Improving the Aerodynamic Stability of a Practical, Low Drag, Aero-Stable Vehicle 2000-01-1577

The aerodynamic drag of future low emission vehicles will need to be low. Unfortunately, vehicle shapes that result in low drag coefficients - of the order of 0.15 - are often aerodynamically unstable in crosswinds. The addition of wheels, transmission, radiators, suspension, steering, brakes, air ducts and wing mirrors can easily increase this drag coefficient to 0.24 and above and produce an undesirable lift distribution. The Aero-Stable Carbon Car (ASCC) is a research project, in conjunction with industrial partners, to design and build a practical 3 to 4 seat low drag car (C_D less than 0.20) with an acceptable lift distribution (front to rear) which is also stable in crosswinds and in yaw through a series of low speed wind tunnel tests performed in the Cranfield College of Aeronautics 8′ × 6′ wind tunnel facility.

This paper contains results, through balance measurements on quarter scale ASCC models (at Reynolds number based on wheel-base of 2 × 10⁶), of the effect that ride height, pitch angle, undertray and bonnet profile, lateral bleed, strakes and roof strips have on the drag and lift distribution at zero yaw and on the lateral stability at yaw angles up to 30°. The results show that the aerodynamic performance of the ASCC is particularly sensitive to changes in ride height and pitch angle with the modifications to the undertray improving the lift distribution significantly and producing an acceptable front to rear lift imbalance. The results also show that the lateral stability was improved with roof strips, strakes and lateral bleed although lateral bleed was not as beneficial to lateral stability as the strakes or roof strips. A drag coefficient of the order of 0.20 was maintained with an acceptable lift distribution and an improved lateral stability.