>>10229599In the 50's, swept wings became the big thing. One of the problems encountered was lateral flow, where wind would not just flow over the wing, but along it as well. This can lower flow energy as it loses it to friction and stall where the lateral flow builds up at the wingtip. Again, many ways to counteract this.
In the earlier days, they added wing fences as a fix; these are the bits you see over wings such as the mig 17 and as you can imagine they block the lateral flow. The problem here is that boundary layers do not like non-smooth angles. The boundary layer on the side of the fence and the top of the wing will compound and does lead to early separation; this is why almost all ww2 planes have a filleted wing root. This is also why no planes have them today, as they have been superseded by better designs. Most planes nowadays have "washout", or a twist of the wing to induce a lower angles of attack at the tip than the root. Vortices also help with the stall problem by energizing the boundary layer. Many planes use dog-tooth leading edges where they need the flow to be straight (so in front of the control surfaces), such as the horizontal tail on the F-15 and the main wing on the F-18.
Note that all of this applies a lot less to the bottom of wings. There is rarely any noticeable adverse pressure gradient, meaning that it's harder for flow to separate here than over the wings. This is why pylons are mounted under-wing, not because, and I quote, "it looks better aesthetically". The tank on the lightning is there because there physically isn't enough space under the wing after the landing gear and existing pylons, and it acted as a fence as well.
