▁hover ing ▁efficiency , ▁but ▁have ▁a ▁limited ▁forward ▁speed ▁of ▁less ▁than ▁ 2 0 0   k not s ▁due ▁to ▁compress ibility ▁effects ▁on ▁the ▁rot or ▁bla de ▁tips . ▁The ▁t ilt ▁rot or ▁concept , ▁found ▁in ▁the ▁V - 2 2 ▁O sp rey , ▁uses ▁large - d iam eter ▁prop ellers , ▁but ▁also ▁has ▁limited ▁top ▁speed ▁to ▁ 3 0 0 ▁kn ots . ▁The ▁til ting ▁du cted ▁fans ▁or ▁shr oud ed ▁prop ellers , ▁such ▁as ▁the ▁Do ak ▁V Z - 4 , ▁the ▁Bell ▁X - 2 2 , ▁were ▁built ▁in ▁ 1 9 5 0 s ▁and ▁ 1 9 6 0 s . ▁However , ▁du ct s ▁designed ▁for ▁static ▁conditions ▁de grad ed ▁in ▁performance ▁at ▁high ▁advance ▁ratio , ▁whereas ▁a ▁du ct ▁designed ▁for ▁ax ial ▁cru ise ▁could ▁reg ain ▁good ▁high - speed ▁performance ▁at ▁the ▁exp ense ▁of ▁static ▁figure ▁of ▁mer it . ▁The ▁Do ak ▁V Z - 4 ▁and ▁the ▁Bell ▁X - 2 2 ▁especially ▁had ▁problems ▁ach ieving ▁high ▁flight ▁spe eds . ▁Therefore , ▁to ▁achieve ▁both ▁high ▁hover ing ▁efficiency ▁and ▁high ▁forward ▁speed , ▁the ▁best ▁way ▁is ▁to ▁use ▁a ▁low ▁disc ▁loading ▁lift ▁fan ▁or ▁fans , ▁in ▁addition ▁to ▁tur bo f ans ▁for ▁forward ▁prop ulsion ▁at ▁cru ise ▁speed . ▁ ▁H over ▁ ▁The ▁ann ular ▁lift ▁fan ▁concept ▁uses ▁two ▁parallel ▁ann