Messerschmitt Bf.109F - German users, part four

The tailwheel of the Bf.109E was also replaced with a semi-retractable one and the undercarriage legs were raked forward by six degrees to improve ground handling. When the first F-1s were put into service, a structural flaw was revealed causing the crash of some aircrafts as the wing surface wrinkled or fractured or by the tail structure failing. One of those accidents claimed the life of JG 2 'Richtofen' Commander Wilhelm Balthasar when he was attacked by a Spitfire during a test flight. When he performed an evasive maneouvre, wings broke away and the aircraft crashed killing Balthasar.
In order to deal with that serious problem, slightly thicker wings and reinforced spars were introduced. Some tests were also performed to see why the tail unit had failed.
It was found that at a certain engine settings a high-frequency oscilation in tailplane spar was overlapped by harmonic vibrations from the engine. So those two combined effects were enough to cause structural failure at the rear fuselage/fin attachment point. Initially, two external stiffening plates were screwed onto the fuselage on each side, and later, the enitre fuselage structure was reinforced.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants#Bf_109F
2. Salamander Books - The Complete Book of Fighters
3. Osprey - Air Vanguard 23 - Messerschmitt Bf 109 E-F series
4. Signal Squadron - Aircraft In action 57 - Messerschmitt Bf-109 in Action Pt.2