Amateur Aerodynamics

It’s a common refrain: “Cars are shaped like wings, so they make lift.” Shockingly, one of these was published by a major automotive magazine. The only problem is, it isn’t true. Cars are entirely different from wings (or 2D sections of wings used for analysis and design called airfoils ). Here’s why. Airfoils Plot of a NACA 0012 airfoil. The numbering system gives us information about the airfoil: it has no camber (curve), there is no position of maximum camber, and its thickness is 12% of its chord. Moving at 60 m/s, and with an angle of attack of just 1.7 °, this symmetric airfoil with a chord (length) of 2.0 m will generate a massive 820 N lift per meter of wing width! The behavior of airfoils is described by a branch of aerodynamics called Thin Airfoil Theory (TAT) that is well-developed and characterized by straightforward math that can, to a surprisingly high degree of accuracy, predict the performance of wings. Theodore von Kármán wrote in 1954:   “Mathematical theories from th

Lots of technical terms in aerodynamics are thrown around by amateurs without a real, working knowledge of what they mean  and look like in practice, and consequently, without clarity of thought (I was certainly guilty of this for a long time). I’ve started to try and address this with a glossary, but here I’ll focus on one term in particular: entrainment . What is entrainment, and what is it good for?   Entrainment   To “entrain” is to cause something to be drawn along or to follow, and this works in fluids due to pressure differences. If I  “push” a fluid somewhere—say, out of a tube or nozzle—so that it moves at some speed and has a lower pressure than the fluid surrounding it, more of that surrounding fluid will want to come along for the ride, moved by the pressure differential. This is entrainment , and it’s a way of getting flow for “free” since you get more flow out than you put in.   Jet Pump   Now, the fun part. You can build a simple device at home to see entrainment in a