While a fairly rough and ready approach will do for much of the design and construction of a trebuchet, model or full-sized, making a good sling which interacts well with the release mechanism is something of a fine art. The sling needs to encapsulate the projectile tightly while the arm pulls it upwards yet not catch the projectile as it releases. A nice way to achieve this is by incorporating folds into a reasonably stiff material, allowing it open out in a predictable way.
Cut out a a rectangle of card slightly larger than the the size required to completely wrap your projectile with. 3 inches by 2 inches is a good size for a model trebuchet that stands 10 to 12 feet high and which uses marbles as ammunition. However this design can be easily scaled up and thicker card used for larger models. All other measurements given also assume a model of this size. Cut the card to double this size and fold it in half if you feel greater strength will be needed.
Fold the rectangle in half lengthwise to form two thinner rectangles, fold each of these in half the opposite way from which you performed the first fold. This should give you an M shape at each of the short ends of the card.
Push the bottom, middle point of the M inwards, pressing the flap of card inwards between the ridges of card on either side. Push the whole corner inwards and squeeze the two ridges on either side around it creating raised triangle of card with a fold in the middle. Do the same at the other end of the card. The card should end up looking a bit like a trough if placed flat on the table.
Unfold the two outer flaps of card and bring them together over the top of the "trough". Cut a hole in each corner just large enough to pass the string through. Unfold the whole thing and wrap it in several layers of duct tape, avoiding the holes. Reinforce the edges of the holes with more tape. Refold the tape wrapped card so that you have the trough with the fold blocking each end and the two flaps raised up above it on either side, just as it was at the beginning of this step.
Push one string down through a hole in one corner of the card, along the back of the flap and back up through the other hole on the same flap. Do the same with the other string. Slide the card cradle along the strings so that it is positioned exactly in the middle.
Tie or otherwise join the two ends of one string to the trebuchet arm. Tie the ends of the other string to the metal ring. The ring will slide over the release mechanism, allowing it to be held in place until the critical moment when it will be pulled free by the outward momentum of the projectile in the sling's cradle. This will cause the hinges made by the folds in the card to open and release the projectile.
Things You Will Need
- Thin card
- Duct tape
- Craft knife
- 2 x 14" lengths string
- 1" diameter metal ring
Large trebuchets can be extremely dangerous, both at the point where the projectile lands and in their general operation. Take extreme care in operating them.