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Apart from Rolling resistance, the Aerodynamic Drag also represent the largest proportion of the overall resistance. It increases in the same way as that of Rolling resistance on movement of the vehicle and arises with increase in driving speed.

Drag Co-efficient:
The aerodynamic drag FL is depending on the following factors.

1. Speed of the vehicle V
2. Size and shape of the vehicle and
3. Air density ρ

The calculation for Drag co-efficient is as shown below.
FL = 0.5 . Cw . A . ρ .

The resistance that arises due to geometry of the vehicle is quantified by the Drag co-efficient (Cw).

The Aerodynamic drag can be controlled by the following measures.

1.Using fully panelled vehicle (Roof spoiler, front Apron and side panelling etc)
2.Aerodynamic shape and
3.Super structure with tightly roped Tarpaulin.

The Aerodynamic drag and Rolling resistance are each 33% for the commercial vehicles with an Engine out put of 301 KW (410 hp), a total weight of 40T and a driving speed of 80 km/h.

BASIC PRINCIPLES
Aerodynamics

Aerodynamics deals with all the processes that can be observed in flows around or through a body.
The drag co-efficient Cw is determined in a wind tunnel. The lower the Cw value the higher the final speed results in lower fuel consumption.

In the light of increasing fuel prices, it has become important for all commercial vehicle manufacturer’s to have favourable aerodynamic design to improve the Cw value. The recent trend is to have fully panelled commercial vehicles.

Aerodynamic Drag

The aerodynamic drag increases with the increase in speed. The raise in aerodynamic drag with increase in vehicle speed is to the power of two. Ie. Double the speed quadruples the Aerodynamic drag. The aerodynamic drag accounted for more than 40% of the total road resistance at a vehicle speed of 85km/h.

There are four proportions of Aerodynamic drag.
1. Pressure drag.
2. Induced drag.
3. Surface drag and
4. Inner drag.

1.Pressure drag: In the commercial vehicles the rear end turbulence zone contributes to this pressure drag. A partial vacuum arises at points where the flow becomes turbulent and this causes pressure drag. Attempts are made to create small turbulence surfaces and thus small partial vacuum zones. This is particularly difficult in the field of commercial vehicles because here the aim is to achieve maximum loading volume.(Box type super structures are preferred).

2.Induced drag: This is a part of pressure drag which caused by the air pressure differences between the top and under side of the vehicle that arises due to the vehicle movement. Pressure drag and Induced drag contributes largest proportions of the Aerodynamic drag ie. to the extent of 50% to 90%.

3.Surface drag: The frictional resistance set by the laminar air flow is called surface resistance. Long vehicles such as semi trailer trucks and buses have strong effect due to this surface drag. It contributes 3 to 30% of the total aerodynamic drag.

4.Inner aerodynamic drag: The vehicle is subjected due to the through flow of air to the drive unit, cooling and interior or drivers cab ventilation to this drag. This amount to 3 to 11% of the total aerodynamic drag.

Aerodynamic Measures:

Vehicle design: In order to optimise the vehicle design with regard to aerodynamic the front section of the driver’s cabin is rounded off in conjunction with the air deflection elements and the use of front apron.

An aerodynamically designed drivers cabin alone, however leads to reinforced air flow to the non optimised superstructure. The total Cw value then reached is even higher than that of the commercial vehicles with a squared drivers cab and non optimised superstructure. The reason for this is that with squared drivers cab the front section of the superstructure is located in a separation zone and hence exposed to lower aerodynamic drag.

Air deflection devices: Air deflection devices and additional shields are provided to achieve good aerodynamic harmonisation of the drivers cab and superstructure. These are divided in to three groups.

1. Air deflection on the drivers cab.
2. Streamlined elements on the superstructure and
3. Side panelling on the chassis.

Fully panelled commercial vehicles achieve best consumption values and hence increase the benefit to the fleet operators.

Air deflector on the drivers Cab: In order to significantly reduce the aerodynamic drag of the commercial vehicle with superstructure, a roof spoiler on the drivers cab with side flaps and roof attachment is incorporated.

Streamlined element on the superstructure: As rounding of the corners and edges of the superstructure reduces the size of the cargo bay and thus the loading volume. Hence an improvement in drag co efficient is achieved by changing the exterior shape of the superstructure. Streamlined elements on the front section of the superstructure reduces the air flow that can enter the separation zone of the air deflectors of the drivers cab.

Side Panelling: The side panelling reduces the aerodynamic drag of the commercial vehicles, which arises due to cross winds.

1.Drivers Cab front section rounded with air deflection element and front Apron.
2.Roof spoiler.
3.Roof attachment.
4.Side flap.
5.Chassis side paneling (Tractor)
6.Streamlined element (Super structure).
7.Chassis paneling (Super structure).

Next Session to follow: Drive Power and Aquaplaning.

Air deflection devices: Air deflection devices and additional shields are provided to achieve good aerodynamic harmonisation of the drivers cab and superstructure. These are divided in to three groups.