 Swept wing

A swept wing has a specified spanwise reference line inclined relative to the direction perpendicular to the aircraft's longitudinal axis. Aft sweep places the outboard part farther rearward; forward sweep places it farther forward. The leading-edge sweep angle and quarter-chord sweep angle can differ, so a sweep value needs a named reference line.^[ref]

A useful geometric construction shifts each spanwise strip of a wing planform fore or aft while retaining its local airfoil chord. If both edges receive the same shift, their separation and hence the integrated wing area remain unchanged. For an original constant-chord example with wingspan $b=12\mathrm{m}$ and $c=2\mathrm{m}$, define $x_{\mathrm{LE}}(y)=|y|\tan 30{}^{\circ }$. The tip leading edge is $6\tan 30{}^{\circ }\approx 3.4641\mathrm{m}$ aft of the root, while $S=bc=24{\mathrm{m}}^2$ and $AR=b^2/S=6$ are unchanged by the strip shift.

Preserving these geometric scalars does not preserve the three-dimensional flow. At high Mach number, the normal Mach number for a swept leading edge helps organize a simplified comparison of the component perpendicular to that edge. Finite-span flow, thickness and interference with the fuselage remain relevant.

The primary 1947 experiment by Richard T. Whitcomb used a different geometric change: it rotated the two wing semispans to obtain zero, forward and backward sweep, then obtained loading and drag information from pressure measurements and wake surveys. The shock-associated changes did not shift by exactly a universal cosine factor.^[ref]

Thus an experimental sweep comparison should identify what changed besides the angle: rotation of an existing wing, a stripwise shear of a planform, and a newly designed swept wing need not retain the same projected area or section arrangement.