Intro: R/C Biplane
Building an RC airplane is a fun project, and a great way to understand how airplanes work! It's very rewarding to build and fly your own airplane from scratch.
The plane that I'll be building in this Instructable is the SIG Smith Miniplane, but the construction methods are similar for most balsa airplanes. The Smith Miniplane is a small, scale biplane, and it flies very well. I've flown this plane a lot and its been one of my favorite airplanes. It's easy to control, and does everything you'd want it to.
Step 1: Preparation
Before you begin building, there are a few things to do first to prepare. If you are building from a kit like I am, most of the materials are already included, so I won't mention those here. Otherwise, if you're building from plans you will need to get all the balsa wood and other materials.
Materials needed for this kit:
-engine, tank, fuel lines, propeller
Tools needed to build:
The Smith Miniplane kit I built had clear instructions on how to build it, and in what order to do things. Before starting the build I read through the manual to first understand the building steps.
There are full size paper plans inside the kit and the airplane is built on top of them. You will need to lay out the plans and then glue the wood pieces directly on top of them, using the plan as a guide.
Step 2: Fuselage Construction
To begin construction of the fuselage, I first laid out the fuselage plan on a large piece of foam board, and pinned it down with wax paper on top (to prevent glue from sticking to the paper). The two sides of the fuselage are built first, and are then joined together.
The fuselage sides are build on top of the fuselage side plan, and you will need to cut 1/4" balsa sticks to match what is shown on the plan. I pinned each stick to the plan and glued it to the others (this is described in the manual). I found that the best way to do this was to use a small razor saw to cut the balsa stick, and then when I got the angle correct I glued it with thin CA (superglue) for an instant bond. This glue made construction very fast.
Once the two sides were completed, I laid them out on the fuselage top plan (shown in photos). I cut pieces of balsa to connect the two halves and glued them in place. It's important to get the alignment right here. I also added the landing gear mounts near the front of the plane.
Step 3: Drill Engine Mounts
Before glueing the firewall to the fuselage, I needed to drill and mount the engine first. For this airplane I used an OS .46 AX glow engine, but this could also be converted to electric. I measured where the engine mount holes are on the black engine mounts, then drilled and tapped it and mounted it with bolts.
After that I measured where the engine mounts line up on the firewall and drilled holes and epoxied T-nuts into the firewall. I also drilled holes for fuel lines and the throttle servo in the firewall.
Step 4: Fuselage Construction
After the glue dried, I flipped the fuselage upright and added formers to the top (image one). Since the firewall was drilled for the engine in the previous step, I epoxied it to the fuselage at this point (image one and two).
After that, I added a plywood sheet to the front of the fuselage. The top wing is mounted to this piece of plywood, and there are two brass tubes included in the kit for the wing mount. I epoxied these tubes onto the sheet of plywood, after screwing them in. This is important to get right as it determines the angle of the top wing.
Step 5: Fuselage Sheeting
Next, I added formers to the top of the plywood wing mount (not shown), and stringers across the formers both in the front and the back of the fuselage. These are the thin sticks running lengthwise down the plane.
After the glue on the stringers dries, I added sheeting to the front of the fuselage. To make the bend, I got the wood wet first. I glued the fuselage where the sheeting will be, then placed it down and used masking tape and pins to hold it down while it dried.
Step 6: Fabricate Cowling
This kit includes a cowling for the engine, and you need to glue the two halves together and drill holes to fit the engine. I also had to cut out cooling holes in the front of the cowling. The third photo shows the finished cowling on the airplane with the engine mounted.
Step 7: Fabricate Landing Gear
After the fuselage is built, I made the landing gear mounts as specified in the instruction manual of the kit. This consists of two wires soldered together, and scrap balsa for a fairing. This mounts to the landing gear blocks on the bottom of the fuselage.
Step 8: Make Wheel Pants
The wheel pants fit over the top of the wheels to make them more streamlined. I built these as specified in the manual, but decided to take them off because they get caught in long grass when taking off and landing the plane.
Step 9: Top Wing Construction
After building up the fuselage, I started on the top wing. Since this is a biplane, I needed to build two wings. The process for building the wing is pretty much same for all balsa RC planes.
To begin, lay out the wing plan on the building board, and pin it down with wax paper on top. Then, pin down the front and back spar on the board where the plans show it to be (these are the long sticks seen in image one and two). The spars are what take most of the load on the wing when the airplane is flying.
Next, there are little airfoil-shaped pieces called ribs, and these are what give the wing its shape. These ribs have a cutout for the wing spars, and they fit right on top of the spars. Line each rib up with where it should be on the plan, and glue it to the spars. Once the wing ribs are in, glue in the top wing spar in the groove on the top of the ribs.
