The x3 has a five-bladed main rotor and a pair of propellers on two short wings.

ECONOMIST: Chop, chop

Its Radmax engine uses two wave-shaped cams that rotate above and below a stationary rotor.

FORBES: How to get 51mpg from an engine the size of a large turkey.

One way to increase the efficiency of a high-bypass jet engine is to turn the fan into an open rotor, a bit like returning to propellers, but using two rings of stubby counter-rotating blades.

ECONOMIST: A new way to build a greener jet engine

This week Korean automaker Hyundai unveiled a multi-rotor flying electric car for congested cities and SolarWorld and PC-Aero announced plans to launch two new solar-powered electric airplanes at an air show in Germany.

ENGADGET: Inhabitat's Week in Green: flying electric car, 3D-printed livers and a two-story-tall bike