Introduction: 8 Ft Wingspan Coroplast RC Piper Cub Flown by 25cc Weed Trimmer

About: Electronics engineer with allot of mechanical design expertise email I dont check here often for messages so email me if you have one

I love Radio controlled airplanes and have built several kinds from balsa to this large scale plastic one.

This one is made from $25.00 worth of plastic I bought locally at a sign company.
The plastic is Coroplast or corrugated plastic, its cheap and builds fast.

You could also use old election signs, you just need to paint them or make a patchwork airplane.

Total cost with radio and motor was around 350 bucks.

This is an extensive build and can be scaled down for a smaller plane if you wish.
A working knowledge of RC airplanes is recommended to build one of these.

Its also a tough model, it can take several crashes without too much problem.

I have built a couple of the Aircore brand fold and glue airplanes and used them to learn how to fly, I strongly recommend starting out with one if you want to get into glow engine airplanes.

Balanced properly its a predictable airframe. Due to the simplified flat wing it will tip stall with you at low speeds, so learn how it stalls at high altitude before slowing down for your landing. The ailerons are large enough for a decent roll rate.

As with all gas powered airplanes please be careful wile starting it and have fun flying.

Zachary M.

Step 1: Specifications:

Semi Scale J4 Super Cub

8 Foot wingspan 16" chord wing with landing flaps
4.6 Foot fuselage without motor, I moved the motor forward to balance the airplane. So max length can be 5.75 feet

25cc motor with 9.5 pounds of thrust.

Total weight with fuel is 15.75 pounds.

Max speed around 45MPH

Step 2: Parts List

Airframe Parts list:

2 4'x8' sheets of 4mm thick coroplast in whatever color you choose I wanted Cub yellow
1 4'x8' sheet of 2mm thick coroplast Used for top of wing and wind screen.
1 Dural landing gear
1 pair 4.5 inch wheels or like kind
1 pair Bolt on Axels
1 Tail Wheel
1 Tail Gear
3 Pair Control horns (large)
1 Pack Pushrods
1 pack 4-40 Pushrods for tail surfaces
1 16oz fuel tank
1 Tygon Fuel Tubing
1 Gas fueler
1 Gas tank conversion
1 Pack Nylon wing bolt 1/4-20
1 pack Blind nut 1/4-20
2 1'x2' sheets of 1/4 inch plywood.
Plastic jug to make the cowling
2 8' pieces of wooden lattice strips to make the wing spar
6" 3/32" piano wire

Radio system for wings with landing flaps.

6 Channel radio reciever and transmitter.
6 Volt receiver battery high current, space for it will not be a concern.
Five standard servos for the Ailerons, flaps and throttle.
Two high torque servos for the tail surfaces.
Two "Y" cables for the wing servos
5 Servo extenders
Two noise filters for the long servo leads, (I had radio jitters during the range check without them).
Radio Switch combination charging jack.

Engine Parts list
18 to 35cc 2 cycle engine (prepared to the engine prep page.)
Engine conversion kit from: Wackerengines
throttle linkage
engine mounting bolts
High torque starter or just start it by hand

Step 3: Engine Selection and Modifications.

You can certainly buy a engine ready to fly in the 25cc range for around 300 bucks, but I wanted to have the whole large scale plane for close to that.
Example: Fuji Imvac 23cc

So I looked around and I had a used 18cc WeedEater brand string trimmer, I took it apart and done some online research. I found a website that sells parts to convert your string trimmer engine to an airplane engine:
His Conversion kit for a Weedeater/Poulan comes with a motor mounting plate, propeller adapter, and a velocity stack for the carb, all for 39.00.

You will notice I started out with an 18cc engine, it flew with this but take offs were very slow, and once in the air it would not do a loop. So I looked for a larger one, You will see pictures of both engines in this article. The 25cc engine puts out 9.5 pounds of thrust and the plane will loop now, still not a speed racer but you can fly for thirty minutes on a tank of gas.

After you get the motor you will notice it has allot of extra metal all over it used to channel air around it and support it in a trimmer housing. You dont need any of that, I cut mine down to the block with a hacksaw and angle grinder. Also make sure your motor has an attached coil, like the one below.

You will also want to get a a NGK resistor type spark plug for it or the ignition will interfere with the radio. I also took the muffle apart and removed the baffles to increase engine RPM's

I cleaned my engine and painted it with silver engine paint, made it look brand new

Step 4: Supplies and Tools Give Ot a Take a Few Items...

