Yes, I am afraid that speed is only one aspect of the problem.
There is very little power available in light winds and with a small machine such as 3ft the friction and iron loss of your dc motor is going to use it all up. With much bigger machines things become easier but even with a 10ft machine you won't get significant power below 7mph. expecting something at 5mph is not realistic so it matters little if your machine doesn't start below 5mph.
Speed depends on tsr. The higher the tsr the faster it runs. It also is a fact that for a given tsr a small rotor will be faster as the tip will not have to go so far in a given time.
Tsr is increased by reducing chord width and also pitch but there is a limit to how far you can go. High tsr blades work better in higher winds and trying for tsr over 7 in low winds probably won't pay off. Profiles are critical and so is surface finish. The blades of such things as the AirX are far more precision than you can hope to achieve but even they produce so little torque in low winds that the AirX doesn't produce anything useful below 10mph.
To get anything from tiny machines in low wind areas you need blades with tsr below 6 and a generator with virtually no losses. The only thing that even stands a chance is an axial machine with no iron loss and even then the choice of bearings is critical.
When you have gone to all this trouble you still only see a few Watts. If you can get a Watt from a 3ft machine at 5mph you have just about achieved the impossible and are near the Betz limit.
Flux
