Original engines lacked sufficient power and were replaced by larger engines. After initial tests, pilot reported the aircraft could fly no higher than 5 feet and flight could be sustained only by maintaining precise angle of attack. Dr. von Karman, who had assisted earlier in the design, was called in and solved problem by making adjustments to trailing edges of elevons. Later, test pilot reported aircraft oscillated in Dutch Roll when moved about vertical axis. Problem cleared by further adjustments to aircraft's configuration. N-1M was plagued by poor performance because it was overweight and chronically underpowered; nevertheless, over 200 test flights made and Northrop convinced General "Hap" Arnold to back more advanced concepts.