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Engine Placement Along Wing On Quads  
User currently offlineLH707330 From United States of America, joined Jun 2012, 1472 posts, RR: 3
Posted (3 years 1 month 2 weeks 20 hours ago) and read 10593 times:

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]

4 replies: All unread, jump to last
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17654 posts, RR: 65
Reply 1, posted (3 years 1 month 2 weeks 14 hours ago) and read 10443 times:

I can think of a few reasons for engine placement:
- Wing bending relief. The more evenly distributed the better. However there is more lift inboard so there will be inboard bias.
- Placement of control surfaces.
- As you say plugs and wingtip extensions on later versoins have an effect.
- FOD avoidance.

The 340 is a bit of a special case since the inboards are in the same place as on the 330. They are pretty much the same airplane after all.

"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinerwessel From United States of America, joined Jan 2007, 2740 posts, RR: 2
Reply 2, posted (3 years 1 month 2 weeks 7 hours ago) and read 10340 times:
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Quoting Starlionblue (Reply 1):
The 340 is a bit of a special case since the inboards are in the same place as on the 330. They are pretty much the same airplane after all.

And much of the structure for the outboard engine mounts is still there on the A330. The A330 tanker mounts the wing refueling pods on the hardpoints for the A340's outboard engines. Fuel, power and whatnot are easily routed there as well.

User currently offlineLH707330 From United States of America, joined Jun 2012, 1472 posts, RR: 3
Reply 3, posted (3 years 1 month 1 week 6 days 9 hours ago) and read 10033 times:

Do the A340 and IL-96 have radically different high-lift systems? It seems weird that they have such different engine placements. Although the 340 shares the wing with the 330, it still looks more similar to other quads than the IL-96 does.

User currently offlineViscount724 From Switzerland, joined Oct 2006, 29469 posts, RR: 24
Reply 4, posted (3 years 1 month 1 week 4 days 20 hours ago) and read 9631 times:

If memory correct the Convair 990 engines were moved slightly further outboard than in the original design to help reduce drag.

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