MAX Update No. 18: Defining Drag, Part 1


| 11/26/2008 3:00:37 PM


Tags: MAX, Auto X Prize, gas mileage, drag, 100 mpg,

If you have a bajillion dollars or a well-equipped university, you can determine a car's aerodynamic drag in a wind tunnel. But you don't, and neither do I, so we'll have to do it on the cheap. Champagne science on a beer budget, that's my motto.

Automotive wind tunnels work by blowing air at a measured speed over a stationary vehicle (or model) and measuring the forces (drag, lift and stability ... doubtless the source of those tailfins that started appearing on American sedans in the '50s) acting on the car via a number of scales under the tunnel floor.

It's a fine way to do things, and is quite comfortable for the technicians, who sit in a room outside the tunnel, who can wander off for coffee, and who don't have to worry about their notes blowing out of their pockets during the test.

For the MAX project, however, we're using a moving car traveling through stationary air, which presents two obvious problems: How do we measure the speed and how do we measure the force?

 

aerodynamic drag 
PHOTO BY KATHERINE LOECK
 

A car speedometer isn't sufficient for speed measuring — too vague and too inaccurate — but modern technology has brought extremely precise speed-measuring equipment to the masses: the handheld hiker's GPS. Not only do they measure speed, they measure position and date and time of day, and best of all, they record it for you. This is important because one disadvantage of doing aerodynamic testing in cars (and trust me, this is also critical when testing aircraft) is that the technician has to pay attention to not crashing, along with attending to the test.

Jack McCornack
12/18/2008 10:50:08 PM

Thanks Mark, I'm glad you find MAX of interest. As far as the articles go, I try to adulterate the fluff with a bit of intellectual input, but I figure that anybody who's going to actually build one will dig deeper and go to our web site for specifics. As mentioned in the article, it's www.kineticvehicles.com The information is available, the KV site has links to a couple of enthusiastic users' groups, it has sources and part numbers for the Kubota conversion, and real info is indeed shared, and shared with great enthusiasm. I haven't been pushing the point because MAX isn't done yet (heck, the rules aren't even done yet) and we don't know exactly how MAX is going to end up when it makes it to the start line, but if its current style suits you and you're satisfied with 70 miles per gallon, you can build a car like MAX right now. But be warned: although we give away the information on how to convert a Locost to turbo diesel power, the folks who provide the how-to-build-a-Locost books (Amazon, for example) are going to try to SELL you a copy. I know there are pdf's of these books floating around the internet, so if you're somebody who feels that charging for intellectual property is unethical, you can probably print up a pirated version and keep the author out of the loot loop.


Jack McCornack
12/18/2008 9:32:13 PM

>Get on with it already! Thanks for the nudge, Mark, I do my best work under pressure. MAX will be ready for the first stage races, but Progressive Insurance and the X PRIZE Foundation say it'll be a year before they drop the flag, and they're the folks putting up the bucks (millions of them) so they get to set the schedule. This blog offers a window to the MAX development process, and right now that process involves endurance testing (and the how-I-logged-6000-miles-in-ten-weeks story is a bit of a yawner). I hope you'll find other steps in the process more interesting, but if all you want to see is the finished product, all you have to do is wait. From now through MAX Update No. 75 or so we'll probably keep improving, testing, demonstrating, and sharing what we're doing, and I'm sure that's not everybody's cup of tea. Meanwhile, anytime somebody says "good enough for me" they can jump in and build a MAX of their own. We'll endeavor to make further improvements retrofitable so nobody is punished for starting too early.


Jack McCornack
12/18/2008 7:53:47 PM

Hah! Looks like there's a limit to how long I can drone on in Comments. Anyhow, 100 mpg is a serious challenge, but it's not yet apparent we won't make the whistle. As far as helping other people working in the field, I do plenty of that when I'm not wearing my writer's hat, and MEN isn't the place for me to be advising other car designers/manufacturers. As far as admitting my mistakes...man, that would fill volumes, there's barely enough space for the stuff I get right. :-)





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