The darling of corner discount stores everywhere is the flex fan. Basically, these fans have blades that flatten out at high rpm. Less blade pitch means less power to turn. When buying flex fans, beware of fans with flimsy blades. For example, some aluminum flex fans have a tendency to flatten out with use, and remain that way. The result? A lack of cooling over a period of time. A better idea is the use of a stainless steel blade. It has a better "memory" than its aluminum counterparts, and because of this, has a tendency to flex and then return to its shape on a regular basis. This stainless steel Moroso model uses nicely set rivets with adequate spacing between the rivet holes and between the edges of the central hub. Some cheap look-alike fans don't have this attention to detail. Finally, keep in mind that some flex fans are made from plastic. I've only had experience with them in drag race applications (where they work great), but I suspect you should disregard them for sustained use.