Many of the test pilots at Edwards praised Armstrong's engineering ability. Milt Thompson said he was "the most technically capable of the early X-15 pilots". Bill Dana said Armstrong "had a mind that absorbed things like a sponge". Those who flew for the Air Force tended to have a different opinion, especially people like Chuck Yeager and Pete Knight, who did not have engineering degrees. Knight said that pilot-engineers flew in a way that was "more mechanical than it is flying", and gave this as the reason why some pilot-engineers got into trouble: their flying skills did not come naturally. Armstrong made seven flights in the X-15 between November 30, 1960, and July 26, 1962. He reached a top speed of Mach 5.74 () in the X-15-1, and left the Flight Research Center with a total of 2,400 flying hours.