China Civil UAV Theory Questions — Questions 669–678

Ground speed is simply the aircraft's speed relative to the ground: a tailwind makes it faster over the ground; a headwind makes it slower. All three options have drift decreasing, so the test point is ground speed. By vector addition, ground speed = airspeed + wind speed. With a wind angle of 0°–90° (quartering tailwind), wind speed WS splits into a crosswind component WS2 perpendicular to track and a tailwind component WS1 along track: there is drift, ground speed exceeds true airspeed, and when wind speed decreases, ground speed also decreases. With a wind angle of 90°–180° (quartering headwind), WS splits into crosswind WS2 and headwind WS1: there is drift, ground speed is below true airspeed, and when wind speed decreases, ground speed increases.

Ground speed is the aircraft's speed relative to the ground: a tailwind makes it faster; a headwind makes it slower. When wind speed increases, drift must increase, eliminating C. Whether ground speed increases depends on wind angle. By vector addition, ground speed = airspeed + wind speed. With a wind angle of 0°–90° (quartering tailwind), wind splits into crosswind and tailwind components, ground speed exceeds true airspeed, and increased wind speed increases ground speed — hence A. (Translator note: the source explanation reuses the decrease-case reasoning from the previous question verbatim; the conclusion is unaffected.)

Ground speed is simply the aircraft's speed relative to the ground: a tailwind makes it faster over the ground; a headwind makes it slower. All three options have ground speed increasing, so the test point is how drift changes. Drift is the angle by which the track deviates from the heading line, ranging from 0 to ±180°. Its size is the angle between airspeed (heading) and ground speed (track).

The position of a UAV's center of gravity (CG) is very important for its stability and safety. The center of gravity is the point at which, if the UAV were hung from that point, it would balance. Trimming a UAV mainly considers the position of the center of gravity along the longitudinal axis. The center of gravity is not necessarily a fixed point; its position depends on how weight is distributed on the UAV. Study note: in flight, changes in speed, center of gravity, and aerodynamic shape inevitably cause moment imbalance, which the trim system compensates for.

In the glide state, the engine is at low throttle, idling, or shut down. Gravity acts vertically downward, but it can be resolved into a component along the flight direction and a component perpendicular to the flight direction. The resultant of the air's reaction force can be approximately resolved into lift perpendicular to the flight direction and drag opposite to the flight direction.

In steady-speed flight, lift balances weight and thrust balances drag: lift = weight, thrust = drag.

In stable motion, an object moves uniformly in one direction, with velocity V a fixed value not equal to 0; at rest, velocity V equals 0. In an unstable state of motion, velocity V changes, so there is acceleration a.

When the angle of attack exceeds the critical angle of attack, airflow over the upper surface of the wing separates severely, lift drops sharply, and the lift coefficient decreases rapidly. Options A and C do not match how the lift coefficient changes beyond the critical angle of attack.

Study note: wing above, tail below. For a conventional-layout aircraft, the wing's center of pressure lies behind the center of gravity, giving the nose a pitch-down moment, which must be countered by a pitch-up moment from the horizontal tail.

Study note: wing above, tail below. For a conventional-layout aircraft, the wing's center of pressure lies behind the center of gravity, giving the nose a pitch-down moment, which must be countered by a pitch-up moment from the horizontal tail.