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Saturday, September 01, 2007

The Majestic Mangonel

So last night my wife discovers that on Tuesday my 6th grade daughter is to have created a catapult or trebuchet for science class, much to my surprise.

Although there weren't many parameters other than that it had to fire a golf ball into a newspaper wall and be about 2 feet tall, we Googled a little to see what we were up against. What we discovered was that there are a few sites that sell plans and a few others that show photos and so we worked from there. In the end we decided to base our catapult on a design called the Mangonel (from the Greek word "magganon" meaning an engine of war) which was descended from the Roman onager (Greek for "wild ass" nicknamed as such because it kicked like a mule when discharged).

In any case we spent about an hour last night planning and then three and half more this morning completing the project and thought we should provide some instructions for those like us who might need to do something similar in the future.

In order to provide the force the mangonel relies on torsion from a twisted cord in which the arm is inserted. We decided to use one of those black rubber straps used to fasten items to a car carrier. We replaced the metal hook with a small drill bit and threaded the bit through both ends of the strap creating a loop.

For the base we used some 3.5" by .5" boards and cut two of them 24" in length. We then drilled a 1 1/8" hole 10" from one end in both boards. After measuring the length of the strap when pulled taught we cut two more boards 16.25" in length. After screwing them all together we had our frame.

When the arm is released it's stopped by a pad and so we simply put one more board on top and fastened another 13" board in the center and used a rubber band to secure a bit of foam to the top.

Finally, we added two boards to the bottom so it would sit up a little along with cardboard wheels to give it that historical touch.

To load the catapult we ran the strap through one of the holes pulling it taught at the end with the drill bit. It took a little force but together we were able to pull the loop end through the other hole and slide a stick of sun block in it. Now the strap (or skein in the technical sense) is pretty tight. From there we slid the arm, which was a wood strip about 1.5" wide and cut to 18", into the middle of the strap at the center. By twisting the drill bit on one side and the stick of sun block on the other you can then create the tension in the strap. After about 8 turns we found the sun block would start turn back so we added a small piece of wood to the top that can be turned to stop it from twisting backwards. At this point the arm wants to swing forward and so we put two nails in the board on which the arm rests and a rubber band between them to hold it down and act as the trigger.

On the arm itself we created a small cardboard cup in which the golf ball rests and used duct tape to secure it.

All that's left is to load the golf ball and fire by releasing the rubber band and the arm swings up and releases the ball as it hits the padded board. It doesn't fire as hard as I had imagined and ends up propelling the ball about five feet on a line. Of course, the more you twist the strap the more force you get.

1 comment:

Carol Gant said...

Pity the child that does not have the family or tools or resources to do this.
Carol from the other myfamily site