Introduction: Build Classic R/C Airplanes With New Methods
Runner Up in the
Things That Fly Challenge
This Instructable will show how to build light-weight, electric versions of a couple of classic R/C airplanes quickly and cheaply using some simple methods. Other than a very simple hot wire foam cutter, which I will show, there are no special tools required. Using these techniques and materials, it should be cheap and quick enough that you can easily experiment with your own versions of your favorites and test many different designs with varying configurations of airfoils, wing dihedral, length, etc.
The impetus for this project was Das Ugly Stik, designed and built by Phil Kraft in the 1960's as a quick to build, semi-aerobatic plane to test his radios in (Here). It turned out to be one of the best flying, most popular R/C planes ever. My goal was to be able to test various air plane designs quickly and cheaply. A good example is the photo above which shows two essentially identical airplanes except that one is shoulder-wing and one is low-wing which were built to settle the debate about which was better for dog-fighting - high wing or low wing.
My design parameters were to have a wingspan of around 4 feet (1220 mm) while keeping the weight below the Park Flyer limit of 2 pounds (907gm/32oz) to be able to use those inexpensive, readily available electronics while having nice, stable semi-aerobatic flight characteristics. Because of that, most of my techniques and materials are focused on minimum weight with adequate structural reinforcement where needed.
Since this Instructable is more about designing and building your own versions of classic planes, rather than building exact replicas, I added a number of useful design links at the end.
- Wingspan: 47.7 inches (1206.5 mm)
- Wing Chord: 9.25 inches (235 mm)
- Wing Dihedral: 1 inch (25.4 mm)
- Wing Angle of Incidence: 1.9 degrees
- Center of Gravity: 3 inches (76 mm) behind leading edge
- Total Length: 44 inches (1118 mm)
- Stabilizer: 18 x 7 inch (457 x 178 mm)
- Rudder: 7 x 5 Inch (178 x 127 mm)
- Motor: Turnigy XP Sk 35-30 1400kv
- Propeller: APC 10 x 7 Slow Fly
- ESC: Castle Creations 36 amp
- Battery: Thunderpower 3S 2100mah LiPo
- Servos (4): Hextronic HXT900 9 gm
- Radio: Airtronics RDS 8000 2.4 GHz FHSS
- All Up Weight: 850 grams (1.87 lbs) (30 oz)
Step 1: Materials
Most of the materials are common items that can be found in a local hardware or DIY store like Home Depot. The exception is the micro-weight fiberglass cloth which I could only find on-line at HobbyKing. This is a great place to pick up servos, batteries, motors, wheels, etc. while you're at it.
- J-B Weld part No. 8265-s. Mainly used on the landing gear. Rather than try to weld the pieces together, I just wrap with thin wire and glue it.
- Plywood 1/8 & 1/4 inch (3 & 6 mm). 1/4 inch for the firewall/motor mount and 1/8 for landing gear and various reinforcement.
- Styrofoam 1/2 and 2 inches (13 & 50mm). 1/2 inch for fuselage and 2 inch for the wing. This is the common low density white Styrofoam insulation. The more dense blue or pink Styrofoam is not necessary and only adds weight.
- 3-M Super 77 Multipurpose Adhesive Spray. Used for gluing the Styrofoam fuselage. It basically melts the foam and is quite sticky and messy, but it is quick. You might want to look for an alternative. Most of the strength in the fuselage construction is from the fiberglass cloth.
- Wire - stiff and flexible. Stiff for the landing gear. Thin, flexible for binding together the landing gear wire and then gluing with J-B Weld.
- Coroplast. For rudder and stabilizer. It's tough and easy to work with but a little bit heavy compared to Styrofoam.
- Formica. For wing jigs. It's thin, strong, and you can sand the edge to a very smooth surface which is important so that the hot wire moves smoothly when cutting. Formica is used in kitchen countertops. It's hard to find small pieces but cabinet makers often have remnants that they will give you.
- Fiberglass Cloth 18gm/m2. Used to cover the fuselage. This is the secret weapon for adding strength with little weight. The only place that I could find this light weight is on-line at HobbyKing.com. Here
- Minwax Water Based Polycrylic semi-gloss. Water-based varnish that will not hurt Styrofoam. Used to apply the fiberglass cloth to strengthen the fuselage. I prefer semi-gloss as it seems to be as strong as gloss but not too shiny.
- Artists water color. The lightest weight option for decorating your airplane. Spray paints and films can add a lot of weight. Just add water to get the appropriate shade. A little goes a long way.
