The next step of our trebuchet project is to test our initial prototype. We printed the arm and attachments from the previous post.
So this morning we got together to do some testing on our trebuchet. We rigged up a sling using string and the curved portion of a 2L soda bottle, and a weight system using a chunk of brass in a plastic bag:
From right to left: Plastic bag with brass counterweight, string to attach counterweight, throwing arm, (hard to see) attachment with finger at 0 degrees.
We then began testing throwing M&Ms. First we looked at which arm ratio worked best, and then which finger angle worked best.
After just a couple throws varying the arm ratio pivot point it was very clear that the 4:1 ratio was the best. This testing was very easy.
We didn’t get much variance between which end piece we used for the finger. We think this means that friction is negligible at this scale and so only extreme angles would have an effect. Because of this we plan to work with an angle of 0 degrees for our final design.
We learned that the way the sling is positioned/resting pre-launch has a large effect. This is something we had not considered prior. The sling should be resting on a surface prior to launch and the strings holding it should be extended fully so that no “jerking” occurs when the counterweight is released. We want everything to be one fluid motion.
The start position of the arm also has an impact on the release point. How high the counterweight is pre-launch has an effect on the amount of potential energy that is available to us for launching the M&M. We decided that we will create a design that will allow us to start the counterweight as high as possible to maximize this potential energy.
We also determined our next iteration of the throwing arm does not need as much material as this one. It will be contoured to provide support where needed but minimize weight everywhere else. This will result in less inertia in the arm and will make it easier to accelerate.
Below is a link to a slow motion video of one of our tests:
As you can see the M&M left at about a 45 degree angle and traveled about 30 feet. These are both very encouraging, especially given the less than ideal circumstances we were testing with.
Heres a picture of an M&M we broke when it hit the wall on the other side of the room (Look out trebuchet #1)
