The main concept of viscous fan was to NOT use up power..
It was a design to add HP ..[over direct link fan]
They free wheel when not called for and have an rpm cut-out besides.
Even at idle , they push a good amount of air , --the problem is they tend to have a high cut-in coupling temp... the makers are trying to get gas mileage to the point of walking the safety line...
Electric must be the best compromise for low rpm, as the newer Benz have gone electric, no more mechanical fans..
Although the electric system has to be up for the job,I think electric is definately a better system...specially at low Rs and
low ground speed ...
An interesting way to look at airflow/cooling sytems is the fact that the aux fans are called for After the coolant temps start to excede the cooling systems normal capacity. They then come to the rescue by adding more airflow..
If airflow was kept constant , regardless of rpm/speed,
temps would be more stable..so an airflow sensed type design
would regulate the fans to keep a/flow the same , regardless of vehicle speed.. seems to be pretty easy to do with electric fans and speed/air flow sensor coupled with ect paremeters..
just another way of controlling engine temps , but coming at it from a different angle... seems we always get into trouble at Low Rs/low speed/traffic conditions.
This would eliminate all those...and BEFORE hand