Tuft (aeronautics)

In aeronautics, tufts are pieces of yarn or string, typically around long, attached to an aircraft surface in a grid pattern and imaged during flight. Their motion can be observed and recorded to locate air flow features such as boundary layer separation and reattachment. Tufting is, therefore, a technique for flow visualization. They are used during flight testing to study air flow direction, strength, and boundary layer properties.
The world's largest bed of tufts was created at NASA Ames Research Center to study air flow fields involving a helicopter's rotor disk.