China Civil UAV Theory Questions — Questions 919–928

The body coordinate system is a very important reference frame, used to describe the positions and motion of aircraft components relative to the aircraft itself. The origin of the body coordinate system is usually chosen at the aircraft's center of gravity, because the center of gravity is the mean position of the aircraft's mass and the center of force and moment action on the aircraft. In aircraft design and flight principles, the plane of symmetry is a key concept, usually referring to a longitudinal plane of the aircraft that divides the aircraft into left-right symmetric halves. The longitudinal and vertical axes are chosen to determine the plane of symmetry. Geometrically, the aircraft's fuselage, wings, tail, and other main components are usually arranged symmetrically along the aircraft's longitudinal direction (i.e., the longitudinal axis). From a flight-dynamics perspective, aircraft motion is mainly rotation about its longitudinal and vertical axes.

Directional stability, also called yaw stability, is the ability of an aircraft in flight, after a small disturbance disrupts its directional equilibrium, to automatically tend to return to its original state of equilibrium once the disturbance disappears. This stability is mainly produced by the aircraft's vertical tail; the directional stabilizing moment and damping moment produced by the vertical tail help the aircraft keep its sideslip angle unchanged and make it act like a weathervane, with the nose automatically aligning with the airflow direction.

Study note: look for the two wings. In aircraft flight, pitch balance is the balance state in the vertical axis, i.e., the aircraft's tendency to pitch up or down. This balance state results from several moments acting together, but the most important moments are usually associated with the wing and the horizontal tail. For a conventional-layout aircraft, the wing produces lift, and this lift produces a nose-down moment about the aircraft's center of gravity, while the horizontal tail, by adjusting its angle of attack, produces a downward pressure that creates a nose-up moment about the aircraft's center of gravity, canceling the wing's nose-down moment.

During a dive pull-out, the aircraft moves along a curve, and the horizontal component of lift provides the centripetal force. Option B, lift equaling weight, is the level-flight case; option C, lift greater than weight but never unchanged, is wrong because lift varies with attitude rather than remaining unchanged.

Balanced external loads mean the aircraft is in stable flight, in uniform straight-line motion, but this motion can be uniform climb, descent, or horizontal motion.

Increasing wing camber increases the curvature of the upper wing surface, speeding up airflow over the upper surface at the wing leading edge. By Bernoulli's principle, faster flow lowers pressure, increasing the pressure difference between the upper and lower wing surfaces, thereby increasing lift. Option A, boundary-layer state, has no direct connection with increased camber; option C, airflow speed at the wing trailing edge, is not the main reason increased camber increases lift.

The vertical stabilizer is the fixed part of the vertical tail. The vertical tail mainly serves directional balance, mainly affecting directional stability. But none of the options here is increased directional stability. The vertical stabilizer does not produce lift, so option A is excluded. Changes in longitudinal stability mainly show as pitch motion, affected by the horizontal stabilizer rather than the vertical stabilizer, so option C is excluded. During aircraft rolling motion, the vertical stabilizer is perpendicular to the rolling direction; increasing vertical stabilizer area increases drag during rolling, thereby increasing lateral stability. So B is chosen.

When sideslipping, part of the aircraft's lift is used to counter the crosswind, so total aircraft lift decreases, causing the lift-to-drag ratio to drop. Options A and B both use sideslip to correct, which lowers the lift-to-drag ratio. Therefore the correct option is C. Key point — crosswind takeoff and landing routes: Sideslip method: sideslip is a commonly used correction method in crosswinds, countering wind influence by banking toward the wind direction to create resistance to the wind. However, in sideslip, part of the aircraft's lift is used to counter the crosswind, reducing total aircraft lift and thus affecting the lift-to-drag ratio. Heading-change method: the heading-change method mainly adjusts the aircraft's heading to maintain a certain angle with the runway, thereby countering crosswind influence. However, relying on heading change alone may not fully correct drift, especially in strong crosswinds, when the heading-change method must be combined with the sideslip method. Combined heading-change and sideslip method: this method combines the advantages of heading change and sideslip. Heading change can align the aircraft with the runway to a certain extent, reducing crosswind influence, while sideslip can further correct remaining drift. By combining them rationally, drift can be corrected while minimizing adverse effects on the lift-to-drag ratio.

Study note: sweep back to reduce noise.

Frost roughens the wing surface, disturbs airflow attachment, causes the boundary layer to separate early, increases stall speed, and makes a stall more likely.