depth

In airfoil geometry, depth describes the maximum vertical distance between the chord line (a straight line connecting the leading and trailing edges) and the curved upper or lower surface of the wing or control surface. This measurement is perpendicular to the chord, not vertical in absolute terms. Depth determines how much an airfoil deviates from a flat plate and directly affects its aerodynamic properties, structural strength, and weight distribution.

Depth is typically expressed as either an absolute distance in millimeters or inches, or as a percentage of the chord length. An airfoil with 12 percent depth means the maximum bulge is 12 percent of the chord length. Common transport aircraft wings run 9 to 15 percent depth; fighter aircraft may use thinner sections around 4 to 8 percent for transonic performance. Fuselage cross-sections have their own depth measurements when discussing non-circular shapes or cargo hold dimensions relative to centerline.

Location and asymmetry

The point of maximum depth does not always occur at mid-chord. Modern supercritical airfoils position maximum depth further aft, around 35 to 40 percent chord, to manage shock formation. The upper surface and lower surface may have different depths; a cambered airfoil has greater depth on the upper side. During maintenance inspections, depth is verified using coordinate measuring machines, dial gauges, or ultrasonic thickness mapping to detect deformation from impact, corrosion, or fatigue.

Damage that reduces depth locally, such as a dent or erosion pit, alters local air flow and can initiate separation or increase drag. Wing leading edge erosion that removes material progressively reduces both the upper and lower surface depth. Repairs must restore the original depth profile; simple filling without reshaping the surface will fail structural or aerodynamic certification.

Depth should not be confused with thickness, which measures the distance between upper and lower surfaces perpendicular to the surface itself, or with camber, which describes asymmetry between upper and lower curves. A thick airfoil may have modest depth if it is nearly flat; a thin airfoil can have significant depth if highly curved. Engineering drawings specify depth at multiple spanwise and chordwise stations to fully define the 3D shape.