Definition
VX is the airspeed that produces the greatest gain in altitude over the shortest horizontal distance. It is the speed at which the difference between thrust available and thrust required is greatest, allowing the aircraft to climb most steeply for a given distance traveled across the ground.
Plain English
VX is the speed that gets you the most height in the shortest distance forward. It is the steepest climb the aircraft can do, used when you need to clear something close ahead, like trees or wires off the end of the runway.
Context Anchor
Seen in takeoff and climb performance discussions, especially when obstacle clearance is a concern.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed. The subscript 'X' is simply a label the FAA uses to mark this specific speed among the family of V-speeds. Knowing the V means 'speed' helps you read all the other V-speeds (VY, VS, VNE, etc.) as members of the same family.
Why Pilots Care
Enables safe obstacle clearance immediately after takeoff when runway distance is limited.
Grounding Statement
Use VX when the main problem is not time, but space: you need to gain height before traveling too far forward.
Intuition Check
Best angle does not mean the highest nose position. It means the speed that gives the steepest climb path over the ground.
Example Sentence 1
With trees off the departure end of the runway, the pilot rotated and held VX until the obstacle was cleared, then lowered the nose to VY.
Example Sentence 2
During the short-field takeoff briefing, the crew reviewed VX to ensure adequate obstacle clearance.