Definition
The airspeed that produces the greatest gain in altitude over the shortest horizontal distance. At VX, the airplane climbs at the steepest possible angle relative to the ground, which is useful when terrain or obstacles must be cleared shortly after takeoff.
Plain English
The speed that gets you the most height in the least distance forward. You climb steeply, but slowly across the ground. It is the speed to use when you need to get up and over something close ahead.
Context Anchor
You will see VX in takeoff and obstacle-clearance discussions, and in the airplane’s operating handbook performance information.
Derivation
The 'V' in VX stands for 'velocity,' which simply means speed. The 'X' is just a label used by aircraft designers to mark this particular climb speed in performance charts. Knowing this helps when reading the POH, because many speeds are labeled V-something (VY, VS, VA, etc.) and each one points to a specific, defined speed.
Why Pilots Care
It maximizes obstacle clearance on short-field or obstructed takeoffs by trading some rate of climb for a steeper flight path.
Grounding Statement
Picture lifting off with trees beyond the runway: VX is the speed that gives the steepest upward path before you pass over them.
Intuition Check
Best does not mean best for every climb. Here it means best for gaining the most altitude in the least forward distance.
Example Sentence 1
With trees off the end of the runway, the pilot held VX until clear of the obstacle, then lowered the nose to a normal climb speed.
Example Sentence 2
The short-field takeoff checklist requires setting flaps and maintaining VX until all obstacles are cleared.