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.

what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.

force required to detach it is given by

charges are taken to be fundamental charge

now we have

so above is the minimum distance at which it will detach