The Trebuchet Project (part 1)

One day, I opened up my work email and read the boring things going on at Wright Patterson Airforce Base and found out about the 5th annual pumpkin chucking fundraiser. Now, this sounded interesting. The contest was to build a catapult or trebuchet that could fling a 6 pound pumpkin a good distance. Unfortunately, this requires a fairly large catapult and is way beyond the scale of anything I can do with my time and money. But, for the first time, is read, there was going to be an amateur class with a 5 x 5 foot size limit and a projectile (pumpkin) of only 2 pounds. This sounded much more reasonable.


So, over the next week, my coworkers and I talked about designs and feasibility of actually competing in the contest. We started with a rotating arm that went round and round until it finally released the pumpkin, but decided that a trebuchet was probably the most efficient method. I visited many websites and absorbed as much knowledge as I could about trebuchets and finally found the most complicated design out there: the floating axel king Arthur trebuchet. Not only are these things incredibly complicated (compared to a regular trebuchet), but they are very efficient for their relatively small size. This means, if we could figure out how to build it, we could get a really good range for a small trebuchet. So, after drawing up some plans based on a wooded FAKA (floating axle King Arthur) trebuchet, I convinced some of my coworkers to actually try and build it.

The FAKA compared to other trebuchets (from the Hurl site):
"The FAKA is a trebuchet design invented in late 2006 by Craig Macomber. It stands for Floating Axle King Arthur. It combines the efficiency of a Floating Arm Trebuchet (FAT) with the increased potential energy of a King Arthur.
Basically, it gets a lot of energy for it's size and weight, and uses it very well, but there are many challenges. A FAKA is a very high end trebuchet form the efficiency perspective. They work great over a huge range of mass ratios. The FAKA is very efficient for its frame height and arm length as well. FAKAs are hard to build, and have have a tendency to go very wrong, and often break things, if the timing is a little off."

Video of other FAKAs:


A regular trebuchet consists of a long arm (usually wood) that can turn on an axle. At the end of the long side of the arm (the long arm or TA) is where the sling is attached. The sling is two ends of a rope with a pouch attached in the middle. You put your projectile in the pouch. When the long arm swings upward, the sling is pulled up and around with it. This creates a whipping motion for the projectile, which is released at about a 45 degree angle so as to get the best distance. The other side of the arm has the short arm and the hanger. The hanger is attached to the short arm and contains all your weight (counterweight). So, to set it up you prop up the weight, stand back, and pull the pin to let it start dropping. The picture below shows the setup.


The next Sunday, Terry and I met at Lowes and purchased all the supplies. We met over at Dan’s and spent the entire day assembling the structure, the track, and the arm. The track gave us a lot of problems. At first, we tried to go with casters on a metal rail, but this didn’t even come close to working. After some redesign and prototyping on the spot with whatever we could find, we finally came up with a rollerblade wheel block for our track. This, so far, seems to be doing fairly well. After a long day, we decided we could try out a dry run (no pumpkin or sling), just to see what it would do. We attached 15 pounds of weight (hoping to get up to 100 pounds by the end of our testing) and propped up the arm. We stood back and pulled the pin, just to watch the counterweights get pulled down by gravity and sheared right out of the wood that was holding them. I guess 2 x 4s are now strong enough to support a little weight. So, back to the drawing board. Our new design for the counterweight arm is made entirely out of iron pipe and should be strong enough to support the 100 pounds. So far, we are up to 30 pounds of counterweight and could throw a 2 pound ball 45 feet. Next weekend we will try to up the weight and see what happens.


Time Lapse Video of the Creation of the Trebuchet


Testing and Tuning the Trebuchet (Sir Launch-a-Lot)