Not sure where your headed with this but the rpm of the fan if directly connected to the armature is a function of output capacity..If the rpm drops off a little the V(o) will too…A mag tachometer can monitor the rpm but the control is a little harder..One way that is parasitic is to place a pretty large wattage lamp in-line to the output ..The lamp regulates the PEP spikes with heat…and you get a fairly even output. The actual wind speed of the turbine can be monitored with an anemometer and read on an analog scale. Just looking at the fan on a calm day even armed with all the pitch elements and sail surface notes won't get you there ..There is just too many variations …If you buy this turbine commercially they should furnish you with a curve that represents voltage/amperage at X air speed…Good luck with the project..from the E