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The single biggest performance limiting factor for a Greenpower car is aerodynamic drag and it is often overlooked where it should perhaps be incorporated during the earliest stages of design.
It is often assumed that rolling resistance is a greater enemy, but even the most aerodynamic cars will spend approximately double the amount of energy fighting aerodynamic drag at 25mph when compared to what is being spent on rolling resistance.
Take a look at the cars on the grid at the International Final, you will no doubt see that the cars that have qualified right at the front will typically be very small, feature very few exposed components and, most interestingly, will be no more elaborate either electronically or mechanically than the cars on the very last row of the grid.
So what is aerodynamic drag? And how does it influence a Greenpower car?
Put simply, aerodynamic drag is the force that an object travelling through air will experience acting against it and is a result of air speed, pressure and the surface finish of your bodywork.
When designing your vehicle, there are two elements that you should keep in your mind
Frontal Area (Fa) – Sometimes referred to as cross-sectional area, Frontal Area is effectively a measurement of the silhouette of the object that is presented to the air it is passing through.
Coefficient of Drag (Cd) – The coefficient of drag is a scaleless measurement that is used to identify the drag generated by the shape of the vehicle regardless of its size.
A vehicle’s Cd can be difficult to measure accurately but as a rule the more angular and bluff the shape of the vehicle is, the higher the Cd will be.
For example the sleek BMW i8 has a Cd of 0.26, whereas the boxy Hummer H2 has a Cd of 0.57, this means that if they were the same size, the Hummer H2 would have to overcome more than double the drag to run at the same speed as the i8.
The total drag of a vehicle is determined by a combination of the Fa and the Cd, both of which play equal parts so one of the main tasks of any Greenpower car bodywork designer is to strike the perfect balance between the size of the vehicle and the overall form.
Sources of inspiration
It’s important to draw inspiration from the right places, Its easy to think that as you are designing a race car you should seek inspiration from Formula One or Sportscars, but they are designed with the aim of carrying enormous speeds through corners that without downforce they simply would not be able to maintain.
Greenpower is a very different challenge, a lot of the circuits are capable of allowing cars to travel much faster than is possible on just 400w.
Below are a few suggestions of good places to seek inspiration for the shape of your vehicle.
Pre-1960’s Racing Cars – This was before downforce made its way in to motorsport, so most cars were very smooth, low slung and narrow.
Land Speed Record Cars – The fastest cars in the world never have to corner hard so they almost always have their wheels inside of their bodywork and are stripped of aggressive wings and any other ancillary parts that may cause drag when exposed to air flow.
Aircraft – Aircraft travel through the air at many hundreds of miles an hour and so low drag is vital. Propeller driven aircraft such as the Spitfire are a great source of inspiration, for example, note how the rivets are flush with the bodywork. Fixings sticking out in the airflow not only generate drag around themselves, but they then send unpredictable, turbulent air further down the vehicle.
Nature – Birds and, believe it or not, Fish are great examples of low drag shapes, study these to help design a low drag body. The Mercedes-Benz Bionic Concept car is a good example of this, it’s shape was based around that of a box fish.
- Luke Horsfall, Aerodynamics Design Engineer, Red Bull Racing Formula 1 Team