Definition
Airfoils whose upper and lower surfaces have identical curvature, so the shape is a mirror image above and below the chord line. A symmetrical airfoil produces no lift at zero angle of attack and generates lift only when tilted to the relative wind, producing equal lift in either direction depending on which side faces the airflow.
Plain English
A wing or blade shape that looks the same on top as it does on the bottom. Because both sides match, it makes no lift when it's flying perfectly level to the oncoming air — it has to be tilted to produce lift, and it can lift equally well either way up.
Context Anchor
Seen in airfoil behavior discussions, aerobatic airplane design, helicopter rotor blade discussions, and some tail surface descriptions.
Derivation
Symmetrical comes from the Greek 'symmetria,' meaning 'same measure on both sides.' Applied to an airfoil, it means the top and bottom surfaces are shaped the same — a useful clue that this airfoil has no built-in preference for one direction of lift.
Why Pilots Care
These airfoils produce equal lift whether right-side up or inverted, which matters for aerobatic and certain training aircraft.
Grounding Statement
Picture slicing a wing from front to back: on a symmetrical airfoil, the top outline and bottom outline match.
Intuition Check
Symmetrical does not mean the whole airplane is balanced left-to-right here. It means the upper and lower curves of one airfoil cross-section match each other.
Example Sentence 1
Aerobatic aircraft often use symmetrical airfoils so the wing performs the same upright as it does inverted.
Example Sentence 2
At zero angle of attack, symmetrical airfoils produce no lift and require a positive pitch attitude to climb.