Directly Measuring Vehicle Efficiency in a Wind Tunnel
Patent US 9,766,153
As a result of inquiring about the possibility of demonstrating the efficiency gains of AERODEFENDER™ wheel fairings within a wind tunnel, it quickly became apparent that current protocols used to measure vehicle drag within wind tunnels would never produce an accurate result. For that reason, we invented a better, more accurate method to determine open-wheeled vehicle propulsive efficiency within a wind tunnel, disclosed in patents US 9,766,153 and EP 3152541.
Instead of measuring drag forces on a vehicle and then only estimating how these forces on various parts of the vehicle actually affect vehicle efficiency, our new method eliminates any guesswork by directly measuring the aerodynamic power efficiency of the vehicle. Instead of restraining the vehicle to the ground inside the wind tunnel and thereby dramatically changing the vehicle propulsive mechanics, our patent US 9,755,153 teaches that the vehicle should be unrestrained and entirely self-propelled as it is on the roadway. The rolling-road is then automatically adjusted in speed for changes in the vehicle speed in order to keep the vehicle centered within the wind tunnel.
In this dynamically stable vehicle operating condition within the wind tunnel, the power being dissipated in drag on the vehicle is necessarily transferred through the ground contact patches of the wheels. This power then is equivalent to the power needed to drive the rolling road supporting the moving wheels. Thus, a measure of the net power driving the rolling road is then an equivalent measure of the actual power being dissipated in drag on the vehicle itself.
Thus, vehicle aerodynamic efficiency is directly measured, incorporating any nonlinear effects of the various wheel surfaces moving in different directions and being magnified in various amounts against propulsive counterforces. There is no longer any need to incorporate engineering modeling simplifications in estimating how drag forces on various rotating wheel surfaces actually affect vehicle efficiency.
For example, wheel surfaces are moving vertically both upward and downward, thereby dissipating power in drag, while not actually imparting a net translational force to be measured on the vehicle. These wheel losses simply go unmeasured in traditional force measure approaches used to measure airplane efficiency, where all surfaces are exposed to the same wind speed.
For this reason, we urge the automotive wind tunnel community to consider upgrading their protocols for measuring vehicle efficiency, especially for the open-wheeled racecars where exposed upper wheel drag is a dominate component of overall vehicle drag. They will likely learn that wheel width could be better minimized as a trade against loss of lateral traction for significant gains in speed in the straight portions of the race course. Thus, selecting proper tire width size will likely play an even more important role for success that will depend largely on the particular event race course, being optimized for whether more dominated by either faster straight sections or slower turns.