Anton Flettner remains one of the lesser known pioneers of rotary wing flight which is remarkable given the fact that his first fully practical helicopter, the Fl 265, was superior to the Focke-Angelis Fa 61 and made its first successful free flight several months before the Vought-Sikorsky VS-300 even began its initial tethered flights. Yet the designers of these other two heleicopters, Heinrich Focke and Igor Sikorsky are thought of as the parents of the modern helicopter. Flettner had been a devotee of the rotary wing since the early 1930’s and was particularly anxious to overcome the torque-reaction problem associated with a lifting rotor driven by a fuselage-mounted engine. This preoccupation was evident in Flettner’s first helicopter design, completed in 1930, that was based on a large two blade rotor powered directly by two 30hp Anzani piston engines and installed on one of the rotor blades to drive a small tractor propeller. The prototype of this helicopter was overturned by a gust of wind and destroyed during 1933 in the course of tethered trials. Fletter then designed a two-seat autogyro as the Fl 184 with enclosed two-seat accommodation, fixed tailwheel landing gear with cantilever main units, a three blade rotor fitted with a cyclic-pitch control system and a powerplant of one 140 hp Siemens-Halske Sh.14 radial piston engine located in the nose to drive a two blade tractor propeller. This machine was built in prototype form but lost in 1936 after suffering an inflight engine fire.
Flettner now turned to the Fl 185 design that was intended to operate as a helicopter when the rotor was powered for vertical flight and as an autogyro when the rotor was unpowered in horizontal flight. The Fl 185 had a tricycle landing gear, a three blade rotor and was powered by a 140hp Sh.14 radial piston engine that was nose mounted inside a long-chord cowling and drove a frontal cooling fan as well as a gearbox that drove the rotor and/or two variable pitch propellers mounted at the tips of outriggers extending from each side of the fuselage. When the rotor was powered, the propellers provided thrust in opposite directions to counteract the torque reaction. But when the rotor was unpowered the propellers absorbed the full power of the engine to provide forward thrust. The Fl 185 made only a few test flights before being scrapped.
By this time however, Flettner had come to his definitive concept of how to obviate torque reaction, namely a side-by-side pair of intermeshing two blade rotors that were mounted at the heads of two outward inclined drive shafts turning in opposite directions so that the torque of one cancelled out the torque of the other. Flettner realized that this system would produce a seriously turbulent airflow pattern, but felt that the problems associated with this airflow would be more than offset by the advantages of the torqueless rotor system and the reduced drag that would result from a design that did away with the need for any external rotor carrying structure. The new rotor system was first used on the Fl 265, which the German Navy ordered six prototypes in 1938. The fuselage of the Fl 265 was based on that of the Fl 185 but revised with intermeshing rotor system but without the two propeller outriggers. The same type of powerplant arrangement was used. To improve the helicopter’s controllability, a conventional tail unit was provided with a trimmable tailplane and a large vertical tail surface incorporating a rudder used to supplement the directional control provided by differential collective-pitch change in the two rotors. The FL 256 V1 prototype made its maiden flight in 5/39 and, despite its loss 3 months later when the rotor blades struck each other, soon proved the general success of Flettner’s design concept. The five other prototypes were used for a number of successful trials in several military applications and in 1940, the Kriegsmarine ordered Flettner to initiate full production of the type.
By that time however, Flettner had moved forward to a more advanced design and the production order was switched to the more capable type, namely the Fl 282 Kolibri. This was designed as a two seat helicopter so that, at the expense of range, an observer could be carried in a rearward facing seat installed behind the rotor assembly. The design was created with sufficient range in the permissible center of gravity position that the helicopter could be flown as a single or two seat without trim changes. Its most obvious role was observation in land and naval applications. The design was completed in 7/40 and work started immediately for no fewer than 30 prototype and 15 pre-production helicopters. Flight trials were scheduled to begin in 1941. The first three prototypes were completed as single seat models with accommodation for the pilot in an enclosed cockpit and the others were completed as two seat models with open accommodation. The Fl 282 marked a departure from previous practice in the location of the engine that was installed in the center of the fuselage with a wooden cooling fan that drew air through slots in the underside of the fuselage and drove a forward mounted transmission unit that turned the drive through 155° to a 65° upward and backward angle to power, via a transmission shaft, the upper transmission unit that drove the two rotor shafts. The two rotor shafts were inclined outward at 12° and forward at 6° and carried the two rotors. These were each of the two blade type, each blade having a steel spar, wooden ribs and a plywood covered in fabric and each blade was attached to the rotor hub by flap and drag hinges. The fuselage was of welded steel tube construction covered over its central portion by light alloy panels and over its rear section by fabric and carried at its rear the conventional tail unit comprising a trimmable horizontal surface and large vertical surface with rudder to supplement differential collective-pitch change of the two rotors for directional control. The pilot was accommodated on an open steel tube structure at the extreme nose and the airframe was completed by the fixed tricycle landing gear. The Fl 282 V1 flew in 1941 and, as additional machines became available and the pace of development was accelerated, the Fl 282 soon proved itself to be an admirable helicopter that combined great reliability with viceless handling characteristics. Operational trials were undertaken from 1942 and so useful did the Fl 282 prove itself that starting in 1943, about 20 of the 24 completed development helicopters were used for the convoy escort role in the Mediterranean and Black Sea. In 1944 the German Air Ministry ordered 1,000 production examples of the Fl 282 from BMW, but no production helicopters were completed due to the disruption caused by the Allied bombing campaign. Only three of the Fl 282 helicopters survived to 5/45, many of the others having been destroyed to prevent their capture by the Allies.