Does fan blades require cyclic pitch when in push configuration inside telecommunication chassis?

It has been realized that fan tray does not function in push configuration in telecommunication chassis when inlet height is smaller than certain value. Originally this was associated with excessive pressure drop due to flow turn inside inlet plenum or basically bottle neck pressure drop through inlet opening it self, however, the problem became worst when higher pressure capacity fans were used. In latter, total CFM increased, as expected, but slot to slot as well as front to back airflow distribution became very uneven, leaving few slots starving. CFD simulations were used to understand this flow behavior, however, they were not capable of duplicating the measured airflow distribution. In this work, this problem is presented and analyzed by means of aerodynamic theory of fan blades. It is shown that high speed flow through small inlet do not turn completely, but rather, enter the fan tray at shallow angle. This causes one side of fans to be ineffective leaving slots over those portions starving. Even the effective sides of fans produce directional jets leaving rear portion of those boards directly above them starving. By bringing knowledge from helicopter aerodynamics to fan blade design, the cause for this mysterious behavior was discovered. Consequently, the concept of fan blade cyclic pitch is introduced as possible solution to this problem. Next generation fans might be designed to have variable blade angle within each revolution. Other implication of this work is to lay out building blocks for better CFD model for fan that include some blade aerodynamic effect. This will be the subject for future work.