Floating Arm Trebuchet

Dylan Woodhead

Projectile Brainstorm

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Before we build our full scale model, or even our small scale, we have to find out what type of launcher we would build. After splitting into groups of three as a class we built mini scale models of each of the brainstormed projectile launchers we had come up with during a class discussion: slingshot, catapult, trebuchet, air cannon, and crossbow. My first thought, largely because it was easy to build, was a slingshot so my group started work.

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Slingshot Build

We started by taking a plywood base and building a wide V shape for the launch. We then attached our rubber band/duct tape sling to the wings to the V and attached a string to the end of the sling. We ran the string over a block of wood on the end and through a hole in the plywood. We attached a nut to the end of a string, and then added weight onto a string with a nut on either end. We ran a pin through all three nuts and attached a string to the end of the pin

Slingshot Results

After completing the slingshot, we launched our miniscale device, along with the other groups launching all of their different designs (the video of our launch is to the left). We then made a decision matrix of all the designs, with categories that included distance (5), accuracy (5), efficiency (4), consistency (3), ease of operation (3), athsetics. (3) [Parenthesis and number tell the weight of the category] With our matrix complete, we decided that a trebuchet was the best option.

Floating Arm Trebuchet (Week 1)

After deciding on a launcher, we started work on a small scale model. Our new group decided to build a floating arm trebuchet, a supposedly more efficient version of a trebuchet. We divided the device into three tasks, base, supports, and launching arm, then assembled it on the final day. We built base and supports out of small blocks of wood and machine screws, while the arm was a thicker piece of wood, a thick metal rod, a thinner metal rod with wooden wheels, and a sling and launching mechanism (to be attached).

To the left and right are steps 1 and 2 of the base of the floating arm

To the right is the support rails while to the left shows the base attached to the support rails

Small Scale Outline

Small Scale Results

Overall, our small scale Floating Arm trebuchet was very successful. It had the highest efficiency in the class, launching a tennis ball around 15 feet with only 5 joules of energy. Below is Logger Pro Analysis of the launch data

Big Scale Week 1

So to start out our Large Scale version, we scaled up our small scale 5x and started work. I was tasked again with building the square base of the Trebuchet. I used 2x4 cut at 45 degree angles and mated them together with Machine screws. I left an inch wide hole in the center for the bar to drop. The two side supports were also made with 2x4s and Machine Screws. The first week was mostly construction and wood building before assembling the trebuchet in week 2.

Big Scale Week 2

After assembling the base, I built the trigger mechanism using an old door lock and a wedge of wood attached to the arm. I also built the roll bar for the throwing arm. After sliding the arm itself through a 5/8 bar of cold roll steel, we welded washers to prevent the bar and the wheels from sliding. After adding the weight bar to the throwing arm, we attached all three parts, the base, arm, and side supports, together. We used PlasmaCut pieces to attach each part to one another, building a very large, sturdy Floating Arm Trebuchet. We then added finishing touches including the rag-made sling.

Launch/Results

In order to launch our trebuchet with exactly 100 joules, we loaded 23.7 pounds to our trebuchet with a stroke of 94.5 cm. After adjusting the launch angle of the trebuchet, we launched.

Analysis

After analyzing our Big Scale data in Logger Pro we determined our initial Y (13.3m/s) and Initial X (12.5m/s) values, totaling an initial velocity of 18.325m/s. Our best launch went a total of 61 feet. We think our trebuchet wasn’t as successful because it was too big. The arm didn’t stop in the swinging motion meaning that all the energy wasn’t completely transferred. Nonetheless, the results from our Floating Arm Trebuchet wasn’t half bad - 55.3% efficiency.

Project Response

Overall I think this project was a fun success. I learned a lot about potential energy, joule calculations, and energy transformation. I also learned a couple informal lessons, including building it bigger is not always better, and learning how to deal with warped wood. If I started from the beginning of the big scale, I probably would have kept the same design, because it was most efficient prototype, but not scale it up 5x: probably 4 or even 3x. I enjoyed this project because It demanded that we work with our hands and improved our construction skills. In the future, I might add more energy than 100 joules not so that the ball launches further, but because it asks more of the strength of the trebuchet, and puts pressure on the students to build a stronger machine.