Flexible bladed fan with increased natural frequency

Claim 1

In a fan comprising a hub and a plurality of fan blade assemblies each comprising: an arm extending radially outwardly from said hub; a flexible, resilient blade having a radially extending leading portion, defined by the direction of fan rotation, connected to said arm, said blade extending transversely from said leading portion to a trailing edge behind said arm, said blade curved from said leading portion to said trailing edge in a downstream direction relative to airflow direction; and a reinforcing member extending radially along said blade leading portion on the upstream side thereof and extending transversely to a trailing edge between said leading portion and said blade trailing edge; said blade leading portion and said reinforcing member in contact along the radial extent of said reinforcing member and spaced apart at a gap behind said leading portion; the improvement in which: said blade and said reinforcing member, with said fan stationary, contact along the radial extent of said reinforcing member at a second position behind said leading portion and said gap, said reinforcing member being curved in an upstream direction behind said blade leading portion at said gap and said blade being bent upstream between said blade leading portion and said reinforcing member trailing edge and contacting said reinforcing member at said second position between said bend and said trailing edge and therebehind said blade being curved in said downstream direction, said reinforcing member at said second position biasing and pre-stressing said blade in a downstream direction; whereby the natural frequency of said blade is raised by the prestressing contact of said blade and said reinforcing member.