AE2111 – Drag Estimation and Breakdown

32 vocabulary flashcards covering key terms, coefficients, factors, and relationships related to aircraft drag estimation and the Raymer drag-breakdown method discussed in AE2111.

Drag Estimation

The process of predicting an aircraft’s aerodynamic drag by analysing and summing the contributions of its various components.

Drag Breakdown Method

A component-by-component approach (after D.P. Raymer) that estimates total parasite drag by evaluating skin-friction drag, correcting with form and interference factors, and adding miscellaneous, leakage, and protuberance drags.

Parasite Drag

All drag not directly associated with lift production; primarily skin-friction, pressure, and form drag acting at zero lift.

Zero-Lift Drag (CD₀)

The drag coefficient of an aircraft when it generates no lift; dominated by parasite drag.

Skin-Friction Drag

Drag generated by viscous shear stresses in the boundary layer over a surface.

Pressure Drag

Drag caused by pressure differences between the front and rear of a body, often due to flow separation.

Form Drag

The component of pressure drag linked to the three-dimensional shape or volume of a body.

Induced Drag

Drag produced by lift generation, appearing as wingtip vortices; not part of parasite drag.

Vortex Drag

Another term for induced drag; energy loss due to the trailing vortex system behind lifting surfaces.

Wave Drag

Additional drag created by shock waves in transonic or supersonic flow, dependent on Mach number and geometry.

Wetted Area (S_wet)

The total external surface area of the aircraft that is in contact with airflow.

Flat Plate Skin Friction Coefficient (C_f)

Coefficient estimating skin-friction drag of a flat plate of the same wetted area; depends on Reynolds number and Mach number.

Reynolds Number (Re)

Dimensionless ratio ρVl/μ indicating the relative importance of inertial to viscous forces; based on stream-wise length.

Mach Number (M)

Flight speed divided by local speed of sound; affects compressibility and skin-friction correlations.

Mean Aerodynamic Chord (MAC)

The chord length that, if placed at spanwise station ȳ, produces the same aerodynamic moment as the actual tapered wing; used as stream-wise length for lifting surfaces.

Form Factor (FF)

Empirical multiplier that corrects skin-friction drag for pressure-distribution effects; depends on thickness, sweep, length-to-diameter ratio, etc.

Interference Factor (Q)

Multiplier accounting for drag changes caused by the aerodynamic interaction of adjacent components; often ≈1.1–1.5 for wing-fuselage-nacelle junctions.

Miscellaneous Drag Coefficient (CD_misc)

Drag due to items not covered in the main breakdown (e.g., landing gear, struts), usually derived from wind-tunnel data.

Leakage and Protuberance Drag (CD_L+P)

Incremental drag allowance (≈2–10 % of parasite drag) for skin leaks, joints, aerials, rivet heads, etc.

Cut-off Reynolds Number

Upper limit of Re beyond which Cf ceases to decrease for a rough surface; approximated as Recut-off ≈ 38.21 (l/ε)¹·⁰⁵³.

Surface Roughness Height (ε)

Average protrusion height above a surface; typical values range from 0.0005 mm (smooth composite) to 0.010 mm (camouflage paint).

Equivalent Diameter

Diameter of a circle having the same cross-sectional area as a non-circular fuselage or nacelle; used in form-factor equations.

Compressibility Drag

Extra drag experienced in transonic flow due to local shock formation; sensitive to thickness-to-chord ratio and sweepback.

Equivalent Flat Plate Area (f)

Product of drag force divided by dynamic pressure; used to convert component drag data (often referenced to frontal area) into CD_misc.

Finesse Ratio

Length-to-maximum-diameter ratio (L/D) of a body such as a fuselage or nacelle; appears in form-factor correlations.

Aspect Ratio (AR)

Span squared divided by wing area (b²/S); influences induced drag, tail sizing, and form-factor terms for lifting surfaces.

Taper Ratio (λ)

Ratio of tip chord to root chord of a wing or tail; affects aerodynamic efficiency and mean aerodynamic chord.

Thickness-to-Chord Ratio (t/c)

Maximum airfoil thickness divided by chord length; influences form factor, compressibility effects, and structural weight.

Sweepback Angle (¼-Chord)

Angle between the quarter-chord line and the aircraft centerline; affects wave drag and form-factor calculations.

Dynamic Pressure (q)

Aerodynamic pressure ½ρV²; used to nondimensionalise drag forces into coefficients.

Parabolic Drag Polar

Simplified relationship CD = CD₀ + kCL², assuming constant parasite drag and quadratic induced drag dependence.

Stream-wise Length (l)

Characteristic length along the flow direction used in Reynolds-number calculation; for bodies of revolution it's geometric length, for wings it’s MAC.