China Civil UAV Theory Questions — Questions 899–908

When the center of gravity is forward and the aerodynamic focus is aft, if the focus moves aft, the moment from the center of gravity to the aerodynamic focus increases, the restoring moment is larger, and longitudinal stability is better.

When the focus lies behind the center of gravity, the aircraft has inherent longitudinal stability. This is because any disturbance that pitches the aircraft up or down produces a restoring moment that returns the aircraft to its original attitude. However, if the focus moves further aft, this stability strengthens, because the restoring moment increases. As the focus moves aft, the pilot must apply greater force on the elevator to overcome the enhanced stability when maneuvering. This means the aircraft's control response becomes more sluggish, because a larger input is needed to produce the same attitude change.

When wing camber increases, the curvature of the upper wing surface becomes greater, the airflow acceleration effect strengthens, and the lowest-pressure point on the upper surface moves forward. At the same time, greater camber makes airflow separate more easily, so the critical angle of attack decreases. Option B is wrong about the lowest-pressure point moving aft; option C is wrong about increasing the critical angle of attack.

Attitude angles include: roll angle, yaw angle, pitch angle. Yaw angle: the angle between the aircraft's longitudinal axis and a datum line on the ground plane (usually the heading). Roll angle: the angle between the aircraft's lateral axis and the ground plane. Pitch angle: the angle between the aircraft's longitudinal axis and the horizontal plane.

When a leading-edge slat opens, high-pressure airflow from the lower surface flows through the slot to the upper surface, delaying airflow separation and increasing the critical angle of attack, thereby increasing the maximum lift coefficient. Option A, increasing the wing angle of incidence, is not its function; option B, increasing aircraft stability, is not its main function.

Study note: lateral axis goes with pitch, longitudinal axis goes with roll, vertical axis goes with yaw. Pitching moment: the moment that rotates the aircraft about its lateral axis. Rolling moment: the moment that rotates the aircraft about its longitudinal axis. Yawing moment: the moment that rotates the aircraft about its vertical axis.

Study note: lateral axis goes with pitch, longitudinal axis goes with roll, vertical axis goes with yaw. Pitching moment: the moment that rotates the aircraft about its lateral axis. Rolling moment: the moment that rotates the aircraft about its longitudinal axis. Yawing moment: the moment that rotates the aircraft about its vertical axis.

Study note: lateral axis goes with pitch, longitudinal axis goes with roll, vertical axis goes with yaw. Pitching moment: the moment that rotates the aircraft about its lateral axis. Rolling moment: the moment that rotates the aircraft about its longitudinal axis. Yawing moment: the moment that rotates the aircraft about its vertical axis.

When the trailing-edge flap is lowered, wing camber increases, airflow acceleration and pressure drop over the upper surface become more pronounced, making airflow separate earlier, thereby decreasing the critical angle of attack. Option B, increasing wing area, mainly affects lift magnitude and has little effect on the critical angle of attack; option C, the slot's function is to delay airflow separation, which would increase the critical angle of attack, contrary to the question.

Study note: to turn left, open the left; to turn right, open the right. A spoiler is a lift-reducing device, mainly used to reduce lift and increase drag, helping deceleration on landing, and usually fitted only on the upper surface of the wing. When using spoilers to maneuver an aircraft into a left turn, left lift must be reduced and drag increased, tilting the aircraft to the left. This can be done by opening the left spoiler upward, while the right spoiler remains unmoved.