a blowing system for controlling the aerodynamics of a ground vehicle comprises a source of compressed air , a valve for regulating the flow of the compressed air , and a plenum for discharging the air at a rear portion of the vehicle . the air discharged through the plenum prevents flow separation and consequently reduces the drag on the vehicle . it may also increase lift , reducing effective weight and thus rolling resistance of the tires , or decrease lift to improve tire traction and handling . the blowing system may have separate plenums for the left and right sides of the vehicle whereby vortex roll - up and flow separation which occur on only one side of the vehicle may be eliminated . asymmetrical blowing of only one plenum will produce rolling and yawing moments to restore the vehicle &# 39 ; s lateral / directional stability when exposed to a side wind . further , the blowing system may have a lower plenum for reattaching flow , generating a negative lift or download to increase traction , braking , and handling and for generating pitch trim . the blowing system may have a number of sensors for detecting various conditions , such as the direction and pressure of a relative wind , turbulence at the rear of the vehicle , braking , or angle of steering . the blowing system may be responsive to one or more of these conditions to selectively adjust the aerodynamics of the vehicle .