Build a cardboard combat model

Raskin, Jef

Not your ordinary paper airplane!

My son Aza and I had long wanted to make an RC model that looks like a conventional paper airplane, but neither of us knew of the other’s interest. I had hesitated to design one because most paper airplanes are “flying wings,” which usually require an electronic or mechanical mixer to control elevons that move up and down together to control pitch and in opposite directions to control roll. Even though affordable Hitec* transmitters with mixers are available, it seemed to run against the simplicity of the airplane to build it with elevons. One day, as I was sketching a paper plane on my shop blackboard, I realized that because the hypothetical rudder was nearly as long as each of the wing halves and was on the bottom of the wing, it would act as both an aileron and a rudder working in the same direction. I call it a “rudderon.” A conventional rudder is too short to have much of an aileron effect and, if it is above the center of gravity, the moderate aileron effect it does have in producing roll works against its yawing effect.

A conventional flying wing uses elevons (Figure 1-A). To bank to the right, the right elevon goes up and the left elevon goes down (1-B). To pull up, the two elevons move up together (1-C).

With a rudderon system (1-D), to bank to the right, you move the rudderon over to the right (1-E). To pull up, the full-width elevator moves up (1-F).

The synergetic turning and banking effect of large, underslung rudders was known long before I was born, but it has seldom been used in models or full-size aircraft. In the WARP 9, it makes the airplane very easy to fly, even for beginners and gives the airplane a very tight turning radius.

As soon as I told Aza about this idea, he told me he had the same dream, and we immediately started building and flying planes we called “WARP 9s.” With two of us building and test-flying, progress was rapid.

THE PAPER AIRPLANE GROWS UP

There are four parts to a WARP 9 airframe: the wing, the keel, the elevator and the rudderon. Typical spans and lengths range from 20 to 27 inches when using small (but not really tiny) RC gear.

After building a few WARP 9s (each one a little different, but all having the same pleasant flight characteristics), we learned that the design had some advantages we hadn’t anticipated: the keel is easy to grasp for launching, and it provides a point to attach a hook for bungee launches; it also protects the radio equipment when landing.

Before you print out computer– drawn plans, add some graphics. There are colorful triangles on my plane, and Aza chose a dragon that we found on the Web. Talk about wowing them at the slope with your color scheme!

For inland flying, attach a rocket motor or a small glow engine to a WARP 9. On the slope, we learned that our soarer is nearly indestructible, which makes it ideal for contact combat. No fussy streamers; just knock the other guy out of the air-if you can.

Since WARP 9s are meant to look like paper airplanes, we build them out of one of our favorite modeling materials: corrugated cardboard. This stuff is just two flat sheets of paper glued together with a wiggly sheet of paper in between. Cardboard is surprisingly light, very inexpensive and strong and tough (which is the reason model planes usually come in cardboard boxes). Most shipping and packaging shops sell sheets of corrugated cardboard, but it is obviously less expensive to cannibalize a discarded cardboard box.

Wherever you get your cardboard, try

for the kind that is 1/8 inch (3mm) thick. In the trade it’s called “B-flute.” The “flutes” are the tube-like openings that run through inside the cardboard. With balsa, it is important to determine how the grain runs; similarly, with cardboard, we are careful about how the flutes run. Incidentally, Aflute is thicker and C– flute thinner than B (the thinnest cardboard I’ve seen is F– flute). You could build a WARP 9 out of foamboard, plastic corrugated material, or even balsa. We’ve found that foam-core board is relatively fragile, plastic is really too heavy and balsa too expensive, but this is a hobby, so your way is just as good as ours. B-flute cardboard comes in three weights. The best for flying is rated “125-pound test.” We called around to local box makers and located cardboard in this weight, even though the heavier 175- and 200-pound-test cardboards are more common.

If you use cardboard with different surface materials on each side (such as a white finish on one side and brown craft paper on the other), or if you leave the finished model in a closed automobile on a hot day, the cardboard wing will take on a distinct spanwise curve-one reason why we call this model WARP 9. Though the plane theoretically would fly perfectly well with a curved wing, the elevator won’t work with a curved hinge line. To “Star Trek” fans (which we are), “WARP 9” is the normal top speed of a Federation starship, and that’s how fast these models look in the air-even though they are not, in fact, speedy flyers.

BUILDING A WARP 9

The shapes for a WARP 9 can be very simple and easy to lay out. If you use the cardboard itself as a hinge, as we do, there are only two parts to cut out! The easiest way is to use a computer drawing program to lay out the shapes, then cut out and glue them to the cardboard using Scotch 77 spray adhesive. We have the computer mark the balance point and the lines for hinging the elevator and rudderon. With a computer, adding fancy graphics is easy, and today’s inexpensive ink-jet printers make it possible to do it all at home.

