I've noticed quite a lot of variation in engine placements along the wing (how far outboard) on quad-jet airliners, and I'd like to understand more about what design considerations determine where exactly they end up. On the first 707s and DC-8s, they seemed to be placed such that the length from wingtip to engine 1 (segment A), between engines 1 and 2 (segment B), and between #2 and the fuselage (segment C) is pretty equidistant:
The 707-300 engines were really far outboard, but this was largely because they added the root insert to add span and fuel:
Then came the 747 with reasonably equal A, B, and C sections:
In the 90s we got the A340 and the IL-96. The 340 has a slightly larger B segment than A segment, and a small C segment, while the IL-96 has a huge section A and the engines bunched together:
What explanations exist for this variation in relative placement of the engines, especially between the IL-96 and A340? I'm aware that placing them further out provides bending relief, but what about the inboard engine? Do the A340s have the inboard engine so close because of the shared architecture with the A330?
Edit: were taxiways in the USSR narrower when they designed the IL86, thus necessitating inboard placement of #1 and #4 and a compensatory outboard movement of #2 and #3?
[Edited 2013-04-13 12:17:27]