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I read several decades ago about how the 747SP's trailing edge flaps were substantially redesigned from those of the "original" 747-100/-200s. The images on the 747SP Wikipedia page clearly show what appears to be an unslotted Fowler flap - contrast with the triple-slotted Fowler flap of the 747-100.

Why the major re-design?

Related sub-question: The Wikipedia page also mentions "... the SP's heavy wings" as one of the factors contributing to its cancellation. But, the SP had the same wing (modulo the trailing edge flaps) as the -100/-200. How could its wing be considered "heavy" in this context? (I'm thinking maybe it has to do with moments of inertia and/or size relative to the fuselage, but hopefully someone else here can clarify this.)

enter image description here Wikimedia Commons (from the Wikipedia 747SP page)

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The wing was relatively too big for the airplane after cutting bits of the fuselage off, which reduced weight by 11,000 lbs. The aircraft did not require the triple slotted high lift devices anymore: if the wing area is large enough the wing requires no flaps & slats but has relatively high cruise drag. All in all the total weight savings on the SP was 45,000 lbs. (Source)

The B747SP could not do completely without flaps, but the simpler flaps of the SP saved weight. The competition at the time was DC10 and L1011, which had lower wing area and one less engine, resulting in better fuel economy.

  • B747SP: 300,000 kg, wing area = 511 m$^2$, wing loading = 587 kg/m$^2$
  • L-1011: 231,000 kg, wing area = 320 m$^2$, wing loading = 722 kg/m$^2$
  • DC-10 : 263,100 kg, wing area = 368 m$^2$, wing loading = 715 kg/m$^2$ Source

In cruise, a smaller wing area means less resistance: higher wing loading gives better fuel economy, but creates problems at low speed which need to be solved with high-lift devices.

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This was primarily done to reduce weight- Simplifying the mechanism resulted in significant weight savings. Also, I'm not sure why the wings were called 'heavy'. While the SP used the same wings as the legacy 747s, the internal structure and construction was modified significantly for weight savings. As this site notes:

While Boeing chose to retain the standard 747 wing on the SP the internal structure was significantly modified to reduce weight. To save weight Boeing employed reduced gauge metals in the wing spars, ribs, skin and stringers in the wing box and center fuselage sections.

In an effort to further reduce the weight of the wings engineers designed a single-slotted variable pivot flap system. ... (which) ... removed 12,000 lbs in weight from the wing structure.

Considering the total weight savings was around 45,000 lbs, this is more than a quarter of it!

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