Wednesday, 13 April 2016

The origins of the concept of "butcher" - Fw190. Part 1. Compiled by German Dzib

The Focke Wulf Fw 190 Würger (English: Shrike) is a German single-seat, single-engine fighter aircraft designed by Kurt Tank in the late 1930s and widely used during World War II. Along with its well-known counterpart, the Messerschmitt Bf 109, the Fw 190 became the backbone of the Luftwaffe's Jagdwaffe (Fighter Force). The twin-row BMW 801 radial engine that powered most operational versions enabled the Fw 190 to lift larger loads than the Bf 109, allowing its use as a day fighter,fighter-bomber, ground-attack aircraft and, to a lesser degree, night fighter.

The Fw 190A started flying operationally over France in August 1941, and quickly proved superior in all but turn radius to the Royal Air Force's main front-line fighter, the Spitfire Mk. V,(1) especially at low and medium altitudes. The 190 maintained superiority over Allied fighters until the introduction of the improved Spitfire Mk. IX.(2) In November/December 1942, the Fw 190 made its air combat debut on the Eastern Front, finding much success in fighter wings and specialised ground attack units called Schlachtgeschwader (Battle Wings or Strike Wings) from October 1943 onwards. In the opinion of German pilots who flew both the Bf 109 and the Fw 190, the latter provided increased firepower and, at low to medium altitude, manoeuvrability.

The Fw 190A series' performance decreased at high altitudes (usually 6,000 m (20,000 ft) and above), which reduced its effectiveness as a high-altitude interceptor. From the Fw 190's inception, there had been ongoing efforts to address this with a turbosupercharged BMW 801 in the B model, the C model with the Daimler-Benz DB 603, and the D model with the Junkers Jumo 213. Problems with the turbos meant only the D model would see service, entering service in September 1944. While these "long nose" versions gave them parity with Allied opponents, it arrived far too late in the war to have any real effect.

The Fw 190 was well-liked by its pilots. Some of the Luftwaffe's most successful fighter aces claimed a great many of their kills while flying it, including Otto Kittel, Walter Nowotny and Erich Rudorffer.

Early development

Between 1934 and 1935 the German Ministry of Aviation (RLM) ran a contest to produce a modern fighter for the rearming Luftwaffe. Kurt Tank entered the parasol-winged Fw 159 into the contest, against the Arado Ar 80, Heinkel He 112 and Messerschmitt Bf 109. The Fw 159 was hopelessly outclassed, and was soon eliminated from the competition along with the Ar 80. The He 112 and Bf 109 were generally similar in design but the 109's lightweight construction gave it performance edge the 112 was never able to match. On 12 March 1936 the 109 was declared the winner. Even before the 109 had entered squadron service, in autumn 1937 the RLM sent out a new tender asking various designers for a new fighter to fight alongside the Bf 109, as Walter Günther had done with his firm's follow-on to the unsuccessful He 112. Although the Bf 109 was an extremely competitive fighter, the Ministry was worried that future foreign designs might outclass it, and wanted to have new aircraft under development to meet these possible challenges.(4) Kurt Tank responded with a number of designs, most based around a liquid-cooled inline engine. However, it was not until a design was presented using the air-cooled, 14-cylinder BMW 139 radial engine that the Ministry of Aviation's interest was aroused. (5) As this design used a radial engine, it would not compete with the inline-powered Bf 109 for engines, when there were already too few Daimler-Benz DB 601s to go around. (6) This was not the case for competing designs like the Heinkel He 100 or twin-engined Focke-Wulf Fw 187, where production would compete with the 109 and Messerschmitt Bf 110 for engine supplies. After the war, Tank denied a rumour that he had to "fight a battle" with the Ministry to convince them of the radial engine's merits. (7) 

Design concepts

One of the first mockups including the Tunnelnabe as well as the wing-root armament
At the time, the use of radial engines in land-based fighters was relatively rare in Europe, as it was believed that their large frontal area would cause too much drag on something as small as a fighter. Tank was not convinced of this, having witnessed the successful use of radial engines by the U.S. Navy, and felt a properly streamlined installation would eliminate this problem. (6) The hottest points on any air-cooled engine are the cylinder heads, located along the outside diameter of a radial engine. In order to provide sufficient air to cool the engine, airflow had to be maximized at this outer edge. This was normally accomplished by leaving the majority of the front face of the engine open to the air, causing considerable drag. During the late 1920s, NACA led development of a dramatic improvement by placing an airfoil-shaped ring around the outside of the cylinder heads (the NACA cowling). The shaping accelerated the air as it entered the front of the cowl, increasing the total airflow, and allowing the opening in front of the engine to be made smaller. (8) Tank introduced a further refinement to this basic concept. He suggested placing most of the airflow components on the propeller, in the form of a oversized propeller spinner whose outside diameter was the same as the engine. The cowl around the engine proper was greatly simplified, essentially a basic cylinder. Air entered through a small hole at the centre of the spinner, and was directed through ductwork in the spinner so it was blowing rearward along the cylinder heads. To provide enough airflow, a cone was placed in the centre of the hole, over the propeller hub, which was intended to compress the airflow and allow a smaller opening to be used. In theory, the tight-fitting cowling also provided some thrust due to the compression and heating of air as it flowed through the cowling. (9)  As to the rest of the design philosophy, Tank wanted something more than an aircraft built only for speed. Tank outlined the reasoning:

The Messerschmitt 109 [sic] and the British Spitfire, the two fastest fighters in world at the time we began work on the Fw 190, could both be summed up as a very large engine on the front of the smallest possible airframe; in each case armament had been added almost as an afterthought. These designs, both of which admittedly proved successful, could be likened to racehorses: given the right amount of pampering and easy course, they could outrun anything. But the moment the going became tough they were liable to falter. During World War I, I served in the cavalry and in the infantry. I had seen the harsh conditions under which military equipment had to work in wartime. I felt sure that a quite different breed of fighter would also have a place in any future conflict: one that could operate from ill-prepared front-line airfields; one that could be flown and maintained by men who had received only short training; and one that could absorb a reasonable amount of battle damage and still get back. 