After the ribs are all glued in, I added a tapered balsa piece to the rear (trailing edge) of the wing. I glued this in and added triangular pieces to support it. The wing should now look like the images above, with all the ribs in place and three spars.
Step 10: Top Wing Sheeting
After building up the top wing, you will need to sheet it with 3/32" balsa wood. The best way I've found to do this is to first get the balsa sheet wet so it bends easily, and then use regular wood glue and glue every part of the wing that the sheet contacts. Then, place the sheet down on the glued wing, and pin it down so it follows the curve of the wing. I repeated this process until the sheeting is done on the wing on both the top and bottom.
Step 11: Complete Top Wing
After the sheeting was done on the top wing, I added wing wing tip plates as shown on the plans, and I added balsa blocks to the cutout in the center of the wing. The balsa blocks were then sanded to fit the shape of the wing.
I added wood filler to any gaps in the wing and carefully sanded it smooth. It helps to have a long sanding block so you can evenly sand the wing.
Step 12: Fabricate the Top Wing Mount
The top wing mounts are created in a similar way as the landing gear, and is explained in the manual. The steel wires fit into the brass tubes that were glued in earlier, and are soldered together. A piece of plywood is epoxied to the wire wing mounts, which will be drilled for wing mounting bolts later.
Step 13: Build the Bottom Wing
The bottom wing is build in the same manner as the top wing, but there are some differences. The bottom wing has ailerons (movable surfaces that roll the plane), and the top wing does not. These ailerons are built as shown on the plans, and torque rods are added so a single servo in the center of the wing can move them. The torque rods and cutout for the servo are shown in the third image.
Step 14: Horizontal Stabilizer and Elevator
The ailerons are what roll the airplane, and the elevator is what pitches the plane up and down. The elevator and stabilizer are both built directly on top of the plans, and are then sanded smooth.
The elevator is hinged to the stabilizer using hinges designed for model aircraft. The hinges will be epoxied in later after the covering is done.
Step 15: Vertical Stabilizer and Rudder
The rudder and vertical stabilizer control sideways movement, or yaw. This is constructed in the same way as the horizontal stabilizer, and is also hinged.
Step 16: Covering Stabilizers and Control Surfaces
Now that the airplane is built, we can start covering it in fabric, just like the full scale airplanes. I used SIG Koverall for this airplane, but an iron on covering like Monocoat would be easier. Koverall gives a really nice and durable finish but it also takes longer to do.
I applied the Koverall using the instructions that came in the package, but I used Minwax Polycrylic instead of aircraft dope. The Polycrylic is safer to use and is water based so cleanup is easier. I've used both dope and Polycrylic on airplanes, and I liked using the polycrylic a lot more.
I also mounted the servos for the throttle, rudder, and elevator in the fuselage at this time. The servos are screwed into blocks which are glued to the inside of the plane.
Step 17: Cover the Wings
I covered the wings in SIG Coverall using the same methods as the previous step.
Step 18: Cover the Fuselage and Paint
I covered the fuselage in Koverall, and then I hung it from the nose and painted it with white Rustoleum spray paint. This paint worked well and is fuel proof. I went with multiple light coats of paint until it was thick enough to be opaque. Make sure to mask off any areas you don't want to be painted.
Step 19: Paint the Sunburst Pattern
After the white paint dried I masked off the wing and fuselage and painted the starburst pattern in red. After painting, I removed the masking tape and added decals to the airplane.
Step 20: Set Up Airplane
Now the Airplane, is built, and all thats left is to install the radio receiver, battery, and make sure everything is working before flight! I also started the engine and made sure is was running well before flight.
Another important step before flying is to check the Center of Gravity (CG) of the plane. The CG is the point that the plane balances longitudinally. The plans show the location of the CG, it's around the thickest part of the wing. I checked the CG by holding the plane by two fingers at the point where it should balance, and if it tilts forward or backwards then you need to add weight to the front or back of the plane. I had to add some weight to the nose of the plane to get it to balance properly.
Step 21: Fly It!!
Finally the airplane is ready to fly! Make sure the battery is charged, the engine is working perfectly, and there's nothing loose on the plane. It's always better to fix a problem on the ground than to have the plane crash because of it! On my first flight the airplane flew very well and I only had to trim it a little bit. The OS .46 AX engine gives it more than enough power for vertical maneuvers. This plane is incredible for slow aerobatics and flies nice and straight.
Thanks for reading and happy flying!