Does a dirty fan spin blade spin slower than a Clean one ? 

your answering all the real quesions!

Surprisingly useful info. well done!

Ive always wanted to do this!

Those guards cause a lot of drag. Test it with the cover /front grill back on. And then without any guards. A few years ago I took all the guards off all my my oscillating fans with plastic fan propellers. It makes such a huge difference but i dont have those fancy tools... rpm tach or air flow/speed indicator. Would be cool to see all the numbers. The motors are pretty weak. only real danger is if the fan blade stops spinning or it gets knocked down or something stalls the engine it could overheat and cause a fire.