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In some multi seat fighters, like the F-111 Aardvark the pilots sit next to each other, like so:

enter image description here

While in other multi seat fighters (most of them it seems) the pilots sit with one in front of the other, like this F-15E:

enter image description here

Why do some fighters have the pilots in a line like that? Is there an aerodynamic or performance advantage that comes with having them in a line? Or maybe it's so they can have a bubble canopy, which perhaps affords a better view? It seems it would be easier to work together when they are side by side.

So why do some multi seat fighters have side by side seating, and others are in a line?

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    $\begingroup$ My naive guess is that a narrower fuselage is aerodynamically more efficient in general. i.e. A large L/D ratio. So I'd go with front & back seating as my default guess for a good design. e.g. The same reason why kayaks mostly have the people in one line & not side by side. Probably doesn't hurt to give a smaller projected cross section to enemy fire though with modern weapons I doubt this matters much. $\endgroup$ – curious_cat Aug 20 '15 at 19:01
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    $\begingroup$ I don't think there are too many side by side seating fighters, honestly. The front/back setup is probably more aerodynamic, whereas the other provides equal visibility for both pilot/CSO. EDIT: on second thought, the pilot probably also has better visibility with no one next to him. $\endgroup$ – Keegan Aug 20 '15 at 19:06
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    $\begingroup$ F-111 is numbered like a fighter, but calling it a fighter is a long shot. It is an attack aircraft/light bomber. Which F-15E is as well, but F-15E is based on F-15, which was designed as interceptor while F-111 was designed as attack aircraft from the start. And F-111 is larger and heavier. $\endgroup$ – Jan Hudec Aug 21 '15 at 9:13
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    $\begingroup$ @JayCarr, Su-34 is not really a fighter either. It has similar role to F-111. Here it shows that the term “figher” is really rather ambiguous. To be more precise, neither aircraft is “interceptor”. $\endgroup$ – Jan Hudec Aug 21 '15 at 12:51
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    $\begingroup$ Another naive guess: having pilots side by side may also increase roll momentum (and decrease maneuvrability) $\endgroup$ – Manu H Aug 21 '15 at 14:34
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The tandem arrangement (one behind the other) is used on aircraft that have single seat variants. The F-15, F-16, and F/A-18 are all primarily single seat aircraft. It's much easier to stretch the nose out a bit to add a second position than to make it wider. This arrangement is also more aerodynamic, requiring less frontal area.

Fighter aircraft designed for a crew of two will often choose the tandem arrangement for aerodynamics. If both seats need forward visibility, there are compromises available. Some aircraft like the F-15E can be flown from both seats even though there is less visibility from the back. It's possible to have a bubble canopy in side-by-side configuration (this is particularly seen in aircraft kits), though it will create more drag than a tandem canopy.

Having the crew sit next to each other can be helpful for crew interaction and sharing instrumentation. The F-111 is one such special case:

The USAF wanted a tandem-seat aircraft for low-level penetration ground-attack, while the Navy wanted a shorter, high altitude interceptor with side-by-side seating to allow the pilot and RIO to share the radar display.

This may have also been easier to design the escape capsule. Of course the Navy changed their mind and went with the F-14 instead for better maneuverability.

