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A Practical Guide to Aerodynamic Modification

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Updated August 15, 2023 Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many peo...
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How to Estimate Your Car's Coefficient of Rolling Resistance

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When two objects in contact with each other slide, a friction force develops resisting the motion. In classical physics, we distinguish between static friction (which resists the onset of motion as some force is applied) and kinetic friction (which resists movement after it has begun). You’ve noticed this before: the last time you tried to push a heavy box across the floor, say, you had to give it a push to get started and overcome the static friction (and if the static friction was great enough, it perhaps tipped over instead of sliding). When you stopped pushing, it didn’t continue sliding indefinitely; it gradually slowed and required some force to keep moving (although not as much as it took to get started). In both scenarios, your pushing on the box was required to overcome its static or kinetic friction. Upright, the fluid-filled can slides when pushed but as energy is dissipated due to friction with the table surface, it quickly comes to a stop. Cars are a bit more complicated b...
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EPA Fuel Economy as a Function of Vehicle Parameters

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Another semester is over and once again, I can’t stop thinking about systems design. Specifically, as I did last semester , I find myself gathering data and trying to apply analysis techniques from aerospace systems design to try and learn something about the physical limits of fuel efficiency in cars.   This time, I started with the EPA’s current list of battery electric (BEV), plug-in hybrid (PHEV), and hybrid cars (HEV)—a comprehensive accounting of fuel economy ratings for several hundred vehicles—to try and get an idea of what the current state of alternative-fuel cars looks like. You can find these at www.fueleconomy.gov . For this analysis, I limited data to MY2025 cars but these included all  2025 BEV, PHEV, and HEV with EPA ratings in the US today with a few exceptions (cars for which I was unable to find other information, such as the Ferrari SF90 and 296, which are plug-in hybrids but for which Ferrari does not publish data such as curb weight). I also excluded ...
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Investigating Separation

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Nearly a year ago, I was sitting in an incompressible flows class when the professor said something I had never considered before. Loss of lift over an airfoil after stall, she claimed, is caused by a pressure increase due to flow separation over the upper surface.   This was contrary to everything I thought I knew about separated and attached flow. Keep flow attached, the conventional wisdom goes, over a tapered shape for greatest pressure recovery and lowest drag (and “zero lift” on a ground vehicle, if you believe some people online, which you shouldn’t); if the flow separates, the pressure drops behind the separation point. Yet in this case, it must be true that separation increases pressure—otherwise, airfoils would not lose lift in stall.   Conventional wisdom, as is so often the case, does not tell us the whole story. The effect of separation on surface static pressures and, consequently, lift and drag can be complicated. To investigate, I decided to run some tests. Yo...
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Why Don't Cars Have Long Tails?

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The drag force cars create as they move through air is generated largely by a pressure imbalance. Cars are bluff bodies as opposed to streamlined; that is, airflow separates over them and, in modern cars, this separation usually occurs most prominently at the very back and creates a large wake (even fully streamlined shapes like wings have wakes, too, but they tend to be a lot smaller). This large wake results in negative gauge pressure acting on whatever body surface is exposed to it which, since it’s the back of the car, tends to be a large vertical surface (we call this the base )—so the low pressure ( base pressure ) results in a force acting almost directly backward.   One way to reduce the drag of cars is to make this wake area and its resulting pressure drag smaller by tapering the rear part of the car in a long tail; this allows the flow to recover pressure so long as it does not detach prematurely and reduces the vertical surface area exposed to low gauge pressure. Alt...
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