Plain English Meaning

A rotor blade that is the same shape on top as it is on the bottom. Because both sides match, the lifting force stays in roughly the same spot on the blade as the blade's angle changes.

Full Definition

Rotor blades whose upper and lower surfaces have identical curvature, so the blade has the same shape above and below its chord line. Symmetrical blades produce no lift at zero angle of attack and have a stable center of pressure that does not shift significantly as angle of attack changes, making them well suited to helicopter rotor systems.

Context Anchor

Seen in rotorcraft aerodynamics when comparing blade shapes and how helicopter rotor blades produce lift.