What kind of airfoil has a mean camber line that is the same as the chord line?

A symmetric airfoil features a mean camber line that coincides with the chord line. This means that the airfoil is geometrically identical on both the upper and lower surfaces. Since there is no curvature above or below the chord line, the lift characteristics of a symmetric airfoil are very controlled and predictable, particularly when in an uncambered state (i.e., at zero angle of attack).

In practical terms, symmetric airfoils are often used in applications such as aerobatic aircraft and some specific types of gliders, where the design encourages equal lift characteristics regardless of the angle of attack, up to a certain limit. This symmetry provides a favorable lift-to-drag ratio and optimizes performance in various flight conditions.

The other options describe airfoils with a distinct mean camber line that either creates a positive or negative curvature relative to the chord line. These airfoils are designed to optimize lift and drag for specific flight scenarios, but they do not have the same mean camber line as the chord line, which is a defining characteristic of symmetric airfoils.