Flight

Flights refered to the stabilising fins found at the rear end of an arrow, bolt or dart. These reduce the tendency of these objects to flip and wobble in the air by introducing drag at the rear of the projectile.

They would be typically made of feathers, but for crossbow bolts they might easily be leather, wood or parchment.

Arrow Flights

The earliest depictions of flights for arrows are either extremely stylised, depicted with a curve that is superficially remensicient of modern parabolic flights or with the traditional parallel-ish design that children might be expected to draw. The triangular flight that we associate with medieval arrows doesn't appear until the mid-13th century. A number of variations on the back of the triangular flights occur.

Visual references sometimes omit this, but written references seem to indicate that all medieval arrows had their flights bound on.