Messerschmitt Bf 109 F-2, After February 1940 an improved engine, the Daimler-Benz DB601E, was developed for use with the Bf 109. The engineers at the Messerschmitt facilities took a Bf 109E-1 airframe and installed this new powerplant, Luftwaffe marking VK+AB, its production number was 5604. The fuselage was cleaned up and the engine cowling modified to provide improved aerodynamics. The relationship to the standard E-1 version was obvious, because the trapeziform wings were taken from the E-1, although this was later changed in the production models of the F version. This adaptation became the prototype for the Bf 109F series. As the DB601E was not yet available in numbers the pre-production F-0 (the only F variant to have a rectangular supercharger intake) and the first production series F-1/F-2 received the 1,175 hp (875 kW) DB601N engine. The 1,350 hp (1,005 kW) DB601E was first used in the F-3 model together with an enlarged propeller for improved performance. Externally the Bf 109F differed from the E-series, resulting from many aerodynamic improvements. The stabilizer struts were removed, the cowling was shaped to be more streamlined, the big underwing radiators were much smaller, the opening for the supercharger was improved to a round, more protruding "snorkel" like form, from the F-1 variants on, the flaps were completely changed, the wingspan was increased to 9.92 m, and the wing tips now were formed elliptically, which supposedly caused some confusions with the Spitfire. The redesigned wing made the internal mounting of guns impractical, so armament was revised. The armament of the Bf 109F consisted of the two MG 17 above the engine plus a Motorkanone cannon firing through the propeller hub: The early F versions were equipped with the MG FF/M cannon, the F-2 got the 15 mm MG 151, and from F-4 on the 20 mm MG 151/20 was used. Several aces, particularly Oberst Adolf Galland, criticised the light armament as inadequate for the average pilot. Major Walter Oesau even refused to fly an F as long as 'Emil's were still available. Only after a lack of spare parts, did he accept an F. Later on, an attachment of underwing 20 mm cannons addressed the issue of fire-power, but at a price to performance. Werner Mölders on the other hand was very much pleased and saw the single centerline gun as an improvement. It is possible that the criticism of the Bf 109F's armament is based on the early F-2 version with the 15 mm MG 151/15 cannon, which was later replaced by the 20 mm version of the same weapon and was seen highly effective by aces like Günther Rall. The first Bf 109F examples were not well tested, consequently, some aircraft crashed or nearly crashed, due to vibrations which caused either the wing surface to curve or break, or caused the stabilizer to break away. In one such accident, the commander of JG 2 "Richthofen", Wilhelm Balthasar lost his life when he was attacked by a Spitfire during a test flight. Making an evasive maneuver, his wings broke away and Balthasar was killed when his aircraft hit the ground. When the wreck was investigated, not a single bullet hole was found. However, the teething problems were subsequently solved, and pilots generally agreed that the F series were the best-handling of all the Bf 109 series.