Explained: how does the V12-engined GMA T.50's fan create downforce?

Myth: the GMA T.50 is a ‘sucker car’ 

Given the fan is so visible on the T.50’s tail, you could be forgiven for thinking it’s there to suck air out from under the car.

After all, its designer, Gordon Murray, gave the world the Brabham BT46B Grand Prix car, so effective it was forced to withdraw after one race. That too had a vertical fan, and it did indeed suck air out the sealed underbody like a vacuum cleaner brush, creating gigantic downforce.

(As a way to circumvent the ‘no moving aerodynamic devices’ rule, Murray actually claimed that it was a cooling fan that just happened to draw from underneath. He always liked to save weight by making one part do two jobs. Anyway...)

The T.50’s fan doesn’t do that. It creates downforce not by its own crude suction, but by enabling the shape of the diffuser and rear wings to do that job better. Like any supercar there’s a diffuser under the rear. Its job is to slow down the rapid airflow from under the forward flat floor so that it can cleanly meet the slower air that goes over the car.

A steeper diffuser does that well, but only up to a point, because if it’s too steep the airflow in the diffuser itself detaches – ‘stalls’ – from the top wall and gets turbulent.

The fan sucks that turbulent air from ducts in the upper surface of the diffuser, so the rest stays linear. At the same time, the twin rear wings pop up, giving vastly more rear downforce and drag for braking – enough, he says, to cut 10 metres off the 150–0mph braking distance. Or they pop up less for a less draggy ‘high downforce configuration’.

But when you want big speed, the wings flatten and the fan draws air from a duct above the rear deck, sending it straight back to form a virtual long tail, minimising turbulence behind. The spoiler drops, and the ducts that draw from the diffuser also partially close, so it stalls, cutting drag even more. That also cuts downforce, so the springs aren’t uncomfortably compressed, and the car needs less power.

The fan is 400mm in diameter and crafted of carbon fibre, driven by a 48V motor at up to 7,000rpm. And it’s very obvious. Actually the McLaren F1 had a pair of small fans hidden in the rear bodywork that, like the T.50’s, pulled air from the diffuser top surface. They also blew air past the car’s electronics, again usefully doing two jobs: aero and cooling.