>>10229536Well I did delete it but here's a general audience version:
RC planes and paper planes operate in the mostly laminar regime; this means that flow is good and beautiful and in straight parallel lines, which is usually very good - until it encounters what's called an adverse pressure gradient: this is the leeward side of a flying golf ball or the latter half of an airfoil. In inviscid (perfect, no viscosity) flow, the static pressure here would increase as you go along with the wind, as the geometry sort of acts like a diffuser (think bernoulli's theorem).
When viscosity is taken into effect, you have to add this thing called a "boundary layer"; this is a small (usually a couple mm to cm in planes) layer starting v=0 at the skin and v=v at the top. Imagine air is sticky and slipping is not allowed. When encountering an adverse pressure gradient, the bottom, low-velocity part of the boundary layer can be pushed in the opposite direction until the whole thing lifts up and separates; in aerodynamics, this is called stall, where wind no long follows the curve of the airfoil and you lose most of your lift. Technically, the separation zone is a low-pressure zone and you do get a BIT of lift from it, but it's at the cost of the rest of your wing-generated lift.
There are a few ways to fix this. One of them introduces a chord-wise vortex over the top of the wing that adds kinetic energy to the air in the boundary layer, making it harder to separate from the wing. You do lose a bit of local lift where the vortex generator is, but this is usually at (traditional VGs) or in front of (strakes) the leading edge. The vortex can be made to be so strong that some planes, namely the delta-winged ones, can fly using entirely the vortex generated at high angles of attack.
