Fly System Counterweight Rig

Batten Load

Scenery load 60 kg

Arbor Trim

Counterweight 95 kg

Rope/pulley friction 8 kg

Rig Readout

Net imbalance (arbor − batten)

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Motion

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Batten height

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How it works

A counterweight fly system is a real Atwood machine: a batten (the pipe scenery hangs from) is tied off on one side of a rope, routed up over headblocks and loft blocks, and down to a weighted arbor on the other side. Whichever side is heavier falls, pulling the other side up — set the counterweight to match the load and the batten "floats", hands-off.

m_batten = pipe(35 kg) + load
m_arbor  = cage(15 kg) + counterweight
a = [(m_arbor − m_batten)·g − f·sign(v)] / (m_arbor + m_batten)

The friction term f is the combined stiction of the ropes and pulley blocks: as long as the imbalance stays under it, the rig holds position even with the rope lock off. Push the imbalance past it and the system accelerates — a small excess is a controllable "hands-off" trim, but a large one is exactly how a real fly system "runs away": an under-weighted arbor lets the batten crash to the deck, an over-weighted one shoots it violently into the grid.

Scenery load — weight hung on the batten out in the house.
Counterweight — bricks loaded onto the arbor to balance it.
Friction — how much imbalance the rig can hold before it moves on its own.
Auto-Trim Arbor — loads exactly enough counterweight to match the current load, the way a real fly crew trims a pipe before a show.
Release Brake — lets the rope lock off so gravity and the imbalance actually move the batten and arbor (in opposite directions, since they share one rope).

Lift / purchase line Scenery load Counterweight bricks

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