Designing an Onager Catapult: Tips and Tricks for Optimal Performance

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SUMMARY

The discussion focuses on designing an Onager catapult optimized for launching tennis balls within a 0.5m x 0.5m x 0.5m cube. Key considerations include the choice between long thin ropes versus short thick ropes for torque generation, achieving a 45-degree sling release angle, and the design of an effective locking mechanism. The user is currently utilizing thick natural fiber ropes and a rectangular arm, and seeks advice on whether a round arm would enhance performance.

PREREQUISITES
  • Understanding of mechanical advantage in catapult design
  • Knowledge of torque and its application in projectile motion
  • Familiarity with materials used in catapult construction, specifically rope types
  • Basic principles of angular release mechanisms in projectile devices
NEXT STEPS
  • Research the effects of rope length and thickness on torque in catapults
  • Learn about angular release mechanisms and their optimal angles for projectile launch
  • Investigate different arm shapes (round vs rectangular) and their impact on performance
  • Explore designs for robust locking mechanisms suitable for catapults
USEFUL FOR

Engineers, hobbyists, and students interested in mechanical design, particularly those focused on historical weaponry and physics applications in projectile motion.

TofuBoy
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Hi! I'm making a onager for a project and so far I'm not having much luck! My Onager has to fit inside a 0.5m x 0.5m x 0.5m cube, and it will be throwing tennis balls.

I've been browsing around a few nights already and found this topic, which has helped me the most by far: .physicsforums.com/archive/index.php/t-155437 (please add "www" in front and ".html" at the end, I can't post links :cry:)

But there are a few things I still don't quite grasp.

1. Long thin ropes vs Short thick ropes, which one would give me more torque?

I'm guessing it's the long thin ropes because most pictures I've seen use those.
I'm using really thick natural fibre ropes right now I can only have two pass-bys on the arm with 3 full-revolution twists on each side.​

2. How would I get the sling to release the at 45 degrees like the one in this picture?
.onager.net/pict12 (please add "www" in front and ".jpg" at the end, I can't post links :cry:)

This just befuddles me

3. How would I make a locking mechanism and where do I place it?
I attempted a few ones but only to end in failure --it was either too weak or I couldn't pull it to release it.

4. Round arm vs Rectangular arm
Would it be better to have a round arm like most of the Onagers I've seen? I'm not sure if it would help with anything.

I'm currently using a rectangular arm with two nails on the side to secure it in the rope​


Any help would be MUCH appreciated!
I'm getting a headache from trying to figure these things out​
 
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