OK, so you just got done watching a TV show with people launching pumpkins with catapults, and decide you want one too, but you think to yourself, those are cool, but I don´t know anything about catapults? that's ok, that is the purpose of this paper. Actually this paper will only cover one type of catapult, the trebuchet, more specifically, the counterweight trebuchet. If you want to make your own, then there is a few things you will have to know, some basic information on the parts, how the different parts work with each other, and how to make a proper trebuchet. I will discus these different things throughout this paper.
So, what's a trebuchet and how does a trebuchet work? Back in the middle ages, these things were huge, and could throw few hundred pound projectiles, hundreds of feet. Today they are much smaller, though you will find some people who have made full sized trebuchets, but in general they are smaller. Big or small, a trebuchet has the same set of parts, and works no differently than one of a different size. The trebuchet is a siege weapon that consists of a base with two A-frames that hold an axle at the top. On this axle is whats known as the throwing arm. The throwing arm has two parts, a short end on one side of the axle with weight on it, and a long end on the other side that has a sling attached to it. The arm is held with the short end up, then when released, the weight pulls the the short end down and the long end up. As the long end goes up, it pulls the sling along with it, eventually the sling whips around and releases a projectile. The projectile then flies through the air, wile the counterweight makes the arm swing back and forth wile it finishes using up its energy. Little things here and there may differ from trebuchet to trebuchet, but overall, thats what all counterweight trebuchets have in common.
Counterweight trebuchets break up into two different categories depending on how the counterweight is mounted. If the counterweight is fixed to the short end of the throwing arm, then the trebuchet is a ?fixed counterweight trebuchet?, if the counterweight is suspended from the throwing arm in a manner that lets it move without moving the throwing arm, then the trebuchet is a ?hinged counterweight trebuchet? also known as a ?hanging counterweight trebuchet?. The difference between the fixed counterweight and the hanging counterweight trebuchet are really a matter in performance. A correctly made hanging counterweight trebuchet will always outperform a fixed counterweight trebuchet of the same size. Why? Because the way the counterweight moves. In the fixed counterweight trebuchet, the counterweight arcs around the axle as gravity pulls it down. Since gravity is a downward force, and the counterweight moves in an arc, much of the energy is wasted. One common way to solve this is to add wheels to the base. Adding wheels to the base allows the trebuchet to move forward wile the counterweight falls, this straightens out the actual path of the counterweight. However, because some of the energy is used to move the trebuchet, you still end up wasting some energy that could be used for throwing the projectile. The hinged counterweight trebuchet avoids the problem that the fixed counterweight trebuchet experiences because the weight isn't fixed to the trebuchet. The weight hangs from the throwing arm in a manner so that it can rotate freely on the throwing arm. This can be done by hanging the weight from a rope or chain, or by having the weight attached to another arm that hangs from the short end of the throwing arm. When the weight drops, it's path doesn't initially arc around the axle, instead it drops relatively straight down, then jerks back towards the trebuchet and swings chaotically back and forth through the trebuchet. Although the hinged counterweight trebuchet is more efficient than the fixed counterweight trebuchet, there is nothing wrong with choosing to make a fixed counterweight trebuchet.
The counterweight, as you have probably guessed is weight. This weight is pulled down by gravity, the energy of the falling counterweight is what fuels the trebuchet. Anything will work for weight, but to save on space, try to get something with a high density. As a rule, the counterweight should be about 1000 times greater than the weight of the projectile that you are throwing. You could go beyond that, but eventually you hit a point where adding more weight doesn't have have a noticeable effect on range.
When picking a projectile, there are a few things to take into consideration. A projectile should be aerodynamic, a sphere shaped projectile is best as it is more aerodynamic than something that has sides and edges. The best projectiles seem to be those with greater densities, so iron will work out much better than wood. Also, use something that wont stick to or snag on the pouch. So the ideal projectile would be a smooth, dense, sphere.