This was the background thinking behind the Focke-Wulf 190; it was not to be a racehorse but a Dienstpferd, a cavalry horse. (10) In contrast to the complex, failure-prone fuselage mounted main gear legs of the earlier Fw 159, one of the main features of the Fw 190 was its wide-tracked, inwards-retracting landing gear. They were designed to withstand a sink rate of 4.5 meters per second (15 feet per second, 900 feet per minute), double the strength factor usually required. Hydraulic wheel brakes were used. (11) The wide-track landing gear produced better ground handling characteristics, and the Fw 190 suffered fewer ground accidents than the Bf 109. (The Bf 109's narrow-track, outwards-retracting landing gear hinged on its wing root structure to help lower weight, but this led to inherent weakness and many failures and ground loops. (11)) The Fw 190's retractable tail gear used a cable, which was guided over a set of pulleys located in the vertical fin, to pull the oleo strut upwards into the lower fuselage. On some versions of the Fw 190 an extended oleo strut could be fitted for larger-sized loads (such as bombs or even a torpedo) beneath the fuselage. (12)
Most aircraft of the era used cables and pulleys to operate their controls. The cables tended to stretch, resulting in the sensations of "give" and "play" that made the controls less crisp and responsive, and required constant maintenance to correct. For the new design, the team replaced the cables with rigid pushrods and bearings to eliminate this problem. Another innovation was making the controls as light as possible. The maximum resistance of the ailerons was limited to 3.5 kg (8 lb), as the average man's wrist could not exert a greater force. The empennage (tail assembly) featured relatively small and well-balanced horizontal and vertical surfaces. (13) The design team also attempted to minimize changes in the aircraft's trim at varying speeds, thus reducing the pilot's workload. They were so successful in this regard that they found in-flight-adjustable aileron and rudder trim tabs were not necessary. Small, fixed tabs were fitted to control surfaces and adjusted for proper balance during initial test flights. Only the elevator trim needed to be adjusted in flight (a feature common to all aircraft). This was accomplished by tilting the entire horizontal tailplane with an electric motor, with an angle of incidence ranging from -3° to +5°. (14)
Fw 190 .. Smile
Another aspect of the new design was the extensive use of electrically powered equipment instead of the hydraulic systems used by most aircraft manufacturers of the time. On the first two prototypes, the main landing gear was hydraulic. Starting with the third prototype, the undercarriage was operated by push buttons controlling electric motors in the wings, and was kept in position by electric up and down-locks. (15) The armament was also loaded and fired electrically. Tank believed that service use would prove that electrically powered systems were more reliable and more rugged than hydraulics, electric lines being much less prone to damage from enemy fire. (13) Like the Bf 109, the Fw 190 featured a fairly small wing planform with relatively high wing loading. This presents a trade-off in performance. An aircraft with a smaller wing suffers less drag under most flight conditions and therefore flies faster and may have better range. However, it also means the wing generates less lift at lower speeds, making it less maneuverable and also reduces performance in the thinner air at higher altitudes. The wings spanned 9.5 m (31 ft 2 in) and had an area of 15 m² (161 ft²). The wing was designed using the NACA 23015.3 airfoil at the root and the NACA 23009 airfoil at the tip. (16) 
Earlier aircraft designs generally featured canopies consisting of small plates of perspex (called Plexiglas in the United States) in a metal framework, with the top of the canopy even with the rear fuselage. This design considerably limited visibility, especially to the rear. The introduction of vacuum forming led to the creation of the "bubble canopy" which was largely self-supporting, and could be mounted over the cockpit, offering greatly improved all-round visibility. Tank's design for the Fw 190 used a canopy with a frame that ran around the perimeter, with only a short, centerline seam along the top, running rearward from the radio antenna fitting where the three-panel windscreen and forward edge of the canopy met, just in front of the pilot.
The eventual choice of the BMW 801 14-cylinder radial over the more troublesome BMW 139 also brought with it a BMW-designed cowling "system" which integrated the radiator used to cool the motor oil. An annular, ring-shaped oil cooler core was built into the BMW-provided forward cowl, just behind the fan. The outer portion of the oil cooler's core was in contact with the main cowling's sheet metal. Comprising the BMW-designed forward cowl, in front of the oil cooler was a ring of metal with a C-shaped cross-section, with the outer lip lying just outside the rim of the cowl, and the inner side on the inside of the oil cooler core. Together, the metal ring and cowling formed an S-shaped duct with the oil cooler's core contained between them. Airflow past the gap between the cowl and outer lip of the metal ring produced a vacuum effect that pulled air from the front of the engine forward across the oil cooler core to provide cooling for the 801's motor oil. The rate of cooling airflow over the core could be controlled by moving the metal ring in order to open or close the gap. The reasons for this complex system were threefold. One was to reduce any extra aerodynamic drag of the oil radiator, in this case largely eliminating it by placing it within the same cowling as the engine. The second was to warm the air before it flowed to the radiator to aid warming the oil during starting. Finally, by placing the radiator behind the fan, cooling was provided even while the aircraft was parked. The disadvantage to this design was that the radiator was in an extremely vulnerable location, and the metal ring was increasingly armoured as the war progressed. (17)