Parasite Drag⚠ unverified
Aerospace / Aerodynamics · Compute the parasite drag force
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| CD0 | CD0 | — | 1.0 | Zero-lift (parasite) drag coefficient (dimensionless) |
| S | S | m**2 | 1.0 | Reference (wing) area |
| q | q | Pa | 1.0 | Dynamic pressure |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | D0 | N | Parasite drag force, in newtons (N) |
The science & history
Understanding the Parameters
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Zero-lift drag coefficient $C_{D0}$ — sums skin friction (viscous shear over the wetted surface) and form drag (pressure imbalance from the body's shape and any flow separation). It is roughly constant with angle of attack, which is why it is called the zero-lift drag. Clean designs (retractable gear, flush rivets, laminar flow) minimise it.
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Dynamic pressure $q$ — parasite drag grows with $q$, hence with $V^2$: it is small at low speed and the dominant drag at cruise/high speed. This is the opposite trend to induced drag (which falls with speed), so their sum has a minimum.
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Reference area $S$ — the same wing area used for lift and induced drag, keeping $C_{D0}$ on the common coefficient basis. (Sometimes wetted area or a "flat-plate equivalent" $f = C_{D0}S$ is quoted instead.)
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Role in the drag polar — $C_{D0}$ is the constant term in $C_D = C_{D0} + C_L^2/(\pi AR e)$; it sets the bottom of the drag curve and, with aspect ratio, the maximum $L/D$ (Lift-to-Drag Ratio).
Derivation (Approaching a Proof)
Parasite drag is just the drag equation (Drag Force) evaluated with the zero-lift drag coefficient. By dimensional analysis, any drag force is dynamic pressure times reference area times a coefficient:
$$D_0 = q\,S\,C_{D0} = \left(\tfrac12\rho V^2\right)S\,C_{D0}.$$
Physically, $C_{D0}$ is built up from skin-friction drag — the integral of viscous shear stress over the wetted area, scaling with the surface's boundary-layer friction coefficient — plus pressure (form) drag from the fore-aft pressure imbalance and any separation. Because neither depends strongly on lift, $C_{D0}$ is treated as constant and the lift-dependent part is handled separately as induced drag.
Dimensional check. $D_0 = C_{D0}\,S\,q = (\text{–})\cdot\text{m}^2\cdot\text{Pa} = \text{m}^2\cdot(\text{N}/\text{m}^2) = \text{N}$ — a force, as required.
History and Development
The separation of drag into a lift-independent "parasite" part and a lift-dependent "induced" part is a foundational simplification of aircraft performance analysis. Reducing $C_{D0}$ — through streamlining, surface smoothness, area ruling, and (later) laminar-flow and riblet technologies — has been a continuous thread of aircraft development, because at cruise speed parasite drag is where most of the drag, and thus fuel burn, lives.
Related Concepts: Induced Drag, Drag Force, Lift-to-Drag Ratio, Dynamic Pressure, Reynolds Number, Lift Force
Notes: Zero-lift drag: skin friction + form drag; ~constant with $C_L$. Grows with $q$ (∝ $V^2$) — dominant at high speed (opposite to Induced Drag). Constant term $C_{D0}$ of the drag polar. $f = C_{D0}S$ = equivalent flat-plate area.