China Civil UAV Theory Questions — Questions 849–858

The elevator, rudder, and ailerons are the primary control surfaces.

Study note: supersonic flow through a converging duct slows down and pressure rises; subsonic flow through a converging duct speeds up and pressure drops. Low-speed airflow through a converging duct increases in speed and decreases in static pressure; through a diverging duct, it decreases in speed and increases in static pressure. Supersonic airflow through a converging duct decelerates and increases in pressure; through a diverging duct, it accelerates and decreases in pressure. This is because at supersonic speeds, changes in cross-sectional area cause air compression, so density ρ in Bernoulli's equation changes.

When a Fowler flap is lowered, it not only deflects downward to increase wing camber, but also moves aft to increase wing area, significantly raising the lift coefficient. Option A, a split flap, only splits from the lower surface of the wing trailing edge; option B, a plain flap, only deflects downward to increase camber, and neither can increase area at the same time, so both are excluded.

When the trailing-edge flap is lowered, wing area and camber increase, so lift increases; at the same time, the change in wing shape increases drag. Therefore, when the flap is lowered, both lift and drag increase, and options A and B are one-sided.

Directional stability, also called yaw stability, is the ability of an aircraft in flight, after a small disturbance disrupts its directional equilibrium, to automatically tend to return to its original state of equilibrium once the disturbance disappears. This stability is mainly produced by the aircraft's vertical tail; the directional stabilizing moment and damping moment produced by the vertical tail help the aircraft keep its sideslip angle unchanged and make it act like a weathervane, with the nose automatically aligning with the airflow direction.

Steady flight is flight in which an aircraft, in a given configuration and flight mass, maintains a state of uniform motion, also called stable flight. A balanced force system is a force system whose resultant force equals zero. In a balanced force system, the vector sum of the forces is zero, i.e., the principal vector of the force system is zero. At the same time, the vector sum of the moments of the forces about any point is also zero, i.e., the principal moment of the force system about any point is zero. In steady level flight, options A, B, and C are all correct. When steady flight is a uniform climb or descent, the horizontal components of lift, drag, and thrust must be considered. Lift and weight, and thrust and drag, are not equal, so option B is more accurate.

Static stability of an aircraft is its ability, when disturbed externally, to automatically return to its original state of equilibrium. When an aircraft with static stability is disturbed externally, it produces a restoring moment opposite to the disturbance direction, making the aircraft automatically return to its original state of equilibrium. When left sideslip occurs, the incoming airflow comes from the left of the aircraft, the vertical tail is affected by the left wind, producing a force moving to the right, so the nose should yaw to the left, aligning with the incoming flow direction.

The wing focus describes the position of the point of application of the wing lift increment. The whole-aircraft focus is the point of application of the whole aircraft's lift increment after considering all aerodynamic components (wing, tail, fuselage, etc.). In a normally configured aircraft, the tail is usually behind the wing and is designed to provide a certain download in flight to help balance the lift produced by the wing and maintain aircraft stability. Because the download produced by the tail actually reduces the whole aircraft's lift increment, and the point of application of this download lies behind the wing, the whole-aircraft focus usually moves aft relative to the wing focus.

An aircraft's directional damping moment is a moment produced during yaw motion, opposite to the yaw direction. It is mainly produced by the aircraft's vertical tail: when the aircraft yaws, the aerodynamic force on the vertical tail produces a lateral force, which in turn forms a moment opposite to the yaw direction, i.e., the directional damping moment. The directional damping moment helps the aircraft quickly return to stable flight after a disturbance, reducing oscillation in yaw motion and improving directional stability.

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.