Counter-rotating propellers
Counter-rotating propellers, also referred to as CRP, are propellers which spin in opposite directions to each other. They are used on some twin- and multi-engine propeller-driven aircraft. The propellers on most conventional twin-engined aircraft spin clockwise (as viewed from behind the engine). Counter-rotating propellers generally spin clockwise on the left engine and counter-clockwise on the right. The advantage of such designs is that counter-rotating propellers balance the effects of torque and P-factor, meaning that such aircraft do not have a critical engine in the case of engine failure. Drawbacks  of counter-rotating propellers come from the fact that, in order to reverse the rotation of one propeller, either one propeller must have an additional reversing gearbox, or the engines themselves must be adapted to turn in opposite directions. (Meaning that there are essentially two engine designs, one with left-turning and the other with right-turning parts, which complicates manufacture and maintenance.) History. Counter-rotating propellers have been used since the earliest days of aviation, in order to avoid the aircraft tipping sideways from the torque reaction against propellers turning in the a single direction. They were fitted to  the very first controlled powered aeroplane, the "Wright Flyer", and to other subsequent types such as the Dunne D.1 of 1907 and the more successful Dunne D.5 of 1910. 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, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration. 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. This can in turn affect performance under extreme conditions and therefore flight safety  certification. Some modern types, such as the Airbus A400M, have counter-rotating propellers in order to meet air safety requirements under engine-out conditions.