I am trying to remember a book series where the main character, a male engineer, is sent back in time to medieval Europe (it is a historical setting with no magic) and applies his knowledge, in the style of a Connecticut Yankee in King Arthur's court. It was a multi-book series published after 1960, and in one of the books he makes an aircraft. It was not a very well known series. At least one of the books' titles was in the format "The Blank Knight" or "The Knight of Blank".
As already answered, this is probably Leo Frankowski's Cross Time Engineer/Conrad Stargard series. But if it isn't, then it could also be a much older book:
I seem to recall that this story sends the modern man (an archaeologist, rather than an engineer) back to ancient Rome, but if you liked one, you'll probably like the other.