The 737 Initial wing was designed for the gross weights that the 737-200 started life at. By the time the aircraft had grown to the Adv models, the wing was overloaded. At cruise, it was flying at a higher lift coefficient than it was designed for, which increased drag.
On the 737 Classic, a new leading edge was added outboard of the engine, that increased chord by 4%. The new leading edge helped abit, allowing the Classics to grow to the highest GW -400, but they were still cruising at high lift coefficients and they were slow (Mcruise=.74 on a good day).
The wing of the 737 NG
is much larger (both chord and span), decreasing the wing lift coefficient, plus the airfoil has been redesigned to have a higher drag divergence Mach number. For this reason, the NG
can cruise faster and climb higher.
Here is an overlay I created of the 737-200 (black), 737-300 (blue) and 737-700 (magenta) where you can see the differences. This drawing was made using the .dwg files available on the Boeing website.
A little comparison (info from airliners.net):
Initial model wing area: 980 feet^2
737-100 MTOW: 110,000 lbs Wing loading: 112 lbs/feet^2
737-200 MTOW: 129,500 lbs Wing loading: 132 lbs/feet^2
Classic model wing area: 1135 feet^2
737-300 MTOW: 138,500 lbs Wing loading: 122 lbs/feet^2
737-400 MTOW: 150,000 lbs Wing loading: 132 lbs/feet^2
737-500 MTOW: 133,500 lbs Wing loading: 117 lbs/feet^2
NG model wing area: 1344 feet^2
737-600 MTOW: 143,500 lbs Wing loading: 107 lbs/feet^2
737-700 MTOW: 154,500 lbs Wing loading: 115 lbs/feet^2
737-800 MTOW: 174,200 lbs Wing loading: 129 lbs/feet^2
737-900 MTOW: 174,200 lbs Wing loading: 129 lbs/feet^2
See how the wing loading on the Initial and Classic models got really high, while the NG started out lower, allowing more room for growth?
[Edited 2005-12-13 07:06:25]