China Civil UAV Theory Questions — Questions 859–868

Study note: lateral and directional stability go together; longitudinal and directional stability are independent. Longitudinal stability and directional stability are independent of each other. Lateral and directional stability need to be balanced and coordinated.

Study note: lateral goes with the longitudinal axis, longitudinal goes with the lateral axis, directional goes with the vertical axis. Lateral stability is rolling motion about the longitudinal axis. Longitudinal stability is pitching motion about the lateral axis. Directional (heading) stability is yawing motion about the vertical axis.

Study note: lateral damping goes with the wing; lateral rolling moment goes with lift. The lateral damping moment mainly affects an aircraft's rolling motion. When an aircraft is disturbed from its equilibrium position, the lateral damping moment prompts the aircraft to roll to return to equilibrium. An aircraft's lateral damping moment is mainly produced by the wing. When an aircraft rolls or sideslips, changes in aerodynamic force on the wing and vertical tail produce a damping moment opposite to the existing roll or yaw motion direction. For example, when an aircraft rolls left, the left wing sinks and the right wing rises, changing airflow speed and pressure distribution on the left and right wings, producing a lift difference that forms a rightward lateral damping moment.

The climb angle is the angle between the aircraft's climb path and the horizontal line, reflecting how steep the aircraft's climb is. Options B and C are not definitions of the climb angle.

Study note: center of gravity goes with longitudinal stability. With the aircraft center of gravity forward and the aerodynamic focus aft, longitudinal stability is good. With the center of gravity aft and the aerodynamic focus forward, longitudinal stability is poor. Further reading: why the aerodynamic-focus and center-of-gravity layout affects longitudinal stability. The focus is the point of application of aerodynamic force, especially where the change in aerodynamic lift produced when the aircraft's angle of attack changes acts. The aircraft's center of gravity is the point of application of the aircraft's weight. When the aircraft's center of gravity lies ahead of the aerodynamic focus, the aircraft is longitudinally statically stable. This is because when the aircraft is disturbed upward (such as by a gust or pilot operation), it produces a nose-down restoring moment that returns the aircraft to its original flight attitude. Conversely, if the aircraft's center of gravity lies behind the aerodynamic focus, the aircraft becomes longitudinally statically unstable. In that case, any upward disturbance causes the aircraft to produce a nose-up divergent moment, making its flight attitude deviate further and further from its original state.

Center-of-gravity position affects the moment from the aerodynamic focus to the center of gravity and the moment from the vertical tail to the center of gravity, thus affecting longitudinal stability and directional stability respectively. Option A is correct.

Study note: center of gravity goes with longitudinal stability. Key point — longitudinal stability: with the aircraft center of gravity forward and the aerodynamic focus aft, longitudinal stability is good. With the center of gravity aft and the aerodynamic focus forward, longitudinal stability is poor. Further reading: why the aerodynamic-focus and center-of-gravity layout affects longitudinal stability. The focus is the point of application of aerodynamic force, especially where the change in aerodynamic lift produced when the aircraft's angle of attack changes acts. The aircraft's center of gravity is the point of application of the aircraft's weight. When the aircraft's center of gravity lies ahead of the aerodynamic focus, the aircraft is longitudinally statically stable. This is because when the aircraft is disturbed upward (such as by a gust or pilot operation), it produces a nose-down restoring moment that returns the aircraft to its original flight attitude. Conversely, if the aircraft's center of gravity lies behind the aerodynamic focus, the aircraft becomes longitudinally statically unstable. In that case, any upward disturbance causes the aircraft to produce a nose-up divergent moment, making its flight attitude deviate further and further from its original state.

Study note: lateral and directional stability go together; longitudinal and directional stability are independent. Longitudinal stability and directional stability are independent of each other. Lateral and directional stability need to be balanced and coordinated.

Study note: lateral goes with the longitudinal axis, longitudinal goes with the lateral axis, directional goes with the vertical axis. Lateral stability is rolling motion about the longitudinal axis. Longitudinal stability is pitching motion about the lateral axis. Directional (heading) stability is yawing motion about the vertical axis.

An aircraft's pitch angle in flight is defined as the angle between the aircraft's longitudinal axis and the horizontal plane. This angle reflects the aircraft's attitude in the vertical plane, i.e., how much the aircraft's nose is pitched up or down relative to the horizontal plane.