flak   Anti-aircraft fire -- from the German word Fliegerabwehrkanone.
Flanker   Su-27. 
flaperon   A combination flap and aileron.
FLIR   Forward-Looking Infrared.
Flogger-B    MiG-23.
Flogger-D, -J   MiG-27.
FLOLS    Fresnel Lens Optical Landing System.  The primary optical
    landing aid aboard an aircraft carrier, called the "ball".
fly-by-wire   A design whereby all the controls of an aircraft are controlled
    directly by a flight computer, which processes input from a control stick. 
flying wing   An aircraft whose entire fuselage generates lift.
Fokker   Anthony Fokker, Dutch aircraft designer who built fighters for the
    Germans during WWI.  After the Armistice, Fokker continued building
    aircraft in Holland.  
fouled deck   An aircraft carrier deck that is not ready to receive aircraft.
Foxbat   MiG-25.
Foxhound   MIG-31.
free-fall bomb    A bomb propelled solely by its own weight.
Frogfoot   Su-25.
Fulcrum   MIG-29.