If you have access to the fan blade outside of the assembly, there is an experimental way of determining the moment of Inertia. It is often used to measure the inertia of motor rotor assemblies. In fact the procedure is described nicely in the NEMA standard for motors. The NEMA standard is MG-7 if you want to look it up yourself, but I'll try to describe it here.<br><br>Suspend the fan with the center shaft oriented vertically using two parallel wires. You may have to make an attachment fixture, but if you keep it's geometry simple, it's added inertia can be calc. and subtracted out in the end. The wires should be attached equally spaced from the centerline of the fan. The ratio of length of the wire(L) to the distance between the wires (D) should be about 10.<br><br>Once setup, rotote the fan a small amount from equilibrium and release. After release, measure the frequency of oscillation. The moment of inertia can then be calc. using the following:<br><br>I = c*m*D^2/(L*f^2)<br><br>I = moment of Inertia about the rotation axis<br>m = fan weight<br>D = distance between wires<br>L = Length of wire<br>f = measured frequency in HZ<br>c = conversion factor depending on units used<br><br>If Inertia is in kg-m^2, with 'm' in kg and "L" & "D" in meters, the c = 6.2e-2<br><br>If Inertia is in lb-ft^2, with 'm' in lb and "L" & "D" in feet, the c = 2.04e-1<br><br>If Inertia is in lb-in-sec^2, with 'm' in lb and "L" & "D" in inches, the c = 7.61e-2<br><br>If desired, don't forget to subtract out the inertia effect of any attachemnt fixture. The procedure works well for motor rotors. I think if you keep the amount of rotation low so it doesn't spin to fast such that the drag forces of the fan blades is not to high, it should give a good value.<br><br>Good Luck.<br><br>