China Civil UAV Theory Questions — Questions 729–738

Study note: negative stability means continued divergence with ever-growing amplitude; other cases mean recovery to the original flight attitude. Static stability of an aircraft means whether, after an instantaneous external disturbance moves it from equilibrium, it has a tendency in the first instant to automatically return to its original state of equilibrium. If the aircraft tends to return to its previous position, it is said to have static stability; if it continues to depart, it is said to be statically unstable (or to have negative stability); if it tends to remain in the disturbed position, it is said to be neutrally stable.

Study note: negative stability means continued divergence with ever-growing amplitude; other cases mean recovery to the original flight attitude. Static stability of an aircraft means whether, after an instantaneous external disturbance moves it from equilibrium, it has a tendency in the first instant to automatically return to its original state of equilibrium. If the aircraft tends to return to its previous position, it is said to have static stability; if it continues to depart, it is said to be statically unstable (or to have negative stability); if it tends to remain in the disturbed position, it is said to be neutrally stable.

An aircraft's center of gravity is the equivalent point of application of all of the aircraft's weight, while the center of pressure is the equivalent point of application of the aircraft's lift. To maintain longitudinal stability, the center of gravity usually needs to lie ahead of the center of pressure. Then, when the aircraft is disturbed upward, the layout with the center of gravity ahead of the center of pressure produces a downward moment that helps the aircraft return to its original flight attitude. A memory example: a shuttlecock or dart, heavy at the front and light at the tail, has good longitudinal stability.

The physical properties of air include its viscosity and compressibility. (1) Viscosity is a physical property of a fluid, expressing the fluid's ability to resist shear deformation. When a fluid flows, fluid layers stick to or drag on each other, and the internal friction produced by this viscous action is the fluid's viscosity. Although air is a fluid of low viscosity, it still has viscosity. (2) The compressibility of air is the property of a gas to shrink in volume under pressure. Because air molecules are relatively far apart, air is much more compressible than a liquid. When gas pressure increases, the average distance between gas molecules decreases, so volume shrinks.

When airspeed decreases, lift at the same angle of attack decreases, so the angle of attack must be increased to raise the lift coefficient and keep lift unchanged, maintaining altitude. Option A, increasing lift more than drag increases, is not the purpose; option C, decreasing the angle of attack, would make lift even smaller and cannot maintain altitude.

Study note: as temperature rises, liquid viscosity falls and gas viscosity rises. Temperature is an important factor affecting fluid viscosity. Note especially that the trend of liquid viscosity versus gas viscosity with temperature is opposite: liquid viscosity decreases as temperature rises, while gas viscosity increases as temperature rises. (1) As temperature rises, the average distance between liquid molecules increases, reducing internal friction in the liquid, so viscosity decreases. (2) As temperature rises, the average speed of gas molecules increases, raising the collision frequency between gas molecules, so gas viscosity increases.

When deflecting the ailerons for a right turn, the left aileron deflects downward, drag increases, and the nose yaws left (adverse yaw). Deflecting the rudder to the right produces a rightward moment that corrects the adverse yaw. Options A and C cannot correct adverse yaw.

When deflecting the ailerons in a turn, the outer aileron deflects downward, its angle of attack increases, and drag increases; at the same time, the outer wing moves faster through the air, so its drag is greater. Therefore outer-wing drag is greater than inner-wing drag. Options A and C do not match the drag distribution.

When deflecting the ailerons for a left turn, the right aileron deflects downward, drag increases, and the nose yaws right (adverse yaw). Deflecting the rudder to the left produces a leftward moment that corrects the adverse yaw. Options B and C cannot correct adverse yaw.

When an aircraft stalls, its angle of attack has exceeded the critical angle of attack; pushing the stick reduces the angle of attack, helping airflow reattach to the wing surface and restoring lift. Option A, pushing the rudder, mainly controls yaw and cannot recover from a stall; option C, pulling the stick, increases the angle of attack and worsens the stall, so it is wrong.