What were the key factors that made the wright brothers’ wind tunnel a useful tool for wing design?

Balancing equipment.

Explanation:

The wind tunnel made by Wright's brother was made up of a wooden box with glasses on it to see the inner instruments work.

The key factor that made the Wright's brother wind tunnel a useful tool for wind design is the balancing equipment used in it.  It was designed to measure lift and drag. Old hacksaw blades and bicycle spokes were used to make the balances which measure the forces of lift and drag action on a wing.

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