[aerohead]

It's the "open-pipe" theory that floats about.The challenge is to get enough air "through" the vehicle,without increasing frontal area,or imposing on occupants or cargo.The ducting must be very complex,and wall friction eats away at kinetic energy available for the wake.The ducting also affects crumple zones,occupant safety in collisions,weight,and is also effected in non-zero-yaw relative-wind conditions.In addition,its a fair-weather system,as,the pressure available at the forward stagnation point of the vehicle finds it difficult to purge or displace rainwater pouring into the discharge perforations.In the wing,its a straight-line path to the trailing edge and the pilots probably don't fly into foul weather.Its a fascinating concept,however plagued with challenges.