A trebuchet is a very dangerous thing, especially if its a big one. If you get hit by the throwing arm or by the counterweight then it will hurt. Also, remember that at one end of the throwing arm is the release pin, that is moving with a lot of force behind it and can seriously harm you. To make the trebuchet a bit more safe you will need a way to fire it from a safe distance. There are many ways to approach this task; one of them is to have a ring on the long end of the throwing arm, this ring lines up with a set fixed on the base, then a pin with a rope is slid through the rings so that it holds the arm in place. Pulling the string will pull the pin out, letting go of the arm and the trebuchet will do it's thing. Another, more simple way to control when the trebuchet fires is to have a tethered board hold the counterweight or, in the case of the hinged counterweight trebuchet, hold the short arm up. Pulling out the board will allow the counterweight to fall. You can find and try other methods, those are just two easy ways to fire the trebuchet from a safe distance.
To function efficiently, everything has to ?fit? with the rest proportionately. These proportions have come from trial and error over time to show what tends to work best. These numbers are flexible because the friction and other miscellaneous forces on each trebuchet will vary. You start with the desired height of the axle. The long end of the throwing should be long enough so that it rests on the ground at an angle around 45-60?. The short end of the throwing arm should be about one fourth to one fifth the length of the long end and if the trebuchet is a hinged counterweight trebuchet, then the distance that the center of gravity of the counterweight hangs should be the same length. Remember to make sure that when the counterweight comes down, it doesn't hit the base, or trough. Half the sling length should be between the length of the longer end of the throwing arm and the distance from where the long end of the throwing arm rests to directly below the center of the counterweight. These numbers are a general guideline just because a trebuchet doesn't follow these numbers doesn't mean its a bad trebuchet.
Now that you know some basic information about the parts of the trebuchet it is time to make one. For materials, wood, metal or anything should that is strong enough to handle all the stresses that the trebuchet will have. The axle will have to be strong to figure out how strong it will have to be, use this equation to figure out how much weight it will have to support (((25^2)/(2*the distance the counterweight falls in ft.))/32)*the weight of the counterweight. For the axle take steps such to reduce friction, as the friction on axle increases, the maximum range will decrease. For the throwing arm, it is best to make the long end as light as possible or at least light enough so that without the counterweight, the throwing arm is balanced. When the counterweight falls, it lifts the long end of the arm, projectile, and sling, the counterweight will fall slower if there is more weight to lift. In some cases, it is better to have supports sticking out from the side of the trebuchet. This will keep the trebuchet upright if it experiences a sideways force, this will happen as the counterweight falls if the arm isn't mounted to the axle at 90?. As for the base, it needs to be big enough so that it will always be under the center of gravity as the counterweight moves.
Lets assume you made your trebuchet, got excited and called everyone you could to come see it fire, but instead of the projectile going far off into the distance, it only goes a few feet. You may ask yourself, ?Is this as far as it will throw?? odds are, no. what you need to do now is go through the process of tunning your trebuchet. First, make sure that the sling doesn't hit or snag on anything before it releases the projectile, then check to see if the arm rotates smoothly. If all that is good, then you can move on to adjusting when the projectile launches. Ideally the projectile should launch at an angle of 45?. The easiest way to tell if its at, or around 45 ?, is to watch the projectiles path. If the path is path is low to the ground, then it launched too late, if the projectile's path goes real high, then it launched too soon. There are quite a few ways to adjust when the projectile fires, you could adjust the sling length, change the weight of the counterweight or projectile, or adjust the pin angle. Shortening the sling length will release the projectile sooner, and lengthening it will cause the projectile to release later. Adjusting the sling length means you have to make a new sling, or cut off rope from a current sling,this may not be the easiest way. You could change the weight of the counterweight, but as you know, you can't just increase weight until the trebuchet throws far, and decreasing the weight of the counterweight will reduce the maximum possible distance, even if you get a better range, you are still losing some distance. Reducing the projectile's weight is pretty easy, just get something different, a heavier projectile will release sooner than a lighter projectile. Using a heavier projectile will mean you will have to add more weight to the counterweight to make up for the added weight. If you were smart and made a release pin that you could adjust, then thats what you should mess with. Adjusting the release pin away from the trebuchet will cause the projectile to launch sooner, rotating the release pin towards the trebuchet will cause it hold onto the sling longer, delaying the release of the projectile. You can use this process to not only achieve the greatest distance possible, but also to hit targets at a certain range within your maximum distance.