Does a dc generator need a flyback diode?
Fly-back.. No. Since you are not switching anything there are no fly-back transients to dissipate. 

However, it is still prudent to include a diode in that same location, in the same direction, to protect your circuit in the event that the generator is driven backwards for whatever reason. 

Some over-voltage protection would also be warranted.







simulate this circuit &ndash; Schematic created using CircuitLab