China Civil UAV Theory Questions — Questions 679–688

UAV-Q0679. When a conventional-layout aircraft stalls, ________.

Explanation

Study note: in a stall, both forces decrease. In a stall, airflow separates from the upper surface of the aircraft, so lift drops sharply (though not to zero). In a conventional-layout aircraft, both the upward lift on the wing and the downward force on the tail decrease as airflow separates. Options B and C do not match how the forces change in a stall.

UAV-Q0680. Viewed from the tail toward the nose, the torque of a clockwise-rotating propeller makes the aircraft ________.

Explanation

A clockwise-rotating propeller exerts a counterclockwise reaction torque on the airframe, making the aircraft roll to the left.

UAV-Q0681. In a level turn at constant angular velocity, when airspeed increases, the turn radius ________.

Explanation

From the centripetal force formula F = m·v²/r (F is centripetal force, v is airspeed, r is turn radius), at constant angular velocity (v = ωr), when airspeed v increases, the turn radius r must increase. Options A and B do not match how the radius changes.

UAV-Q0682. When speed increases and induced drag decreases, ________.

Explanation

As speed increases, induced drag decreases while parasite drag increases, and skin friction drag increases. Induced drag decreases with increasing speed, while profile drag (form drag and friction drag) is proportional to the square of speed V and increases with speed.

UAV-Q0683. When a UAV's angle of attack is the critical angle of attack, ________.

Explanation

When the angle of attack is the critical angle of attack, airflow over the upper surface of the wing is about to separate but has not yet fully separated, and the lift coefficient reaches its maximum. Beyond this angle, separation worsens and the lift coefficient drops rapidly. Option A, greatest flight speed, is unrelated to the critical angle of attack; option C, smallest drag, usually occurs at the favorable angle of attack, not the critical angle of attack.

UAV-Q0684. For a UAV with flaps, when the flaps are lowered, the aircraft's stall speed will ________.

Explanation

When flaps are lowered, the wing lift coefficient increases, so the aircraft can produce enough lift at a lower speed, and therefore the stall speed decreases. Stall speed is the minimum speed at which the aircraft can maintain flight, and a larger lift coefficient lowers this speed. Options A and C do not match how stall speed changes when flaps are lowered.

UAV-Q0685. For a UAV with flaps, when the flaps are lowered, the aircraft's lift will ________ and its drag will ________.

Explanation

Lowering the flaps increases wing area and camber, increasing lift; at the same time, interaction between the wing and the air strengthens, so drag also increases. The lift and drag combinations in options A and C are incorrect.

UAV-Q0686. For gliders and aircraft in a descent, the relationship between lift and weight is ________.

Explanation

For gliders and aircraft in a descent, lift and weight are not exactly equal: L = Wg·cos α. However, for dive or climb angles generally under 10°, the two are not much different.

UAV-Q0687. In control-surface remote control, in level flight, when the elevator stick is pulled slightly back, the aircraft's angle of attack ________.

Explanation

Study note: pushing forward slightly decreases the angle of attack, pulling back slightly increases it. In control-surface remote control, in level flight, pushing the elevator stick slightly forward deflects the elevator downward. The tail then produces an upward force, and the nose pitches down accordingly. As the nose pitches down, the aircraft's angle of attack (the angle between the wing chord and the relative airflow) decreases. (Translator note: the source explanation describes the forward-push case; the pull-back case is its opposite, giving the correct answer A.)

UAV-Q0688. In control-surface remote control, in level flight, when the elevator stick is pushed slightly forward, the aircraft's angle of attack ________.

Explanation

In control-surface remote control, in level flight, pushing the elevator stick slightly forward deflects the elevator downward. The tail then produces an upward force, and the nose pitches down accordingly. As the nose pitches down, the aircraft's angle of attack (the angle between the wing chord and the relative airflow) decreases.