- Scotch Shipping/packing tape Heavy Duty. Used for aileron reinforcement and hinges. Actually, the cheap thin stuff is preferable as it bends and forms better.
- 5-minute epoxy. For gluing wing halves together.
- 3M 2-inch strapping tape & Scotch 893 1-inch strapping tape. This is the tape with the fiber in it. Used to protect wing leading and trailing edges.
- Gorilla Glue (not pictured). Used to glue support spar in the wing. Light, strong, and fills gaps. It really expands (foams up) so don't use too much. Moistening the spar helps to activate the glue.
- Blue painters Tape (not pictured). Used for holding the fuselage together while gluing. Also used to cover parts from over spray of the 3-M Super 77 spray glue.
Step 2: Tools
The only tool that I will go into detail on is the hot-wire cutter. The rest are just common hand tools that most people have around.
1: Hot-Wire Foam Cutter
Safety Warning: This uses high voltage mains power on the input and you should take care to properly insulate and cover open connections. If you are unsure, get someone qualified to help. Be careful on the output and cutting wire as well as it is very hot and could shock you if you accidentally touch it. Also, do your cutting in a ventilated area as there will be fumes from the melting Styrofoam.
The picture above shows the layout of a simple hot-wire cutter. Pictured is the small 12 inch one for general cutting of foam and I also have the same thing in 32 inch size for cutting wings. It is handy (and safer) to have the small one for cutting the fuselage and small parts rather than having to control the large bar :
A: Standard household light dimmer switch to control current to the hotwire.
B: UET 150 Transformer. Input 120/240V, Output 24V 50VA. This is powerful enough to heat the 32 inch cutter and pretty safe on the output end but you still need to be careful. I ran standard 14-18 gauge automotive wire to the cutting frame. Make the wire longer than you think you need because you will need to freely maneuver the cutting bow while cutting.
C: Cutting Bow. This can be made of any non-conducting material. I used old scraps of wood because it is easy to screw together.
D: Rubber band to maintain wire tension. This is enough for the small cutter but for the larger cutter you may want something stronger like a thin piece of rope and a spring to maintain tension. Maintaining tension is critical because the hot wire will expand (lengthen) when heated and you need to keep it tight or the wire will drag behind in the center and the cuts will be crooked.
E: I attached the hot wire to the cutting frame using eye-screws and to the power wires using alligator clips. You will want a system that is easy to change wires because you will break them and need to replace. Generally the hot wire is nichrome which heats well with low stretching. You can find it at hobby stores but it is expensive so I went to a craft/sewing store and got a 25 yard spool of 34 gauge wire for less than a dollar and it works well.
F: Jigs for making straight cuts. You will need these to make straight cuts with the smaller bow for the wing dihedral and for the front on the fuselage where the motor mounts plus miscellaneous parts.
G: Soldering Gun. Used for cutting the slot in the wing for the support spar, cutting out pockets for servos, and miscellaneous. It is best to have something with a flat tip and an on/off trigger to control the heat. Basically you are just melting the foam but it is a lot easier than trying to cut with a knife for certain jobs.
2: Cutting Foam
The actual cutting of foam is a bit of trial and error so the best way to do it is just jump in and try with some scrap foam. You will need to experiment with power settings and speed of cut. I try for a speed of about 3-5 seconds per inch when cutting a 24 inch wing panel. Set your heat to a temperature that will cut well without melting too wide of a kerf. More power is required when you are cutting a 24 inch wing than a couple inch fuselage side so keep that in mind when you are testing power settings on scrap pieces.
Step 3: Wing 1 - Cutting
As I mentioned above, cutting foams wings is kind of an art but it is so much fun and really simplifies and speeds up the building process so it is worth the learning curve. There are lots of good sites and Instructables with different techniques.
A: Find a wing foil that you like from a plan and scale it to the chord that you want. In this case I was looking for 9 1/2 inches. Print it out and trace it on to a piece of Formica with a Sharpie. Make sure to turn the pattern over for the second side if your wing is not fully symmetrical. After you cut out the jigs, make sure to sand the ends as smooth as possible as any unevenness will cause your hot-wire to hang up and probably ruin your wing. I marked the jigs in 1/2 inch increments as I used one person on each side of the hot-wire bow and that way we could call out numbers and stay even as we cut.
B: Jig is attached to the foam using galvanized roofing nails. These seem to stick in the foam best.
