Definition
VS1 is the calibrated power-off stalling speed, or the minimum steady flight speed at which the aircraft remains controllable, in a specified configuration. The configuration is defined by the manufacturer and is typically a clean configuration (flaps up, gear up if retractable). VS1 is shown on the airspeed indicator as the bottom of the green arc.
Plain English
The slowest speed the airplane can fly, with power off, in a defined clean configuration, before it stalls. Below this speed, the wing can no longer produce enough lift and the airplane stops flying normally.
Context Anchor
Seen in aircraft performance charts, operating handbooks, and discussions of stall speed margins for a particular aircraft setup.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed — used throughout aviation for standard speed references. The 'S' stands for stall, and the '1' identifies this as the stall speed in a specific (usually clean) configuration, distinguishing it from VS0, which is the stall speed in the landing configuration.
Why Pilots Care
Pilots use VS1 to set safe margins above stall during maneuvers and to calculate appropriate approach and departure speeds.
Grounding Statement
VS1 marks the edge of controllable slow flight for the specified aircraft setup.
Intuition Check
Do not treat VS1 as one universal stall speed for the airplane. It applies to a stated configuration and is given as calibrated airspeed, not necessarily the exact number shown on the cockpit airspeed indicator.
Example Sentence 1
During slow flight practice, the student gradually reduced power and pitched up until the airspeed approached VS1 and the stall warning began to sound.
Example Sentence 2
In the handbook performance section, VS1 was listed for the clean wing configuration.