Counter-rotating propellers

(Meaning that there are essentially two engine designs, one with left-turning and the other with right-turning parts, which complicates manufacture and maintenance.)

In designing the Lockheed P-38 Lightning, the decision was made to reverse the counter-rotation such that the tops of the propeller arcs move outwards (counterclockwise on the left and clockwise on the right), away from each other.

The counter-rotating powerplants of the German World War II Junkers Ju 288 prototype series (as the Bomber B contract winning design), the Gotha Go 244 light transport, Henschel Hs 129 ground attack aircraft, Heinkel He 177A heavy bomber and Messerschmitt Me 323 transport used the same rotational "sense" as the production P-38 did – this has also been done for the modern American Bell Boeing V-22 Osprey tiltrotor VTOL military aircraft design.

The following German World War II aviation engines were designed as opposing-rotation pairs for counter-rotation needs: The aerodynamics of a propeller on one side of an aircraft change according to which way it turns, as it affects the P-factor.

Some modern types, such as the Airbus A400M, have counter-rotating propellers in order to meet air safety requirements under engine-out conditions.