Amateur Aerodynamics

Measuring Changes in Drag (NB: I conducted this testing early last summer but, due to a busy fall semester, have only got around to posting it now...). Before I construct a full tail mockup, I decided it would be worth my while to do some additional testing on the partial buck I already have: There are a few reasons I decided this. First, I want to know how much this board and side plates—effectively, a large spoiler at this point—reduce the drag of the car. Second, I want to check my process for measuring changes in aerodynamic drag on this car, which has an electronically controlled throttle that does not allow for throttle-stop testing.   I’ll elaborate on that process below. As far as the first reason: not only will measuring the drag change at this point allow me to better predict the change from a full tail, but if this spoiler turns out to reduce drag as much as my design requirement, then I can stop here (if I want; I won’t, because I want to go through the process of ...

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 fro...

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...