Parasite Drag

Parasite Drag Buildup (aerodynamics/drag-polars/parasite-drag)

Use when the task is estimating the zero-lift (parasite) drag of an aircraft by the component buildup method: flat-plate skin friction, wetted areas, form factors, and interference factors for the wing, fuselage, nacelle, and tail.

Domain quick reference

• Parasite drag is the drag at zero lift: skin friction, pressure drag from separated or thick boundary layers, and miscellaneous excrescences. It is the CD0 term of the parabolic drag polar CD = CD0 + k * CL^2.

• The buildup method sums per-component drag coefficients:

  CD_i = Cf * FF * Q * S_wet_i / S_ref
  

  Cf is the flat-plate skin-friction coefficient, FF the form factor, Q the interference factor, S_wet_i the wetted area of the component, S_ref the reference area, and CD_parasite = sum of all CD_i.

• Flat-plate skin friction depends on the Reynolds number Re = rho * V * L / mu (density, speed, length, viscosity): laminar Cf = 1.328 / sqrt(Re); fully turbulent Cf = 0.455 / (log10(Re))^2.58. With transition at Re_tr the mixed value is Cf = Cf_turb(Re) - (Re_tr/Re) * (Cf_turb(Re_tr) - Cf_lam(Re_tr)).

• Wetted area is the surface actually in contact with the flow. A first-order wing estimate is S_wet = 2 * S_exposed * (1 + 0.2*t/c); fuselage and nacelle wetted areas come from body dimensions.

• Form factors inflate the flat-plate value for the component shape: wing/tail FF = 1 + 2*(t/c) + 100*(t/c)^4; fuselage FF = 1 + 60/(l/d)^3 + 0.0025*(l/d); nacelle FF = 1 + 0.35/(l/d).

• The interference factor Q >= 1 accounts for drag added where component flows interact (wing-body, nacelle-pylon, tail-body junctions); Q = 1.0 for an isolated component.

• The equivalent skin-friction coefficient Cf_e = CD_parasite * S_ref / S_wet_total condenses the whole buildup to one number; clean subsonic transports fall near 0.0025 to 0.004.

Workflow

1. Gather the flight condition (rho, V, mu) and the reference area S_ref; set the transition Reynolds number (commonly 5e5).
2. For each component (wing, fuselage, nacelle, tail): estimate the wetted area S_wet, the characteristic length L, and the geometry (t/c for airfoils, l/d for bodies).
3. Compute Re = reynolds_number(rho, v, l, mu) and the flat-plate Cf with cf_flat_plate_laminar, cf_flat_plate_turbulent, or cf_flat_plate_mixed.
4. Compute the form factor with form_factor(kind, t_over_c=..., l_over_d=...) and choose the interference factor Q.
5. Convert each component with component_parasite_drag(cf, ff, q, s_wet, s_ref) and sum with total_parasite_drag.
6. Back out the fleet-average friction with equivalent_skin_friction and compare against the typical band for the aircraft class.

Pitfalls

• Using the fully turbulent Cf for a short-chord component whose flow is still laminar: the laminar and mixed values are markedly lower.
• Forgetting the (Re_tr/Re) weighting when applying the mixed formula: the transition correction scales with the wetted length fraction.
• Applying an airfoil form factor to a body and vice versa: wing and tail use t/c, fuselage and nacelle use l/d.
• Using the planform area instead of the wetted area, or forgetting the factor of two for two-sided surfaces like the wing and tail.
• Dropping the interference factor: Q = 1 everywhere understates the drag of a real installed configuration.
• Dividing by the wrong area: CD_i must be normalized by the same S_ref everywhere, while Cf_e uses the total wetted area.
• Accepting negative or zero Reynolds numbers, t/c at or above 0.5, or fineness ratios at or below 1: the formulas are undefined there.

Behavior contract (gate 3)

The buildup logic is exercised by the gate 3 contract test: scripts/test_parasite_drag.py against scripts/parasite_drag_logic.py (stdlib unittest, offline). Run: python3 scripts/test_parasite_drag.py

Compliance

• NACA Report 824 (Abbott, von Doenhoff, Stivers) is US government work in the public domain and is cited as a validation reference only: physics values and formulas are paraphrased here, never copied.
• 14 CFR Part 25 (FAR-25) is US government work in the public domain; this leaf cites it as reference only for the performance and drag context of transport-category certification. No text from proprietary standards is reproduced anywhere in this leaf.
• compliance: STANDARDS-REF, gated: false.