I think the main reason is that many of these planes were produced before winglets were common
Look at a list of USAF planes and see what years each one were produced (I am not including every plane, but most of the common ones)
C-5: 1968–1973, 1985–1989
C-17: 1991-2015 (has winglets)
KC-46A: 2013-present (...
Air flow follows pressure gradients, and the lift-creating suction over the upper wing pulls in more air, not only from ahead of the airplane (which explains the induced angle of attack), but also from the side.
The explanation of induced drag on the Boldmethod page is actually quite good; only gems like the wingtip vortices curve up and around ...
All the discussions of engineering difficulties and cost-effectiveness are valid arguments but based on my USAF career in program offices and in operating commands, including 4 years in TAC, it boils down to money and priorities.
Once the a/c is operational the program office is concerned with sustainment. They need to provide the engineering, tools, and ...
In the lens of potential theory, we can explain and predict the efficacy of winglet through how much it dilutes the trailing vortices strength. Just like we can show that the Elliptical lift distribution produces the least induced drag for a given span length on a flat planform, we can equally derive the optimal lift distribution for any wing+winglet ...
No prior knowledge of this design choice, but the design direction seems pretty clear here.
The original design reduces load towards the wingtip for efficiency. The textbook spanwise loading for maximum efficiency is the elliptical loading, however when tip devices, as in the original design, are employed, the tip load should not be zero. With a tip device, ...
For example, look at the wingtips of the P-8. They don’t look like the commercial variant (because the commercial variant will snap off if you hang out in icing conditions for hours), but the wingtips it has essentially work the same way.
In general though, the answer to your specific question is that there is no “aftermarket”. Air Force and Army ...
Twist, morphing sounds torsionally flexible. Add mass to a soft spring and you get all kinds of resonances and eigenmodes. Better put the engine mass to a stiff and strong structure that can deal with its mass and forces.