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The whole purpose of an ER model is to increase range and by doing that you need to increase the amount of fuel stored in the wings. EX: Boeing 737-900, Boeing 737-900ER. Is the wing area increased or is their another way to increase fuel capacity.enter image description hereSource:(https://en.wikipedia.org/wiki/Boeing_737_Next_Generation#/media/File:Lionair_737-900_ER_first_flight.jpg)

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    $\begingroup$ Weird persepctive in that photo: the right wing and stabilizer look half-size. $\endgroup$ – David Richerby Sep 21 '15 at 22:53
  • $\begingroup$ My understanding is that in most airplanes, fuel capacity is not limited by volume but by weight. The non-ER model already has plenty of space to store as much fuel as the ER model, or more; but that would make it too heavy to fly. The ER model might be designed with stronger structures, more powerful engines, etc, that allow it to not only hold the fuel but also lift it. $\endgroup$ – Nate Eldredge Sep 22 '15 at 0:48
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    $\begingroup$ @NateEldredge, in most aircraft loading cargo to MZFW plus full tanks will get you over MTOW, but full tanks alone won't by far. So the ER models do have use for larger tanks. $\endgroup$ – Jan Hudec Sep 22 '15 at 5:02
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    $\begingroup$ @JanHudec Yeah, if full fuel tanks alone would get you over MTOW, then the fuel tanks are probably just too big. - haha $\endgroup$ – reirab Sep 22 '15 at 5:04
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    $\begingroup$ @DavidRicherby - that's a demonstrator plane - it's got the standard wing on the right side, and the larger, ER wing on the left side. ;) $\endgroup$ – FreeMan Sep 22 '15 at 14:21
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Not all fuel is stored in the wings. Some is also stored in the fuselage. The ER variants have a larger, or additional, centre (fuselage) tanks at the expense of cargo bays volume.

The centre tank is located between the wings to minimize the change in centre of gravity as the fuel is consumed. The wing spar, which must go through to provide sufficient structural strength, does not allow the cargo bay to be continuous in most designs anyway.

The ER models may be also fitted with more powerful engines to increase the maximum take-off weight (engine out performance is usually the limiting factor), so the plane can still take reasonable amount of payload when the tanks are full.

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    $\begingroup$ Indeed engine performance is definitely an issue in the particular case of the 737-900ER. They already had to extend the landing gear in order to fit larger engines under the wing for it and it still has an insanely long takeoff roll (9,843 ft ! ) $\endgroup$ – reirab Sep 22 '15 at 5:04
  • $\begingroup$ @reirab that surely isn't the takeoff roll but the the take-off run required / accelerate stop distance required $\endgroup$ – DeltaLima Sep 22 '15 at 9:09
  • $\begingroup$ @reirab, it would seem to me that the long take-off run is more due to the rather high take-off speed, which would be due to the relatively small wings that were originally designed for much smaller aircraft. The take-off distance required grows with square of take-off speed. $\endgroup$ – Jan Hudec Sep 22 '15 at 9:59

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