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A wind-tunnel study was carried out to determine the influence of some tractor and trailer geometries on the distribution of aerodynamic drag of tractor-trailer trucks. Also investigated were the effects on the drag distribution of drag-reducing devices and of exhaust stacks. The measurements were made on 1/8th-scale models at a Reynolds number of 106. The shape of the tractor strongly influenced the drag on the trailer. Streamlining the tractor could increase the drag on the trailer to the point that the drag on the tractor-trailer combination increased. A relatively streamlined conventional tractor was responsible for between 45% and 57% of the total aerodynamic drag at 0° yaw, whereas a relatively unstreamlined cab-over-engine tractor was responsible for between 73% and 82% of the zero-yaw drag. A streamlined fairing mounted on the tractor increased the drag on the tractor, but decreased the drag on the trailer substantially more than the drag on the tractor increased.

Procedures for conducting full-scale coast down tests for the purpose of determining the aerodynamic drag forces on tractor-trailer trucks are presented. Tests were conducted on the baseline vehicle and on the baseline vehicle modified with drag reduction devices. Techniques for the evaluation of mechanical drag forces prior to the coast-down tests are given and a description of the instrumentation is included.

A data reduction procedure is presented and sucessfully used to analyze coast-down data obtained in a windy environment to provide a measure of the aerodynamic drag on a full-scale tractor-trailer combination as a function of the yaw angle of the vehicle. Full-scale drag coefficients were evaluated for a vehicle operated in the baseline mode, and following the addition of four different drag reducing combinations, over a yaw angle range of from -10° to 10°. Comparison with wind tunnel measurements suggests that the wind-tunnel provides a reasonable simulation of the effects of winds on vehicle drag. The full-scale drag reductions measured in the presence of winds were generally lower than those found in wind-tunnel tests, except in situations where a vertical gap seal device was present. In this case, there was good agreement between the results of the coast-down tests and the wind-tunnel tests.

A study has been carried out to determine the reduction in the aerodynamic drag of tractor-trailer trucks brought about by fairings mounted on the cab roof and a vertical seal mounted across the gap between the tractor and the trailer. This study was conducted using one-eighth-scale models in a wind tunnel at a Reynolds number of about 25 percent of that of full scale trucks at 50 mph. Reductions of up to 35 percent of the zero-yaw drag coefficient, and of over 25 percent of the wind-averaged drag coefficient were obtained. The drag reductions obtained with roof fairings, with and without gap seals, significantly exceeded those obtained with a commercially-available roof deflector, with and without a vortex stabilizing device. Comparison with other model tests and a full-scale test indicates that the results obtained in this study should be directly applicable to full-scale trucks.