Plain English Meaning

The way a soft fabric wing puffs out and changes shape when air pushes against it in flight, similar to how a sail fills with wind.

Full Definition

The flexing, ballooning, or change in shape of a flexible wing surface (such as a hang glider sail or paraglider canopy) under aerodynamic load, where the fabric bulges outward between its supporting structure as air pressure pushes against it.

Context Anchor

Seen when studying flexible wings, such as parachute-style canopies or other wings that get part of their shape from airflow.

Derivation

From the Old Norse 'belgr' meaning a bag or bellows, which evolved into 'billow' meaning a large swelling wave or . The image is of fabric swelling outward like a wave or a filled sail.