Well Evan, that kid's stuff has been proven full-size. You could try yourself with a single-engine Cessna (it helps if it's one with a big prop). Run it up statically, change flap deflection and watch the left-tendency changing as well. Precession of the helical propwash isn't limited to twins.

The first drawing from the book shows what happens pressure-wise and speed-wise and why the propwash is constricted. The "outside" pressure should make for constriction and centripetal force.

The second drawing shows what happens on a blade. Lower left shows axial (thrust) and tangential (spin) speed difference caused by the blade. I wouldn't expect that on the tips, though, and put streamers inside the propwash.

Third drawing shows maximum effect at 75-80% of radius, zero at tips.

How far behind the prop the slipstream vanishes is another question, as well as how it's spoiled by wing and fuselage. Anyway, I respectfully stay away behind an airplane running up its engines. By the way, the picture of the Hercules shows what happens to the propwash when it's "cut" and deflected by the wing.