High Quality Content by WIKIPEDIA articles! The Northrop YF-17 (nicknamed "Cobra") was a prototype lightweight fighter aircraft designed for the United States Air Force`s Lightweight Fighter (LWF) technology evaluation program. The LWF was initiated because many in the fighter community believed that aircraft like the F-15 Eagle were too large and expensive for many combat roles. The YF-17 was the culmination of a long line of Northrop designs, beginning with the N-102 Fang in 1956, continuing through the F-5 family.