Focke-Wulf Fw 61

The Focke-Wulf Fw 61 (indicated Focke-Achgelis F later 61 ) was a experimental Hélicoptère which marked the beginning of the career of helicopterist of Henrich Focke in 1932. It had been carried out in two specimens in the years 1935-1936 by the company Focke-Wulf (Focke-Wulf Flugzeugbau AG) which it had founded in 1923 with Georg Wulf 1923. It proved quickly that it was the first model of entirely functional helicopter in the world.

Genesis of its design

Following the experience gained within the framework of manufacture under license of the Autogiro S Cierva C.19 and Cierva C.30 and of the not very conclusive tests carried out with its prototype Fw 186, Henrich Focke included/understood en1932 which their defects could be eliminated only by producing a genuine helicopter. He saw the problems quickly to be solved. There were first of all the question of stability and of maneuverability but it was also a question of finding a means to in full safety bring back the apparatus on the ground in the event of breakdown of the engine or rupture of the driveshaft. With regard to the characteristics of the rotor, it could be based on work of the Spaniard Juan of Cierva which worked in Great Britain like on those of Glauert and Lock. It was however necessary first of all to carry out many calculations to be able to build models which would be then tested in Soufflerie.

Focke sliced in favor of a concept with two contrarotating rotors ensuring a perfect symmetry of the orders. A first model been driven by a two-stroke engine provides first promising results in 1934. It is exposed today to the Deutsches Museum with Munich. February 9th, 1935, Focke accepted ministry for aviation (RLM) an order to produce a prototype officially indicated Fw 61 but that Focke itself however always called F 61. The order of the exemplary second had placed on December 19th, 1935 by the aeronautical engineer staff (Fl.Hauptstabsing.) of the RLM Roluf Lucht.

In-flight tests

The first prototype (registered V 1 D-EBVU, n° of manufacture machines 931) was already practically finished and the engineer Ewald Rohlfs who had joined the Focke-Wulf company on May 1st 1935, accomplished the first trial flights, the captive apparatus, being connected on the ground by cords in a hall with Hemelingen where Focke had been banished by the “new leaders” of its company. The first entirely free flight took place only the June 26th 1936, because Rohlfs had had to be repatriated of South America thanks to the insistence of Ernst Udet, where the direction of the company - which disapproved the project had sent it on mission to spring 1936. Until the beginning of August, the tests had sufficiently progressed to make it possible the official services of the flight test center (`' Erprobungsstelle'') of Rechlin to remake the same tests to check the performances, tries which was entrusted to the engineer Walther Ballerstedt. This one had a certain experiment of the flight on Autogire C.30. Focke, which was more than précautionneux, held so that its first flight is done on captive apparatus. Ballerstedt was filled with enthusiasm by qualities of flight of the aircraft, considering only that the ordering of variation height, ordered by the throttle hand grip and consequently by the variation of mode of the rotors, was very sensitive. Udet, which had succeeded meanwhile the general Wilhelm Wimmer was filled with enthusiasm and made pressure to present the helicopter in public as quickly as possible. The second prototype (registered V 2 D-EKRA, n° factory 1789) could make its first flight in spring 1937, with again Rholfs with the orders. The May 10th 1937, this one carried out the first landing in Autorotation driving decree.

Records

One month later (June 25th and 26th 1937), Rohlfs had beaten on this apparatus all the records established previously. At one time when the best performances of the other helicopters had remained unchanged during years (lasted of flight of 10 min by D' Ascanio, distance maximum lower than 1 km by Pescara and 18 m height by Etienne Oehmichen and that Bréguet had carried in November 1936 respectively to 1 H2 min, 44,69 km in closed circuit and 158 m), Fw 61 reached an altitude of 2439 m, were maintained in the air during 1:20 min 49 S, reached a speed of 122,553 km/h on a distance of 20 km and covered 80,604 km in closed circuit.

The new business

The difficulties which the leaders of its old company made him pushed Focke to be entirely separated some and to found the April 27th 1937 with the famous pilot of aerobatics Gerd Achgelis a new business called Focke-Achgelis & Co GMBH whose factories Weser Flugzeugbau held also shares. The technical follow-up of the two helicopters which as all the new developments remained the state-owned property, was transferred to the new business. Rohlfs having decided to remain at Focke-Wulf, Focke engaged on the councils of Udet the engineer Carl Bode to continue the tests. This one had belonged to the gaining team of the competition Deutschlandflug (turn of Germany) of 1933 as a member of Akaflieg Hannover and joined from now on Focke-Achgelis after having worked in the flight test center of Rechlin and at Arado. The two others pilot being pleased to fly on this helicopter were the engineer Carl Francke CEV of Rechlin and Hanna Reitsch, which did not have however an authorization. It even failed crasher during its first flight because it had underestimated the difficulties of the piloting of a helicopter. She however learned how very quickly to control it. During a flight of Bremen to Berlin - Tempelhof (October 25th 19937), which it had to stop twice, initially because of an overheating then with Stendal due to bad weather, it pushed the record of distance in straight line to 109 km. Bode beat this record on June 20th, 1938 by crossing 230,248 km by rejoining Berlin - Rangsdorf since Fassberg. On the prototype V 2, it beat also the January 29th 1939 with 3427 m the record of altitude held previously by Rohlfs.