Assuming that you will choose cardboard, the best way to put a WARP 9 together is with a glue gun. Hot glue dries quickly, is easy to use and very tough, and its flexibility matches that of cardboard. It is also easy to unglue using a hot-air gun (the same kind as most of us use to tighten covering); you can easily fiat any mistakes you might make. Typically, CA does not work well on cardboard, and most epoxies are too rigid. White glue is an acceptable, though slower drying, alternative.

The openings that run through the cardboard (the “flutes”) should be oriented spanwise, from wingtip to wingtip. A steel straightedge and a sharp blade make quick work of the outline. A scroll saw is a good alternative.

Cut out the keel, with the flutes perpendicular to the wing, i.e., running up and down, not along the length of the keel.

BALANCING THE WARP 9 WITH THE RADIO GEAR

If you make your own planform for a WARP 9, the best way to find the proper balance point is to make a model of your design using stiff paper, balsa, or thin sheets of foam plastic. I use sheet foam from supermarket food trays. Keep the elevator the barest smidgen up, 1/32 inch or less, and add modeling clay to the nose until the model glides well. Notice where the model balances, then balance your WARP 9 at the same relative location. If you use the provided design, you’ll see we’ve already marked the balance point.

Start by temporarily positioning the battery pack with masking tape so that the plane is just a bit tail-heavy, then position the receiver and servos until the balance is right. The control rods should not be so heavy as to materially affect the balance.

If your planform is an isosceles triangle, the balance point will be easy to find: it will be halfway between the front and the back!

The next step is to hinge the surfaces. My favorite method for making hinges is to cut the cardboard away except for one surface and to use the paper itself as a hinge (Figure 4). My paper hinges have lasted for years (I made my first one in the early 1970s), and if one does tear, a piece of clear packaging tape fixes it in a jiffy.

After you’ve made the elevator and rudderon hinges and are sure that they work freely, draw a centerline on the bottom of the wing. Place the wing upside-down on your work surface and, using a T-square or other right angle to make sure that the keel is perpendicular to the wing, glue the keel to the center of the wing.

INSTALLING THE RADIO GEAR

You can either affix the radio gear to the underside of the wing using hot glue or double-sided tape, or put it inside the keel by building a cardboard box for it. My favorite method is to cut holes in the keel for the gear, and then attach a servo to each side of the keel just behind the receiver. None of this is critical so long as the WARP 9 balances at the right spot. The control rods should be dead-straight runs. I use old aluminum arrow shafts with short, 1/16-inchdiameter steel-wire ends hot-glued in-fast to build, stiff and light. Pieces of arrow shaft are available at archery shops that make custom arrows. I cut mounting positions for the nylon control horns into one layer of the cardboard, and then hot-glue the horns into place. Putting the horn on the side of the hinge that was cut away makes the surface less sensitive than if you glue it to the other side.

ON THE SLOPE

The WARP 9 is not a high-performance sailplane, nor is it a floater for those light– wind days. At our flying sites, it needs a good, straight-on breeze to stay aloft. But given some decent wind at the slope, it is a stable, easy-to-fly machine. It will hover like a kite and turn on an amazingly small radius. Because of the rudderon, inverted flight is nearly uncontrollable (we’ve done it, but it isn’t fun). Loops are possible, but they require death-defying dives to get up enough speed. Aerobatics are not the WARP 9’s “thing.” But combat is another story ….

The WARP 9 loves combat. Its very high maneuverability and quick recovery into straight and level flight from almost any attitude makes it easy to recover from a hit. Keep the models in close because that makes contacting the enemy a lot easier. Because of the WARP 9’s turning ability, you can fly several in a tiny area, which can be very exciting.

As I’ve said, with rudderon control, this model is not particularly aerobatic. But if you use elevons, it heats up a lot. With elevons, WARP 9 rolls require no preparation and are fast. With elevons, inverted flight is as easy for the plane as is upright flight (assuming you have the requisite trained thumbs). These models have low drag and pick up speed very quickly in a dive; because of their inefficient airfoils, they lose speed quickly in a climb, so plan accordingly.

Now, go ahead; build a pair of WARP 9s in a single evening, and fly “cardboard combat” the next day. * The addresses of the manufacturers mentioned are listed alphabetically in the Index ofManufacduers on page 142.

Sopwith Baby Sea Scout

Stretch: conference acknowledgements