the friction generated by the wind turbines would decrease the flight efficiency and distance as well. Not to mention the gain from it would not be 100% efficient. I'm pretty sure more = less at the end.
What you alluding to here falls under the category of "perpetual motion machines"... As the previous commenters have pointed out, the turbines would add a higher penalty in terms of drag, weight and thermal loss than they could ever contribute in terms of "scavenged" energy.
I like your thinking, though. I've been trying to develop some approaches to energy harvesting in flight that might be more practical.
Regenerative braking - With some additional electronics, it is possible to convert the drive motor from a electricity consuming motor to a power generating alternator. The idea is to run in "motor mode" to gain altitude, then switch to "alternator mode" , drop into a shallow dive and coast to a reasonable altitude while pushing energy back into your batteries., then repeat. The electronics would be a bit complicated, you'd have to take 3-phase AC and convert it to a useful level of DC voltage. It's doable, but might be overly complicated/heavy for a minimal gain in power.
Mech Power scavanging - you could install peizo devices that could convert vibration, wing flex, fuse flex etc... into power. Very small levels though. Likely not worth the trouble.
RF Power scavenging - there are methods by which stray RF energy can be harvested and converted into useful power. This is a tough one and the power levels are low. Also there is the issue of what effects these devices have on desirable RF emmissions... ie, if your harvester couples to your video or telemetry transmitter, you could significantly reduce the emmitted power, hurting you ability to receive data from the UAS.
Solar - I have some PowerFilm solar cells that I want to ahere to the upper surface of a Skywalker wing and use their harvested power to drive perhaps at least the video transmitter and some other electronic devices on the aircraft in lieu of carrying an additional battery (or allowing the use of a much smaller battery) or drawing off the primary battery.
Based upon simplicity, light weight, low effort level I think that solar is actually a feasible approach.