Providing it is not too big (causing lots of drag)-- you are already going through the air, so there is airflow there! In old aircraft (like my 1940 Luscombe) there was a standard type wind-generator that was mounted forward of wing strut on bottom (as I rememvber)-- but was mounted in different places on many brands of aircraft! This produced
enough electricity to run radio (sparingly-- remember these were vacuum tube radios, and pulled a lot of juice. It also kept on board lead acid battery charged to use for navigations lights landing light etc,-- if the aircraft even had any! Now if you use a "high lift" slow fly8ing aircraft (long wide wings like a glider), you may be able to economize, and increase charge by shutting down motor, and riding the "updrafts"-- the little peopeller keeps right on turning you know! NASA has built a Solar powered aircraft (not for people passengers)-- with wingspan around 100' plus, and about 8 electric motors, which has flown to around 60,000 feet, but had to come down when sun went down. You can probably find that one on internet search engine!