Originally Posted by
Chu Chu
I found the following paragraph in a National Materials Advisory Board report:
To minimize structural weight and thus maximize payload capability, the Air Force elected to use 7178-T6 aluminum in the lower wing skins as well as in other locations in the aircraft along with 7075-T6 aluminum. The commercial 707 used 2024-T3 aluminum in the lower wing skins at about two-thirds the stress level.
https://www.nap.edu/read/5917/chapter/6#87
And a bit more - some of the KC fuselage skins were spot welded- but later replace due to fatigue issues early on, and at one time a ' laminate" of aluminum and ' cold BONDING of aluminum was used- later found to cause problems due to entrapment of moisture- it was the late 60's before the COLDWORK process was developed (which involved prestressing the fastener holes via a disposable sleeve and an expansion mandrel pulled thru ) the inventor was Lou Champoux ( sat next to me in the 60's ) working with a local vendor later to become Fatigue technology . There is often confusion between the cold BONDING process, and the coldWORKING process.
And a lot of the tooling for the KC135 was also used for 707- and vice versa via a complicated lease arrangement...