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    $\begingroup$ Also, the F-111 was not really a fighter - more of a light bomber. $\endgroup$ – SSumner Aug 20 '15 at 22:32
  • $\begingroup$ Also side by side allows for training situations, such as the ct114 tutor used in Canada. All fighter pilots must be trained in this jet first. $\endgroup$ – Mark Aug 21 '15 at 5:33
  • $\begingroup$ Well, some fighters (e.g. F-4 or F-14) did not have any single-seat version and still used tandem seating for aerodynamic reasons. $\endgroup$ – Jan Hudec Aug 21 '15 at 9:15
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    $\begingroup$ @SSumner - The F-111A, used by the USAF, was a light bomber in the vein of the F-105, but there were plans for an interceptor version. The radar systems of the F-14, and its AIM-54 Phoenix missile, were all originally designed to be carried by the F-111B naval interceptor, but when the Navy added in the ability to dogfight, the F-111 fell short of expectations and was passed over for a new design (the F-14) $\endgroup$ – KeithS Aug 21 '15 at 14:51
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    $\begingroup$ To be perfectly fair, the F-14 chosen instead was still a massive plane that, while it turned better than the F-111, could still have circles flown around it by light fighters in both Soviet and NATO arsenals, because the design still had to incorporate the interceptor role into this new profile of an "air superiority fighter". The F-15 got the same treatment as the USAF's direct answer to the MiG-25 interceptor. It wasn't until the high costs of both programs necessitated something cheaper to supply the bulk of both air fleets that the LWF competition produced the F-16 and F/A-18. $\endgroup$ – KeithS Aug 21 '15 at 19:11
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The main reason for having tandem seating in fighter aircrafts is that it offers better performance compared to the side by side version. This is why most of the high performance fighters have tandem seating arrangement.

Su 30
"Sukhoi Su-30LL demonstrator flying along the runway at Zhangjiajie Hehua Airport less than 1 metre off the ground"
by Xu Zheng - http://www.airliners.net/photo/Russia---Air/Sukhoi-Su-30LL/1025605/L/. Licensed under CC BY-SA 4.0 via Commons.

Another advantage in tandem seating arrangement is that conversion from single to double seat (and vice-versa) variant is relatively easy and requires minimal design change compared to the side by side configuration. Su 27
"Su-27 05" by Unknown - DefenseImagery.mil ID DD-ST-88-09314. Licensed under Public Domain via Commons.

The main advantage of the side-by-side seating arrangement is that it allows for better work sharing between the crew and reduces the need for duplication of flight instruments. The crew comfort is also (comparatively) better for long range flights, reducing fatigue.

Su 34
"Sukhoi Su-34 at the MAKS-2013 (03)" by Doomych - Own work. Licensed under Public Domain via Commons.

Also, the customers (Air Force/Navy) may also decide which arrangement to use. For example, the initial postwar British trainers used side by side seating as RAF preferred this arrangement.

In case of B-52, the prototypes (and even the initial production aircraft) had a tandem seating arrangement. YB 52
"YB-52sideview" by Original uploader was Sf46 at en.wikipediaLater version(s) were uploaded by Nobunaga24 at en.wikipedia.(Original text: U.S. Air Force) - Transferred from en.wikipedia; Transfer was stated to be made by User:Nobunaga24.(Original text: USAF Museum Website). Licensed under Public Domain via Commons.

However, Gen. LeMay decided that a aside by side arrangement utilizes the copilot better and improved instrumentation and the arrangement was changed from tandem to side by side. B 52
"Boeing B-52D-40-BW (SN 56-0695) and GAM-72 Quail decoy missile and trailer 061127-F-1234S-010" by US Goverment. Licensed under Public Domain via Commons.

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    $\begingroup$ (Photo #1) Sergi: Um, Igor, shouldn't the gear be down before we park? $\endgroup$ – FreeMan Aug 21 '15 at 13:40
  • $\begingroup$ @FreeMan This Sukhoi is still on the runway. Look at the Sukhoi parked on the tarmac in the background. This one's gear are down. $\endgroup$ – Manu H Aug 21 '15 at 14:39
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    $\begingroup$ @ManuH - Actually, that first shot is of a famous Russian aerobatics pilot making a high-speed, low-altitude pass (less than one meter off the ground). He is actually too low in this shot to extend his gear. $\endgroup$ – KeithS Aug 21 '15 at 14:54
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One other point not mentioned here is that the F-111 was designed during the time frame that the Air Force and its contractors were experimenting with the lifting body principle. At that time, a lot of work was going into widening the fuselage and making it act like a wing. The main benefit the Air Force was looking for was that additional lift equals additional weapon payload. The contractors also explored that a lift-creating fuselage reduced structural stress on the wings and increased fuel capacity (increasing range).

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