Amateur Aerodynamics

Systems engineering has a long history despite not becoming a formalized discipline until after World War 2. What is systems engineering? Basically, it is the overarching management of large or complex design projects: identification of customer needs and market, development of requirements, control of subsystems and integration, etc. For example, how is a modern airliner built? First, a preliminary design study is completed, often lasting a few years, before requirements are finalized and the basic design of the vehicle is established; then, detailed design work can begin, which usually takes several years to complete. Finally, verification and certification testing are completed, after which the vehicle can be released. All these processes are overseen by systems engineers. The evolution of automotive systems engineering in one picture. As time goes on, the product is more technically complex and requires more rigorous control of various inputs in the design process. Similarity Analy

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

Reading through a Youtube post recently on the perceived roadblocks to aerodynamic modification, I was disheartened to see many commenters espouse the idea that effective mods require huge expenditures of money and time, lots of trial and error, or won’t be effective at (legal) road speeds and therefore aren’t worth doing. These attitudes couldn’t be more wrong: aerodynamic modification of road cars can be done very simply and cheaply, and the results can be dramatic.   Myth: To effectively modify your car’s aerodynamics requires a powerful computer and CFD or thousands of dollars’ worth of wind tunnel time.   Reality: CFD and wind tunnels are the two test environments most amateurs are somewhat familiar with and put a lot of faith in. However, there are huge problems with both of these, as I’ve intimated before in several posts .   CFD, or computational fluid dynamics, uses mathematics to model airflows. This can be a very useful tool in the initial development phase of a new ca

Curvature Now that I know from my first round of testing which panel orientations have attached flow, how the pressure behaves with changes in angle, and a rough prediction of pressure drag reductions from all that information, I’ll move on to a larger test buck that will start to approximate a full tail. This buck is almost as long as my maximum length requirement and, rather than a flat panel like my first board approximations , has some curvature in it. Specifically, the extension here bends from an angle of about 20° from horizontal at its front to 23° at its trailing edge, in between the shallowest and middle angles I tuft- and pressure-tested: “Conventional wisdom” says to bend the tail in a convex curve like this for lowest drag. But is that “wisdom” correct? Only testing will answer that. I’ll test this buck in this configuration as well as with an added spoiler that, when affixed to the tail, is horizontal: More fun with the miter saw. All in, I’m still sitting at $0 inve