Additional Vortex Lift
Stalling of conventional wings, such as those configured for high-subsonic civil aircraft, occurs around the angle of attack, a, anywhere from 14 to 18 deg. Difficult maneuvering demanded by military aircraft requires a much higher stall angle (i. e., 30 to 40 deg). This can be achieved by having a carefully placed additional low – aspect-ratio lifting surface – for example, having a LE strake (e. g., F16 and F18) or a canard (e. g., Eurofighter and Su37). BWB configurations also can benefit from this phenomenon.
At high angles of attack, the LE of these surfaces produces a strong vortex tube, as shown in Figure 3.42, which influences the flow phenomenon over the main wing. Vortex flow has low pressure at its core, where the velocity is high (refer to aerodynamic textbooks for more information).
The vortex flow sweeping past the main wing reenergizes the streamlines, delaying flow separation at a higher angle of attack. At airshows during the early 1990s, MIG-29s demonstrated flying at very high angles of attack (i. e., above 60 deg); their transient “cobra” movement had never before been seen by the public.