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Wind tunnel experiments, with emphasis on cross-wind effects, have been used to evaluate the effectiveness of a number of commercially available devices for reducing the aerodynamic drag of a tractor-trailer combination. The evaluations included consideration of the effects of tractor type, trailer height, and the bluffness of the tractors and/or trailers. A wind-averaged drag coefficient was introduced to interpret the basic data for the prediction of average drag in a highway environment. The average drag of the base-line vehicles was found to be a strong function of the bluffness of the tractor and/or trailer, and a weak function of the tractor type. Estimates of the average fuel savings that would result from the application of the various drag reducing devices ranged from -400 to 3300 gallons per 100,000 miles, depending on the combination of tractor, trailer and device that is used.

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.