How BMW ran a blown rear wing back in Japan 2009

quoted from my autosport article last year

BMW brings back innovation

By creating a full-width slot, BMW can run a steeper wing without fear of stallingThere have been few genuinely innovative ideas since the start of this season, with teams taking inspiration from each others’ designs. But in Japan, BMW-Sauber appeared with a radical rear wing aimed at creating higher levels of downforce.

For many years, rear wings have been limited to just two elements – the greater number of elements, the more slots are created in between flaps. These slots send fast-flowing air underneath the wing to prevent the airflow from separating and stalling the wing. With two elements, the wing is limited in how steep an angle it can be set at and therefore how much downforce it can create.

At high-downforce tracks, some teams make use of a loophole in the rules to add a narrow 15cm slot in the middle of the wing. But having such a narrow exit behind the wing, the effect of the slot is limited.

BMW has widened the exit of the slot to the full width of the wing, which effectively gives the wing a third element. Air enters the slot above the wing and a higher pressure is created by the humps to the side of the inlet and the small wing above it. Air is ducted internally to exit in a slot spanning the entire wing and the flow exiting this slot prevents the separation and stalling problems of a steeper wing.

The rules demand the two elements to be a “closed section” to the side of the middle 15cm, so BMW’s wing meets the wording of the regulation as there is no inlet in these areas.

If BMW can create a steeper wing without the problem of stalling, teams that have struggled with high-downforce set-ups could follow this trend.