ular ▁fans ▁counter - rot ating ▁about ▁the ▁central ▁fus el age .( see ▁plan ▁view ) ▁The ▁two ▁fans ▁are ▁incorpor ated ▁in ▁an ▁ann ular ▁du ct , ▁separated ▁by ▁a ▁b aff le , ▁but ▁couple d ▁by ▁the ▁g ears ▁in ▁the ▁b aff le ▁to ▁counter - rotate ▁and ▁eliminate ▁tor que . ▁During ▁V TO L , ▁the ▁outer ▁fan ▁is ▁driven ▁p neum atically ▁or ▁mechan ically ▁by ▁the ▁jet ▁engines ▁in ▁the ▁outer ▁wing . ▁As ▁the ▁ann ular ▁fan ▁area ▁can ▁be ▁made ▁large ▁enough , ▁low ▁disc ▁loading ▁and ▁high ▁hover ing ▁efficiency ▁can ▁be ▁achieved ▁in ▁hover . ▁ ▁According ▁to ▁the ▁CF D ▁simulations , ▁at ▁the ▁disc ▁loading ▁of ▁ 2 8 . 3 7   lb / ft 2 , ▁the ▁maximum ▁power ▁loading ▁of ▁ 9 . 0 3   lb / hp ▁is ▁achieved ,( see ▁power ▁vs ▁disc ▁load ings , ▁green ▁star ) ▁very ▁close ▁to ▁the ▁theoretical ▁limit . ▁As ▁the ▁disc ▁loading ▁increased ▁to ▁ 9 1   lb / ft 2 , ▁the ▁power ▁loading ▁dro ps ▁to ▁ 4 . 6 5   lb / hp .( see ▁power ▁vs ▁disc ▁load ings , ▁lower ▁red ▁star ) ▁However , ▁when ▁the ▁aircraft ▁is ▁within ▁one ▁aircraft ▁radius ▁to ▁the ▁ground ▁plane , ▁due ▁to ▁very ▁strong ▁ground ▁effect , ▁the ▁power ▁loading ▁increases ▁almost ▁ 5 0 % ▁to ▁ 6 . 7 7   lb / hp ,( see