# Aerodynamic Road Test Techniques To Measure Lift and Drag Forces

In this post, we’ll look at how to use aerodynamic road test methods to calculate lift and drag forces and improve a vehicle’s performance.

## Hill Rolling Method to Measure Lift and Drag Forces

Hill rolling method comprises of allowing vehicle to roll down on a hill of uniform slope up to a fixed terminal. The terminal velocity of a vehicle is a function of all forces acting on the vehicle. The forces other than aerodynamic force is determined from a force balance mounted in a vehicle and hence drag force can be determined.

The disadvantages of this method is that, hill of constant  slope without traffic is difficult to achieve and also terminal velocity may not be reliable and the wind is present during the test hours. Airplanes defy the laws of physics to only for airfoil sections is the identify structures used to measure the drag coefficient. The force starting to act at a higher level compared of motion through the air is remembered as lift. Differences in air pressure produce lift. A mechanical force is drag.

The interaction and touch of a solid body with a fluid generates it (liquid or gas). It’s not produced by a gravitational field in the form of a gravitational or electromagnetic field, in which one object will influence another without physically touching them. The solid body must be in contact with the fluid to produce drag. There is no drag if there is no fluid.

The difference in velocity between the solid object and the fluid causes drag. Between the object and the fluid, there must be motion. There is no drag if there is no movement. If the object moves through a static fluid or the fluid moves past a static solid object makes no difference. Drag acts in the opposite direction of movement.

## Constant speed test to measure Lift and Drag Forces

Here the vehicle is allowed to maintain a constant road speed. The force required to maintain constant speed being measured by a torque meter mounted on drive axle.

## Method of variation of gross vehicle weight to calculate Lift and Drag Forces

This method is based on the assumption that, at a constant vehicle speed, the increase in total drag force is directly proportional to increase in Gross Vehicle Weight (GVW). The test procedure is to determine the total drag force at different Gross vehicle weight at constant vehicle speed. Of course, nothing else is available; the value of raise is drag.

## Coast Down Test to measure Lift and Drag Forces

In this technique, the vehicle is accelerated to certain speed in a gradient less road. After attaining the test speed, the engine is turned off and the vehicle is allowed to run by without applying brake and without negotiating any curve. Now the deceleration period for every 10 kmph speed is noted down and the coefficient of drag is found by substituting the values.

## Mathematical Analysis method to calculate Lift and Drag Forces

This method is based on the simple mathematical analysis of Coast down data. The method doesn’t use any instrumentation to determine the drag and it allows separation of Aerodynamic and Rolling resistance.

Among the above said method Coast Down Test is easiest one and it is mostly used to calculate Lift and Drag forces.

### Test Procedure

The tyre pressure should be checked first, then the test should be conducted in a straight road without gradients. To be more practical, the test is done in both direction to nullify any gradient present. Also care is taken to choose the test site without any traffic disturbances and also the site with zero wind velocities. The weight of the vehicle together with the weight of the occupants is determined, using a weigh bridge.

The car is started, the speed of the car is accelerated to 80 kmph. When the vehicle is moving steadily at or above 80 kmph, the ignition switch is switched off while the gear is changed to the neutral position. At the same time, stop watch is started. Due to aerodynamic drag and rolling resistance, the car is decelerated from 80 kmph to lower speed. The time for deceleration of vehicle at every 10 kmph is noted. The time should be noted up to the vehicle speed reaching 10 kmph. Then the vehicle is stopped. Because, at very low speeds, aerodynamic drag is negligible.

Let the vehicle is tested in both the direction between the fixed terminals. This is because to nullify the effect of any gradient. The trial is repeated twice.

The Coast Down Test is conducted for the following two conditions of the vehicle.

• With windows opened
• With windows closed

The wind tunnel provides the most convenient method for investigating the drag changes produced by various modifications of a vehicle configuration. However, the size limitations of the available wind tunnel, the tests are usually performed with scale models of vehicle rather than with full scale vehicle. This allows the relative merits of various vehicle modification to be assessed in most cases.

However, direct application of drag changes measured on models in wind tunnels to the prediction of the fuel saving on full scale vehicle moving on the highway is not likely to produce agreement with consideration of Froude model and Reynolds number simulation of the turbulence generation by winds and other vehicle on the highways is the major contribution to the source of disagreement comes from wind engineering studies of the effects of turbulence on the flow around bluff and contoured shapes and from some basic studies on the effects of turbulence on the flow.

One of the few drawbacks, full scale wind tunnel apart from being quite costly to construct in the absence of relative motion between the wind tunnel floor and tested object.

On the other hand, in the case of Coast down test, the test is conducted in road and hence there is no need for the correcting factor.

### Some other advantages of Coast Down Test are:

• Simple Instrumentation
• No adaption needed to new vehicles
• Conceptually simple

# 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:

Px – Force of air drag in the direction of motion with wind angle along the longitudinal axis

Py – Cross wind force

Pz – Aerodynamic lift

## Longitudinal Air Drag to measure Aerodynamic Forces

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

Px = Cx ρ A V2 / 2

where,

Cx – Longitudinal wind force dimensionless coefficient

ρ – Air density in kg/m3

V – Velocity of wind in m/s

A – Cross sectional area of the vehicle viewed from the front in m2

## Cross Wind Force to measure aerodynamic forces

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

Py = Cy ρ A V2 / 2

where,

Cy – Cross wind force dimensionless co-efficient

## Aerodynamic Lift to measure aerodynamic forces

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

Pz = Cz ρ A V2 / 2

where,

Cz – 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:

Mx – Rolling moment

My – Pitching moment

Mz – Yawing moment

## Rolling Moment to calculate Aerodynamic Forces

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

Mx = Py a = Cmx ρ A L V2 / 2

where,

a – Height of centre of thrust above CG

Cmx – Rolling moment coefficient

L – Reference length

The rolling moment affects 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 Py or the longitudinal force Px. The pitching moment My is given by

My = Px b = Cmy ρ A L V2 / 2

where,

b – Distance between CG and CP

Cmy – Pitching moment coefficient

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 Py. The yawing moment Mz is given by

Mz = Py c = Cmz ρ A L V2 / 2

where,

c – Distance between CG and CP

Cmz – Yawing moment coefficient

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.