.e. ailerons) rather than microservers, as those will not be strong enough to pull the plane.

We bought most of our electronics on the Hobby King website.

The receiver will have specific plug spots for each servo and the ESC. We bought a 5-channel receiver even though we only needed 4 channels: ESC, rudder, elevator, and ailerons (the ailerons come together using a y-cable). As shown in the diagram above, the battery plugs into the ESC, the ESC plugs into the motor and the receiver and the servos plug into the receiver.

The installation of the servos is detailed in the wings and Ailerons sections, the motor installation is talked about in the Putting It All Together section, and the other electronics just need to be secured in the inside of the fuselage.

Now that all of the parts are created, you can start assembling the plane.

1. Glue the elevator and rudder servos to the fuselage while they are attached to their respective control surfaces.

2. Glue the horizontal and vertical stabilizers to the fuselage. Make sure that they go on straight or your plane will pull to the left or right.

3. Prepare the upper-half of the fuselage for mounting the wings. Our design allows us to remove the wings between flights to access the battery and electronics.

a. Drill holes for four bolts into the top of the fuselage where the wings will rest, the bolts will be glued to the fuselage to prevent them from unscrewing during flight.

b. Drill holes in the bottom of the wings that align with the holes in the fuselage.

c. Drill two sets of holes into the side of the fuselage under the wings. Through these holes thread two wooden dowels that stick out of each side about 1”-2” past the fuselage and glue them in place.

d. Screw the bolts through the fuselage and slide the wings onto them. Use wingnuts or regular nuts to secure the wings.

e. Wrap rubber bands around the wings and dowels, holding the wings to a lower part of the fuselage.

f. Remove wings to access electronics and complete the fuselage.

4. Position and glue the motor mount on the lower half of the fuselage.

5. Connect the motor to the ESC with the motor outside the plane and the ESC inside.

6. Use epoxy to attach the two halves of the fuselage together. While doing this, slide the top half onto the motor motor mount and glue it in place.

7. Fasten the motor to its mount.

8. Fasten the propellor to the brushless motor.

9. Reattach the wings.

10. Ready for liftoff!

Here we'll include information that isn't directly about the construction of the plane, but was part of the overall project. It is primarily to document our process and the steps involved in actually the project coming to fruition. If interested, keep reading.

A big obstacle we had to overcome in this project was funding. To solve this, we decided to put our engineering skills to use. Several weeks prior to the annual Holiday Fest at our high school, we began designing our own ornaments and earrings. These ranged from snowflakes to dala horses to earrings with our high school’s initial. We then spent a couple of days laser cutting and 3D printing our designs, creating a sizeable stock. We used the school’s laser cutter but our own 3D printers and our own materials. On Holiday Fest weekend, we had our own booth where we sold our ornaments and earrings to the shoppers. We set up our 3D printers and had them running so people could come see how they work and how we made our products. It was really cool to see people’s faces light up with interest when they saw the 3D printers printing away, and most of our day was involved in teaching people about how 3D printers work. We also enjoyed telling people about our RC Airplane Project when they asked about why we were there. In the one weekend, we made over $500, more than enough to fund our entire project. We made sure to tell people that any leftover money would go into our college savings, so it wasn’t going to waste!

We built this plane as our Senior Project in high school. All three of us were in Engineering 1, 2, and 3 for our Freshman, Sophomore, and Junior years. This project took us a semester. We all have had awesome experiences in the Engineering Program at our high school and are planning to pursue Engineering at college.

We used all of these links in our project.

https://hobbyking.com/ -- We used Hobby King to buy most of our electronics. It's a great, reliable website.

https://www.rc-airplane-world.com/rc-airplane-cont... -- Airplane Information

https://www.instructables.com/id/Design-Build-Your... -- Helpful Instructable

https://www.instructables.com/id/Complete-Guide-to...

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.ht... -- Wing lift calculator

http://airfoiltools.com/plotter/index

https://www.instructables.com/id/Foam-RC-Plane-Bui...