China Civil UAV Theory Questions — Questions 769–778

Study note: for a stall, look for the critical angle of attack. An aircraft stall means that during flight, for some reason, its lift suddenly decreases so it cannot maintain normal flight, usually caused by too low a speed or an attitude that makes the angle of attack too large. When the aircraft's angle of attack (the angle between the wing and the incoming flow direction) increases to a certain point, airflow over the upper surface of the wing begins to separate and lift drops sharply; this angle of attack is called the critical angle of attack. Once the angle of attack exceeds this value, the aircraft enters a stall.

Study note: for a stall, look for the critical angle of attack. 'Stall angle of attack', i.e., 'critical angle of attack', means the angle of attack at which the aircraft's lift coefficient is greatest. When the angle of attack reaches this value, airflow over the upper surface of the wing begins to separate; if the angle of attack continues to increase, the lift coefficient drops sharply and the aircraft enters a stall. Options A and B are unrelated to the definition of the stall angle of attack.

A dirty wing surface causes air flowing over the wing surface to experience greater friction, increasing friction drag and the drag coefficient. A dirty wing surface, because of increased surface roughness, causes more turbulence and separation in the airflow over the wing surface, reducing the wing's effective area and lift coefficient. To obtain the same lift coefficient, a dirty-surfaced wing needs a larger angle of attack.

Study note: ideal, incompressible, no energy exchange. The three conditions of Bernoulli's equation are: (1) ideal fluid (no viscosity); (2) the fluid must be incompressible; (3) no energy exchange with the outside during flow.

Study note: ideal, incompressible, no energy exchange. The three conditions of Bernoulli's equation are: (1) ideal fluid (no viscosity); (2) the fluid must be incompressible; (3) no energy exchange with the outside during flow. High-speed gas is compressed, so Bernoulli's equation does not apply.

High humidity and temperature-difference changes accelerate corrosion; changes in air pressure do not directly take part in electrochemical corrosion and do not affect the corrosion rate.

Air density is an external environmental factor, not a property of the wing itself, and does not affect the lift coefficient.

The chord is the length of the line from the leading edge to the trailing edge of the wing, an important parameter of the wing cross-section. Option A is how wingspan is measured; option B is not how chord is measured.

Wingspan is measured as the straight-line distance from the left wingtip to the right wingtip, reflecting the lateral extent of the wing. Option B is how semi-span is measured; option C is how chord is measured.

A laminar-flow airfoil is designed to keep laminar flow over a large area of the wing surface to reduce drag. Compared with ordinary airfoils, a laminar-flow airfoil's maximum thickness is positioned farther toward the trailing edge, its leading-edge radius is smaller, and its upper surface is fairly flat, allowing the wing surface to maintain laminar flow as much as possible, thus reducing friction drag. The basic principle of a laminar-flow airfoil is to keep accelerating the airflow over as long a distance as possible before it reaches the pressure-rise region near the trailing edge of the wing, delaying transition from laminar to turbulent flow.