Amateur Aerodynamics

Formulating Requirements   Before I begin testing (and especially before I begin construction!), I want to identify the requirements my tail should meet. “Requirements” specify the objectives a design must fulfill and how it should achieve them. Requirements can be split into two categories: technical (or engineering) and stakeholder. Stakeholder requirements lay out the needs or desires of all stakeholders in a project (in this case, that’s just me). Technical requirements specify the performance objectives a design must fulfill in exact language with specific, measurable goals.   TECHNICAL REQUIREMENTS   Length   I wrote in my first post that tails are a good option for reducing drag if you don’t care about the length of your car—since a tail requires length to function, adding length to your car is unavoidable if you decide to build one.   But how much length you’re comfortable adding is something you’ll have to decide. My car still has to fit in my garage (...

Now that the semester is almost over and I have no plans for the summer, I decided to revisit a modification I’ve cursorily stabbed at before, in a not-very-smart manner: the drag-reducing tail. I’ll elaborate on these poor attempts as I walk you through this project and how I’m approaching it differently this time. But before all that, first we need to visit some theory and look at how tails are (often incorrectly) characterized in online discussions, what their purpose is, and how best to design one.   Streamlining   First, some basic definitions. A lot of the simplified aerodynamic theory you’ll encounter online is based on analysis of airfoils, which is an important part of aeronautics but not always applicable to cars. One of the reasons for this is that cars are not streamlined shapes. Even the most perfectly shaped production car has a large area of separated flow at its back end: …which you can see here. My Prius has attached flow over most of its upper and side su...