Focke-Wulf Fw 61

Focke-Wulf Fw 61
	; Hanna Reitsch flies the Fw 61 V2
Type:	Experimental helicopter
Design country:	German Empire
Manufacturer:	Focke-Wulf Flugzeugbau AG
First flight:	June 26, 1936
Commissioning:	1936
Production time:	1935 to 1936
Number of pieces:	2

The Focke-Wulf Fw 61 (later referred to as Focke-Achgelis Fa 61 ) was a test helicopter that Henrich Focke had already started to design in 1932. Based on his designs, Focke-Wulf Flugzeugbau AG, founded by him and Georg Wulf in 1923, built two copies in 1935 and 1936 using the principle of the side rotors . They quickly emerged as the world's first serviceable helicopters.

Emergence

By working with the gyroplanes Cierva C.19 and C.30 built under license by Focke-Wulf, as well as the subsequent less successful in-house development Fw 186 , Henrich Focke came to the conclusion in 1932 that their inadequacies and limited usability were only due to a real helicopter to be eliminated. He soon became aware of the problems to be overcome. Questions of stability and controllability had to be solved; Above all, however, a way had to be found to be able to bring the helicopter safely to the ground in the event of an engine failure or a break in a drive shaft. Useful results were available on the properties of a rotor both from the inventor of the gyroplane, the Spaniard Juan de la Cierva , who worked in England, and from the work of Glauert and Lock. However, extensive calculations were initially necessary in order to be able to build models that could then be examined in the wind tunnel.

Focke opted for the arrangement of two counter-rotating rotors that would provide symmetry in every relationship. A free-flying model powered by a small two-stroke engine brought the first promising results in 1934. It can be seen in the Deutsches Museum . On February 9, 1935, Focke received an order from the Reich Aviation Ministry (RLM) to build a prototype, which was given the designation Fw 61, but which Focke himself only ever called "F 61". It was Flieger-Hauptstabs-Ingenieur Roluf Lucht from the Technical Office of the RLM who arranged for the order to be expanded to include a second aircraft, which happened on December 19, 1935.

testing

The first prototype, the V1 D-EBVU , Wnr. 931, was completed at the end of 1935. Ewald Rohlfs began the first - still tied - test flights in a hall in Hemelingen , where Focke had been "banished" by the authoritative gentlemen of his own company. Rohlfs was only able to make the first free flight on June 26, 1936, because Rohlfs had to be brought back from South America under pressure from Udets , where the company management, who opposed the whole project, had sent him in spring 1936. By the beginning of August, the tests had progressed so far that the Rechlin E-point could think of flying again, which Walther Ballerstedt was commissioned to do. This brought some experience of flying with one of the gyrocopter C.30 with. Nevertheless, the overly cautious Focke insisted on letting him fly tied up at first. Ballerstedt was very impressed by the properties of the new device, only found the height control, which was only done by operating the throttle and thus by changing the rotor speed, as too sensitive. Udet, who in the meantime had taken over the technical office from General Wilhelm Wimmer , was enthusiastic and urged that the helicopter be presented to the public as soon as possible. The meanwhile also finished V 2 D-EKRA , Wnr. 1789, again with Rohlfs, was able to take off for the first flight in spring 1937. On May 10, 1937, he succeeded in making the first autorotation landing with the engine off.

Record flights

In June 1937 the public learned that Rohlfs had brought all helicopter world records to the German Reich with this aircraft on June 25 and 26, 1937. The best performances of other helicopters were for years with a flight time of 10 minutes ( d'Ascanio ) or with no more than 1 km flight distance ( Pescara ) and 18 m flight height ( Œhmichen ); it was not until November 1936 that Breguet increased them to 1:02 h, 44.69 km in closed orbit and 158 m with his helicopter. The Fw 61 reached a height of 2439 m, a flight time of 1 hour 20 minutes 49 seconds, a speed of 122.553 km / h over a distance of 20 km and a flight distance of 80.604 km in a closed runway. Flight captain Dipl.-Ing. Carl Bode (1911–2002) on June 20, 1938 with 230.248 kilometers by a flight from Faßberg to Rangsdorf again considerably; With 3427 m he also set Rohlf's old height record on January 29, 1939 with the V2.

A new company

Fw 61 in the helicopter museum Bückeburg (replica in original size)

Persistent difficulties with his old company prompted Focke to break away from it and to found the Focke-Achgelis company on April 27, 1937 together with the well-known aerobatic pilot Gerd Achgelis , in which Weser Flugzeugbau was involved. The further care of the two helicopters, which like all new developments were imperial property, was transferred to the new company. Since Rohlfs decided to stay with Focke-Wulf, Focke hired Carl Bode to continue the experiments on the advice of Udets. As a member of Akaflieg Hanover, he was part of the winning team of the Germany flight in 1933 and has now moved to Focke-Achgelis after working for the E -stelle Rechlin and Arado . The next two pilots who had to fly the new device were Carl Francke from the E -stelle Rechlin and Hanna Reitsch ; the latter actually illegally. She almost ended her first flight with a break because she underestimated the difficulty of helicopter flying. During a flight from Bremen to Tempelhof , which she had to interrupt first because the oil temperature was too high and then again in Stendal because of bad weather, she was able to increase the distance record on a straight line to 109 km on the flight from there to Berlin on October 25, 1937 .