C: After I cut my first wing, I realized that I needed to add a small "landing strip" piece to the front and rear of the jig to give good support for starting and ending.
D: After the wing is cut, I put it back in the lower cut-off piece so it stayed level and then cut the dihedral using the small bow and the jig. If you want a 1 inch dihedral, just raise the outer end of the wing 1/2 inch and make a straight cut.
Step 4: Wing 2 - Gluing and Reinforcing
With proper reinforcement, the light weight Styrofoam makes a strong wing. I am a fairly tame flyer, but my nephew really wrung it out and it held up with no problems.
A: Glue the halves together using 5-minute epoxy. Lay one half flat, and prop the outer end of the other half up the height of the desired dihedral, in this case 1 inch. Use modelers pins to help hold it aligned while the glue is drying.
B: The support spars are 1/8 inch (3 mm) wide x 1/2 inch (12 mm) deep cedar. Any wood will do. They can be quite thin as the main force on the wing is up and down. I overlapped the spars about 1 inch (25.4 mm) in the center. The spar is placed at the high point of the wing and only needs to go about 3/4 of the way out to the wing tip. To make the slot for the spar I used a soldering gun, marked the depth on the tip, and used a board to follow a straight line while cutting. The spar is glued in with Gorilla Glue. It really foams up so don't use too much and I would recommend to put a piece of painters tape over it while it cures to keep from foaming out of the slot as much as possible.
C: The wing can be painted before or after you add the support spar but it needs to be done before you add the support tape as the paint does not stick to the tape. Choose a bright, rich color. Just add water to the artist watercolor paint. Paint only the top of the wing so that you will have contrast when you are flying and can tell whether the plane is upside down or upright.
D: (1) Use strapping tape (with filament) to protect the leading and trailing edges from damage. don't cover the ailerons with strapping tape. (2) The center of the wing is covered with fiberglass and Polycrylic. I used a standard autobody weight fiberglass rather than the light weight for this. The purpose is to strengthen the area where the rubber bands holding the wing on go. (3) The ailerons use packing tape (without filament) for reinforcement and hinges. This will be covered more in the next step.
Step 5: Ailerons
For the ailerons I considered using balsa wood but then decided that the Styrofoam would be strong enough if I wrapped it with packing tape. This allowed me to also use packing tape to make an easy, gap-less hinge.
A: Mark and cut out the aileron. I use a fine X-Acto X235 saw with the back taken off. Here
B: Cut about a 45 degree angle on the aileron. I used a stationary belt sander but a hot wire should also work. Next wrap the aileron completely in packing tape (except outside edges). This strengthens it and allows a good surface for the hinge tape to stick to. Also wrap the cut-out area on the wing where the aileron will be taped (hinged) to.
C: Hinge the aileron to the wing using packing tape. I taped the top and bottom but probably just the top would be enough.
D: Make a plastic or plywood holder for the servo. Cut pocket in the wing using a soldering iron and glue in using 5-minute epoxy. Hot glue might also work well.
Step 6: Fuselage 1 - Cutting and Gluing
The fuselage is made of 1/2 inch Styrofoam with some thicker bulkheads. Plywood reinforcing is added for the landing gear, motor mount, and for the dowels for mounting the wing with rubber bands. The whole fuselage is covered in ultra light fiberglass coated with Minwax Polycrylic water-based varnish.
A: This is the basic plan that I used. The planes tend to be tail heavy with the lighter electric motors so you might want to lengthen the front by an inch or so.
B: Draw the plan on 1/2 in Styrofoam using sharpie. Make sure to add center-lines as they will be important for lining it up when you glue and also for cutting the wing saddle and elevator/rudder slots at the proper angles. Cut out the pieces using the hot wire. Don't worry if your cuts are a little ragged as the Styrofoam is very compressible so it won't be a problem when you glue it.
C: Clamp together the two fuselage sides and then use the wing jigs to cut out the wing saddle with the hot wire.. Try to get the wing incidence as close as possible. The center line that you drew earlier helps. You can always shim the wing later but it's best to get it close.
D: Check the fit of the wing and fuselage sides.
E: Glue the center and front bulkheads and tail end together using 3-M Super 77 spray adhesive. Don't worry if the edges in front and back don't line up perfectly as you will cut or sand them down later. It is important to make sure that you get the fuselage straight.
F: Cover the areas where you don't want to get glue on. One drawback of the 3-M Super 77 spray glue is that it is hard to place it precisely. Cover your spraying table with newspaper, you don't want to be trying to clean the glue off of it.
