During the Fw 190's design, Tank believed that through the fighter's service life, it would get progressively heavier. So the team had to allow for this. He took the landing gear as an example.
"For the design weight and estimated landing speed of the prototype aircraft, we calculated that the undercarriage should withstand a sinking speed of 2.5 meters/second [8.5 ft./sec.] to be sufficient. But if the aircraft was developed [over time], its maximum speed, weight and landing speed would all increase. That would result in considerably higher forces on the undercarriage during landing. So in the original stress calculations, we allowed for a sinking speed not of 2.5 m/sec. but of 4.5 m/sec. [15 ft./sec.]. And then we designed the undercarriage to be strong enough to take that. The move paid off. During its life, the maximum loaded weight of the Fw 190 rose from 2.75 tons to more than twice that figure, but with few changes, the undercarriage remained adequate. I have used the undercarriage as an example, but in fact several parts of the original structure were a great deal stronger than the minimum necessary."
When the Fw 190 began flight tests in the late spring of 1939, the test pilots began to establish its performance envelope. Tank, a qualified pilot, took part in the tests.
"Hans Sander did the initial testing, and then I flew the aircraft and found that it handled beautifully in the air. The work we had put into the flying controls had produced the results we wanted. I have always believed that a pilot should not have to use a great deal of muscle power to get an airplane to do what he wants. If the controls were properly designed, he should be able to conduct most maneuvers with only a finger and thumb on the stick. In combat, a high rate of roll is essential for a fighter so that the pilot can make rapid changes in his direction of flight. The aileron stick forces, however, had to be kept below a maximum of about 3.5 kilos [eight pounds] because a man's wrist cannot exert a force much greater than that. We succeeded in getting the stick forces down and, finally, I had the aileron controls as I wanted them. The aircraft followed the movement of the stick immediately and precisely with no initial tendency to yaw. Compared with the ailerons, the other flying controls were relatively easy to design; the stick forces were not so critical for the elevators, and the highest forces of all could be taken on the rudder pedals because a man's legs are far stronger than his arms."
Once the controls had been correctly balanced, it was important that they stayed that way over a wide range of speeds. Tank did not want a fighter pilot to have to re-trim the aircraft each time he moved the throttle. The team was so successful in this that movable trim tabs were considered unnecessary on the Fw 190. Small fixed trim tabs were fitted to the ailerons, the elevators and the rudder. These were adjusted on the ground after a fighter's initial test flight to compensate for the manufacturing variations inevitable with a mass-produced aircraft. The only system for re-trimming the aircraft in flight was in the elevator sense, and that was achieved by using an all-moving horizontal stabilizer.
To reduce the drag from the radial engine, the prototype was fitted with an unusual ducted spinner. The air was compressed as it entered the duct and then heated and expanded as it passed over the cylinders, before being ejected rearwards under pressure. Thus the system resembled a low-powered ram jet and provided a little thrust to cancel out some of the engine cooling drag. Flight tests soon revealed, however, that the ducted spinner produced very little thrust. The benefits of the installation were not worth the extra complication, and early in the test program the airplane was fitted with a regular radial engine cowl.
Although the prototype Fw 190 flew well enough, there were problems with its BMW 139 engine: the rear cylinders of the two-row radial often overheated. By then, BMW had its new BMW 801 engine undergoing bench testing. Quite apart from an extra 50hp, rising later to 200hp, the new engine was more reliable and less prone to overheating than its predecessor. Shortly after the first fight, Focke-Wulf received a Luftwaffe contract to modify the fighter to take the BMW 801. The resultant aircraft, the Fw 190 V5, flew for the first time early in the spring of 1940.
"Although the extra 50hp was useful, we found that the extra 160 kg [350 lb.] of engine weight plus the additional structure necessary to carry it, and the weight of armor and the additional equipment the Luftwaffe now wanted, had increased [the fighter's] all-up weight by about a quarter. The wing loading rose from the 1.6 kg/m2 [38 lb./sq. ft.] of the first prototype to 1.9 kg/m2 [46 lb./sq. ft.], and the turning performance deteriorated accordingly. To restore the aircraft's previously pleasant handling characteristics, we enlarged the wing by extending each tip by just over 50 centimeters [20 inches] and reducing the amount of taper so that the outer sections were somewhat wider. In this way, we increased the wing area by just over 3.25 m2 [35 sq. ft.] and lowered the wing loading to a more reasonable 1.5 kg/m2 [35.8 lb./sq. ft.]. Later, to maintain the correct relationship between the wing and the tail plane, we made a proportional increase in the area of the latter. The wing and the tail plane of the low- and medium-altitude versions of the Fw 190 then remained unchanged throughout the remainder of the development life of the aircraft."
The BMW 801 engine suffered from some cooling problems, though these were not as serious as those with BMW 139. And with modification, the difficulties were reduced to within acceptable limits. More serious were the troubles with the automatic engine control system, the Kommandogerät, fitted to the new engine. Tank explained:
"This was a rather clever device intended to save the pilot from having to worry about the optimum relationship between altitude and fuel flow, fuel mixture, propeller pitch setting, ignition timing, engine revolutions and the selection of the correct supercharger gear. The pilot had simply to move one control—his throttle—and in theory the Kommandogerät did the rest. I say in theory because at first, the device did not work well at all. All sorts of things went wrong with it. One of the more disconcerting was the rather violent automatic switching in of the high gear of the supercharger as the aircraft climbed through 2,650 meters [8,700 ft.].
"Once I was carrying out a test with an early version of the Fw 190 that involved a loop at medium altitude. Just as I was nearing the top of the loop, on my back with little airspeed, I passed through 2,650 meters, and the high gear of the supercharger cut in with a jerk. The change in torque hurled the aircraft into a spin with such suddenness that I became completely disorientated. And, as there was a ground haze and an overcast and my artificial horizon had toppled, I had no way of knowing which way was up. Indeed, I never did find out whether it was an upright or an inverted spin. After considerable loss in altitude, I managed to recover from the spin. But the incident had given me a lot to think about. As soon as I landed, I was on the telephone to the BMW. I told them that if they did not sort out their engine and its terrible Kommandogerät, I would do all in my power to see that somebody else's engine went into the Fw 190! The Kommandogerät was made to work, and it worked very well, but it took quite a battle on our part."
When it went into action in the summer of 1941, the Fw 190 demonstrated a clear margin of superiority over the Spitfire Mark V, the fastest fighter in RAF service. With its powerful radial engine, the Focke-Wulf Dienstpferd had the edge over the opposing racehorse. This situation lasted for nearly a year, until the Spitfire Mark IX became operational in quantity, powered by the Merlin 61 engine with a two-stage supercharger. In terms of performance, the Spitfire IX and the Fw 190A-3 were almost neck and neck.
Altogether, about 19,500 Fw 190s were built, of all versions, and the type remained in full production until the very end of the War.