China Civil UAV Theory Questions — Questions 709–718

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 pushed, the angle of attack briefly decreases, lift decreases, and the flight path turns downward. When the flight path is in a steady descent, the angle of attack regains its original size, and lift and weight balance again. This question refers to the angle of attack in a steady descent, which remains greater than zero.

An aircraft can take off successfully only when wing lift reaches the lift needed for takeoff; with lift coefficient, air density, and other conditions the same, the aircraft must reach a fixed airspeed V. Speed of a moving object is a vector: airspeed + wind speed = ground speed. When accelerating to the same ground speed, the headwind case reaches the required takeoff airspeed first, so the required ground roll distance is smallest.

Study note: smaller angle of attack, center of pressure aft; larger angle of attack, center of pressure forward. The angle of attack is the angle between the wing chord and the relative airflow direction, and it affects airflow distribution and lift production on the wing. When the angle of attack decreases, the difference between airflow speed over the upper surface and under the lower surface shrinks, so lift decreases. The center of pressure, as the equivalent point at which lift acts on the wing, moves as the pressure distribution on the wing changes. When the angle of attack decreases, the lift contribution near the leading edge relatively decreases, so the equivalent point of lift application (the center of pressure) moves aft.

Study note: smaller angle of attack, center of pressure aft; larger angle of attack, center of pressure forward. The angle of attack is the angle between the wing chord and the relative airflow direction. As the angle of attack increases, the difference between airflow speed over the upper surface and under the lower surface grows, so lift increases. The center of pressure, as the equivalent point at which lift acts on the wing, moves as the pressure distribution on the wing changes. In most cases, when the angle of attack increases, the center of pressure moves forward, because as the angle of attack increases, the lift contribution near the leading edge relatively increases, moving the equivalent point of lift application forward.

When an aircraft is in ground effect, airflow under the wing is blocked by the ground, its speed increases and pressure decreases, increasing the pressure difference between the upper and lower surfaces of the wing, so lift increases; at the same time, the ground obstructs downwash, so induced drag decreases. Options B and C do not match the aerodynamic changes in ground effect.

When an aircraft enters ground effect on landing, induced drag decreases and lift increases. To maintain a normal landing state, power must be reduced to balance the decrease in drag. Option A, a nose-up change, is not a necessary result of ground effect; option C is wrong because, since lift increases, there is no need to increase angle of attack to maintain lift coefficient.

In an aircraft turn, the horizontal component of lift acts as centripetal force, making the aircraft follow a curved path. Option A, aircraft thrust, overcomes drag and does not provide centripetal force; option B, the aerodynamic force on the rudder, controls yaw and is unrelated to centripetal force.

In an aircraft turn, the outer aileron deflects downward, its angle of attack increases, and lift increases; at the same time, the outer wing moves faster through the air, so its lift is greater. Outer lift greater than inner lift causes the bank angle to tend to keep increasing. Options A and C, about drag differences, are not the main cause of the increasing bank angle.

In a turn, the vertical component of lift decreases as bank increases; increasing the angle of attack increases total lift, keeping the vertical component unchanged to balance weight and maintain altitude. Options B and C are not the reason for increasing the angle of attack.

Study note: rolling left and right with strong lateral and weak directional stability means Dutch roll; weak lateral and strong directional stability means spiral instability. (1) Strong directional stability and weak lateral stability lead to spiral instability; (2) weak directional stability and strong lateral stability lead to Dutch roll.