Why do the “stretch” versions of the same aircraft have shorter ranges?

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I do not have any particular aircraft model numbers in mind, but I know when watching youtube videos about passenger aircraft models, manufacturers tend to have “stretch” version of an aircraft where all else being equal, they basically tack on extra length somewhere in the fuselage and allow for more seats. This always results in a reduced range. This, on the surface, is logical as the, now heavier, plane requires more fuel to go the same distance. But why can’t they include an additional fuel tank in the belly of the aircraft which was just added? Seems like this would allow a longer aircraft to also have a longer range. And to add merit to my thought, the 737-700ER is no different than the 737-700 except that it has nine auxiliary fuel tanks increasing its range by 65%.

Edit: Turns out I was way oversimplified the 737-700ER. It is a much more complicated update.

Answer TL;DR: Increased Angle of Attack

In: Engineering
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As far as I know, fuel in passenger planes is always stored in wing tanks because it reduces the risk of barbequeing the passengers. Adding more tanks would have so many safety implications, you’re almost better off scratch-redesigning the aircraft.

Another issue is that adding one piece of fuselage is going to shift your center of mass, and to make up for that you’d have to adjust your trim. The trim tabs are draggy. It isn’t a lot, but it’s one more thing that affects your range. Adding balanced fuselage sections, in front of and behind the center of mass, could get around that problem but now you’re making probably more-than-twice as many changes to the aircraft and now the risks are piling up.

All things being equal the wings still have the same load capacity. It is not just enough to strengthen the wing roots but the increased weight requires increased angle of attack to get more lift which means more drag and lower fuel efficiency. So stretching the fuselage not only increases the weight but you can not even load the fuel tanks full as this would overload the airplane.

The 737-700ER is very different from the 737-700. In order to have the weight capacity for the added fuel tanks they have installed the wings and wheels from the 737-800 which are significatly bigger. So compared to other airplanes it have a small fusulage and big wings, similar ratio to the 737-300ER.