The first explanation of lift that I came across as a middle school student was the theory of “Equal Transit Times”. Keeping this definition in mind, let’s investigate some popular theories about how aircraft create lift.

The diversity of phenomena the theory can explain, including phenomena that other theories could not explain.

The simplicity of its assumptions, ideally as few as possible.

In science, the simplicity of a theory is a hallmark of its elegance. According to Einstein (or Louis Zukofsky or Roger Sessions or William of Ockham… I give up, who knows), “everything should be made as simple as possible, but not simpler.” Hence, the strength of a theory is related to: In fact, the theory that requires the simplest assumptions and least abstraction is typically considered to be the most useful.

This is not necessarily because more advanced theories are harder to understand or require a lot maths. Most importantly, the equal transit time theory, explained further below, is taught in many school textbooks and therefore instils faulty intuitions about lift very early on. Just throughout my time in school and university, I have been confronted with several different explanations of how wings create lift. Given its importance to flight, it is surprising how many different and oftentimes wrong explanations are being perpetuated online and in textbooks.

How airplanes fly is one of the most fundamental questions in aerospace engineering.

Assumptions in thin airfoil theory