[slightknack]

> He identified the four forces which act on a heavier-than-air flying vehicle: weight, lift, drag and thrust. [...] He also designed the first glider reliably reported to carry a human aloft. He correctly predicted that sustained flight would not occur until a lightweight engine was developed to provide adequate thrust and lift. The Wright brothers acknowledged his importance to the development of aviation.

— https://en.wikipedia.org/wiki/George_Cayley

I think by the time the Wright brothers came around, the general theory for powered flight was in place. The Wright Flyer was precision engineered, compared to most 'kites' that came before it. It's not every day you see an internal combustion engine on a kite.

[notahacker]

> It's not every day you see an internal combustion engine on a kite.

Yup. The engine was more important than the understanding of the principles of propulsion too: even if the four forces had been identified by Aristotle, that wouldn't have been much use in achieving sustained powered flight to civilisations whose closest approximation of a propellor powered by a turbine engine was a waterwheel.