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How to Think About Car Aerodynamics: A Very, Very Basic Overview

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The study of aerodynamics is complicated. If anyone tries to tell you otherwise, run the other direction—it’s a sure sign they don’t know what they’re talking about.   Over the last two years especially, my thinking about aerodynamics and appreciation of its complexity has changed dramatically—a result of my going back to school to get another bachelor’s degree, this time in aerospace engineering where a good working knowledge of airflows is required and education in not just general fluid mechanics but also aircraft aerodynamic design forms a core part of the technical curriculum. I'm in the midst of my last semester now and to clarify my thinking at this point I decided to put some things in writing in the hopes they might help someone else as well as myself, specifically focused on car aerodynamics. A word of warning: I've tried to minimize the amount of math below, but some mathematical relations are unavoidable if you want to build an understanding of fluid flows. If anyth...
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Explainer: Tuft Testing

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Tuft testing is an experimental methodology most amateur aerodynamicists are familiar with, even if the vast majority of them have never actually tried it. This is too bad, since tuft testing can give us a lot of good information about the quality of the flow over a car.   We all know that this involves taping bits of yarn to your car, like what the actors playing Carroll Shelby and crew do to the Ford GT in a ( comically reductive ) scene in Ford v. Ferrari (2019). The tufts as depicted in that film are pretty much useless for trying to determine what the "engineers" were attempting to ascertain (magnitude and direction of front lift), but in real life they can be tremendously instructive if applied with purpose and interpreted correctly. So, what is the physical explanation behind tuft testing, why does it work, and what can it actually tell us? Stress   To explain tuft testing, we first need to define some terms and concepts, beginning with stress.   Divide force b...
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Measuring and Modifying Wheel Drag – Part 1: Overview

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My background now is in aerospace engineering, where landing gear and wheel drag are sort of an afterthought in the academic curriculum (I mean this seriously—it isn't discussed specifically until the last year of the program I recently completed). That's understandable because it is approachable as a combination of areas of study—materials, structures, mass properties, configuration, performance, and aerodynamics.   Hoerner includes a short section on landing gear and wheel drag in his Fluid-Dynamic Drag (yes, with a hyphen if you want to look for a copy. His estate still publishes the book, available by special order). After specifying reference area (the simply projected area of the tire as width times height), Hoerner goes through the drag characteristics of various landing gear configurations, fixed and retractable. He also notes something interesting: wheel drag can account for as much as a 50% increase in zero-lift drag or C D,0 on commercial and general aviation ...
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Measuring and Improving Cooling System Performance – Part 8: Modification

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Production car cooling systems are always beholden to styling considerations, packaging issues, and cost constraints (design cost, materials cost, manufacturing cost, etc.). Because of this, most leave room for improvement if your goals and requirements are different than the manufacturer's. We've seen so far that a few simple measurements on your car will give you a lot of information about how well its cooling system works and where it can be made better. If you don't mind getting your hands dirty, you should be able to reduce cooling drag on your car while maintaining or even increasing cooling capacity. How big should inlets be? Outlets? How should they be shaped? How much will internal smoothing reduce drag? How much will mass flow increase if we reduce system losses? Don't try to modify any of these things by guessing! The engineers behind this 2019 Porsche 911 certainly didn't. Modifying Cooling System Flow   As an example, I'll modify the cooling system ...
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