737 wheel well drag. why aren’t more a/c door-less?
 

With just a less-that-smooth paintjob meaning a few-percent increase in fuel burn, how much penalty do these cause? What would have been the weight of the doors? How about exposing the gears to elements, that must cost extra in maintenance. No doubt the math comes to show that it is cheaper the way they are - so why not all aircraft? What other a/c incorporated the feature?

It's the corrosian that gets them. Boeing has thought of a Mod to add the doors. Its not the best way to do it. :)

The weight of the doors and associated mechanics is considerable, a trade off against the disadvantages of not having them.

With my experience of the B727, with doors, and B737, without doors, I have never seen any more corrosion in the wheel bays and/or gear assemblies or deterioration of gear seals/fitments/seals/hoseing etc. on the doorless 737.

The gear and wheel bays on both 727 and 727 are covered under the CPCP - corrosion prevention and control programme which requires removal and corrosion treatment at maximum 10 years installed/or overhaul whichever comes first (in parallel with the maintenance programme hours or cycles limiter which is usually reached earlier), this covers both the gear mountings in the bays and the gear assembly itself, the wheel bays themselves are usually covered under a variety of heavy check (BMPD), CPCP, AD and structural inspection tasks.