China Civil UAV Theory Questions — Questions 889–898

Study note: two balances. When an aircraft flies in the air, its state of equilibrium is determined jointly by the external forces and external moments acting on it. Force balance: when an aircraft is in equilibrium, the external forces in three directions (longitudinal, lateral, and vertical) must remain balanced. This means the aircraft's thrust (or pull) must equal drag, lift must equal weight, and any lateral force must also remain balanced. Only then can the aircraft maintain a stable flight speed and altitude without accelerating, decelerating, climbing, or descending. Moment balance: external moments are the rotational effects produced by external forces acting on the aircraft. When an aircraft is in equilibrium, the moments produced by all external forces must also remain balanced. This ensures the aircraft does not rotate about its longitudinal, lateral, or vertical axis.

The textbook landing stages are descent, flare, float, touchdown, and landing roll — four stages; splitting float and touchdown gives five stages, so B is correct.

When trailing-edge flaps are lowered, wing area and camber increase, raising the wing lift coefficient, so the aircraft can still maintain enough lift at low speed, ensuring a safe landing. At the same time, lowering the flaps also increases drag, helping deceleration. Options A and B are not the main functions of trailing-edge flaps at landing.

MAC, also called mean aerodynamic chord, is the reference for both aircraft center-of-gravity and focus positions. The position of the center of gravity relative to MAC is a commonly used expression method, given as the percentage of MAC total length represented by the distance from the center of gravity to the MAC leading edge.

Study note: longitudinal goes with the lateral axis and recovery to the original attitude. Aircraft longitudinal stability mainly concerns how an aircraft's pitch angle changes after external disturbances such as vertical convection, and the aircraft's recovery ability after the disturbance disappears. Such disturbances usually cause the aircraft to pitch about its lateral axis. Option C, automatically recovering its original flight attitude, matches the description of longitudinal stability.

Study note: for longitudinal, look for horizontal. The longitudinal damping moment is a moment used in aircraft flight to stabilize aircraft attitude and prevent unnecessary oscillation. The horizontal tail is the aircraft's main stabilizing and control surface in the longitudinal (i.e., pitch) direction. When the aircraft is disturbed, for example by a gust causing the nose to rise or fall, the elevator on the horizontal tail automatically adjusts, producing an opposing moment that returns the aircraft to its original flight attitude.

Constant-speed straight-line level motion shows the aircraft is currently in equilibrium. Options A, B, and C are all correct, and C is relatively the most complete. When an aircraft flies in the air, its state of equilibrium is determined jointly by the external forces and external moments acting on it. Force balance: when an aircraft is in equilibrium, the external forces in three directions (longitudinal, lateral, and vertical) must remain balanced. This means the aircraft's thrust (or pull) must equal drag, lift must equal weight, and any lateral force must also remain balanced. Only then can the aircraft maintain a stable flight speed and altitude without accelerating, decelerating, climbing, or descending. Moment balance: external moments are the rotational effects produced by external forces acting on the aircraft. When an aircraft is in equilibrium, the moments produced by all external forces must also remain balanced. This ensures the aircraft does not rotate about its longitudinal, lateral, or vertical axis.

Study note: angle of incidence goes with the longitudinal axis; quarter-chord goes with sweep angle; sideslip angle goes with the plane of symmetry; climb angle goes with the climb path. The sideslip angle is the angle between the airspeed vector and the aircraft's plane of symmetry, describing how far the aircraft's plane of symmetry deviates from the relative airflow direction. Option A is the definition of pitch angle; option B is the definition of climb or descent angle.

The lift formula is L = 1/2 ρv²SCl, and the drag formula is D = 1/2 ρv²SCd (where S is wing area). When wing area S increases, both lift and drag increase accordingly. Options A and B do not match how drag changes.

Option A is wrong: a stall is caused by an excessive angle of attack leading to a drop in lift coefficient, and increasing engine power alone cannot overcome it. Option B is wrong: in subsonic flight, too low a speed can also cause a stall because the angle of attack becomes too large; it is not only high-angle-of-attack stalls that occur. Option C is correct: a high angle of attack easily exceeds the critical angle of attack, and in high-speed flight, improper operation making the angle of attack too large can also cause a stall.