Drill and bits
Jig saw or band saw
Several sizes of hole saws
Exacto knife and many blades
Heavy duty scissors
Needle nose pliers
Dremel tool
C clamps allot of them!!!
small wood saw
metal yard stick
tape measure

Epoxy 30-minute
Epoxy 5 minute type
Thick "CA" glue (big bottle)
Thin "CA" glue
Styrofoam blocks
Yellow spray paint
Black spray paint
masking tape
Wood glue

Step 5: Wing Spar Construction

Begin the wing construction by gluing the two 8' long pieces of lattice wood together. I used Gorilla glue but wood glue would be OK.

I got my lattice from Home Depot, be sure and select the straightest pieces you can find.
I also used allot of clamps to make sure it was firmly glued.

Step 6: Wing Spar Continued

next bevel cut both ends of the glued spar leaving the top at 7' 11.5" long this will leave the spar just shorter than the plastic we will glue it to. The angle I used was 45 degree's.

Set it aside we must now ready the bottom of the wing.

Step 7: Wing Assembly

Now on to the wing, I first began the wing by cutting a 16" wide 8' long piece of 4mm coroplast This will be the bottom of the wing, the flutes or hollow cavities in the coroplast need to go across the wing for strength.

Lay the spar on the plastic with the beveled tips up and measure how much you need to cut off to make it the same length as the bottom of the spar. (The wing bottom will be a few inches shy of 8').

Now a word on preparing coroplast for gluing

You cant just glue to pieces together, the joint will fail. The plastic is extruded from a machine and has a light oily surface. The best and fastest way to remove it is to burn it off with a torch.

Its easy to do, but i suggest practicing on the scrap left over from the above step.
You simply get the torch close to the plastic and keep it moving without melting the plastic, you will see a very light smoke come off the plastic in front of the flame, Very Light.
You will also have to take care and move the torch very fast on the 2mm coroplast.

After you do this when you apply CA glue the plastic around the joint will fail not the joint!

This has to be done on all glued joints that involve the coroplast.

Glue the spar 4.5" back from the leading edge of the wing using SLOW CA glue, lots of it.
Make you torch it first!

Let dry a few hours for a good bond.

Step 8: Making Hinges in Coroplast

An advantage of coroplast airplanes is that you make the flaps out of the wing itself, the plastic makes a great hing when you cut the top of one flute and leave the bottom intact.

All control surfaces are completed this way. Like torching the plastic I recommend you practice on a few scraps of plastic first. I used an exact-o knife to cut the flute out, you can also buy a tool for splitting the plastic at a sign shop.

You also have to make the hinge just like the illustration below.
After you feel good at making hinges continue to the next step.

Step 9: Cutting the Wing Control Surfaces

I like having landing flaps, its a heavy airframe and needs some slowing down.
I have had limited landing space before and because its so east to add them I recommend it.
It will fly OK without them so the diagram below shows how I would do it for each type of wing.

I made the hing at 10 flutes in from the trailing edge of the wing. That means the surface it self has 9 flutes.

Tip: mark the flute to be cut with a pen and use a small C clamp as a depth gauge for your exact-o blade and only let a small portion of the blade show. So you can cut only the top of a flute. Remember to practice. Also if you make a mistake you can make a patch from scrap plastic, just remember to torch the plastic a little!

Step 10: Glueing the Top of the Wing On

The top of the wing is made of 2mm coroplast and the flutes run from front to back.
This is to stiffen the wing and keep it from twisting.

Cut two pieces of 2mm coroplast to a size of 20" x 4 feet.

Then glue one of them at a time to the bottom of the leading edge, don't forget to scorch the plastic.
I glued them 3/4" under the wing as shown in the picture below. The wing tops need to be longer than the top to cover the top of the spar.

Also use the slow CA for this step its a large piece you will need the time.
Try to get it as square as possible.
Let the glue set up good.

Next you will have to score the plastic so it will fold easily, you also need to choose the leading edge design you want, I went an easy route and made the leading edge pretty sharp, its a little more streamlined for speed but you lose a little lift.

If you want to go with this type of airfoil score it only once on the inside next to were the thick plastic meats the thin. I used a pizza cutter to crush the plastic and make it bend better. You will have to flex it back and forth several times for it to lay down.
For a more curved leading edge, score it three times about 3/16" apart then fold each of those joints one at a time. Use the metal yard stick as a strait edge.

