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Shock waves occur as parts of the car reach supersonic speeds – not necessarily all at the same time. They cannot be avoided, but a great deal of work is put into minimising them, and managing their effects. All the planning and simulation in the world cannot predict every eventuality though, at 1000mph this is unchartered territory


Minimising frontal area is crucial but, as in the case of Sonic Wind, packaging of the driver, engine and fuel tanks dictate how small this can be


Silver Bullet RV1 features a distinctive ‘wasp waist’ in an obvious application of the Whitcomb Area Rule. Twin tail fins are designed to aid yaw stability at speed by ensuring the centre of pressure is behind the centre of gravity


vehicle. The transonic region is therefore where additional power becomes necessary to overcome the wave drag. But because of the way in which shock waves initiate on different parts of the vehicle, and the way the shock waves themselves move as speed increases, it is also where aerodynamic instabilities occur. The idea is therefore to use


aerodynamic techniques to try and prevent additional shock waves forming along a supersonic body, and this is where lessons learnt in other areas of technology come into play. Shapes such as the transonic minimum drag ‘Sears Haack body’ (see figure 1), and nosecone


variants based on high velocity bullet shapes, such as the ‘von Karman ogive’ and the ‘Power Series’ (especially x1/2 power). Also, the application of the ‘Whitcomb Area Rule’, also known as the Transonic Area Rule, or


and very gradually changing cross sectional area would not generate any additional shock waves along the length of the body, but is not a practical shape for a ground vehicle of this type. But the general principle


“an altered reality with different physics“


just the Area Rule. ‘Fineness’, or length-to-diameter ratio is also a key weapon in the supersonic aerodynamics armoury, larger ratios reducing wave drag. A shape like the Sears Haack


body, for example, with its cylindrical section, pointed ends


of making very gradual, gentle changes to the cross sectional area still applies in reducing the number of locations from where additional shock waves might propagate, even though the final shape will necessarily involve compromises.


Canard wings are used to trim the vehicle and maintain balance


But it is not just from where shape changes occur that shock waves might emanate, they can also originate where there are changes in cross-sectional area, and this is where the Whitcomb Area Rule comes into play. Essentially, this states that if a shape is designed to have the same cross-sectional area distribution along its longitudinal axis, as the Sears Haack body, it will generate similar wave drag, largely independent of its actual shape. So where the packaging of internal components requires bulges or other shape changes, if the body is thinned elsewhere around that part so that the cross-sectional area is


January 2012 • www.racecar-engineering.com 65


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