Читать книгу The Double Dangerous Book for Boys - Conn Iggulden - Страница 12

FLYING MACHINES

Rubber-band aeroplanes are as old as, well, rubber bands. You could use the skills in this chapter to make a simple one, with a propeller. That’s part of the reason why we’ve put this in the book: it’s astounding what you can do with balsa wood, superglue and ordinary thread. The other reason is that an ‘ornithopter’ flaps its wings. They’ve been around since the 19th century as toys, but they’re still impressive. You can buy a kit or a complete plastic one easily enough, but it is immensely satisfying to make. At the time of writing, the world record for indoor rubber-band-powered flight is 21 minutes, 44 seconds, held by Roy White of America. Our ornithopter will fly for just a few seconds, but it’s still worth it.

YOU WILL NEED

 Needle-nose pliers

 Superglue

 Thread

 Elastic bands

 A razor blade or art knife

 Tissue paper or a supermarket plastic bag

 Wire. Paperclips will do, but we used 1/16in brass rod, because it felt a little stronger

 Electrical wire (for the plastic covering)

 Oil or grease

 ⅛in aluminium tubing

 A few coffee-shop wooden stirrers (or ice-lolly sticks)

 A hard plastic bead

 Spars of balsa wood

Note: ‘Super’ or ‘crazy’ glue refers to the group of fast-setting cyanoacrylate glues. Although it is apparently a myth that they were developed to close battlefield wounds, they are sometimes used in that way today, to seal cuts where it would be difficult to stitch. In other words, superglue sticks well to balsa wood, but it sticks amazingly well to skin. In the course of making this ornithopter, we stuck our fingers together many times – and our fingers to other things, including the tube of glue itself. Acetone is your friend when it comes to removing superglue, but please try to avoid pulling your fingerprints off. They do grow back, however. There is clearly potential for injury here, so find a dad and make him help, but don’t let him do all the tricky bits. Remember: scars are fine. Having to go through life attached to a superglue tube is not.

In balsa wood, you’ll need a couple of spar pieces to act as the body and a couple of thin spars to form the backbone of the wings. Hobby shops sell balsa in all shapes and sizes. It’s not expensive stuff, so you should be able to get a selection for a few pounds. The point of having more than you need is (a) for when you tread on a wing spar and snap it, and (b) because, as you’ll see, balsa is really useful. It won’t be wasted.

We formed the main body with four pieces. The thin spars are ⅛in thick, ½in wide and 7in long (4mm × 13mm × 17.5cm). The thick ones are ½in × ½in square and 2in long (13mm × 13mm × 50mm). We added blocks of ½ × ½in batten beneath as well, to hold the rubber band later.

Up to this point, it’s all symmetrical. Choose an end to be the front and cut three pieces of aluminium tube. You could use brass or plastic. It should be the right size to allow your wire to pass through.

With a razor blade or a hacksaw, we cut all three tubes the same length – ⅝in (16mm). As a general principle, it’s worth sanding the rough edges. Two tubes go on the top, parallel to one another, while the third goes at the bottom. It’s vital they don’t come loose, so we cut a groove for the bottom one, glued them all in place, then wound cotton thread around the entire assembly. Cyanoacrylate glues essentially form a hard plastic as they set, with a little heat given off. The result is impressive and incredibly strong. Honestly, the application of superglue on balsa was very satisfying. Once or twice when fiddling, a crack was heard. A dab of glue on the hairline crack, good as new.

The crank is the only tricky part of making this, so here it is. The rubber band will be held by one end and wound at the front. (We did try a rear-winder on the first attempt, with a cork attached. That actually worked pretty well, but we’ll keep this simple.)

The problem is how to turn that rubber band rotation into an up-and-down motion for the wings. A spinning disc doesn’t work – it’s impossible to get the wings to act together. What works is a staggered crank, which lifts two rods at slightly different times.

The wire comes out of the bottom tube, leaving enough room for a plastic bead (fake pearls work well). With the needle-nose pliers, make a 90-degree bend to the left, looking at it from the front. It should be as sharp as you can make it – you do not want that bead to work its way around the corner. After ¼in (6mm), make another 90-degree bend, heading away from the main body. Leaving enough space for a small wooden batten – say ¼in – turn back in a 90-degree bend, parallel with the first bend.

So far so good, but that upside-down ‘U’ has to be twisted to create an angle of 60 or 70 degrees.

Looking down the crank, it will look like this:

If you bend it the wrong way, the wings will not work together, no matter what you do. It took us a ridiculously long time to figure that out.

On to the wings. You’ll need two narrow 8in (200mm) balsa spars for this. We’ve heard of one built entirely of wooden coffee stirrers, which isn’t a bad idea, though balsa is perfect. Lightness is crucial – the less weight the better.

The two wing wires are bent twice at 90 degrees. One end will go into the twin tubes from before, as shown below. More thread and glue will keep the wires stable. We cut the wires so that a little bit poked out at the back of the tubes. With the pliers, you can twist those ends up a fraction. It stops the wings falling out all the time.

