Longitudinal Axis: The longitudinal axis, also known as the roll axis, is an imaginary line that runs from the nose to the tail of the aircraft, passing through its center of gravity. This axis is perpendicular to the wingspan and the lateral axis (roll axis) and parallel to the aircraft’s fuselage. Rotation about the longitudinal axis is associated with changes in pitch, which is the up-and-down motion of the aircraft’s nose.
Q means dynamic pressure
M means mass
U0 means airspeed
Cd means drag coefficient