Two Young Girls Posing With The Old Airplane

Since the 1940s, propellers and propfans with swept tips or curved "scimitar-shaped" blades have been studied for use in high-speed applications so as to delay the onset of shockwaves, in similar manner to wing sweepback, where the blade tips approach the speed of sound. The Airbus A400M turboprop transport aircraft is expected to provide the first production example: note that it is not a propfan because the propellers are not mounted direct on the engine shaft but are driven through reduction gearing.
Other less common power sources include:
Electric motors, often linked to solar panels to create a solar-powered aircraft.
Rubber bands, wound many times to store energy, are mostly used for flying models.