# Aerodynamic Forces and Moments acting on the Vehicle Body

In order to analyse the various aerodynamic forces and moments acting on the vehicle body, consider the vehicle as a mass having six degrees of freedom. Now the various aerodynamic forces acting on the vehicle body can be summarized as follows:

P_{x} – Force of air drag in the direction of motion with wind angle along the longitudinal axis

P_{y} – Cross wind force

P_{z} – Aerodynamic lift

## Longitudinal Air Drag

The longitudinal component of the resultant of pressure distribution is called as “Longitudinal Air Drag”. The magnitude of this component can be represented by,

P_{x} = C_{x} ρ A V^{2} / 2

where,

C_{x} – Longitudinal wind force dimensionless coefficient

ρ – Air density in kg/m^{3}

V – Velocity of wind in m/s

A – Cross sectional area of the vehicle viewed from the front in m^{2}

## Cross Wind Force

Cross wind force is formed by the asymmetric flow of air around the vehicle body when the wind angle is not equal to zero. The cross wind force can be given as

P_{y} = C_{y} ρ A V^{2} / 2

where,

C_{y} – Cross wind force dimensionless co-efficient

## Aerodynamic Lift

Aerodynamic lift is the vertical component of the resultant of the pressure distribution over the vehicle body due to flow of air around it. The aerodynamic lift can be represented as

P_{z} = C_{z} ρ A V^{2} / 2

where,

C_{z} – Lift co-efficient

The lift will tend to reduce the pressure between the wheels and the ground, which causes losses of steering on the front axle and the loss of friction on the rear axle. The magnitude of this lift and its distribution over the front and rear is a function of ground clearance, the contours of the body and the underbody and the angle of attack of the air on the vehicle body.

Since these factors are not acting at the centre of gravity (C.G) of the vehicle body but at the centre of pressure, they create the following three aerodynamic moments:

M_{x} – Rolling moment

M_{y} – Pitching moment

M_{z} – Yawing moment

## Rolling Moment

This movement is caused by the cross wind force P_{y} about the longitudinal axis. The magnitude of this rolling moment is given by

M_{x} = P_{y} a = C_{mx} ρ A L V^{2} / 2

where,

a – Height of centre of thrust above CG

C_{mx} – Rolling moment co-efficient

L – Reference length

The rolling moment effects the weight distribution on the wheels. This effect is dangerous for tall vanes where the side force acts much above the CG. The only near solution to reduce rolling moment is to increase the wheel track.

## Pitching Moment

Pitching moment is caused about y axis by cross wind force P_{y} or the longitudinal force P_{x}. The pitching moment M_{y} is given by

M_{y} = P_{x} b = C_{my} ρ A L V^{2} / 2

where,

b – Distance between CG and CP

C_{my} – Pitching moment co-efficient

L – Reference length of the wheel base

The pitching moment is usually negative I.e. nose down and this moves. The rear axle lifts off the ground and further reduces the available traction.

## Yawing Moment

Yawing Moment is caused about z axis by cross wind force P_{y}. The yawing moment M_{z} is given by

M_{z} = P_{y} c = C_{mz} ρ A L V^{2} / 2

where,

c – Distance between CG and CP

C_{mz} – Yawing moment co-efficient

L – Reference length

These moments adversely affects the directional stability of the vehicle at high speed. The use of stabilizer fins at the rear of the vehicle gives a very good reduction in yawing moment.

Note:

Centre of Gravity is the point where the whole mass of a system is assumed to be act

Centre of Pressure (CP) is the point where the total pressure acts on the system.