Hi Guys,
back in the 80's I was between jobs with no money for my hobby but I needed a plane so I decided to try a cardboard plane. The first thing I did was to treat the cardboard with a water based polycrylic that is thinned to 60-80%
with water. This adds almost no weight but it helps to seal the cardboard against humidity which can reduce the strength of the wing and the treated cardboard is easier to cut cleanly. I used a heat gun to dry the cardboard and then placed a board with weights on it overnight to prevent the cardboard from curling.
Cardboard comes in different strength grades so use the lightest you can find.
Make a pattern of the wing skin using taped together poster paper and transfer
it to the cardboard and cut it out. Then you use a roller tool that is blunt and has a roller wheel about a 1/16" thick. I think I used a screen door a spline tool. starting a the center of the leading edge, use a straight edge and roll a crease on the back side of the cardboard.
repeat the crease in 1/4" increments till you reach the spar at the thickest point of the wing. Then,gently bend the cardboard on the leading edge section into a mostly smooth curve. The creases you rolled on the backside of the cardboard makes this possible. So no diamond airfoil is required.
My plane was around 625 sq" and I can't remember if I used 6 or 8 ribs per panel. Anyway, go to your plans and copy 6 or 8 equally spaced ribs for each panel and then, change the airfoil to be flat behind the thick point of the wing. Then, reduce the rib outline by the thickness of the cardboard wing skin. Glue copy of rib pattern to cardboard and cut out with the flutes running vertically.
The spar is cardboard with the flutes running vertically topped with a 1/4" sq. balsa sticks on the top and bottom and re-enforced with plywood an fiberglass in the center.
The ribs and spar are slotted "eggcrate style" so they just slip together and are the topped with with the 1/4" balsa. I cut lightening holes in the ribs just like a balsa rib. Brush a small amount of titebond (or etc.) glue onto the rib edges and let dry. This re-enforces the edges to give a firm bond of the rib to the wing skin.
To assemble: join ribs to spar and add the 1/4" strips top and bottom. Apply plywood joiner in the center and re-enforce with fiberglass. I then installed some recycled controls.
Shim the front of the wing skin to keep the wing warp free with the trailing held flat on the on the bench. glue the wing skins together on the flat rib section. Then CA glue the balsa trailing edge to the skin. Next, CA glue in the rib/spar assembly to the flat section making sure that the controls still move freely. I used A-grain 1/16" x 1" by 3+" scraps CA glued on the inside of one panels leading edge area. This helps keep both panels even as the panels are joined. I then CA glued the bottom of the leading edge section of each rib to the curved cardboard skin.Next, Glue down the top skin. Finally, glue an 1/8" balsa cap onto the trailing edge which seals off the raw cardboard edge. Then, cut a few round lightening holes in the wing between the ribs and you are done building.
I used Monokote to finish the wing which was the most expensive item on the plane. I bought the roll for about 9 bucks so I guess I had a total cost of about 5 bucks for the wing. The flaps and tail were built up from my scrap balsa pile and monokoted. The price of "moneytcote" has gone up sine back then.
For the fuselage,I prebuilt the entire motor crutch assembly including slide in tank compartment with all formers and plywood doublers. fuel proofed and painted. Cut the fuselage sides with the flutes running vertically. Line the inside edges on each cardboard side with 3/16" sq. balsa longerons except of course where the crutch assembly glues in. Glue the sides to the crutch assembly and clamp the join the tail with a balsa tailpost just like a balsa fuselage. Add cross pieces of 3/16" sq balsa sticks to the top and bottom of the fuselage.
Cut the top and bottom cardboard pieces. Glue paper to cover the flutes on each side of the top and bottom and let dry.Glue on the cardboard top and bottom to make a simple square box assembly. Then re-enforce each corner with epoxy and fiberglass tape. I used a leftover spraycan of white primer to paint the fuselage and rudder and a leftover spraycan of white epoxy refrigerator paint for a fuelproof coat.
So, I ended up with a full size stunt plane for under $10.00. I used a strong .40 engine I already owned to power it. The plane did the whole pattern and I probably put 800+ flights on it over 4-5 years. I smacked it hard enough a couple of times that it would have shattered balsa plane but the cardboard plane just dented. Yes, that plane was square and butt-ugly and built out of a scrapbox, but it gave me many hours of pleasure and kept me flying when money was short.
PS.A cardboard plane would make an ideal trainer. it's cheap and durable.
Guys, I'm sorry to run long but I just had not thought about that plane for a long time and it brought memories.
Pat Robinson