fourenginejock,
The difference between a -120 and a -138 was about three frames in the fuselage, the -138 was that much shorter, which made its dutch roll characteristics even more interesting.
The -138s were delivered with JT-3 "straightpipes", but, along with many early B707/720, converted to fan engines. In the -138B, the fan was a JT3D-MC6. The B707-138 was the first Boeing 707 delivered to a non-US customer, they were built for Qantas, the first non-US operator of any Boeing jet. -X38 is the Boeing constructor number for Qantas, -X36 was BOAC, and -X21 was originally PanAm.
As I recall, the major difference between a -120 and a -720 was the -120 had a centre wing tank. Some -720 also had freon air conditioning, instead of air cycle machines.
As for the "full size" 707, compared to the original -300 Intercontinental (using Boeing trade names, not necessarily the same as the name on the type certificate) the 707-320 had a heavily revised wing, with full span leading edge devices, and a different training edge and flap system, that increased the wing area. All weights were up, but the revised wing gave much improved field performance.
Most of the -300 had JT-4 engines, a few had RR Conway (BOAC B707-436? and, from memory, Air France), most -320 had a JT3D-3B, but a few had up to and including the much higher thrust JT3D-7 (SAA).
The only major variants I haven't flown, in about 8000h on 707, is any -200, the Conway powered aircraft, and the -138. A great old aircraft, and the original -300 was, in my opinion, the nicest handling, even if that bad tempered little Welshman, D.P Davies would not agree with me. The -320 could be a bit touchy, particularly on approach.
Last edited by LeadSled; 25th Apr 2015 at 06:36.