I just finished reading an interesting book from the early days of aviation - The Aeroplane Speaks, Horatio Barber, published in 1917. The book attempts to explain the theory of flight in simple terms to prospective pilots and aircraft mechanics. I was surprised to learn how much of the theory of flight they had already figured out by 1916, when the very long and detailed prologue was first published as a series of magazine articles.

Some of the terms have changed since 1917 (e.g. what we know as “drag” was then called “drift”), but they had already figured out induced drag and profile drag, the importance of high aspect ratio to improve lift to drag ratio, the value of streamlining to reduce profile drag, how optimum camber varies with speed, etc. They had a pretty good handle on lateral and directional stability, but the understanding of longitudinal stability was still somewhat superficial (or maybe the author had a better understanding, but the description in the book was overly simplified to better meet the intended audience).

The copy I read was found by one of my coworkers in a used bookstore, but the complete book, with illustrations, can be viewed online.