What determines the longitudinal stability of an airplane?

The longitudinal stability of an airplane primarily depends on the location of the center of gravity (CG) in relation to the center of lift (or center of pressure) on the wing. When the CG is ahead of the center of lift, the aircraft will naturally tend to pitch down when disturbed, and conversely, if the CG is behind the center of lift, it will tend to pitch up. This pitching tendency creates a restoring moment that helps to stabilize the aircraft.

A well-placed center of gravity ensures that the aircraft can handle disturbances smoothly and return to its original flight attitude without excessive control input from the pilot. The design and configuration of the aircraft must maintain appropriate CG limits to facilitate safe and efficient flight, therefore enhancing overall stability.

While the effectiveness of control surfaces like the horizontal stabilizer, rudder, and trim tabs are essential for control and maneuverability, they complement the inherent longitudinal stability established by the relationship between the CG and the center of lift, rather than determining it. The balance of thrust and lift against weight and drag is crucial for overall flight performance but does not specifically address stability in the longitudinal axis.