The Focke-Wulf Fw 61/ Focke-Achgelis Fa 61 was the first practical helicopter in the world, and was a twin-rotor machine that made quite an impact when it was flown through the Deutschlandhalle in February 1938.
The Fw 61 was developed by Professor Heinrich Focke and Gerd Achgelis, a stunt pilot who became a flying instructor and then a test pilot, holding the post of chief test pilot at Focke-Wulf from 1933.
The Fw 61 made its maiden flight on 26 June 1936 as a Focke-Wulf aircraft. In 1936 Focke was forced out of Focke-Wulf, either because the Nazis believed him to be politically unreliable or because they wanted to use the Focke-Wulf factories to build the Bf 109. If Focke did fall out of favour it didn't last, and the success of the Fw 61 led to the formation of a new Focke-Achgelis company on 27 April 1937.
Focke gained experience of rotary-wing aircraft by building the Cierva C.19 and C.30 autogyros under licence. These aircraft used a rotor blade to provide lift and a normal propeller to provide forward movement. Production of the Cierva aircraft began in 1933 and the Fw 61 owed much to their design. The C.19 was the first autogiro to be controlled by altering the characteristics of the rotor blades, the same control method that would be used in the Fw 61. It also had a similar looking fuselage, based on a standard light aircraft of the period.
The Fw.61 used the fuselage of the Focke-Wulf Fw 44 training aircraft. The horizontal tail surface was moved to the top of the tail, and the undercarriage was connected to the new rotor blade supports (see below). Power was provided by a Bramo Sh 14A radial engine. There was a very small air screw attached to the front of the engine. This was only used to provide a flow of air over the engine, which would normally operate at much higher speeds.
Two prototypes were built - Fw 61 V1, which was given the registration code D-EBVU and Fw 61 V2, registration D-EKRA.
The Fw 61 had two side by side (transverse mounted) counter-rotating three-bladed rotors, each mounted on a set of struts. Two connected the rotor to the undercarriage. Three connected the rotor to a point on the side of the fuselage just behind the engine, with the middle of these struts carrying a shaft that connected the engine to the rotor. Finally another strut led to a connection mounted just above the centre of the fuselage. The counter-rotating rotors cancelled out each others torque, and so the Fw 61 didn't need a tail rotor.
On modern helicopters height is controlled using collective pitch, where the angle of all propeller blades are adjusted by the same amount at the same time, increasing or decreasing the amount of lift they generate and causing the helicopter to rise or fall. On the Fw 61 height was controlled by using the throttle to alter rotor speed.
Horizontal movement was producing using cyclic pitch, where the angle of each blade changed as it rotated. If no other controls are in use then each blade on both rotors would be at the same angle at the same positions. Adjusting cyclic pitch means that the rotor produces a different amount of lift in different places on the disc, causing it to tilt, and thus generate movement.
Differential cyclic pitch was used to spin the helicopter. Here the two rotors would be adjusted in opposite ways - if the lift was increased at the front of the right-hand rotor then it would be decreased at the front of the left hand rotor. The two rotors would tilt in opposite directions and the Fw 61 would spin.
Differential collective pitch was used to tilt the helicopter to left or right. Here the angle of every blade on one rotor would be altered by one amount and every blade on the other rotor by a different amount. The two rotors would thus produce different amounts of lift, and the Fw 61 would spin.
The Fw 61 V1 made its maiden flight on 26 June 1936 with test pilot Ewald Rohlf at the controls. Focke recorded the test flight as lasting for 45 seconds, while other records said 28 seconds.
The Fw 61 was flown by a number of test pilots, include the famous Hann Reitsch, Rohlfs, Karl Bode and Karl Franke.
The Fw 61 established a whole series of rotor craft records, although by the time this began the aircraft had been redesignated as the Focke-Achgelis Fa 61.
On 25 June 1937 Ewald Rohlfs set a height record of 8,000ft and stayed in the air for 1hr 20min.
On 26 June 1937 Rohlfs set a straight line distance record of 10.19 miles, a closed-circuit speed record of 76.15mph and a closed-circuir distance record of 50.09 miles.
On 25 October 1937 Hann Reitsch raised the straight-line distance record to 67.67 miles (From Breman to Berlin).
In February 1938 Hanna Reitsch flew the Fa 61 inside the Deutschlandhalle in front of a large crowd. Most people in the crowd were impressed by the flight, but were unaware of its true significance, which was that the Fa 61 was considered to be reliable enough and controllable enough to fly over a large crowd in a confined success. The flight signalled the success of the Fa 61 to the wider aeronautical world.
In 20 June 1938 Karl Bode raised the straight line record yet again, this time to 143.05 miles.
On 29 January 1939 Bode raised the altitude record to 11,240.5ft. This was the last official record set before the outbreak of the Second World War.
The success of the Fa 61 led to an order for a new passenger carrying version of the helicopter, and the eventual development of the Focke-Achgelis Fa 266 and military Focke-Achgelis Fa 223. Focke also had plans for a two-seat sports version, the Focke-Achgelis Fa 224, but this was abandoned after the outbreak of the Second World War.
Engine: Bramo Sh.14A seven cylinder radial engine
Rotor diameter: 22ft 11.5in each
Length: 23ft 11.5in
Height: 8ft 8.25in
Empty weight: 1,764lb
Maximum take-off weight: 2,094lb
Max speed: 76mph
Cruising speed: 62mph
Service ceiling: 8,600ft
Absolute ceiling: 11,240.5ft
Range: 143 miles