Continuing the analysis of my Solar Racing Basics Poster (see this tag), perhaps the most important issue in solar car racing is aerodynamic efficiency. In the Challenger class, average power is limited by the size of the solar panel to about that of a microwave oven. This is about one hundredth of the engine power in a typical small car.
Solar cars must therefore eliminate as much aerodynamic drag as possible. Aerodynamic drag is the most important limiting factor on speed.
The aerodynamic drag force, which is given by the formula F = ½ Cd A ρ v2, acts to slow the car down. At maximum speed, the drag force (plus also other factors) exactly balances the force of the motor, which of course acts to speed the car up. In the formula, v is the speed, Cd is the drag coefficient, A is the frontal area of the car, and ρ is the density of air (which we can’t change).
If Fmax is the maximum force that the motor can deliver, then the maximum speed is given by rearranging the formula: vmax = sqrt (2 Fmax / (Cd A ρ)). One way of speeding up the car is by making the shape more aerodynamic (that is, by reducing Cd). Challenger class teams should be aiming at drag coefficients Cd under 0.1. In the Cruiser class, values under 0.2 would be appropriate (for comparison, Cd in ordinary cars ranges from 0.25 for a modern streamlined sports car to 0.6 for an SUV).
Click to zoom / Image credits: Anthony Dekker (Electrum from Michigan, Green Lightning from Top Dutch)
We can also speed up the car by making it narrower (that is, by reducing the frontal area A). In 2019, the 1-metre-wide Electrum from Michigan finished ahead of the 1.2-metre-wide Green Lightning from Top Dutch, in spite of having had problems (79.6 km/h compared to 78.4 km/h). On the other hand, if you make the car too narrow, it will roll over (which means that the car fails pre-race scrutineering). Alternatively, we can reduce the frontal area A by making a “gap” for the air to flow through. If we can make both the drag coefficient Cd and the frontal area A one sixth of the values for a typical small car, then we can travel at 60% of the speed of that car, even though we have only one hundredth of the engine power.
In the past, successful Challenger class designs have included:
- Catamarans: double hulls, one of which holds the driver, held together by a “wing” on which the solar panel is fixed, with a “gap” between the hulls (this design has won every race since 4 wheels were made mandatory in 2013)
- Monohulls or “bullet cars”: long, narrow cars with a tapering rear like Electrum or Green Lightning (this design came second in both 2017 and 2019)
- Outriggers: like monohulls, but with the wheels outside the main body and more widely spaced for stability (this design has not performed quite so well, because of aerodynamic drag from the wheels)
Three wheels are now allowed in 2021. This allows slightly better monohulls (with two wheels inside the front of the body and one wheel just inside the tapering rear of the body). It also allows noticeably better outriggers (only two wheels need to be outside the main body). In addition, there are a number of new designs that teams have been thinking feverishly about for several months.
During design, aerodynamics is typically assessed using computational fluid dynamics software, but “ground truth” is a wind tunnel or an actual race. In the illustration on the poster, Belgian team Agoria is using a green laser to reveal airflow around their car in a wind tunnel (see also the video here).