Parkerdog is right, although it's better explained mathematically.
In any system, power in and power out will be roughly equal. I say "roughly", because we (man) can't build anything perfectly. There will always be some inherent losses in the transition from one power medium to another.
What is suggested is that one generates electricity from forward motion of a body, by using airflow through a propellor to spin a generator.
Let's examine that; You would be forcing air through a propellor in order to turn linear airflow into rotary motion......there would be some power losses in that, due to inefficiencies of the prop. We would then be rotating a winding, suspended in bearings, within a magnetic field. We'll lose some power from bearing friction, we'll have to generate the magnetic field, unless we're using a permanent magnet generator, and we'll have to hope that there's 100% efficiency in just the generator output. (Which there won't be.)
What we end up with is something less, (usually considerably less), than the total energy that went into the effort. We suffer losses through heat generation, friction, and worse from transforming one energy source into a different medium. (Linear wind energy into rotary motion, then into electrical energy.)
I'm not suggest you don't try it, as it does work to a degree. What the end result will be is an extended range before battery power gets too low to sustain travel.
As an example, early Cessna private planes had a simple generator on a pivot that was cable controlled from the cockpit. The electrical system of the plane was initially powered by an onboard battery, and once airborn, the pilot would allow the generator to drop into the airstream. Being as the generator had it's own prop mounted to the input shaft, electricity was generated, which powered the plane's instruments and lighting for a longer flight time than was possible by battery alone.
It does work, to a point, but it will not be 100% self sustaining.
Roger