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Textbooks say an aircraft in flight changing its pitch attitude rotates about its center of gravity. It seems to me that it would rotate about its center of pressure (lift). Which is it?
The aircraft will rotate about its center of gravity. For an airplane in balanced flight, the center of lift for the airplane is located exactly on top of the center of gravity. If it were not, the airplane would be in constant rotational motion about the center of gravity. In order to change the pitch attitude of the airplane, the center of lift is momentarily moved away from the center of gravity, causing the airplane to momentarily execute a pitching motion about the center of gravity. In order to stop the pitching motion at some other angle of pitch, the center of lift must then be brought back to act precisely at the center of gravity, thus stopping the pitching motion. This is done through the use of control surfaces, such as the elevator on the tail.
