China Civil UAV Theory Questions — Questions 909–918

A lift-reducing device is equipment fitted on an aircraft mainly to reduce wing lift or slow the aircraft. Common lift-reducing devices include spoilers and speed brakes. Spoilers are usually fitted only on the upper surface of the wing, and they not only increase drag but also reduce lift. Speed brakes are located on the upper and lower surfaces of the wing and on the fuselage, and only increase drag. The aileron is one of the aircraft's primary control surfaces, used to control rolling, not lift reduction. The leading-edge slat is a lift-increasing device, used to raise the lift coefficient, especially at low speed, not lift reduction.

Lift-increasing devices raise the lift coefficient, including leading-edge slats, leading-edge flaps, trailing-edge flaps, and others. The leading-edge flap is a lift-increasing auxiliary control surface. Option A, the spoiler, is a lift-reducing device; option B, the aileron, is a primary control surface used to control roll, not a lift-increasing device.

Study note: range goes with cruise speed. Level-flight cruise speed is the best speed chosen for long-distance flight, to cover the greatest distance on a given amount of fuel.

Study note: endurance goes with favorable speed. At the level-flight favorable speed, the lift-to-drag ratio is greatest, flight efficiency is highest, and endurance is longest.

When a spoiler opens upward, drag increases, and it can shorten the ground-roll distance on landing, so statement A is correct. The aileron is a primary control surface, and the spoiler is only an auxiliary control surface. The aileron is specially designed to change the lift distribution of the left and right wings, thereby achieving aircraft roll and lateral control. A spoiler's main function is to increase drag and reduce lift, not to control aircraft lateral motion as precisely as an aileron. Therefore statement B is correct and statement C is wrong.

In flight, or during takeoff, landing, or ground movement, forces exerted on the aircraft by other objects are called aircraft external loads, including gravity, lift, thrust, drag, and other aerodynamic forces. Gravity is the Earth's gravitational force acting on the aircraft; aerodynamic force is the force of relative air motion acting on the aircraft. Inertial force is not actually a real force, but an object's tendency, due to inertia, to maintain its original state of motion. In aircraft force analysis, inertial force is usually not regarded as an external load, but is used to describe the effect produced when an aircraft's state of motion changes.

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: for longitudinal, look for horizontal. The horizontal tail is the aircraft's main stabilizing and control surface in the longitudinal (i.e., pitch) direction. When an aircraft flies level, the horizontal stabilizer produces no additional moment on the aircraft; when the aircraft is disturbed to pitch up, the aerodynamic force acting on the horizontal stabilizer then produces a nose-down moment, returning the aircraft to level flight; likewise, if the aircraft pitches down, the moment produced by the horizontal stabilizer pitches the aircraft up, until level flight is restored. Increasing horizontal-tail area can increase longitudinal stability.

With the aircraft center of gravity forward and the aerodynamic focus aft, longitudinal stability is good. With the center of gravity aft and the aerodynamic focus forward, longitudinal stability is poor. Further reading: why the aerodynamic-focus and center-of-gravity layout affects longitudinal stability. The focus is the point of application of aerodynamic force, especially where the change in aerodynamic lift produced when the aircraft's angle of attack changes acts. The aircraft's center of gravity is the point of application of the aircraft's weight. When the aircraft's center of gravity lies ahead of the aerodynamic focus, the aircraft is longitudinally statically stable. This is because when the aircraft is disturbed upward (such as by a gust or pilot operation), it produces a nose-down restoring moment that returns the aircraft to its original flight attitude. Conversely, if the aircraft's center of gravity lies behind the aerodynamic focus, the aircraft becomes longitudinally statically unstable. In that case, any upward disturbance causes the aircraft to produce a nose-up divergent moment, making its flight attitude deviate further and further from its original state.

Normal circling flight is flight along a circular path, requiring constant altitude and circular motion. Option C, maintaining constant-speed straight-line flight, is unrelated to the circular-motion character of circling flight, so it is correct.