With both types of wings the plastic needs to fold over the top of the wing and go past the trailing edges a little. you will trim off the excess later.

I started by bending it over and gluing it to the top of the spar first, I used anything heavy to hold it in place wile the glue set up. As always torch the plastic first.

After the spar joint has set up, glue it down to the trailing edge and the flaps. I used my metal yard stick and some clamps to hold it down. The pic below has all the places you will want to glue down.

Glue were its blue and the flaps will not be attached to the top of the wing, after the top is dry you can fold the flaps down and trim out the top of the wing around the hinges and glue those pieces to the top of the flaps and ailerons, this will stiffen them and make them the same thickness as the wing.

Step 11: Wing Tips and Wraps

The center of the wing is reinforced by a wrap of 2mm coroplast.

Its 6" wide and 32" long and centered on the wing. The flutes of the wrap run the same direction and the wings bottom.

Start by gluing it to the center of the wing bottom and go all the way around the wing using allot of slow CA.

Next cut the wing tips in a curved shape as in the picture, I used my heavy scissors for this and used the scrap cut off as a guide for the other tip so they would match.
Then glue some 4mm coroplast in the bottom of the wing tips, I used 5 Minute epoxy for this because the joint lacks surface area.

You can hide the joints with strips of plastic cut from the coroplast.
See the second pic

Step 12: Finishing the Wing

Cut a 2" diameter hole on the bottom of the wing just behind trailing edge of the spar, this is for the servo wires.

Next cut holes for the four servos on the bottom of the wing on the trailing edge of the spar, center each hole with the flap or aileron.

The front of each servo can be screwed down to the wing spar but the front servo holes need a little more plastic to attach to, I glued some scrap solid plastic to the inside of the servo holes were the back of the servo would screw down.

I also pre-drilled for each servo screw so that it wouldn't split the spar.

Attach the servos to the four extensions and tape the electrical joints together with black tape.

Run the wires threw the hole you cut in the bottom of the wing and threw the noise filters.

Put on the control horns and use the smaller push rods like the pic below.

Step 13: Fuselage Support Structure

The fuselage Support structure is made of 1/4" plywood and stiffens the plastic so it can support the wing, engine and main landing gear.

The fire wall is made 1/2" thick by doubling up the 1/4" plywood. and is 4" tall and 5 3/8" wide.

The bottom plate for the landing gear is 1/4" x 5 3/8" square and has scrap wood epoxied to it to strengthen it.

The Front wing mount is 3/4" x 2" wide 5 3/8" long solid wood, screwed and glued in place.

The rear wing mount is a scrap of 3/4" square solid wood glued and screwed in place, its also narrower than the rest of the fuselage support structure and sets the taper of the tale, its 5" long and will need to be shaped to fit.

After cutting out the sides of the fuselage supports, stack them together and cut several holes out with your hole saws, this is to save some weight. If you can land an RC airplane good, make these as large as you can.

Glue and screw the firewall, bottom plate rear wing mount and front wing mount in place. You will also want to fuel proof the fire wall front and back with epoxy. The oil residue will soak into the wood if you don't seal it.

Step 14: Fuselage Sides Top and Bottom.

Next you can cut the left and right sides of the fuselage out of 4mm coroplast with the flutes running from front to rear of the aircraft.

I then glued them to the fuselage support sides with slow CA and let dry for several hours. Be generous with the glue in this step.

You can now see the taper of the fuselage, lay the assembly over some 4mm coroplast flutes running the length of the airplane. Trace the top and bottom and cut out.

Next glue a strip of coroplast 1/2" tall to the inside top and bottom of the plastic fuse halves, but not flush with the edge, recess it 4mm so the top and bottom of the fuselage sits in on top of it. This strengthens the joint. CA the bottom in place now and let dry.

Dont glue the top of the fuselage down yet, you will need to put in the rudder and elevator next.

let dry wile we make the tale feathers.

Step 15: Tail Construction

The tail is made up of 4mm coroplast cut like below, its also double up for strength, so its 2 flutes thick or 8mm total. Flutes run left to right and up and down as they are installed on the airplane.

The hings are also to the center of the surfaces like the second diagram in this step.

I also let the rudder come threw the rear wing and epoxied it on the top and bottom.

To mount the tail onto the fuselage you need to cut two flutes out of each side close to the middle of the tale.

Once satisfied with the fit, Epoxy it in place with a liberal amount of glue.