That is the hardest part over with, believe it or not. There might be some fiddly bits to finish, but the main job is done.

JOINING THE CRANK AND THE WING SPARS

This is where the wooden coffee stirrers or ice-lolly sticks cut lengthways come in. You’ll need to drill a tiny hole at each end. Remember to use scrap wood underneath. Our first stick had holes 2¾in (70mm) apart for the inner batten, with a fraction more – 72mm – for the outer one. The plastic stoppers are vital to stop it all falling apart. Cut a piece of electrical wire and use pliers to tug out the copper core, leaving only the plastic sheath. You can then cut that into small pieces and wiggle them on.

The inner batten sits at the bottom of the ‘U’ on the crank – the first position. Line up the highest position of the crank with a wing position of no higher than 40 degrees up from horizontal. Any higher and it can be unstable. Mark your holes and drill. Once attached, you should be able to turn the crank and watch one wing rise and fall through its entire motion. If for some reason it doesn’t work – which happened to us on the second run-through – the problem will lie with the length between the holes or the angle of the wire crank. Adjust – half of making something is fiddling around when the thing just refuses to work and you can’t see why.

Once you have one wing rising and falling on the crank’s turn, you can attach the second. This outer batten sits on the part of the crank furthest from the main body – the second position. You’ll need more bits of electrical plastic sheath to hold it in place. Line up both wings so that they’re symmetrical at the midpoint of the sequence, mark your holes and drill. Be prepared to drill a line of holes and try each one if it doesn’t quite work. Keep your temper. You will be proud you did – and ashamed if you didn’t.

Once you have the wings moving smoothly, you’ll need to attach a hook at the back to hold the elastic band. The crank will wind it, so this is just a piece of wire, held with thread and glue. Something like this:

For the tail, we first tried a short wide one, which was completely useless. This bird is front-heavy, so it needs a long tail to give it a chance of stability in the air. With a main body 7in long, our best tail was 8in (20cm). We bent a narrow wire into a shape of an isosceles triangle with an apex of 45 degrees, and fixed coffee stirrers to it with glue and a few turns of thread. The top wire was then fixed with glue and thread to the main body. It’s trickier to wind the thread with the wings attached, so the tail should go on first.

To cut out the tail, just put the plane on a flat surface upside down, use sticky tape to hold the plastic in place, mark it with a felt-tip pen and cut it. Superglue will hold the tail to the wooden struts.

Our first wings were made from white paper, which looked fantastic, like something Leonardo da Vinci might have dangled from his workshop ceiling on a thread. If you’re after a model, that will look great. The trouble is that the paper tore really easily – and even though they were easy to fix, with strips of more paper and a dab of glue, we ended up cutting up a supermarket plastic bag and making a new tail and wings out of that instead. Fiddling and improving is the key here.

THE WINGS

We tried two wing shapes – curved and later a triangle. Both seemed to work. They should be taut, however. Leaving them to flop about just leads to rips.

The first wings were formed as a big semicircle of 16in radius (40cm). We used a circular tray to trace it on tissue paper. We then cut out a smaller semicircle at the centre point, to keep the mechanism clear. When you are finished, it is a thing of beauty and enormously satisfying to use. Photograph it immediately, before you snap it in two.

When you wind the elastic for the first time, it’s quite possible it will jam. The band pulls tight on the front assembly and friction is the enemy. The plastic bead on the outer side of the crank will help, but a drop of oil on every moving part is vital.

The elastic is two bands doubled and doubled again. We tried various kinds, but thin ones seemed to provide better power than thick ones. Buy a box of them and put oil on the bands to ease friction.

That’s it. Wind it up forty or fifty turns, find a high place and let it fly. Fetch it back, raise the tail a little and let it fly again. If there is a bad landing, be prepared to fix it on the spot with superglue and thread. The mechanism is a wonder to behold – it moves a little bit like the mandibles of an insect. Making a rear winder is easy enough, with a cork to help turn the wire and a groove to lock it, ready for take-off.

FINAL THOUGHTS

After half a dozen flights, our first ornithopter snapped almost entirely in half while we were wrestling with the crank. One of the wing spars broke at the same time. The amazing glue-and-thread combination restored it all well enough to flap again! No damage is so great that it means the end – though it will get heavier.

One bead works better than two. Flapping speed is vital, so you might consider purchasing a range of rubber bands.

Remember, the purpose of this is to build a flying machine, yes, but also to introduce you to the combination of balsa, thread and glue. Using the same set-up, you could get a small plastic propeller from a model shop and glue it to a hook instead of a crank. An epoxy glue would be more suited to holding the propeller in place. If you add a fixed wing made from either balsa sheeting or paper, it shouldn’t be too hard to make a small prop plane. Pictures of planes you have made can be sent to the authors care of the publishers – and are very welcome. Good luck.