= Swept wing
{wiki}
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.<Applied Computational Aerodynamics by W. H. Mason>[Appendix A, pp. A-27–A-29, planform coordinates and sweep relations]{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 $x_{\rm LE}(y)=|y|\tan30\,{}^{\circ}$. The tip leading edge is $6\tan30\,{}^{\circ}\approx3.4641\,\mathrm m$ 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.<An Investigation of the Effects of Sweep on the Characteristics of a High-Aspect-Ratio Wing in the Langley 8-Foot High-Speed Tunnel by Richard T. Whitcomb>[NTRS author abstract, geometry change, measurements and comparison with cosine scaling]{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.
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