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AERO AT 1000mph


Aerodynamics at 1000mph


If you want to break the World Land Speed Record, you now have to go supersonic. So what’s involved? And what exactly is supersonic?


It’s only a concept at present, but the New Zealand-based Jetblack borrows from modern fighter jet practice in its overall aero package


U


nderstanding what air does around a racecar at ‘normal’ speeds is complex enough, but reaching


the speed required to set a land speed record, and go on to 1000mph, involves an altered reality with different physics. Airflows become compressible, unstable shockwaves develop and move along the vehicle with changing speed, and ultimately the vehicle travels faster than the very noise it generates. The fundamentals


superficially sound simple. Clearly, any vehicle intended purely to go as fast as possible needs low aerodynamic drag, which comes from shaping and


BY SIMON MCBEATH


minimum frontal area (and, to some extent, minimum ‘wetted’ or external surface area). It requires stability, which means there should be no significant changes in its aerodynamic performance over the large speed regime it will encounter, and also that it is as insensitive as possible to cross winds. It will probably require neutral lift or very modest downforce to maintain more or less constant wheel loads across the large speed range. And it should have benign response to any changes of attitude, which one hopes will be minor. As with any vehicle though, the basic requirements have to


64 www.racecar-engineering.com • January 2012


be met while also packaging the driver, the drivetrain, the wheels, the suspension and the control systems. And then there are the special requirements involved in passing through the transonic region and into the supersonic speed regime. And these requirements have direct influence on the fundamental shaping of the vehicle.


TRANSONIC AERODYNAMICS The speed regime is said to be transonic when the airflow over some parts of the vehicle accelerates to supersonic speed, while over others it is sonic or subsonic. While with aircraft the transonic speed range is typically Mach 0.8 to 1.2 (roughly


600-900mph), with ground vehicles the range is likely to be somewhat bigger because of the necessarily smaller details like wheel support struts and the wheels themselves, around which the air will locally accelerate significantly. Where the airflow reaches sonic speeds, shock waves form. These contain a lot of energy and generate significant additional drag – up to four times subsonic drag in magnitude, and known as ‘wave drag’. As the vehicle climbs through the transonic speed regime, shock waves will first be generated around the smaller detail areas, then increase until the airflow is sonic or supersonic over most of the


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