Definition
CD is a dimensionless number representing the drag characteristics of an airfoil or aircraft at a given angle of attack. It relates the drag force produced to the dynamic pressure of the airflow and a reference area, allowing drag to be compared across different speeds, altitudes, and aircraft sizes.
Plain English
CD is a single number that captures how much drag a wing or aircraft makes at a particular angle. A higher CD means more drag for the same airflow conditions.
Context Anchor
Seen in discussions of lift/drag ratio, aircraft performance, and how airplane shape or configuration affects drag.
Derivation
The 'C' stands for coefficient, meaning a ratio or multiplier. The 'D' stands for drag. Together, CD is the drag coefficient -- a number that scales with the actual drag force when combined with airspeed and wing area.
Why Pilots Care
Lower CD values improve glide range, fuel efficiency, and maximum speed.
Intuition Check
CD is not the drag force itself. It is a number used to represent and compare how much drag is being produced.
Example Sentence 1
As angle of attack increases toward the critical angle, CD rises rapidly, which is why drag climbs sharply just before the stall.
Example Sentence 2
At high angles of attack the CD rises sharply, increasing total drag.