I wish I had taken some photos of the underside of the 40". There is no room under there. It's a 3 blade design and the spindle housings are 9.375" in diameter leaving only 2" between the spindle housings and the baffles. Add to that the short diameter of the baffles and you've got a real problem under there.
The 36" is a two blade design and there is just so much more room for air to move under there.