Quote:
Originally Posted by
pattern_is_full 
the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe).
That part sounds a bit like the aeronautical equivalent of pulling yourself up by your own bootstraps.
Re directing the exhaust forward is accelerating it in the opposite direction. That acceleration (A) is the same as in F=MA which produces a force (F). That force slows the aircraft.