So the Aztecs/Incas/Mayans or some other such society existed in S. America before the conquistadors
disease wipes out a huge tract of the populations, especially those in Urban centres, years before the Spaniards get there.
the spaniards find ancient Flying machines and other stuff, they have no idea what it is, but they bundle it int their ships and take it back to Europe
spain is doin it inquisitions at the time, so anyone who might have an inkling as to what this stuff is stays quiet fr fear of bein tortured as a heretic
Stuff gets buried in a royal palace basement somewhere and remains there till the spanish civil war
Germans then lay their paws on it, realise what it is, smuggle it back to the fatherland and work on replicating the technology.
suddenly We have Rockets, Jet engines and Delta Wing aircraft, and supposedly Nazi flying Saucers at a base in Antartica