A family of five four-blade fans were investigated during this project. Two fans had flat spiders and were tested only statically to check the initial development of the finite element models. The other three fans were operational fans. These fans were tested under simulated operational conditions to determine the resonance frequencies, mode shapes and corresponding critical speeds associated with the first-order flapping and twisting modes. These operational test results were compared with the corresponding results predicted by the current version of ASHRAE Standard 87.1-1983 and with the corresponding finite element results.