Design Tips – Aerodynamic Drag
Aerodynamic drag is a major barrier to achieving high speed in any vehicle, being an invisible barrier to going faster. Here we will explain where drag comes from, and using this knowledge, how we can try to minimise its effect to build a fast car.
Drag acts against the car's thrust, and the balance of these forces determines whether you can accelerate quickly, or not at all! Friction acts with the drag to try to slow the car down.
There are several factors that affect the drag of a vehicle. We can break this down into an equation, so we can see exactly how each factor plays its part. The equation for drag is:
Or using common notation, this reads:
There are quite a few different terms there, so let's see how these affect the drag in different circumstances:
The density of the air is important, how weather means lower density and hence less drag.
Note that drag is affected by speed times itself, i.e. speed squared. This means that if we double the speed the drag goes up by four times. Three times speed means nine times the drag, four times the speed means sixteen times the drag, etc, etc.
The frontal area of the vehicle.
This special number characterises the aerodynamic design of the vehicle. It encompasses many physical features and can be reduced by good design of your car.
Now let's see how drag acts as a wall that prevents you from going any faster. Below we plot how drag increases with speed squared. The green line indicates the maximum available thrust from the engine. We're neglecting the friction effect for now to make things simpler.
The red dashed line is for a high drag vehicle - and the maximum attainable speed can be deduced by where the maximum thrust line crosses the drag line. This is indicated by the vertical red arrow.
The blue solid line is for a vehicle with half the drag of the previous one. The important thing to note is that for the same engine thrust it can achieve a much higher speed. As we go to higher and higher speed the difference between the low drag and high drag vehicles increases - the two curves are diverging.
News feeds