the T.50 has a fan, and it is used for aerodynamic benefit, but it does so by helping speed up air through the under car diffuser, which allows them to use a more aggressive diffuser than would otherwise work. It all adds up to a modest downforce improvement and/or drag reduction.

The McMurtry by comparison is more like the old F1 fan car, in that it is literally sucking itself down to the road, with tons of force, with a skirt and so on.