Actually, the curvature of the blades is supposed to have the convex part going forward, it's to reduce the fan noise. I notice the older MTH and all the Lionel impellers had the convex side in the proper direction, now the new MTH impellers have them facing what I believe is the wrong way. I think someone re-specified the fan design not understanding why the convex blades were used. 
If you spin the fan CCW, the airflow will be reduced.
I'm also amused by the holes in the base of the impeller. What's the purpose of reaching the screws through the impeller when you have to take it off anyway to remove the motor? I'd love to know what that designer was smoking. 
