1433 Underbody floor height versus aerodynamic lift and drag

(Figs 14.21(a and b) and 14.22) With a large underfloor to ground clearance the car body is subjected to a slight negative lift force (downward thrust). As the underfloor surface moves closer to the ground the underfloor air space becomes a venturi, causing the air to move much faster underneath the body than over it, see Figs 14.21(a) and 14.22. Correspondingly with these changing conditions the air flow pressure on top of the body will be higher than for the under-body reduced venturi effect pressure, hence there will be a net down force (negative lift) tending to increase the contact pressure acting between the wheels and ground. Conversely a further reduction in underfloor to ground clearance makes it very restrictive for the underbody air flow (see Figs 14.21(b) and 14.22), so that much of the airstream is now compelled to flow over the body instead of underneath it, which results in an increase in air speed and a reduction in pressure over the top to cope with the reduction in the underfloor air

Upthrust

Upthrust

Higher stagnant Slower moving air Direction Drag air pressure slight reduction of motion resistance in pressure

Higher stagnant Slower moving air Direction Drag air pressure slight reduction of motion resistance in pressure

(a) Large ground clearance (negative lift downthrust)

Small reduction in air speed

(a) Large ground clearance (negative lift downthrust)

Small reduction in air speed

(b) Small ground clearance (positive lift upthrust)

Fast air flow

Fast air flow

movement. Thus the over and under pressure conditions have been reversed which subsequently now produces a net upward suction, that is, a tendency toward a positive lift.

14.3.4 Aerofoil lift and drag

(Figs 14.23(a-d), 14.24(a and b) and 14.25) Almost any object moving through an airstream will be subjected to some form of lift and drag. Consider a flat plate inclined to the direction of air flow, the pressure of air above the surface of the plate is reduced while that underneath it is increased. As a result there will be a net pressure on the plate striving to force it both upwards and backwards, see Fig. 14.23(a). It will be seen that the vertical and horizontal components of the resultant reaction represents both lift and drag respectively, see Fig. 14.23(b). The greater the angle of inclination, the smaller will be the upward lift component, while the backward drag component will increase, see Fig. 14.23(c and d). Conversely as the angle of inclination decreases, the lift increases and the drag decreases; however, as the angle of inclination is reduced so does the resultant reaction force. If an aerofoil profile is used instead of the flat plate, (see Fig. 14.24(a and b)), the airstream over the top surface now has to move further and faster than the underneath air movement. This produces a greater pressure difference between the upper and lower surfaces and consequently greatly enhances the aerodynamic lift and promotes a smooth air flow over the upper profiled surface. A typical relationship between the CL, CD and angle of attack (inclination angle) is shown for an aerofoil section in Fig. 14.25.

14.3.5 Front end nose shape (Fig. 14.26(a-c)) Optimizing a protruding streamlined nose profile shape influences marginally the drag coefficient and to a greater extent the front end lift coefficient.

Fig. 14.22 Aerodynamic lift versus ground, floor height

With a downturned nose (see Fig. 14.26(a)) the streamlined nose profile directs the largest proportion of the air mass movement over the body, and only a relatively small amount of air flows underneath the body. If now a central nose profile is adopted (see Fig. 14.26(b)) the air mass movement is shared more evenly between the upper and lower body surfaces; however, the air viscous interference with the underfloor and ground still causes the larger proportion of air to flow above than below the car's body. Conversely a upturned nose (see Fig. 14.26(c)) induces still more air to flow beneath the body with the downward curving entry gap shape producing a venturi effect. Consequently the air movement will accelerate before reaching its highest speed further back at its narrowest body to ground clearance. Raising the mass airflow in the space between the body and ground increases the viscous interaction of the

0.6 0.8 1.0 h/b air with the under body surfaces and therefore forces the air flow to move diagonally out and upward from the sides of the car. It therefore strengthens the side and trailing vortices and as a result promotes an increase in front end aerodynamic lift force.

The three basic nose profiles discussed showed, under windtunnel tests, that the upturned nose had the highest drag coefficient Cd of 0.24 whereas there was very little difference between the central and downturned nose profiles which gave drag coefficients Cd of 0.223 and 0.224 respectively. However the front end lift coefficient CL for the three shapes showed a marked difference, here the upturned nose profile gave a positive lift coefficient CL of +0.2, the central nose profile provided an almost neutral lift coefficient CL of +0.02, whereas the downturned nose profile generated a negative lift coefficient CL of —0.1.

Fig. 14.22 Aerodynamic lift versus ground, floor height

(a) Reaction force on an inclined plate (b) Lift and drag components on an inclined plate

Large lift component

Large lift component

Resultant reaction

Small drag component

Resultant reaction

Small drag component

Smaller lift component

Smaller lift component

Resultant reaction

Larger drag component

Resultant reaction

Larger drag component

(c) Small angle of inclination (d) Large angle of inclination

Fig. 14.23 (a-d) Lift and drag on a plate Inclined at a small angle to the direction of air flow

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