Drag⚠ unverified
Physics / Fluids · Compute the aerodynamic drag force
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| drag_coefficient | dragcoefficient | — | 1.0 | Drag coefficient, dimensionless |
| density | density | kg/m**3 | 1.0 | Fluid density |
| velocity | velocity | m/s | 1.0 | Free-stream flow speed |
| area | area | m**2 | 1.0 | Reference area |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | D | N | Drag force, in newtons (N) |
The science & history
Understanding the Parameters
-
$C_D$ — not universal; depends on shape, Reynolds Number, Mach Number, surface, and which area is used. Sphere, car, airfoil each have different charts.
-
$\rho$, $v$ — enter through $q = \tfrac12\rho v^{2}$; drag rises with $v^{2}$.
- $A$ — must be the same reference used when $C_D$ was measured.
- $D$ — force opposite the free-stream velocity (for the usual definition).
Derivation (Approaching a Proof)
Dimensional analysis: force $\sim \rho v^{2} L^{2}$. Define $C_D$ so that
$$D = C_D\cdot\tfrac12\rho v^{2}\cdot A.$$
The factor $\tfrac12$ matches dynamic-pressure convention. $C_D$ absorbs viscosity, separation, and wave drag — it is measured or computed, not derived from first principles for arbitrary bodies.
History
Newtonian impact ideas and 19th–20th century aeronautics standardised the $C_D q A$ form; it is the universal engineering expression for fluid resistance.
Related Concepts: Lift, Dynamic Pressure, Reynolds Number, Mach Number, Thrust
Notes: Registry calculator drag (unverified). Empirical $C_D$; steady free-stream model.