G: Blue painter's tape works well for clamping the parts.
Step 7: Fuselage 2 - Finishing
The finishing of the fuselage involves 1) cutting the front square for the engine mounting and squaring up the tail and cutting a slot for the tail using the hot wire, 2) cutting a slot for the rudder using the soldering gun, 3) adding plywood reinforcements for motor, landing gear and wing attachment dowels, 4) painting, 5) covering with fiberglass, and 6) adding decoration if desired.
A: I used 1/4 inch (6 mm) plywood for the firewall motor mount. I also drilled holes and added 1 inch (25.4mm) long dowel for added strength.This is important as it is not uncommon to nose-in on takeoff or landing with a tail-dragger. Wing dowel reinforcements are 1/8 inch (3 mm) plywood.
B: Landing gear reinforcements are 1/8 inch (3 mm) plywood. I cut the slots for these using the hot wire with straight board clamped on either side for depth control.
C: Before gluing on the top, add the rudder and elevator control rod tubes.
D: Once the reinforcements are in and the top is glued on, sand the corners. It's amazing how much more finished it makes the plane look. Here you will see the defects in your cutting and gluing techniques are not that important. I used a stationary belt sander. The sander takes a lot of material off quickly so practice with a test piece first. Make sure that you have it hooked up to a dust collection system. You don't want to be breathing or cleaning up fine Styrofoam dust.
E: Paint with artists water color thinned with water.
F: Drape the ultralight fiberglass cloth over the airplane as smoothly as possible. It takes 1 piece (1/2 m2) to cover the fuselage.
G: Apply the Polycrylic starting from front to back, smoothing as you go. Then turn the plane over and do the bottom. Let it dry and add additional coats. You will need 3 or 4 coats.
H: Use a Sharpie marker and stencils to name your creation.
Step 8: Tail
I used Coroplast for the tail. Carefully cut out one side of a single corrugate and you have an instant gap-less hinge. It is a very slick material so I drilled holes where it will be glued to the plane so that the 5-minute epoxy would fill in and hold better.
If you have an artist in the house, a little tail art is always nice.
Step 9: Landing Gear
I made all of the landing gear by bending very stiff wire, tying it together with thin wire and gluing it with J-B Weld.
A: Completed front gear.
B: Front and back parts of gear secured by wrapping with thin wire and gluing with J-B Weld glue. This was much easier than trying to weld or solder them.
C: Washers on both sides of the wheels to hold them on. I used thin cardboard as temporary spacers while gluing.
D: Medium wire for the tail gear. This was a little bit light as it can bend on hard landing and I need to bend it back.
E: I drilled a small hole for the post and used a screw in the middle to hold it straight.
Step 10: Electronics Installation
Even the installation of electronics can be pretty fast if you are just gluing it in. A couple of key points are that you want your elevator and rudder servos secure so that they don't come loose in flight and you want your two antenna wire angles close to perpendicular to each other for best reception. Also, it is best to mount the antennae and the receiver as far as possible from the ESC and motor.
A: The motor mount is just screwed to the plywood. If you did a good job of trimming with the hot-wire cutter in the previous step, you shouldn't need much shimming.
B: The servos are attached to a plywood holder and then glued in to a Styrofoam support. I used 5-minute epoxy to make it as secure as possible.
C: The receiver is stuck to one side with double sided foam tape. You can't see it well, but there are a couple of small plastic tubes glued in front of it to run the antenna wires into to hold them at the correct angle. The ESC is stuck to the opposite side and forward to keep it as far as possible from the receiver.
D: The battery is held in by a Velcro (hook and loop) strap glued to the fuselage. This system makes it quick and easy to change batteries between flights. The battery is as far forward as possible because, as mentioned before, the light weight of electric motors compared to gas tend to make these designs tail heavy. The blue piece is a short ring of PVC to protect the hole where the power end of the ESC comes out.
Step 11: Useful Links
Below is a set of links for design calculators and plans that I have found helpful when customizing my airplanes.
- Calculator for power and flight time based on motor, prop, and battery. brantusa
- Calculator for center of gravity and stability for various wing/tail configurations. adamone
- Airfoil analysis tool. xflr5
- Airfoil design website for model planes. airfield
- Download free plans in CAD or PDF. starcadplans
- Download free vintage plans. outerzone
- Goldberg Falcon plans. This is the wing template that I used. Falcon
- Das Ugly Stik history and original publication. das Ugly Stick
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