I was mucking about with a bathroom fan I was supposed to be installing the other day, and noticed something amusing. If I blocked the inlet or the exit while the thing was running, the little "squirrel-cage" centrifugal fan would actually speed up frantically, then slow down with a happy sigh when I
allowed the air to flow through it. In trying to justify my time-wasting cruelty-to-electrons, I came up with a way to use this effect to assist automobile brakes.
To start (or should I say "to stop"?) stick a centrifugal fan or two in the car, driven off the wheels or the drive shaft. Use a belt or gears, as you please.
Arrange the ducting to suck air from under the car, and to blow air upward. That part is similar to _Brake_assist_ by [Twizz] (see link).
The key here is that the fans are spinning at all times, but the airflow is blocked off by a flapper similar to a throttle in a carburetor or to a crazy man's hand. The flapper is linked to the brake system, and opens when brakes are applied. When the brakes aren't engaged, the fan spins freely in its vortex.
On braking, the flap opens, air moves, and the fan instantly begins working, with no spin-up time and no clutches (the weaknesses in [Twizz]'s idea). The wheel or drive shaft the fan is driven by is slowed as the fan begins to perform work, and the car is pulled and pushed downward.
This system delivers a braking assist and an aerodynamic assist simultaneously. But, as an adjustable aerodynamic method, it won't be allowed on Formula One cars.
It may not be allowed on any cars for several reasons: I don't know how much energy is consumed by a blocked-off fan, but it is something, at least at this state of the art. Next, a centrifugal fan that is pumping air doesn't consume all that much horsepower--one big enough to slow the car may be bigger than the car.
Plus, I wouldn't want to be in a car with a giant fan spinning away, no matter how good the housing. Squirrels might like it, though.