Lift generation on Delta wing aircrafts

We are well aware that in aircraft with straight and unswept wings, flow separation results in a poor ratio of lift to drag, and vibration due to instability of flow. However in aircraft with swept wings(Delta wings also) the separated flow will roll up into a pair of stable cone-shaped vortices. Unlike the bound vortex of a conventional wing, which merely represents the circulatory tendency, these are real vortices of swirling mass of air like in whirlwind.

the flow separates in the leading edge and rolls up into pair of vortices

The separated vortex flow represents an alternative method of lift generation. The airspeed in the vortex is high, and so the pressure is low. Thus, lift is still produced by exposing the upper surface to a low pressure than the lower surface; this low pressure is produced due to the vortex motion above it.

At low angle of attack the generation of lift is same as in the convention wing the effect of vortex lift comes into play only in high angles of attack.

Cl vs angle of attack (note that the Cl increases in the high angles more compared to conventional wings)

The Slender delta-winged Concorde was designed to fly with separated conical vortex flow in normal flight conditions. The leading edge is sharp in order to favour flow separation even at small or moderate angles of attack.

The strong conical vortex forms over the leading edge of the slender delta wing Concorde can be seen by vapour condensation

This conical flow may be thought as an controlled separation. When lift is generated this way, the wing will not stall in the conventional sense, and the lift will continue to increase for angles of attack up to 40 degrees or so. At the higher angles, the vortices start to break down, and the lift falls off.

This method of lift generation is used in supersonic flights and it also have other advantage of postponing the sonic drag rise.

"Separated vortex flow is the reason for keeping paper darts flying across, this shows that this method is also suitable for producing lift in low speed flights and interestingly the slender delta aircrafts, based no doubt on the paper darts"