Contra-rotating propellers

Aircraft equipped with contra-rotating propellers (CRP), coaxial contra-rotating propellers, or high-speed propellers, apply the maximum power of usually a single engine piston powered or turboprop engine to drive a pair of coaxial propellers in contra-rotation. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear or spur gear transmission.

Source: Wikipedia — Contra-rotating propellers (CC BY-SA 4.0)