China Civil UAV Theory Questions — Questions 699–708

The angle between the wing's direction of advance (equivalent to the airflow direction) and the chord (which differs from the fuselage axis) is called the angle of attack, also called the angle of incidence.

Study note: lateral goes with the longitudinal axis, heading goes with the vertical axis, longitudinal goes with pitch. Static stability categories: (1) longitudinal static stability: prevents the aircraft from pitching too high or diving too low; (2) lateral static stability: prevents the aircraft from rolling too fast about the longitudinal axis; (3) directional static stability: prevents the aircraft from yawing too fast about the vertical axis.

When an aircraft enters a spin, it rolls about the vertical axis. Applying rudder opposite to the spin direction produces an opposing moment that stops the roll, and this is the conventional method to stop a spin. Option A, pushing the stick to recover from the stall, is an action after a stall, not the direct method to stop a spin; option C, increasing throttle to gain speed, does little to stop a spin.

The main cause of an aircraft spin is that after the aircraft stalls, one wing stalls first or airflow separation is asymmetric, producing a lateral rolling moment that causes the wing to autorotate, putting the aircraft into a spin. Options A and B are operating causes that may lead to a stall, but they are not the direct cause of the spin.

A vertical upward gust acting on the wing increases the static pressure under the wing, increasing lift.

As bank angle increases, lift tilts, its vertical component decreases and its horizontal component increases. The vertical component must balance weight, so as bank increases the angle of attack must be increased to keep the vertical component unchanged. Options A and C do not match how the vertical component changes.

As bank angle increases, the horizontal component of lift increases, providing greater centripetal force for the turn, so the turn radius decreases or the turn rate increases. Options B and C do not match how the horizontal component changes.

Study note: the angle of attack is greater than zero in both climb and descent. Normally, the angle of attack is greater than zero. The angle of attack is the angle between the wing's direction of advance (airflow direction) and the chord. When the stick is pulled, the angle of attack briefly increases, lift increases, and the aircraft begins to climb. When the flight path is in a steady climb, the angle of attack regains its original size, and lift and weight balance again.

Study note: for a stall, look for the critical angle of attack. The fundamental cause of an aircraft stall is an angle of attack beyond the critical angle of attack, at which point airflow over the upper surface of the wing separates severely and lift drops sharply. Option A, speed too low, may cause the angle of attack to increase to maintain lift and thus exceed the critical angle of attack, but low speed itself is not the direct cause of a stall; option B, speed too high, generally does not directly cause a stall unless improper operation makes the angle of attack too large.

In a level turn, a larger bank angle requires greater lift to provide centripetal force, so stall speed increases (stall speed is the minimum speed that produces enough lift). Options A and B do not match how stall speed changes as bank increases.