On-Road Aerodynamic Drag Analysis by Simultaneous Linear Inversion of the Equation of Motion

The objective of this work is an investigation of the aerodynamic improvements by e.g. the application of underfloor panels that might be masked by traditional, fixed-ground wind tunnel testing. Of prime importance is the simplicity and practicability of the developed analysis method, with particular emphasis on industrial application. A coastdown method with minimal instrumentation effort is chosen to determine drag coefficients on the road. Usually such experiments demand the measurement of vehicle velocity, airspeed, yaw angle and some additional environment characteristics like temperature and air pressure. However, in principle it is only required to measure the vehicle's speed-time history during the test, from which the road load results can be derived by mathematics of inversion. In this work several underfloor configurations of the test vehicle are investigated by only considering the motorcar's speed data retrieved from the control area network data bus during the coastdown. The velocity data of all configurations are reduced simultaneously by constrained linear inversion of the equation of motion. The differences in rolling resistance are kept minimal during the inversion, which is reasonable for the relative comparisons of the aerodynamic configuration changes. The results of this procedure are compared with the balance of energy method for the analysis of coastdown data and with wind tunnel investigations. Both analysis methods for the on-road data result in consistent aerodynamic drag differences for the various configurations, even though they are based on different models. The coastdown experiments tend to result in lower drag coefficients than the tunnel shows but the relative error is less than 3 % for all configurations.