▁in ▁Belg ium . ▁Their ▁advantage ▁compared ▁to ▁par ach ute ▁assault ▁was ▁that ▁the ▁troops ▁land ed ▁in ▁one ▁place , ▁rather ▁than ▁being ▁dispers ed . ▁ ▁The ▁Sl ings by ▁T . 1 8 ▁H eng ist ▁was ▁designed ▁by ▁John ▁" Jack " ▁F rost . ▁It ▁was ▁a ▁ 1 5 - se at ▁gl ider ▁designed ▁to ▁meet ▁Spec ification ▁X . 2 5 / 4 0 ▁( in ▁accord ance ▁to ▁O . R . 9 8 ), ▁issued ▁in ▁February ▁ 1 9 4 1 . ▁The ▁specification ▁was ▁issued ▁in ▁case ▁sufficiently ▁powerful ▁t ugs ▁were ▁not ▁available ▁for ▁the ▁larger ▁Air speed ▁H ors a . ▁Four ▁pro tot ypes ▁were ▁ordered ▁in ▁late ▁ 1 9 4 0 , ▁the ▁first ▁prototype ▁D G 5 7 0 ▁flying ▁in ▁January ▁ 1 9 4 2 , ▁to wed ▁by ▁an ▁Arm strong ▁Whit worth ▁Wh itle y . ▁ ▁A ▁total ▁of ▁ 1 4 ▁production ▁H eng ist ▁M k ▁I ▁were ▁delivered ▁to ▁the ▁Royal ▁Air ▁Force ▁( RA F ) ▁between ▁February ▁ 1 9 4 3 ▁and ▁March ▁ 1 9 4 4 . ▁One ▁of ▁F rost ' s ▁unique ▁design ▁touch es ▁was ▁to ▁fit ▁the ▁H eng ist ▁with ▁a ▁rub ber ▁bag ▁as ▁a ▁landing ▁device . ▁A ▁strength ened ▁under car riage ▁was ▁used ▁on ▁the ▁production ▁aircraft ▁and ▁was ▁called ▁the ▁M k ▁III , ▁but ▁this ▁name ▁was ▁not ▁carried ▁over