The tale is also a little over sized so if you need to cut some off to make the airplane balance its easy to do. Just remember to cut the same off each side.

I also recommend gluing in at least two 4mm thick bulkheads equally spaced down the tale, this will stiffen it considerably and cut holes in your bulkheads to allow for your hands to run the servo wires.

Now you can fit the top piece in, you will need to cut an 8mm slot to allow for the rudder, after satisfied with the fit, glue with thick CA.

You can now hide all the joints with strips of plastic cut from the coroplast, I cut it using my exacto knife and some practice.

Step 16: Join the Elevators

Using the 3/32" piano wire, make a large "U" shaped piece that will fit in the elevators like the diagram below.

You will need to drill two 3/32" holes in the elevators and epoxy the wire in place.
To make it look good mount the wire on the bottom of the elevator.

Step 17: Covering the Joints

You can now hide all the joints with strips of plastic cut from the coroplast, I cut it using my exact-o knife and some practice.

Start by cutting the coroplast in 1" wide strips and cutting it down the center and trimming off the flutes so that you are left with two thin sheets of plastic.

Bend it in the middle all the way down length wise to make a perfect cap to hide the joints, use FAST CA to glue them on.

Also fit and glue a piece of 4mm coroplast to the bottom front of the fuselage, you will later mount the throttle servo there. Hide its joints the same as the tale joints.

Step 18: Tale Servo's

Just like the wing, cut holes in the tale for the servo's, I used high torque servos because the surfaces are so large. Reinforce the coroplast around the servo's with hard plastic and pre-drill for the screws.
You can see were to mount them from the pics, also use the larger push rods here.

You will need the other two servo extenders here as well.

Step 19: Joining the Wing and Landing Gears

Drill and mount the main and rear landing gears just like you would a balsa airplane using nylon bolts so if you crash you will shear the nylon bolts, not your airplane.

Make matching holes in the bottom of the fuselage and use 1/4-20 blind nuts to mount the gear.
Now that its fitted you can remove it and paint it if you wish.

The rear gear is installed by its included instructions.

The Main wing is mounted the same way, by drilling out the mounts and using 1/4-20 nylon bolts to blind nuts on the airframe.

To make this step easy drill the holes on the airframe first then sit the wing on it and measure from the tale to the wing tips and center the wing perfectly, then using a fine tip marker reach in the airframe and put the pen up threw the holes you just drilled. Then drill out the wing and its ready to screw down.

Step 20: Mount the Engine

I installed the engine two degrees to the right to counter the engine torque. Simply add a few washers to the left side between the firewall and the engines back plate.

Bolt the motor and wing on and using the center of the wing spar as the balance point.
Mine was tale heavy so I used longer and longer bolts to hold the motor and a wood spacer between it and the firewall.

You will have to play with this to get it right, also you may want to mount your radio battery in the nose for more weight there without moving the engine as far. I used four 10/32 x 3" bolts with nylon lock nuts to hold the motor in place.

Shown below is the 18cc engine, the 25cc engine weighs more so it didn't need to go out as far.

After the engine is mounted and the airframe balances drill two 1/4" holes in the fire wall on the side of the carb. One hole is for fuel in and the other is for overflow and returns to the tank.

Then mount the throttle servo behind the engine like in the second pic.

Step 21: Fuel System.

Unlike a glow engine this type of motor has a pump to pull fuel from the tank.
So you have the tanks "clunk" attached to the fuel in side of the carb and the fuel out to the tank.

The tank also has a vent that is ran to the out side of the airplane, I ran mine strait down so if I over fuel it, it will just run on the ground not into my airplane. You must remember to plug the vent after you run the airplane or fuel will leak out and stink up your house...

I used a 16oz tank converted to gasoline as listed on the parts page.

You build the tank as if it was for a glow engine with one exception, you add the third vent line.
I mounted the tank just above the main gear, it fits very nicely between the nylon bolts that hold the gear on.

To fuel the airplane I used great planes gas Fueler
Just follow the enclosed instructions on how to hook it up.

Step 22: Wind Shield

The wind shield is 2mm coroplast that you will paint a little later.

Just cut fold and screw down with four small screws.

Don't glue it because you may need to get at the fuel system or radio battery at some point.

Step 23: Cowling

I made the 18cc engine cowling out of a gallon jug that windshield washer fluid comes in.
Its seen before painting in the first picture in this step.

The 25cc engine needed a different one, I made it out of a Johnny cat kitty litter jug.
Its in the second pic.

