A study of caravan unsteady aerodynamics

The high speed stability and handling characteristics of car-trailer combinations are affected by both road and aerodynamic forces. While the tyre-to-road interaction is well understood the action of gusts, passing large vehicles and even small steering inputs will disturb the symmetry of flow and generate aerodynamic forces and moments which are sufficient to affect the handling of the system. Although accidents caused by high speed instability are relatively uncommon a better understanding of these aerodynamic effects will improve safety. In this paper a series of wind tunnel investigations using scale models are presented. Steady state investigations were used to measure the aerodynamic properties of the car-caravan pair while a novel technique was developed to measure the aerodynamic damping derivatives in yaw and side force for a caravan model. These damping derivatives were shown to be destabilizing in most cases of sideslip and stabilizing in yaw although it was demonstrated that high damping derivatives were attained at certain frequencies of excitation.