Flettner Fl 282
The Flettner Fl 282 "Kolibri" ("Hummingbird") is a single-seat intermeshing rotor helicopter, or "synchropter", produced by Anton Flettner of Germany. According to Yves Le Bec, the Flettner Fl 282 was the world's first series production helicopter. Design and development. The Fl 282 "Kolibri" was an improved version of the Flettner Fl 265 announced in July 1940, which pioneered the same intermeshing rotor configuration that the "Kolibri" used. It had a 7.7 litre displacement, seven-cylinder Siemens-Halske Sh 14 radial engine of mounted in the center of the fuselage, with a transmission mounted on the  front of the engine from which a drive shaft ran to an upper gearbox, which then split the power to a pair of opposite-rotation drive shafts to turn the rotors. The Sh 14 engine was a venerable, tried-and-true design with low specific power output and low power/weight ratio (20.28 hp/L, 0.54 hp/lb) which could (anecdotally) run for up to 400 hours without major servicing, as opposed to the more powerful 27 litre displacement, nine-cylinder BMW/Bramo "Fafnir" 750 hp radial engine powering the larger Focke Achgelis Fa 223 helicopter, whose higher output (27.78  hp/L, 0.62 hp/lb), more modern design required moderate maintenance as often as every 25 hours (such as changing spark plugs, etc., well within the norm for modern radial engines of that era). While such a heavy and low-powered engine would work well in a very small craft like the Fi 282, to try and scale it up and use an engine of equivalent power/weight ratio in the 700-1000 hp class would result in a massive and heavy engine leaving little excess capacity for cargo or passengers. 750 hp was the lowest rating that the Fafnir  was available in - indeed, it was a low-power, low maintenance design compared with many other engines of this era. The Fl 282's fuselage was constructed from steel tube covered with doped fabric, and it was fitted with a fixed tricycle undercarriage. The German Navy was impressed with the "Kolibri" and wanted to evaluate it for submarine spotting duties, ordering an initial 15 examples, to be followed by 30 production models. Flight testing of the first two prototypes was carried out through 1941, including repeated takeoffs and landings from a pad mounted on the German cruiser "Köln". The first  two "A" series prototypes had enclosed cockpits; all subsequent examples had open cockpits and were designated "B" series. In case of an engine failure, the switch from helicopter to autorotation was automatic. Three-bladed rotors were installed on a test bed and found smoother than the vibrating 2-blade rotor, but the concept was not pursued further. The hover efficiency ("Figure of Merit") was 0.72 whereas for modern helicopters it is around 60%. Intermeshing rotors were not used on a mass production helicopter until after World War Two. Operational history. Intended roles of Fl 282 included ferrying items between ships  and reconnaissance. However, as the war progressed, the Luftwaffe began considering converting the Fl 282 for battlefield use. Until this time the craft had been flown by a single pilot, but by then a position for an observer was added at the very rear of the craft, resulting in the B-2 version. Later the B-2 proved a useful artillery spotting aircraft and an observation unit was established in 1945 comprising three Fl 282 and three Fa 223 helicopters. Good handling in bad weather led the German Air Ministry to issue a contract in 1944 to BMW to produce 1,000 units . However, the company's Munich plant was destroyed by Allied bombing raids after producing just 24 machines. Towards the end of World War II most of the surviving Fl 282s were stationed at Rangsdorf, in their role as artillery spotters, but gradually fell victim to Soviet fighters and anti-aircraft fire.