Definition
A pure number that has no units of measurement attached to it because the units in its underlying ratio cancel out. Nondimensional numbers are used in aerodynamics and engineering to compare physical situations regardless of the size, speed, or system of units involved.
Plain English
A number that is just a number, with no units like feet, pounds, or seconds attached. It comes from dividing one quantity by another so the units disappear, leaving a clean figure that can be compared across different aircraft, sizes, or conditions.
Context Anchor
Seen in aerodynamics and performance discussions when a text compares aircraft or airflow behavior without tying the comparison to a specific unit of measure.
Derivation
From 'non-' (not) and 'dimensional,' meaning 'having measurable extent or units.' A dimensional quantity carries units (like 200 knots or 5,000 feet); a nondimensional one does not. The term simply means 'no units attached.'
Why Pilots Care
These numbers let engineers predict how air will behave around an aircraft regardless of the aircraft's actual size.
Analogy
If you divide 10 feet by 5 feet, the feet cancel and the answer is simply 2. That 2 is a nondimensional number because it has no unit attached.
Intuition Check
Do not read “dimension” here as physical size only. In this context, a dimension is a type of measurement unit, and “nondimensional” means the units have canceled out.
Example Sentence 1
Mach number is a nondimensional number because it is the ratio of true airspeed to the local speed of sound, so the units cancel.
Example Sentence 2
Wind-tunnel tests use nondimensional numbers to compare model results directly with full-scale aircraft performance.