system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do fan and propeller blades often have different shapes and angles of attack? Why does the number of blades vary, shouldn't there be an optimal design?
gpt
There *is* an optimal design... for any given purpose and set of operating conditions.  The problem is that such things as expected air pressure, the speed at which it's moving forward (in the case of a propeller), the amount of driving power available, whether or not the fan is ducted, and various others, will change the optimal design.  Hence, different designs for different purposes.