145 Aerodynamic lift control

14.5.1 Underbody dams (Fig. 14.35(a-c)) Damming the underbody to ground clearance at the extreme rear blocks the underfloor airstream and causes a partial pressure build-up in this region, see Fig. 14.25(a), whereas locating the underbody dam in the front end of the car joins the rear low pressure wake region with the under-floor space, see Fig. 14.35(b). Thus with a rear end underfloor air dam the underfloor air flow pressure increase raises the aerodynamic upthrust, that is, it produces positive lift, see Fig. 14.35(a). Conversely a front end air dam reduces the underfloor air flow pressure thereby generating an aerodynamic downthrust, that is, it produces negative lift (see Fig. 14.35(b)). Experimental results show with a front end dam there is a decrease in front lift (negative lift) whereas there is a slight rise in rear end lift (positive lift) as the dam height is increased, and as would be expected, there is also a rise in drag as the frontal area of the dam is enlarged, see Fig. 14.35(c).

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