Whipper trebuchet

A three-degree-of-freedom launch mechanism developed through multibody dynamics, structural checks, detailed design and physical testing

Year: 2023
Role: Dynamics, simulation and detailed design
Tools: Python, SciPy, Pymunk, SolidWorks

Assembly drawing and bill of materials for the whipper trebuchet — Complete SolidWorks assembly and 26-item bill of materials

Model

I rebuilt the mechanism as a three-degree-of-freedom model with one rigid main arm, a floating counterweight link and a massless tension-only sling. The Python simulation integrates the power stroke, selects a feasible release state and switches the projectile to a quadratic-drag flight model.

With the documented mass assumptions, the model predicts a 30.5 m/s release and 43.5 m of range. The report states each estimated component mass so measured or CAD-derived values can replace it.

30.5 m/sModelled release speed

143 ftDrag-inclusive simulated range

625 NPeak modelled pivot reaction

Power stroke0.00 s

30.5 m/s release speed33.9° release angle43.6 m range with drag625 N peak pivot reaction

Geometry

Long arm0.914 mShort arm0.183 mCounterweight link0.635 mSling length0.823 mPivot height1.079 mProjectile diameter0.067 m

Masses

Long-arm mass0.55 kgShort-arm + hub mass1.10 kgCounterweight-link mass0.40 kgCounterweight mass13.6 kgProjectile mass0.057 kg

Release and environment

Initial arm angle-140 °Initial link angle-70 °Initial sling angle20 °Gravity9.81 m/s²Air density1.23 kg/m³Drag coefficient0.55

Release timing

Manual release time0.357 s

Projectile speedm/s

ProjectileRelease

Mechanism anglesdegrees

ArmCounterweight linkSling (attached)Release

Sling tensionN

TensionRelease

Attached-model energy errorpercent

Numerical errorRelease

The browser solves the multibody equations and drag-inclusive flight again whenever a parameter changes. The post-release arm and counterweight continue under their own two-degree-of-freedom dynamics.

Document

Dynamics and design calculations

Open PDF Download

Detailed design

Recovered joint reactions replace the earlier scalar acceleration estimate. The model gives a 625 N peak pivot reaction and a 34.3 N m peak long-arm root moment; these feed the shaft, stress and deflection checks.

The SolidWorks package includes the full assembly, exploded views, component drawings and a 26-item bill of materials. It preserves adjustment at the release and counterweight interfaces.

Exploded technical drawing of the trebuchet arm assembly — Exploded arm assembly and release components

Document

Trebuchet drawing package

Open PDF Download

Physical test

We tested the manufactured mechanism with tennis-ball projectiles. Friction, flexibility and release behaviour exposed the limits of the clean numerical model and provided a basis for calibration.

Completed whipper trebuchet outdoors before testing — Completed mechanism prepared for testing

Trebuchet test footage

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