This is the trebuchet that I built for a contest between me and a friend. The rules were that we had to make it so that it would fit into a 12" box with out the arm. Then see who's could launch the farthest. The hardest part of making this was getting the sling to hold the ball, and let go at the right moment. If it lets go too soon, the ball will go straight up, too late, and it pounds into the ground a foot away.
Launching Distance: 30ft max
Arc Heigh 8ft max
The steel ball shown above next to the minifig is the projectile it launches. The weight shown below is filled with steel balls, and weighs about 3.5lbs. It is 32 x 4 x 12 studs (excluding the support braces.)
As seen by the pictures above and below, the arm had to be reinforced so that it would not bend. If the arm is bent, then it has less flinging power.
The arm and sling (above) need to be the right length. The brown piece on the end of the arm is where the other side of the sling attaches. It is the releasing mechanism. I played with about 4 different designs until I came up with this one that seems to give the ball enough of an arc to launch it far, but not so much as to go straight up. The sling is just a rectangle of fabric folded over a few times.
Above and below, The Axle is not a legoŽ rod. I had to replace it with the long side of an allen wrench. The legoŽ rods kept bending under the weight. Also the Reinforced A arms of both sides are critical to keep the Trebuchet from falling over once the weight is released.