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.
Plain English
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.
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 surge. The image is of fabric swelling outward like a wave or a filled sail.
Why Pilots Care
On flexible-wing aircraft, the amount of billow affects the wing's shape, lift characteristics, and handling. Designers build a specific amount of billow into the sail to give the wing its flying shape and stability, so understanding the term matters when reading about how these aircraft are rigged and flown.
Analogy
Think of how a sailboat's sail puffs outward into a curved shape when the wind fills it. A flexible aircraft wing does the same thing as it moves through the air.
Intuition Check
Billowing does not just mean “moving in the wind.” Here it means the fabric is swelling outward because air pressure is shaping it.
Example Sentence 1
As the hang glider picked up speed, the sail began billowing into its proper flying shape.
Example Sentence 2
Proper rib stitching prevents unwanted billowing that could distort the wing shape in flight.