CHAPTER 2 : Trebuchet

PROJECT SIZE: Small
SKILL LEVEL: ★★✩✩

SOMETIMES THE EVIL GENIUS LIKES to go for that retro-look when creating his projects, and it’s hard to get much more retro than a medieval siege engine.

The trebuchet uses a weight attached to an arm with a sling on the end of it to throw objects. In its time, the projectiles varied, but were often big rocks or dead and rotting horses, thrown into the enemy fortifications during sieges. The Evil Genius’ trebuchet will not throw much more than a tennis ball, but it will throw it a decent distance, and it’s a good project to try if you like woodworking.

Figure 2-1 shows the trebuchet ready for action. It’s a simple design that should only take a few hours to construct and needs little in the way of special tools or equipment. Trebuchets are elegant machines that convert the potential energy stored in a counter-weight into kinetic energy in the projectile. Unlike the coil gun of the previous chapter, trebuchets do this in an efficient manner. The action of a trebuchet is shown in the sequence of diagrams in Figure 2-2.

The trebuchet has a sling attached to the throwing arm. So, initially the projectile is almost underneath the weight (Figure 2-2A). As the weight falls, the throwing end of the arm rises, pulling the sling and projectile along the slide on the base of the trebuchet. At some point, the projectile leaves the slide (B) and is swung round in a wide arc as the weight keeps falling (C).

The sling has one end attached to the top of the throwing arm, and the other end attached to a ring that fits over a hook on the end of the throwing arm. As the arm passes the vertical mark, at some point the sling will slacken and the ring will slip off the hook, releasing the projectile (D).

WARNING

Be careful when cocking the trebuchet, and stand well clear of it when it fires.

What You Will Need

You will need the materials listed in the Parts Bin to build this project. All of these can be easily obtained from hardware stores, and in the case of the plastic container, a supermarket.

Note that lumber is sold in different standard sizes in different countries, so you may not be able to get exactly the same size. This should not matter, and if in doubt, use thicker, more solid wood. In addition, you will also need the following tools:

Assembly

The cutting list for this in Table 2-1 includes all the wood you will need for the project.

NOTE: The author used inches in this project; it seemed appropriate for such an ancient design. So, those measurements are the more exact figures.

Step 1. Make the Frame Sides

The first step in this project is to build the frame sides. These are constructed from pieces of wood, which are arranged in an “A” shape. The apex of the “A” is held together by a metal plate, through which the bolt should be attached, which acts as a pivot. So, before attaching the plate, drill a hole right in the center, big enough for the bolt to pass through.

Figure 2-3 shows the plan for one of the A-frames. One of the A-frames is shown in the photograph of 

Figure 2-4. The sections of wood are just screwed together with a pair of screws at each joint.

Step 2. Build the Throwing Arm

Figure 2-5 shows the construction of the throwing arm. Drill one large hole to fit the bolt used, and two smaller holes for the weight ropes and for attaching the permanent end of the sling. Straighten out the metal hook (Figure 2-6) and screw it into the end of the throwing arm. 

You will need to use pliers to grip the hook while you screw it into the end of the throwing arm.

Step 3. Build the Base

The structure of the base is shown in Figure 2-7. We start by fixing the two A-frames together at the base, using two lengths of wood (J and K in the cutting list) placed under the A-frames. There should be a gap of about 7 inches (180mm) between the A-frames at the base. Also attach the central strut (L) that will hold the side braces, and piece (M) that runs down the center of the base to support the runway board and trigger mechanism.

At the top of the frames, fit the bolt through one plate, a nut, and then the throwing arm and the second plate (Figure 2-7). Fit a nut on the inside of the bracket before the second plate. The nuts are going to hold the two A-frames apart against the side bracing.

Step 4. Attach the Side Braces

The side bracing is formed by a cross piece under the middle of the A-frames (L) and two struts (O and P) from the bottom of the strut up to the apex of the A-frames (Figure 2-8). The struts and braces are both fixed into place using screws. Drill the struts at an angle first and use long screws. More adept woodworkers may elect to cut the ends of the struts at angles so they fit better.

Step 5. Assemble the Weight

The weight is constructed from a plastic box designed to hold breakfast cereal. The box has a 10-pint (5 L) capacity, which when filled with wet sand will have a weight of about 18 pounds (8 kg). Before filling the container, we need to drill four holes near the rim (Figure 2-9). 

Cut two 24-inch (610mm) lengths of the nylon rope and thread them through the holes drilled in the food container and the hole at the weight end of the throwing arm. The rope should be just long enough to allow the weight to stay away from the rotating end of the throwing arm, without being so long that the weight hits the ground at its lowest position.

