Definition
VS1 is the stalling speed, or minimum steady flight speed, of an aircraft in a specified configuration — typically with the landing gear and flaps retracted (the 'clean' configuration). Below this speed, the wing can no longer produce enough lift to support the aircraft in level flight at 1G, and a stall will occur.
Plain English
The slowest speed the airplane can fly without stalling when the gear is up and the flaps are up. Go any slower in this configuration and the wing stops producing enough lift to keep flying.
Context Anchor
Seen on airspeed indicator markings and in aircraft performance information when discussing stall speeds and the green arc.
Derivation
The 'V' comes from the French word 'vitesse,' meaning speed. The 'S' stands for stall. The subscript '1' indicates a specific configuration — in this case, clean (gear and flaps up). Knowing 'V = velocity/speed' helps decode the whole family of V-speeds (VS, VSO, VS1, VFE, VNE, etc.).
Why Pilots Care
Determines safe operating margins during climb, cruise, and approach planning when the aircraft is in its cleanest aerodynamic state.
Grounding Statement
With the landing gear and flaps retracted, the airplane needs at least about VS1 to keep flying steadily without stalling.
Intuition Check
Do not read the “1” as meaning “first speed to memorize.” It identifies a particular stall-speed condition: gear and flaps retracted in this context.
Example Sentence 1
After cleaning up the airplane on climbout, the pilot kept airspeed well above VS1 to maintain a safe margin from the stall.
Example Sentence 2
The performance section lists VS1 as the reference for calculating the clean-configuration stall margin at different weights.