Small antique aircraft do this already: on the bottom of the fuselage is mounted a little "windmill" propeller connected to a small generator. When flying, the windmill spins the generator and electrical power is carried into the fuselage with wires that are connected to a battery, which hence remains charged whenever the plane is in the air. Radios, lights, etc. are then connected to the battery.

Using a ram air turbine to spin a generator would be complicated and not very efficient. The right way is by gearing a generator right onto one of the plane's engines.

Small antique aircraft which did not possess electrical systems originally do this already: on the bottom of the fuselage the owner can mount a little "windmill" propeller connected to a small generator. When flying, the windmill spins the generator and electrical power is carried into the fuselage with wires that are connected to a battery, which hence remains charged whenever the plane is in the air even though the engine does not possess a generator. Radios, lights, etc. are then connected to the battery.

Using a ram air turbine to spin a generator would be complicated and not very efficient. The right way is by gearing a generator right onto one of the plane's engines.