Focke Wulf Fw 190

Design & Development: Concepts

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.

The hottest points on any air-cooled engine are the cylinder heads, located around the circumference 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.

Tank introduced a further refinement to this basic concept. He suggested placing most of the airflow components on the propeller, in the form of an 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, an internal 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.

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 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."
– Kurt Tank

An Fw 190F's tailfin, showing the triangular hinged panel for access to the tailwheel retraction mechanics inside of it. 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. The wide-track undercarriage 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.) The Fw 190's retractable tail gear used a cable, anchored to the "elbow" at the midpoint of the starboard maingear's transverse retraction arms, which ran aftwards within the fuselage to the vertical fin to operate the tailwheel retraction function. The tailwheel's retraction mechanical design possessed a set of pulleys to guide the aforementioned cable to the top of the tailwheel's oleo strut, pulling it upwards along a diagonal track within the fin, into the lower fuselage — this mechanism was accessible through a prominently visible triangular-shaped hinged panel, on the left side in the fin's side sheetmetal covering. On some versions of the Fw 190 an extended tailwheel oleo strut could be fitted for larger-sized loads (such as bombs or even a torpedo) beneath the fuselage.

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.

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°.

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. 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.

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 aircraft has a higher stalling speed 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.

Earlier aircraft designs generally featured canopies consisting of small plates of perspex (called Plexiglas in the United States) in a metal "greenhouse" framework, with the top of the canopy even with the rear fuselage - this was true of the IJNAS Mitsubishi A6M Zero, whose otherwise "all-around view" canopy was still heavily framed. 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.