To help hold up the cowling I made kind of a hood support using scrap coroplast.
This will keep it from flapping in the wind.

Both cowling are held in place with small screws.

Step 24: Radio Installation

I used a 6 channel radio receiver and packed it in some foam (see pic).

One trick I like with coroplast is running the wire antenna threw the fuselage flutes, just make a small hole in the inside of a flute and its a natural antenna tube.

Remember to follow your radios set up instructions and make sure all the flaps move the proper way.

Control throws are as follows:
Flaps down to 45degrees
Rudder 3" from center
Elevator 1.5" from center on low 2.5" high rate
Ailerons 1" on low 1.5" on high rate

I glued some foam to the airframe and put the radio in it, then hooked up the 6v battery and all the servo's.

I installed the power switch where the window will be painted so you cant tell its there after painting.

Remember to do a range check, your radio must be jitter free at 100 feet with your transmitters antenna down.

Step 25: Painting

I painted the cowling and main gear with fuel proof paint from the hobby store.

I then taped off the windows and lightning bolt and painted them black.

The tale numbers were meant for a boat and came from walmart.

You could also paint a star burst type paint job on the top of the wing, I have seen a few real Cubs with that scheme.

Attach all the wheels and your set to go!

Step 26: Tuning and Trimming

The first step in tuning is peaking the motor out for maximum thrust.
I took the airplane with out the wing on it and tied the tale to a fish scale and tied that to a tree. That way the engine thrust in pounds can be measured. At full throttle tune the high speed needle so that you get max thrust.

When I started the 25cc engine gave me 7.5 pounds of thrust after tuning it gave me 9.5.

You will also need to make the idle as slow as you can get it by tuning the slow speed needle on the carb. I had to run mine richer to get it to idle slow, and it wont idle that slow until the engine is warm, I always warm it up for several minutes before a flight.

If you have used a single needle carb you must juggle low speed idle and max thrust to get a balance point. Thats why I used a two needle carb.

You will also notice that you don't have to adjust the needles very far to see a big difference in power output. Also once set you don't really have to adjust them often, unlike glow engines.

Second I trimmed all the control surfaces level, except the elevator I like it down just a tick or two, this gets the tale up fast and makes take offs smoother. After your in the air simply readjust your trim.

The radio I used has a engine cut off switch, it closes the throttle all the way to shut down the engine as fast as possible.

Third do a ground radio check, if your using a FM radio one with a long antenna keep the antenna down on your transmitter and walk at least 100ft away from the airplane, you should have jitter free operation, if you don't, fix it before you fly. Do the test with it running and move around the airplane to make sure you dont have a blind spot.

If your using a new digital radio, follow the manufactures recommendations.

Step 27: FLIGHT!!!

Although this airplane goes together faster than balsa its still a treat to take one out for first flight.

You also need to do a full aircraft check over and check your balance, a tail heavy airplane doesn't fly well at all.

Im going to assume you used the 25cc engine or better and describe how mine flies.

On gravel takeoff at full throttle with no flaps is around 65 feet, with 35% flaps around 45 feet.
On pavement take ten feet off of those measurements.

My first flight I ran up the throttle and took to the air and climbed out slowly, made a slow turn and noticed that it drops more in a turn than any balsa airplane. I didn't use any flaps.

Up to a safe altitude I then adjusted the radio so that it flew level and didn't roll any.
because its a plastic airplane I find I have to do this every flight, although not very much.

Flying around in huge figure eights for some time I got the feel for it. The airplane is very reactive at full throttle, I pulled it back to 3/4 throttle and cruised.

I dropped the flaps and it slowed to a crawl, a very unstable crawl, you have to keep on top of it every millisecond with the flaps all the way down. I made a few passes to simulate landing and then retracted the flaps some, it flew good that way so I made a real landing.

The landing went well, although I spun out at the end, tail draggers are tricky on gravel....
taxied back and turned it off .

I flew around thirty minutes and used 3/4 of a tank of fuel.
After you get used to its size and flight characteristics you can do slow rolls, hammer heads and loops. Just make sure your high in the sky.

I hope you find this instructable useful, I look at the new airplanes and most of them are electric, that's fine except the 15minute flight time on one 150.00 battery that is very finicky doesn't appeal to me. I must also remind everyone that this is a large RC airplane and could seriously hurt someone in a crash so fly accordingly.

Safe Flying,
Zachary M.