Check the travel of the whole mechanism, and make sure that there is enough clearance between the A-frames for the weight. Figure 2-10 shows how the container will eventually be attached. Note that it is shown here filled with sand, but it is better to wait until everything is assembled before you fill the container.

Step 6. Assemble the Sling

The sling (Figure 2-11) is made from 64 inches (160mm) of the rope with a square patch of cloth, 8" 10" (200mm 250mm). A reasonably strong material like denim is ideal. The Evil Genius’ minions can often be found wearing jeans with large patches of cloth removed. The Evil Genius tells them that this is the latest fashion and the minions are pleased.

Lay the rope across the cloth as shown in Figure 2-12 and fold the edges over to make a seam enclosing the rope. Sew the seams up, or if you prefer, apply a row of staples down each side. Sewing will be a lot more durable than staples.

Tie a loop into one end of the rope. This will fit over the hook of the throwing arm. 

Step 7. Create the Runway

The runway is a rather grand name for the smooth panel of hardboard (N). It is along this that the projectile will be pulled before it is lifted by the rotating arm.

It fits on top of piece M (Figure 2-13) that sits across the base and doubles as the point to attach the trigger mechanism.

Step 8. Fashion the Trigger Mechanism

The trigger mechanism is quite unsophisticated. It comprises a nail on a string and a hole that goes through the throwing arm and the piece of wood M (Figure 2-14). To fire the trebuchet, simply pull the
nail out.

Fire!

A tennis ball makes a good projectile. It has the advantage of being fairly tough and not being damaged by a collision with a target (minion). To test the trebuchet, a suitable open space should be found and minions dispatched to a reasonable distance in front of the trebuchet. Fit the ball into the sling and pull down the throwing arm until it can be pinned by the trigger nail. Stretch out the sling in the runway, with the ball at the far end, so that the rope is straight and the loop is over the pin on the end of the throwing arm.

Stand clear of the trebuchet and pull on the string to release the nail holding the throwing arm in place. The ball should sail off into the distance. Dispatch a minion to retrieve it and do it again, because it’s fun.

Tuning the Trebuchet

There are various things you can do to the trebuchet to squeeze the best performance out of the engine. You will likely need to make a few adjustments to the pin before this works well. If the ball flies off too low to the ground, then bend the pin back a little so the sling is released earlier. Or, if the ball is released too early and flies straight up into the air, bend the pin forward a little.

This book’s web site (www.dangerouslymad.com) contains links to videos showing the trebuchet in action. These may be useful to refer to.

Theory

The trebuchet takes its energy from the weight that falls as the arm swings. This “potential” energy is transferred to the arm and sling of the trebuchet and is released as kinetic energy in the tennis ball. As we know the energy stored in the weight and know how far the tennis ball can be thrown, we can calculate both the energy going into the system and the energy released into the ball by the system. This will allow us to calculate the efficiency of our trebuchet.

The input energy can be calculated using the formula:
E =  mgh
where m is the mass of the weight, g is the gravitational acceleration on Earth (9.8), and h is the height.

So the energy in joules is approximately:
E = 8 kg * 9.8 * 0.5m
   = 40 joules

On the other hand, we can calculate the approximate amount of energy that was transferred into the ball on the basis of the distance that it traveled and its weight.

E    =  ½ (mv^2)
d     = (v^2/g)

v^2 = dg

E =  ½ (mdg)

The tennis ball weighs about 60 g and a good throw will send it 25m.
E =  ½ * 0.06 * 25 * 9.8
   = 7.35 joules

From this, we can calculate the efficiency of our trebuchet at 7.35/40 18%. This figure of 18% neglects the fact that the tennis ball has to travel through air and is probably not launched at the optimum angle. So, the real figure is probably closer to 25%.

This does not sound great, but actually is quite typical for a homemade trebuchet. These very rarely exceed 30%.

The following are a few things you can do to improve the efficiency of the trebuchet:
■ Vary the length of the sling; start by making it longer.
■ Decrease the weight of the throwing arm by reducing the amount of wood at the sling-end, or cutting it into a wedge shape.
■ Vary the pin angle. Just in terms of increasing the range, without changing the efficiency, increasing the weight is an easy win. You could use a bigger container, or use gym weights. You will, however, quickly need a stronger frame and throwing arm as you increase the weight.

Summary

If you want to see what happens when you scale up a design like this, take a look at YouTube. It shows some truly enormous trebuchets, which can throw people, pianos, barrels of flaming gasoline, even cars. Now that’s evil! 

In the next chapter, we will continue with the theme of weapons and make ourselves a ferocious minigun.