Structural matters
The following provides a simple overview of the behavior of fan blades under live loads. The blade is subjected to inertial and aerodynamic forces, which act in concert to produce the loads on the system. The total loads are the result of steady state as well as unsteady loads. Structurally, steady state is relatively simple to account for, however unsteady loads are always of interest; cyclic loads cause high frequency fatigue to accumulate.
following are gif files of blade bending and torsion, with (i) a standard fan blade moving from no load to normal maximum RPM and thrust and (ii) a modified blade moving from no load to the same thrust load, at a lower RPM.
The deflection of the blades and the stresses are shown as follows:
