China Civil UAV Theory Questions — Questions 739–748

Symmetric wing: a symmetric airfoil is one with zero camber, i.e., upper and lower arcs symmetric. When a symmetric wing pitches up by an angle of attack relative to the incoming flow, the airflow strikes the lower surface of the leading edge, so the stagnation point moves toward the lower surface.

The larger the lift coefficient, the greater the lift. To simplify calculation, aerodynamicists combine all the very complex factors of wing shape and trim into the lift coefficient, to describe the lift produced by an aircraft or any of its parts. The larger the lift coefficient, the greater the lift it can produce.

Study note: for a stall, look for the critical angle of attack. The direct cause of a stall is an excessive angle of attack, beyond the critical angle of attack, causing airflow over the upper surface of the wing to separate and lift to drop sharply. Whether in low-speed or high-speed flight, once the angle of attack exceeds the critical angle of attack, a stall occurs. Options A and B are only conditions that may cause an excessive angle of attack, not the direct cause of a stall.

The favorable glide speed is the speed at which an aircraft in a glide obtains its maximum glide distance. At the favorable glide speed, glide drag is smallest and the lift-to-drag ratio is greatest. This speed is calculated from aircraft performance, weight, atmospheric conditions, and other factors, aiming to let the aircraft cover the greatest distance safely and economically before landing.

Study note: changing angle of attack controls lift, airspeed, and drag. Thrust is mainly produced by the aircraft's power system, and weight is an inherent (or slowly changing) property of the aircraft; neither changes with angle of attack. In flight, the angle of attack is the angle between the wing chord and the relative airflow. By adjusting the angle of attack, the remote pilot can affect several flight parameters. (1) Aircraft lift is mainly produced by the wing, and wing lift is directly related to angle of attack: as angle of attack increases, the wing lift coefficient also increases, increasing lift. (2) Drag includes several parts, one of which is induced drag produced by the wing. When angle of attack changes, induced drag changes accordingly. In addition, a larger angle of attack may bring the aircraft close to or into a stall, at which point drag increases sharply. (3) Airspeed is the aircraft's speed relative to the air, usually determined jointly by aircraft power and flight attitude (including angle of attack). However, changing the angle of attack affects the lift-to-drag ratio, thus affecting flight performance at a given airspeed.

A simple memory aid: in fluid flow, low-speed regions have high pressure and high-speed regions have low pressure. Since the mass of fluid flowing into and out of the duct per unit time is the same, in the converging section where the cross-section narrows, the flow velocity V must increase to discharge the same mass of liquid, and when flow velocity increases, the pressure of the liquid in that region decreases.

By elimination: 'convert potential energy to kinetic energy and dive nose-down' — the aircraft loses altitude and falls rapidly to the ground. 'Convert the remaining kinetic energy to potential energy, maintaining a nose-up attitude and a safe altitude' — once kinetic energy is exhausted, the aircraft ultimately cannot maintain a safe altitude.

Study note: elevator — lateral axis; rudder — vertical axis; aileron — longitudinal axis. A UAV's rudder is usually located at the tail, on either side of the vertical stabilizer. When the pilot operates the rudder, it deflects left or right, producing a lateral force that yaws the nose left or right. This yaw motion is movement about the aircraft's vertical axis.

Study note: elevator — lateral axis; rudder — vertical axis; aileron — longitudinal axis. A UAV's elevator is a movable surface on the trailing edge of the horizontal stabilizer at the tail. When the pilot operates the elevator, it deflects up or down, changing the aircraft's pitch attitude, i.e., the nose pitches up or down. This pitch motion is movement about the aircraft's lateral axis.

Study note: elevator — lateral axis; rudder — vertical axis; aileron — longitudinal axis. A UAV's ailerons are small controllable surfaces on the outer trailing edges of the wings. When the pilot operates the ailerons, they deflect in opposite directions, one side up and the other side down. The resulting force difference causes the aircraft to roll to one side. This roll motion is movement about the aircraft's longitudinal axis.