Following these successes, Udet made pressure so that the series production started at once. To be useful such an apparatus was however to be able to transport two people (instructor more raises pilot) and especially a payload and it was thus necessary to produce a two-seater apparatus before considering a mass production. Consequently, one began the development of a two-seater successor called Fa 224 Libelle, which would have been motorized with an engine Hirth HM 508 of 280 cv (206 kw) or an engine Argus As 10 C of 270 cv (198 kw). The project did not exceed however the stage of the model and had to be abandoned at the beginning of the Second world war with the profit of the Fa 223 '' Drache '' which was also under development.

Presentation in the Deutschlandhalle

Udet was filled with enthusiasm so much by the new apparatus which it advanced another idea. Towards the end of the year 1937, Berlin prepared a colonial exposure entitled Kisuaheli which was to be held in the Deutschlandhalle. It obtained that the new helicopter was presented in flight during the review scheduled for February 1938. Bode and Hanna Reitsch began the February 10th the preparations to fly on the prototype V 2, transported in Berlin by the train, first tests which occurred first of all without tear. But the February 13rd an accident took place in the presence of the Secretary of State Erhard Milch. During the second flight that Bode carried out this day, the Cardan joint right broke in consequence of tiredness of materials and the helicopter rocked towards the line. The rotor struck the ground while breaking but Bode could leave unscathed apparatus D-EKRA laid down on the back. This accident should in itself have justified prohibition to fly in a room archi-roof. But all the scruples were swept. The prototype V 1 was transported to Berlin and given in-flight status in one night and a workday for the First which was to take place the February 19th. Registration D-EBVU was replaced on the fuselage by the word Deutschland (Germany). Thus, Hanna Reitsch presented the very recent technical realization of aeronautics at the time of the First and then each day, 18 times on the whole, without incident. Udet was very disappointed because the public did not show a particular enthusiasm of it, even if Hanna Reitsch at the time of its descent of the apparatus were each highly applauded time. The reasons of the mitigated reaction of the spectators were easy to guess. There was first of all the dust raised by the rotors in spite of a meticulous cleaning of the ground and that spectators of the first ranks (the most expensive places…) received in the eyes, and in addition the fact that during the flights the windows were to be open and the ventilation pushed to the maximum in full February. Hanna Reitsch had indeed noted at the time of one of the last trial flights which it was to push more than usually the control stick of gases which made it possible to control the height. This loss of obvious power of the engine was due to the quality of the ambient air. Even the specialized press remained reserved vis-a-vis what had been announced like a sensational event. In its review Flugsport (aeronautical sport), Oskar Ursinus did not devote more than one line to him and half… Henrich Focke accepted in October 1938 the Lilienthal medal out of gold (Goldene Lilienthal-Medal) and the November 12th it was high with the rank of doctor honoris causa by the technical university of Hanover where it had studied.

The prototype V 2 (renamed F 61) was useful then for tests until in 1941, in particular in the research center aerodynamics AVA of Göttingen. It was then transferred to Ochsenhausen close to Biberach where the company had its new seat following the allied bombardments of Hoykenkamp and probably fell to the hands from the French troops towards the end from the war. The prototype V 1 on the other hand, which was exposed to the entry of the factory of Bremen, was destroyed during a bombardment in the night from June 3rd to 4th 1942.

Technical description

For the two prototypes, Fw 61 V 1 registered D-EBVU (n° 931) then Fw 61 V 2 registered D-EKRA (n° 1789), Focke had used a fuselage of Focke-Wulf Fw 44 Stieglitz slightly modified. It had only one opened cockpit, preserved its drift above which was fixed a horizontal plane in T connected to the drift by props but compensable, acting as elevator. It moreover was equipped with a nose gear fixes and of the caster of tail of origin the wheels of the principal train had brakes. The bearing pressure was ensured by two contrarotating three-bladed rotors laid out at the end of side beams out of weld steel tubes. The motorization consisted of a Radial engine with 7 cylinders Bramo 314 E nonducted, fixed in the nose cone and equipped with a small propeller being used only with its cooling and not for the propulsion. The major part of the horse-power was transmitted, through a clutch and of a gear case, with trees placed in the side beams and which attacked the Cardan joints on the level of the hub of the rotors. The blades of the rotors, of trapezoidal form doubles and whose cord was maximum with the third their length, consisted of a steel tube (member) and veins of wood whose angle of chock decreased while going towards the end to obtain a optimal twist. The profile of the blade was determined by an leading edge out of wooden (laminated) whereas the part postpones was cloth-lined. The blades had articulations of beat and trailed on the level of the hub of the rotors. Piloting around the axis of rolling was carried out while varying differently the collective angle of incidence of each rotor. To ensure displacement in translation before and back as well as the movement of gyration (around the yaw axis), the blades were ordered from each rotation so that the slope of the resulting push generates the necessary force. The requests of the pilot were transmitted to the blades by the means of cables and pulleys placed under the hub of each rotor. A particularly important device was that making it possible to land in full safety even in the event of engine failure or of rupture of a driveshaft. A lever made it possible to reduce the not of all the blades and to allow a descent in autorotation. This order ensured at the same time the disengagement of the engine. The landing was done then in planed flight. This once started operation was however not reversible. Once the activated lever, the landing could be done only in autorotation.

Literature

Steve Coates, Helicopters off the Third Reich , editions Ian Allan
Henrich Focke, Mein Lebensweg , Mitteilung 77-01 DER DGLR (Memories of Focke)

External bonds

http://www.luftarchiv.de/hubschrauber/fw61.htm
http://www.fw61.de/ homepage zur Focke-Wulf FW-61

to also see the List of the helicopters

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