An Intro to Drag

Aerodynamic Basics

This document is a super simplified look at aerodynamics relevant to the MSV competition. Aerodynamics is a complex subject so all we will do is give a very simplified overview to point you in the right direction. Refer to texts and other publications for a more detailed analysis.

The aerodynamic drag force is trying to slow your vehicle. The drag force can be calculated using the formula below.

Drag Force = ½ × Air Density × Drag coefficient × Frontal Area × Velocity²

There are only two parameters in this formula that you can change. They are:

Area, which is frontal area. The area that is pushed through the air as the car runs forward. (We will ignore the effect of wind which could be coming from any direction) Frontal area is relatively easy to control. Just make your vehicle as small as possible within the regulations and functionality.

If you can halve the frontal area you will halve the drag force.

Drag coefficient, which is related to the shape of your car. Typically, smooth rounded curves with an aerofoil type shape will give a low drag coefficient. Refer to texts for details.

All the above shapes have the same air drag

Sharp corners / changes in area cause flow separation with low turbulence & low pressure.

The rear of your vehicle is important, aim for a shape that eliminates flow separation or at least limits it to a small area. Flow separation, the turbulence and low pressure it creates is a significant factor in air drag.

Some figures from wind tunnel testing of models follow:

Wind tunnel testing of 2 models, one a simple box the other an aerofoil shape gave us aerodynamic drag figures for these shapes. Photos are of models in the wind tunnel.

Block model has 2.6 times the drag of the shaped model below.

Shaped model, drag difference 2.6 times less than block.

The models below were also tested in the wind tunnel. The first model is in the shape of an airship. Both models have the same frontal area and surface area.

The airship shape exhibited drag of 8 times less than the blunt ended model.

Airship model 92.2 mm diameter by 395 mm long air drag 8 times less than block below.

Block model 92.2 mm dia 252 mm has air drag 8 times that of airship shape.

The expected effect on car performance of aerodynamics can be evaluated using the mathematical simulation of a car race available on the web site.

Related to aerodynamics is the common suggestion, that useful lift to reduce wheel load (and hence rolling resistance), or down force (to hold the car onto the track) can be generated aerodynamically by spoilers.

It is true these forces exist and are used to great effect on formula 1 and other race cars, but one should question the usefulness of them in Model Solar Car racing. For a typical car, considering the car plan area together with the maximum expected velocity, these forces will generally be so low as to be ineffective. Remember: there is always an unwanted drag force associated with lift.

Any increased air drag is detrimental to car performance.

In any case, down force has no use in the car competition as it increases rolling resistance by increasing wheel load. This is unnecessary for this competition as the car guiding is achieved not by wheel grip to the track but by guides running on a rail.

Lift force will reduce wheel load and consequently rolling resistance, but the effect is slight. In the extreme, lifting the car will result in loss of guidance leading to a crash.