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Viscous clutch fans on start up run runabout 3k rpm. When you hear them get quiet, the thermostatic valve is allowing more slippage and the fan speed drops down to 1500 rpm until things heat up. Then it closes the slippage valve and the fan speed increases. When aero was not a concern the airflow past the radiator would probably drive the fan at 1500 rpm at 70 mph, even if it was just a free wheeling fan, so the losses would be much lower.
The bimetallic spring in the front of the clutch senses the temp of the air flowing through the radiator. It's a very reliable system. When the clutch is shot, I used to grab the fan, WITH THE ENGINE RUNNING, and spin it backwards to show the customer it was defective!
regards
mech
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