David, several years ago, I stumbled onto a one-day presentation given by a Ken Hellevang. It was excellent.

One of the handouts was this publication: http://www.ag.ndsu.edu/pubs/plantsci/smgrains/ae701-3.htm

Table 10 which is down the page a ways will give you some idea of just what a lousy fan an axial is, when it is confronted with a bit of back pressure. As you can see, the inline centrifugal holds its own quite nicely when put up against a HS centrifugal and even though it may be similar in appearance to an axial, the performance is much better.