The DC-8 is one of only a few jetliners that can safely use reverse thrust in flight; even so, reversing the inboard engines (the outboard engines can only be reversed on the ground) while flying still produces extensive airflow disruption over the wing’s upper surface in the vicinity of the reversed engine(s), as detailed in this excellent PDF documenting a series of flight tests carried out by NASA on their DC-8-70. A significant area of separated flow extends from leading to trailing edge (wider at the trailing edge) whenever the inboard engines are reversed below approximately mach 0.88, and becomes very large (covering over a third of the upper wing surface) below approximately mach 0.6; additionally, at mach numbers of 0.6 and below, an area of full-on reverse airflow appears over the trailing edge, in line with the inboard engine.
This would make perfect sense if the engines extended aft of the trailing edge, in which case the reversers would blow air forward and over the wing, but that isn’t the case for the DC-8; instead, the DC-8’s tailpipes (even on the DC-8-50 and -60, with their long, cigar-shaped JT3Ds) are placed under, or just behind, the wing’s leading edge. If anything, one would expect the use of reverse thrust on a DC-8 to strengthen the airflow over the wing, instead of disrupting it, due to all that forward-blowing air piling up around the engine and under the leading edge and helping to partially block air from passing under the wing, forcing it to go over the top instead. So why does in-flight thrust reversal on a DC-8 instead produce large areas of airflow separation (and, at lower speeds, even airflow reversal)?