China Civil UAV Theory Questions — Questions 789–798

Leading-edge icing on an aircraft increases surface roughness, causing more turbulence and separation in the airflow over the wing surface, so the lift coefficient at the same angle of attack decreases. Option A is wrong because the weight of ice has a small effect on the aircraft; option B is wrong because icing decreases the critical angle of attack, making the aircraft more prone to stall.

Study note: pressure-difference drag goes with body shape and frontal area; interference drag goes with fairings, relative positions of components, and rational layout; induced drag goes with aspect ratio and winglets; friction drag goes with surface finish. Fairings streamline the disturbed, irregular airflow at fuselage junctions, reducing interference drag.

Friction drag is mainly produced by the friction between air flowing over the aircraft surface and the aircraft surface. This friction depends on air viscosity, aircraft surface roughness, and the area of aircraft surface in contact with the air.

Friction drag is mainly produced by the friction between air flowing over the aircraft surface and the aircraft surface. This friction depends on air viscosity, aircraft surface roughness, and the area of aircraft surface in contact with the air.

The whole-aircraft center of pressure is the intersection of the line of action of the total aerodynamic force on the aircraft with the aircraft's longitudinal axis, also called the aerodynamic focus.

Lift is proportional to density ρ, proportional to velocity squared v², proportional to wing area S, and proportional to the lift coefficient.

Since the lift formula and the drag formula are similar, the lift-to-drag ratio can be regarded as a comparison of the lift coefficient and the drag coefficient, and angle of attack affects both the lift coefficient and the drag coefficient. Because lift L = 1/2 × ρ × v² × S × Cl and drag D = 1/2 × ρ × v² × S × Cd, the lift-to-drag ratio L/D = Cl/Cd.

Study note: of two short options and one long one, choose the longest. Airflow over the upper and lower surfaces of the wing creates a pressure difference, producing lift; therefore the direction of lift is perpendicular to the relative airflow velocity.

Stall speed is the minimum speed at which an aircraft maintains flight. The greater an aircraft's load, the greater the lift needed to overcome weight; at the same speed, a greater angle of attack is needed, making it easier to reach the critical angle of attack, so stall speed increases. Option B, 'the greater the angle', is unclearly worded, and stall speed is related to weight, so option C is wrong.

Spoilers are an important control surface on an aircraft, usually located on the upper surface of the wing. When spoilers extend, they disrupt airflow over the upper surface of the wing, thereby reducing lift and producing additional drag.