5:1 is a lot of gearing. If you have a typical mountain bike put it in high gear (likely 3 1/2:1 so, still a far cry from 5:1). Lift the back wheel and see how hard it is to turn - after you take the pedals off and have only the drive axle to twist. A windmill with enough torque to start (without the alternator or heavy flywheel) will have a blade design limiting rpm to dozens instead of hundreds. See water pumping windmills for an example of those torquey low speed blades. Going that route requires even more gearing etc. etc.
Perhaps you could devise electronics to bring the transmission (and flywheel if you think you need it) into play after the windmill got up to speed?
You mentioned this was to be small windmill. You'd likely need close to 8' of swept diameter to utilize the potential of an average alternator. Small by many standards but a big project none the less (and the tower is an even bigger challenge - 30 ft above anything within 300 ft rule of thumb).
I've been pondering in a similar direction for some time now so I'm interested in what you come up with as your idea evolves.
Cheers,
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