How can some stretched derivative airplanes have longer range while most have less?

Question

On the one side A321 neo has almost the same range as the A320 neo (250 NM difference), and a LR version of A321 neo will have an even longer range. The same goes for the Dassault Falcon 7x and the longer and longer-range Falcon 8x.

On the other side Boeing 737 MAX line shows a clear tendency: the longer the fuselage the shorter the range. The same tendency can be observed on most aircraft types.

I understand that fuel is generally in the wings which keep the same volume on a stretched derivative, but fuselage can accommodate fuel tanks and range can be a crucial point for aircraft. So why don't/can't aircraft manufacturers propose equivalent range stretched aircraft in general, as the A321 neo does?

Answer 1

When aircraft are stretched, they usually just receive extra fuselage plugs on both sides of the wing: wing redesigns are incredibly expensive and take a long time. So the longer aircraft has a higher empty weight, and due to the unchanged wing design the fuel capacity remains the same - this reduces range. The aircraft is fuel limited. Conversely, if fuselage plugs are removed the same amount of fuel can take the aircraft further, as the B707-138 demonstrated, Qantas' first jetliner.

The A321NeoLR fixes the fuel limitation by adding extra fuel tanks in the cargo bays as this article explains, so that the aircraft now becomes weight limited. The extra fuel means that there can be less extra revenue from cargo. The B737-9 cannot follow suit, because it is has a shorter landing gear and has rotation limitations: from this article:

737 MAX9 is not suitable for stretch to an international version, not because the wing is not good enough but because the MAX9 cannot bring the wing to an angle at take-off where it can work efficiently; the landing gear is too short.

The longer fuselage limits the rotation angle before a tailstrike occurs, so max. TO weight has a hard limit that cannot be managed by trading cargo weight for fuel weight. Above photo is from this site, which also contains a table with geometric angle limitations for the different 737 versions: the -9 has a tail strike pitch attitude of 10 deg, and has increased flap deflection of 25 deg to limit lift-off attitude to 6.8 degrees. A small variation in centre of gravity at a high TO weight can cause a tail strike.

Boeing has launched the B737-10 which will address the rotation limit by a redesigned landing gear.