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AERODYNAMIC LIFT & PITCHING MOMENT

Vertical component of the resultant of the pressure distribution Lift. General vehicle profile same effect as aerofoil. Streamline body higher velocity at the upper part & lower velocity at below the vehicle. Aerodynamic lift is applied through the center of pressure of the body profile and, since this point does not correspond with the centre of gravity, it creates a pitching movement about the lateral axis. Influence of force Px on Pitching moment is usually small, as the vertical separation between CG & CP is not great.

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Both Lift & Pitching moment have undesirable effects. Lift tend to reduce the pressure between wheels and ground.

Loss of steering on the front wheels Loss of traction on the rear axle.

Pitching moment is usually negative i.e nose down.

Rear axle is lifted off the ground further loss on traction.

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EFFECT OF FAIRING If a fairing is used to cover the cockpit there will be

an increase in both lift and pitching moment.

However if a fairing is not used there will be an advantageous effect on lift and pitching movement but increase in the drag coefficient CX

With the fairing fitted, the large area of negative pressure is toward the rear of the car.

It is this negative pressure which causes the increase in lift and negative pitching moment.

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EFFECT OF CROSS WIND

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It indicating that the lift coefficient increases parabolically with the increase in the wind angle, up to two or three times its value when there is no side wind.

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EFFECT OF VEHICLE PROFILE

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Three box construction has the greatest spread of lift coefficients (from 0.4 to 1.0)

Flat fronted type of vehicle has the smallest range (0.15 to 0.55)

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Saloon cars can reach a value of 100 kg, or 8 to 10 per cent of the total weight.

Sports or racing cars the lift can reach values of 130 kg, which is 15 to 25 per cent of the total weight.

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SIDE FORCE, YAWING MOMENT AND ROLLING MOMENT

Side force is formed by asymmetric flow round the vehicle body when the wind angle is not equal to zero. This force acts at CP & creates moment about CG-Yawing moment(MZ)about Z-axis and rolling moment(Mx) about the X axis It try to turn the vehicle away from the direction of motion-loss of directional control.

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Fig 1 and 2 shows the pressure distribution for two wind angles around two different vehicles, measured at a constant height from the ground.

The diagrams clearly shows that the maximum change in pressure occurs at the front and rear of the vehicle(causing large changes in the yawing moment)

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FIG:1

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FIG:2

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FIG:3

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Values of the cross wind coefficient CY are shows in above fig 3. for different vehicle body shapes.

The lowest values of the cross wind force coefficient are obtained with the streamlined bodies of sports cars.

Saloons, vans, the pseudo-aerodynamic vehicles(styles with fore and aft projection wings)

And lastly, old models with box-like bodies have increasingly large values.

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FIG:4

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The above fig 4 gives values of Cmz Obtained from wind tunnel tests.

It is worthy of note that vehicle bodies with good aerodynamic direct force coefficients (low values of Cx CY and CZ ) have the largest values of Cmz

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The vehicle with poor aerodynamic properties will have a better( that is, lower) yawing moment coefficient.

A side wind will produce a yawing moment tending to turn the vehicle away from the direction of the motion.

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FIG:5

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FIG:6

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The use of stabilisers or fins at the rear of the vehicle gives very good results. (Fig 5 and 6)

Without a stabiliser (curve a) the vehicle is unstable.

Simple stabiliser (curve b) reduces the yawing moment coefficient and, at large cross wind angles, actually provides a stable condition.

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The center of the aerodynamic forces is usually above the center of gravity so that the cross wind force PY will produce a rolling moment Mx about longitudinal axis.

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Rolling moment generated by cross winds has a sizeable effect on the weight distribution on the wheels.

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FIG:7

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Fig 7 shows that the wheel load on the same axle can vary by up to 100kg.

This effect is dangerous for coaches and particularly for tall vans, where the side force acts a long way above the center of gravity.

The only real solution here is an increase in wheel track

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