when Indonesia made F-5IDs,many country that was the enemy of US interested of buying it,including the USSR,in 1980,Indonesia agreed and gives 2 F-5IDs to Soviet,the Soviets were impressed with the flying performance,but pretty dissapointed with the stability but they still agree that the plane is worth the price,other than just using it for making better planes,they experiment by mounting MiG-21 F-13ID wings,but since these Fishbeds wings is larger than F-5 wings,they need to put the main wing to back,and the back wing to front,and they made F-5-21,a experimental plane that was pretty weird,though the plane never made it into service,it still mounts M39A3ID 20mm and few hard points with AIM-120ID and AIM-9ID in it,and surprisingly its combat capable,but was too expensive to build since it uses the most expensive and best avionics Soviet had at the time