▁project ▁during ▁the ▁Second ▁World ▁War . ▁Three ▁wings ▁were ▁mounted ▁radial ly ▁as ▁a ▁rot or ▁on ▁a ▁rot ating ▁section ▁of ▁the ▁fus el age ▁and ▁driven ▁by ▁small ▁jet ▁engines ▁on ▁the ▁wing ti ps . ▁The ▁aircraft ▁was ▁to ▁be ▁prop elled ▁by ▁this ▁wing ▁rotation . ▁For ▁take off ▁and ▁landing ▁it ▁would ▁fly ▁vert ically ▁as ▁a ▁hel ic opter , ▁then ▁t ilt ▁over ▁hor izont ally ▁to ▁fly ▁as ▁a ▁self - prop elled ▁wing ▁generating ▁both ▁lift ▁and ▁thrust . ▁The ▁contemporary ▁He inkel ▁L er che ▁project ▁had ▁an ▁ann ular ▁wing ▁forming ▁a ▁du ct ▁around ▁a ▁conventional ▁prop eller , ▁and ▁in ▁the ▁transition ▁from ▁vertical ▁to ▁forward ▁flight ▁the ▁lift ▁would ▁have ▁transferred ▁to ▁the ▁wing . ▁ ▁The ▁S NE C MA ▁Col é opt ère ▁was ▁a ▁post war ▁French ▁development ▁of ▁the ▁ann ular ▁wing ▁concept , ▁but ▁power ed ▁by ▁a ▁tur bo jet . ▁It ▁fle w ▁in ▁both ▁vertical ▁and ▁horizontal ▁flight ▁modes ▁but ▁never ▁achieved ▁the ▁transition ▁between ▁the ▁two . ▁ ▁Meanwhile , ▁the ▁USA ▁was ▁experiment ing ▁with ▁prop eller - dri ven ▁design ▁configurations ▁with ▁relatively ▁conventional ▁fixed ▁wings . ▁The ▁Con v air ▁X F Y ▁P ogo ▁had ▁a ▁delta ▁wing ▁with ▁cru c iform ▁tail ▁and ▁successfully ▁demonstrated ▁the ▁full ▁transition ▁between ▁flight ▁modes , ▁while ▁the ▁Lock he ed ▁X F V ▁Sal mon ▁had ▁a ▁straight ▁wing ▁with ▁X ▁tail ▁but ▁never ▁achieved ▁the ▁flight