further increases in Reynolds number are likely to yield only small decreases in xb/C. In the foregoing, emphasis has been on the onset of pronounced undulations/breakdown for the vortices emanating from the apex of the planform. It is, however, possible to discern the onset of vortex breakdown of the wing root vortex at higher values of Reynolds number. For example, see α = 7d eg,Re = 3 × 10 4 . This onset of breakdown at α = 7d eg appears to move closer to the wing root for an increase of Reynolds number Re to 4 × 10 4 . VI. Angles of Inclination of Wing Root and Apex Vortices Another indicator of the effect of Reynolds number is the angle of inclination of the vortex emanating from the wing root. It is pos- sible to determine an effective sweep anglerv of this vortex core, as defined in the schematic of Fig. 5. As indicated, it is the angle be- tween a line drawn through the center of the vortex core (or, in cases where the core exhibits a spiral, through the center of the spiral) and a line that is perpendicular to the plane of symmetry of the wing. In other words, this effective sweep angle is equivalent to the defi- nition of sweep angle of the leading edge of a delta wing. The dye images of Figs. 2a and 2b indicate that, at lower values of Reynolds number extending up to Re = 2 × 10 4 , the value of the effective sweep anglerv varies substantially, whereas at Re = 3 × 10 4 and 4 × 10 4 little change is evident, indicating that an asymptotic value has been attained. The plot of Fig. 5 clarifies this trend, and em- phasizes that the anglerv is a mild function of angle of attack α. This observation contrasts with the strong effect of α on the onset of pronounced undulations/breakdown xb/C indicated in Fig. 4. Re- garding the values of sweep angle of the vortices from the apex of the wing, it is predominantly a function of Reynolds number. At a given Reynolds number, over the values of α = 4-13 deg, the range of values of sweep angleav of the apex vortex are as follows. For Re = 10,000, 14 ◦ ≤ � av ≤ 17 ◦ ; for Re = 15,000, 15 ◦ ≤ � av ≤ 20 ◦ ; for Re = 20,000, 17.5 ◦ ≤ � av ≤ 20 ◦ ; for Re = 30,000, 21 ◦ ≤ � av ≤ 23 ◦ ; and for Re = 40,000, 27.5 ◦ ≤ � av ≤ 32 ◦ .