In 1937 Flettner began to design the first helicopter to use intermeshing contra-rotating synchronized rotors. The following year the German Navy gave Flettner an order for six of these single-seater helicopters, powered by a 7-cylinder air-cooled engine to drive its two intermeshing two-bladed rotors and with an inertia damping system to reduce the shake of the control stick.
The Fl 265 first flight took place in May 1939. During this flight the blades struck each other and the helicopter was destroyed. A similar fate overtook the second one, because the pilot had for-gotten to fill his fuel tanks. The remaining four Fl 265s in the original contract were extensively tested on the deck of a cruiser with such encouraging results that work was speeded up on the Fl 282, a second intermeshing rotor helicopter to embody the experience acquired during the tests of the Fl 265.
P.Lambermont "Helicopters and Autogyros of the World", 1958
Through a gradual process of working on various rotating-wing schemes, Anton Flettner arrived at his celebrated scheme of intermeshing rotors first employed in the Fl 265 helicopter. This scheme, though viewed with suspicion by many at the time, dominated Flettner's helicopter work a couple of years prior to, and then throughout, the war. When, in 1930, Flettner first turned his attention to helicopter problems, he designed a helicopter having a single, torqueless, rotor, the absence of torque being achieved by applying the drive directly at the rotor, two 30hp Anzani engines driving small airscrews, being attached to the rotor blades. This helicopter was unfortunately destroyed in 1933 during tethered tests when it was overturned by a gust of wind.
Flettner then turned to the design of a straightforward two-seat autogyro, the Fl 184. This machine was scheduled to carry out trials with the German Navy to ascertain its suitability for reconnaissance and anti-submarine work, such a machine offering distinct advantages over naval fixed-wing aircraft which required catapult-launching and special recovery procedures. The Fl 184 had a fully-enclosed fuselage and tail surfaces, and the 12m diameter rotor employed cyclic pitch control. At the nose was mounted a 140hp Siemens-Halske Sh 14 radial engine driving a two-blade wooden propeller. During 1936, before this design could be evaluated, the only prototype, D-EDVE, caught fire in flight and was completely destroyed.
In the next design, the Fl 185, the machine was arranged to act as a helicopter when the rotor was powered or act as an autogyro when the rotor autorotated in forward flight. A 140hp Siemens-Halske Sh 14A engine was mounted in the nose and was provided with a cowl and frontal fan for cooling. Behind the engine was a gearbox from which the drive was taken to the rotor and to two variable-pitch airscrews mounted on outrigger arms extending from the fuselage sides. When the rotor was powered, to counteract torque the airscrews provided thrust in opposite directions, but, when the rotor was freely autorotating, the pitch of the airscrews was altered to give thrust for forward flight when they also took up the full power of the engine. Again, only one prototype, D-EFLT, was built, and this was only given a few tests near the ground before being abandoned.
During 1938, the German Navy placed a contract for six Fl 265s, the design of which Anton Flettner had begun in 1937 as the first to use intermeshing contra-rotating synchronized rotors. The single-seat Fl 265 had a very similar fuselage and tail surfaces to the earlier Fl 185, and, once again, the engine was mounted at the nose with a cowl and cooling fan. The 160hp Bramo Sh 14A radial engine provided the power for the two two-blade rotors, which had inclined shafts mounted close together and had an inertia-damping system to reduce the vibration reaching the control column.
The Fl 265 VI, D-EFLV, made its first flight in May 1939, and its first autorotative descents were made the following August, but this machine was eventually destroyed in flight when the rotor blades struck each other. Because of this accident, the Fl 265 V2 was the first to be used in a series of naval trials in the Baltic and Mediterranean in which Fl 265s operated from platforms fitted to cruisers and even made landings on U-boat decks. Despite the fact that one Fl 265 was lost due to its refuelling being overlooked, the trials were a great success and augured well for the machine's place in naval reconnaissance and anti-submarine work. Other roles were also evaluated when an Fl 265 was used in exercises with Wehrmacht troops and performed such work as towing dinghies across a river and lifting bridge sections during construction. Although the Fl 265 had performed its duties well, had flown in adverse conditions and had no trouble going into and out of autorotation, natural doubts were expressed concerning its vulnerability to aerial attack. Consequently, a test was made in which a Messerschmitt Bf 109 and a Focke Wulf Fw 190 fitted with camera guns made determined simulated attacks on an Fl 265 for 20 minutes but failed to score one hit because of the helicopter's manoeuvrability. During the war, German fighters made similar but genuine attacks on a British rotorcraft but with the same lack of success.
The outcome of all these successes was that Flettner received instructions in 1940 to proceed with quantity production of the Fl 265, but, by that time, the design of a more advanced two-seat derivative of the Fl 265, the Fl 282, had been completed and the programme was switched to the new type. Thus, only the six prototypes of the Fl 265 were completed.
J.R.Smith, Antony L. Kay "German Aircraft of the Second World War", 1972
The pioneering work carried out in the field of rotary-wing aircraft by a German, Anton Flettner, is often overlooked and, perhaps for that reason, is particularly interesting. Seeking a way to overcome the torque induced when a rotor is driven from an airframe-mounted power source, Flettner explored the idea of putting a small engine and tractor propeller on each blade of a two-bladed rotor. This prototype helicopter made a successful tethered flight in 1932, but was destroyed shortly after on the ground when it overturned during a gale.
Flettner then built a two-seat Flettner Fl 184 autogyro with a three-bladed auto-rotating rotor and power provided by a 104kW Siemens-Halske Sh 14 radial engine driving a tractor propeller. This, too, was destroyed before it could be evaluated and the prototype Fl 185 followed, this being a combined autogyro/helicopter. Its Siemens-Halske engine, mounted at the fuselage nose, could be used to drive two variable-pitch propellers mounted on outriggers, one on each side of the fuselage, but the main rotor was powered only when required for operation in a helicopter mode. When flown as an autogyro the propellers on the out-riggers were both set to act as pushers and the main rotor auto-rotated. For helicopter flight the main rotor was powered from the engine and the outrigger propellers set so that one acted as a tractor and the other as a pusher to offset rotor torque.
The Fl 185 was only flown a few times before Flettner began construction, in 1937, of his Fl 265 VI prototype (D-EFLV), first flown in May 1939. This was of similar airframe configuration to the Fl 185, but dispensed with the outriggers and propellers, and introduced two two-bladed counter-rotating inter-meshing and synchronised main rotors which, because they were rotating in opposite directions, each cancelled the effects of the other's torque. To simplify control problems a tail unit incorporated an adjustable tailplane for trimming purposes, and for steering a large fin and rudder to augment the use of differential collective-pitch change on the two rotors. The aircraft was lost in an accident some three months later when the counter-rotating blades struck each other, but the Fl 265 V2 was used successfully for a variety of military trials. In all, six prototypes were built under contract to the German navy before, in 1940, an order was placed for quantity production. By then, however, Flettner had de signed a more advanced two-seat helicopter and it was decided instead to proceed with the development and manufacture of this improved aircraft.
D.Donald "The Complete Encyclopedia of World Aircraft", 1997
Technical data for Fl 265
Engine: 1 x Bramo Sh.14A 7-cylinder radial pistone engine, rated at 119kW, rotor diameter: 12.3m, take-off weight: 1000kg, empty weight: 800kg, speed: 160km/h