China Civil UAV Theory Questions — Questions 779–788

Here the polar curve is the curve of aircraft lift coefficient versus drag coefficient. The highest point's ordinate on the curve represents the maximum lift coefficient. A line parallel to the ordinate tangent to the curve gives the minimum drag coefficient and its angle-of-attack value. Option A: the maximum angle of attack usually means the largest angle the wing can reach before stalling, which has no direct relation to the tangent point from the origin. Option B: the favorable angle of attack usually means the angle of attack corresponding to the maximum lift-to-drag ratio, and this point is related to the tangent to the polar curve from the origin, because the tangent point corresponds to the maximum lift-to-drag ratio, i.e., the least drag encountered when the aircraft produces a given lift. Option C: the minimum angle of attack has no clear definition and is usually not a meaningful point in the context of a polar curve.

When speed v decreases, to balance lift against weight, the lift coefficient must be increased, which can be done by increasing the angle of attack to raise the lift coefficient.

When fluid flows in a duct, where flow speed increases, static pressure decreases; conversely, where flow speed decreases, static pressure increases.

Study note: when you see high speed, choose low static pressure. When fluid flows in a duct, where flow speed increases, static pressure decreases; conversely, where flow speed decreases, static pressure increases.

When the angle of attack reaches the critical angle of attack, airflow over the upper surface of the wing begins to separate, and lift suddenly decreases greatly. At the same time, airflow separation increases the pressure difference fore and aft on the wing, so pressure-difference drag rises sharply and total drag increases rapidly. Options A and C do not match how lift and drag change at this point.

Study note: when you see high speed, choose low static pressure. When fluid flows in a duct, where flow speed increases, static pressure decreases; conversely, where flow speed decreases, static pressure increases.

Microburst airflow (micro downburst airflow) causes vertical wind shear, making the aircraft suddenly rise or fall, which may result in too low an altitude or a stall, affecting takeoff and landing safety.

Because the upper and lower surfaces are asymmetric, a cambered airfoil still has lift at zero angle of attack. Because of the airfoil's camber, the flow characteristics of the upper and lower surfaces differ, showing as streamlines over the upper surface being denser than those over the lower surface, with faster flow speed.

Study note: small negative angle. Asymmetric airfoil: an airfoil whose upper and lower surfaces are not symmetric. Angle of attack: the angle between the relative airflow and the wing chord. Zero-lift angle of attack: the aerodynamic zero point of the airfoil, the angle of attack at which no lift is produced. Because the airfoil's upper and lower surfaces are asymmetric, at zero angle of attack, affected by the airfoil's camber, airflow over the upper surface is faster than over the lower surface, so lift is positive. To make lift zero at this point, the angle of attack must be turned negative. For a symmetric airfoil, the zero-lift angle of attack is zero angle of attack.

Zero-lift angle of attack: the aerodynamic zero point of the airfoil, the angle of attack at which no lift is produced.