Building a trebuchet can challenge your understanding of carpentry, physics and mechanics while at the same time teaching your kid those same elements. Trebuchets are siege engines, developed in the medieval period, that rely on the force of a counterweight to activate the throwing arm. The throwing arm then whips a sling forward, which carries the projectile. Trebuchet models can be adopted to different size scales depending on your needs, desires or spatial considerations.
Obtain a design. There are plans either on the Internet or available through retailers or you can draft one yourself. The critical thing is that you have all five major components (throwing arm, sling, counterweight, guide chute and supportive frame) and that the weight ratios are set properly for propulsion.
Build the throwing arm and sling. Outfit the top end of the sling cord with either hooks or other hardware to receive the cords from the sling (one on each side). The bottom end needs a pair of threaded rods, one for the counterweight pivot and the other for the fulcrum. Trebuchet builder Kevin A. Geiselman suggests 50 cm (20 inch) sling cords for a throwing arm of roughly 100 cm (40 inches) in length. Equip the upper end of the arm with an eye hook to receive the trigger. with an eye hook to receive the trigger.
Fashion the counterweight. This will likely resemble a bucket design held to the contraption through the counterweight pivot. Therefore, it needs two holes at its top to receive the threaded rod and throwing arm assembly. The rest of the design can be made from treated wood sides and a pine board bottom. The counterweight materials (like lead balls) can then be placed in another vessel (like an old soda bottle) and put inside the wooden counterweight housing.
Construct the frame. Also made from wood, the frame is a tower that allows the throwing arm to rest in a downward position, connected to the tower by a string trigger. The counterweight should be suspended near the top of the tower without causing the assembly to tip over. Attach the throwing arm-sling-counterweight assembly through the fulcrum screw at the top of the tower.
Set the trigger. Place another eye hook at the base of the tower on the sling side. Now take a piece of string and attach one end to the tower hook and another end to the tower base. Add the counterweight to the counterweight receiver. The unit should balance if designed properly.
To test, take the trebuchet to a safe area. Place a projectile (foam ball, golf ball or other) in the sling. Make sure the counterweight is in position. Remove the string from the tower hook while maintaining the tension. When ready, release the string and watch the projectile soar.
Always ensure that when testing a projectile-firing device the space is devoid of potential dangers; this includes unaware bystanders or the possibility of property damage. Always have all parties stand to the rear of the trebuchet after you have loaded the sling with the projectile.