Video

Coefficient of drag has nothing to do with size | Know it All with Jason Cammisa

by Hagerty
23 July 2021 1 min read
Coefficient of drag has nothing to do with size | Know it All with Jason Cammisa

For decades, plenty of car makers have obsessed about the coefficient of drag. It was once seen as a powerful message of engineering integrity that could help sell cars – with the likes of Audi even going so far as to equip every 100 to leave the production line with a sticker proclaiming a drag coefficient of 0.30 – even if it was no better than plenty of cars already on the road.

But as Jason Camissa sets out to explain in this episode of Know It All, drag coefficient describes a car’s shape, not its size. Think of it as a voucher code that reduces or increases a car’s effective frontal area, the measure of how big the air sees it.

A low Cd can make a huge difference in a vehicle’s overall aerodynamic drag. A Tesla Model X has 1.6 times the frontal area of a Lotus Elise, and yet its overall drag is lower. To the air, the tiny Lotus is actually a bigger car.

For more colourful, humorous insights on the definition of the coefficient of drag, hit ‘play’ and watch along.

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Comments

  • JP Deyst says:

    (previous comment garbled. I’m sorry. Below, I cleaned it up; might have added a word or too. Hope okay. BR)
    Hi Jason,
    thank you for your engaging videos.
    At risk of being chucked off a building, I had comments I hope might be useful in some way:
    1. Would you consider making a video about how Cd is function of Reynolds#= L*v*rho/nu? yes there is L which is characteristic size! Uh-oh. But before you throw me fro roof, I’d plead that velocity (v) is the big deal, i.e., drivers accelerating 0-60mph run from Re=0 (where Cd is high, goes as 1/v, “skin” drag, laminar flow), up to Re>1e6, separated flow, turbulent BL. So they operate through 3-4 types of flow regime, and Cd is varying in each regime. And i’d bet someday our cars’ controllers and aero surfaces will be able to find and spoil/foil/adapt to operate in each regime, likre lowering skin drag even if thats were im driving; And to shape the car to be in a supercritcal “gully”(?) like you see in many Cd(Re) curves*at about 0.5-1E6 and hopefully can be higher.
    *https://en.m.wikipedia.org/wiki/File:Drag_coefficient_on_a_sphere_vs._Reynolds_number_-_main_trends.svg .
    2. And the aero surfaces will help in cross&headwinds, and buffeting from semis; and help us draft or platoon behind others safely and effectively, and help us in hard corners and black ice. I guess we’ll arrays of air jets or suction, that use maybe stored air from turbo waste gate or sucked from nose stagnation point. Air Jets, with AI, machine learning, will learn to optimize airflow. adapt to weather, bed contents, tailgate down, tonneau, etc. And suction arrays (that stay clean), can reduce BLs. Computers&actuators getting cheaper.. and on $55K+pickup might be worth offering foils, airjets etc, if you get 30mpg highway, and Saudis&Russians charging $150/bbl.

    3. I’ll bet five big plates of my undercooked vegetables, that a full.size Ram pickup doesn’t ever get to Cd=0.36. No way.. standard trim&height? With that underside? Nose? Bed? Mirrors? Back of cab?window, rear? No way… “With aero you never know” but im sure you gotta eat the veggies. E.g., Airshaper.com using CFD measured the ’21 F-150 at Cd=0.46. Can a Ram be so much lower? At standard trim/height/stance?

    4. Seems like the undersides of pickups and cars will get smoother almost by accident, as battery cases get added; and air intakes maybe smaller as engines get smaller. Air/hydraulic suspension on pickups really useful, and can reduce flow underneath on highway.. mirrors smaller as cameras augment them. Aero wheel covers will come soon. Computation, control, and Varying foils and airjets in the future.

    5. It still seems way too hard likre impossible to get real aero data and real highway economy measurement for US cars and especially pickups. nThere are very few wind tunnels that fit a whole car let alone a pickup or semi. most controlled by automakers. And while I read research papers do scale model work but don’t usually get the Reynolds numbers to match; instead this thing where they hunt for anomalies changes in their Reynolds region, and if none decide it is okay? The EPA, DoT, and DoE have I think exaxtly zero wind tunnels for testing automobile perfomance and efficiency “for” real, especially with “moving ground” beneath, Instead we have the EPA and its iffy coastdown data, and it’s plus no-wind simulated load estimations. (And no idling data either)
    6. I think British may be going to real-life outdoor economy testing; post-adjusting for traffic and conditions.
    7. I hope you might like this paper, see Figure 3 if nothing else*. Talk about drag gullies! Maybe our cars should be like spinning balls.
    Thx for your work. (*http://baseball.physics.illinois.edu/AJP-Nov03.pdf)

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