Story starts with discovery of wreckage on a riverbed, but with unknown or futuristic alloys - is a wreck of a strange airship.
The story involves what happened to that craft, and a hidden base somewhere where people are making flying saucers. There is an episode at a castle where Nazis(?) are hunting men. At some point a character is horribly burned.
Perhaps time travel involved .. not sure.
Anybody recognise this?