China Civil UAV Theory Questions — Questions 869–878

Study note: of two short options and one long one, choose the longest. In aircraft flight, the compressibility of air is related to two main factors: the aircraft's flight speed (airspeed) and the local speed of sound where the aircraft flies. (1) Aircraft flight speed (airspeed): when the aircraft's flight speed approaches or exceeds the speed of sound, air compressibility becomes especially significant. This is because when the aircraft flies at near-sonic speed, air molecules ahead of the aircraft do not have enough time to move aside, forming a compressed air region, causing significant changes in air density, temperature, and pressure. (2) Local speed of sound where the aircraft flies: the speed of sound differs under different conditions (such as temperature and pressure). For example, at higher altitude and lower temperature, the speed of sound is lower. Therefore, the local speed of sound where the aircraft flies also affects air compressibility.

Aircraft directional stability mainly concerns how an aircraft's heading changes after external disturbances such as crosswind, and the aircraft's recovery ability after the disturbance disappears. A crosswind disturbance usually causes the aircraft to yaw about its vertical axis. Option B, automatically recovering its original flight attitude, matches the description of directional stability.

Study note: lateral damping goes with the wing; lateral rolling moment goes with lift. The rolling moment in an aircraft's lateral balance comes from the lift difference between the left and right wings. When left lift is greater than right lift, a right-rolling moment is produced; when right lift is greater than left lift, a left-rolling lift moment is produced. Wing drag is a force opposite to the aircraft's direction of advance and has little effect on lateral rolling. The horizontal tail is usually not a main factor in lateral rolling moments. The horizontal tail mainly affects nose-up and nose-down motion, not rolling motion.

Aircraft lateral stability concerns how an aircraft's rolling motion changes after a disturbance, i.e., rotation about its longitudinal axis, and the aircraft's recovery ability after the disturbance disappears. Option B, automatically recovering its original flight attitude, matches the description of lateral stability.

The aerodynamic focus, also called the aerodynamic center or neutral point, is the point of application of the resultant of the lift increments produced by the wing, tail, fuselage, and other aerodynamic components when the aircraft's angle of attack changes. 'Point of application of lift' describes the position where the lift acting on the whole wing applies. Because of the wing's shape and airflow, the wing produces lift, and this lift acts over the whole wing surface rather than concentrated at one point. Option A is imprecise in meaning. 'Point of application of additional lift' describes the wing focus's role more specifically. The focus is a specific point on the wing, indicating the position of the line of action of additional lift when the wing's angle of attack changes. Option B is correct.

The aerodynamic focus, also called the aerodynamic center or neutral point, is the point of application of the resultant of the lift increments produced by the wing, tail, fuselage, and other aerodynamic components when the aircraft's angle of attack changes. For a given aircraft shape, this point is fixed and unchanging.

In a constant-speed dive pull-out, the magnitude of speed is unchanged but its direction changes, because centripetal force (the horizontal component of lift) changes the direction of velocity, so option C is correct. Option A is wrong because the direction of velocity changes, so velocity changes; option B is wrong because under balanced external loads, the aircraft should move in uniform straight-line motion, which does not match the curved motion of a dive pull-out.

Study note: shorter is better. Liftoff speed is the minimum speed at which an aircraft can leave the ground during the takeoff roll. Ground roll distance is the runway length needed for the aircraft to accelerate from rest to liftoff speed. The smaller the liftoff speed, the shorter the distance needed to accelerate from 0 to liftoff speed at a given acceleration, reflecting better takeoff performance.

The climb angle is determined by excess thrust (thrust minus drag) and aircraft weight. The greater the excess thrust and the smaller the aircraft weight, the larger the climb angle. Options A and B alone cannot determine the size of the climb angle.

Study note: altitude, fuel consumption, and fuel quantity all affect level-flight range, with fuel quantity having the greatest effect. An aircraft's range is directly limited by the fuel it carries. In theory, the more fuel, the farther the aircraft can fly. Therefore, the amount of usable fuel for level flight has a decisive effect on range. Altitude mainly affects air density and the aircraft's lift-to-drag ratio, thus affecting fuel efficiency. Engine fuel efficiency relates to how long or how far the aircraft can fly on a given amount of fuel. An engine with high fuel efficiency can provide a longer range on the same fuel.