After the success of Rohlfs, Reitsch and Bode, Udet urged the immediate start of series production. However, the designer was able to convince him, with reference to the single-seat design and the lack of the possibility of instruction by a second pilot as well as the missing payload, that a successor model with at least two seats had to be built and tested before series production could be considered. As a result, work began on a two-seater successor called Fa 224 Libelle , which was to be powered by a Hirth HM 508 with 280 hp (206 kW) or an Argus-As-10-C engine with 270 hp (198 kW) . The draft did not get beyond a dummy and was finally discontinued when the war broke out in favor of Fa 223 Drache, which was also in the works .

The demonstration in the Deutschlandhalle

Udet was so enthusiastic about the new aircraft that he immediately came up with another idea. Towards the end of 1937, a so-called colonial show with the title Ki sua heli was prepared in the Deutschlandhalle in Berlin . He ensured that the new helicopter should be demonstrated in flight in the revue planned for February 1938. With the V2, which had been brought to Berlin by rail, Bode and Hanna Reitsch began preparations on February 10th, which initially went off without any difficulties. During Bode's second flight on February 13 - in the presence of State Secretary Milch and Udets - the right universal joint broke due to fatigue , the helicopter tilted to the right, the rotor splintered on the ground and Bode had to wriggle out of the D-EKRA lying on his back . Actually, this incident would have been reason enough to stop flying in the fully occupied hall. However, all concerns have been pushed aside. The V1 was quickly brought to Berlin and prepared for the premiere on February 19th in day and night work. The D-EBVU mark on the fuselage was removed and replaced by the inscription "Germany". Hanna Reitsch was able to show the latest technical achievements in aviation at the premiere and from then on every day - a total of 18 times - without any further incident. To Udo's disappointment, the audience was only moderately enthusiastic, even if Hanna Reitsch personally enjoyed a lot of applause every time after getting out. The reasons for the weak reaction of the audience were easy to see. On the one hand it was the dust that the rotors blew into the eyes of the spectators, especially in the front (expensive) rows, despite the previous thorough cleaning of the floor, and on the other hand the fact that the windows had to be opened during the flight demonstration (in February) and the ventilation has been brought to full capacity. The reason for this was that Hanna Reitsch had discovered during one of the last test flights in the hangar that she had to push the throttle lever, which was used to control the altitude, much further than was normally necessary. This obvious drop in engine performance was attributed to the poor air conditions. The expected enthusiasm of the audience did not materialize. Even the trade press reacted very cautiously to the supposed sensation. Oskar Ursinus was just 1½ lines worth of the report in his Flugsport magazine . Henrich Focke received the Golden Lilienthal Medal in October 1938 and an honorary doctorate from his old Technical University in Hanover on November 12th .

Whereabouts of the machines

With the V2 referred to as Fa 61 , tests were carried out until 1941, partly at the AVA in Göttingen. It was then stored in Ochsenhausen near Biberach , the company's new headquarters after the bombing in Hoykenkamp and possibly fell into the hands of the French who were moving in at the end of the war. The V1, which was exhibited at the entrance to the Bremen plant, was destroyed in the bombing raid on the night of June 3rd to 4th, 1942.

In 2009 one of the rotor heads of the Fw 61 was found again in Ochsenhausen. The original can now be viewed in the helicopter museum in Bückeburg .

Original rotor head of the Fw 61 in the helicopter museum in Bückeburg

description

For the two helicopters, the Fw 61 V1 D-EBVU (Wnr. 931) and then the Fw 61 V2 D-EKRA (Wnr. 1789), the fuselage was taken over from the Fw 44 Stieglitz for the sake of simplicity and only slightly changed. He now only had an open driver's seat, kept his normal vertical stabilizer , on which a fixed but trimmable horizontal stabilizer was attached in a T-shape , and got a rigid nose wheel landing gear , whereby the tail roller was retained. The main wheels could be braked. The buoyancy was provided by two counter-rotating three-bladed rotors, which were mounted on side arms welded from tubular steel. They were powered by a 7-cylinder radial engine Bramo 314 E , which was attached to the nose of the fuselage without disguising and which was also cooled by a small propeller. Most of the engine power was transmitted to the rotors via a clutch and a transfer case through shafts mounted in the boom struts and via the cardan drives located in the rotor heads. Its leaves, with a double trapezoidal plan and the greatest profile depth about a third of the length, consisted of a steel tube as a carrier, onto which wooden ribs with outwardly decreasing angles were pushed to achieve the necessary twist . At the front, a plywood nose formed the shape of the symmetrical profile, while the rear part was covered with fabric. The blades were hinged to the rotor head with flapping and swivel joints. It was steered around the roll axis through different setting angles on the two rotors. For the forward or backward movement, but also for the rotation around the vertical axis, the blades were controlled with each revolution in such a way that the inclination of the thrust resultant on each rotor produced the desired control forces. The control inputs were transmitted to them via cables and sheaves attached under the rotor heads. It was particularly important to have a device that would enable a safe landing in the event of an engine failure or a broken drive shaft. With the flick of a lever, the setting angles of all blades were reduced to such an extent that they enabled autorotation . At the same time, the engine was automatically disengaged. A gliding landing followed. The process was irreversible. After actuating the lever, landing had to be carried out with autorotation.

Technical specifications

3-sided view

Parameter	Data
crew	1
length	7.29 m
Rotor diameter	2 × 7.0 m
height	2.64 m
Top speed	122 km / h
Service ceiling	3200 m
Range	230 km
Engine	Bramo 314 E , 118 kW (160 PS)

literature

Steve Coates: Helicopters of the Third Reich. Ian Allan publisher.
Henrich Focke: My path in life. Communication 77-01 of the DGLR.

Web links

Commons : Focke-Wulf Fw 61 - Collection of images, videos and audio files