Aerodynamics in Racing Vehicle Design

In this article, I will look into the objective of aerodynamics, aerodynamics formulae, wing-tip vortex generation, vortex rotational sense, diffuser and venturi effect, aerodynamic cooling and varying the cooling air flow. Please feel free to skip to the parts most relevant to you.

What is the Point of Aerodynamics?

If you Google some pictures of some of the Formula Student cars University teams produce, you will see that some teams decide to adopt aerodynamic devices while others ignore them. So, bearing this in mind, what is the point of aerodynamics?
  • Drag reduction - Devices such as the diffuser help in reducing the pressure wake behind the car reducing the overall drag of the car.
  • Downforce production and balance - The maximum speed a car can go around a corner is depending on two factors primarily: coefficient of friction between the road and tyre and the normal force on the road. The normal force can be produced through downforce generation.
  • Efficient engine/brake and other cooling systems - Aerodynamics can be used to increase performance by cooling the engine and brakes better to enable them to reach higher temperatures safely.
  • Lateral Stability - By tweaking aerodynamics, stability and balance can be controlled for the car.

Aerodynamic Formulae

Drag, D = ½ v2 CD A      
Lift, L = ½ v2 CL A
Rolling resistance, F roughly equals k(mg - L) where k is the coefficient of friction between tyre and road

Thus makes it clear that has the speed of the car increases, the rolling resistance will stay pretty constant until centripetal force of the wheels cause the rolling resistance to increase at around 100km/h. Aerodynamic drag is squarely proportional to speed so that if you double the speed, the drag will quadruple.

Combination of lift and drag force
Increasing the weight will not improve cornering because the centripetal acceleration required also increases directly with mass. By using aerodynamic downforce will help cornering because it will help provide better tyre adhesion.

It is important to remember that the force created by an aerodynamic device is not perpendicular to the motion that the device is moving in. This is because although the device will produce a perpendicular lift force to motion, there will be a parallel force parallel to the motion of the aerodynamic device. Therefore, the resultant will be the combination of both the forces which will be at an angle to the perpendicular to motion axis.

Wing-tip Vortex Generation

Any wing will had some degree of vortex generation. This is when vortices are created at the tip of the wings  (also known as trailing vortices). This is why end plates are used on the end of wings so that the trailing vortices and starting vortex (when the trailing vortices connect perpendicularly) are reduced in magnitude decreasing the drag on the aerodynamic device

Vortex Rotational Sense

Vortex Rotational Sense
Another common feature that cars face is vortex rotational sense. This is the phenomenon which causes the flow of air to the side of the car to either move outwards of the car or inwards of the car (depending if the car has aerodynamic devices on them or not). (a) clearly shows a car without any aerodynamic devices creating outward vortices whereas the lift produced by pushing the air vertically up in (b) sucks air from the sides so inwards vortices are produced.

The Diffuser

The diffuser uses a venturi effect to create low pressure under the car. This is because of the following equation:

Ps + ½ v2 + gh = constant

This makes it clear that as the velocity of the fluid increases and height above ground stays constant, the pressure of the fluid must decrease, creating the suction that is desirable in motorsport.

Aerodynamic Cooling

On a petrol engine vehicle running efficiently, the energy available in the fuel is roughly spent as:
  • Power output of the engine - 33%
  • Heat to cooling air (via radiator) - 33%
  • Heat to the exhaust and via radiation - 33%
This makes it apparent that the engine output is roughly equals to the heat lost in cooling the air.

Cooling systems are usually designed to work without fan assistance except at low speed. This is because, at high speed, the fan has little effect to the cooling. F1 and many other racing cars have no fan: the cooling is done by ram air alone. This is why it is dangerous when F1 cars are on the starting line since they have nothing cooling the engines at all. The ram air enters into the cooling system via ram intakes.

Varying the Cooling Air Flow

Varying the outlet (or inlet area) of the cooling system to give only the required amount of cooling under any given condition means that the cooling drag and fuel consumption are kept to a minimum. At a steady cruise, the cooling requirements are usually relatively small.