China Civil UAV Theory Questions — Questions 719–728

Study note: positive below, negative above. Bernoulli's principle states that in fluid flow, where flow speed is high, pressure is low, and where flow speed is low, pressure is high. When an aircraft moves forward, the wing cuts through the air; the upper surface of the wing has greater curvature, so airflow over it is fast and pressure decreases, while the lower surface has less curvature, so airflow over it is slower and pressure is higher. Therefore, the positive pressure on the lower surface of the wing and the pressure difference with the upper surface together produce lift.

In level flight, an aircraft's total lift equals its total weight, where total lift is written L and total weight = mass W × gravitational acceleration g, giving the formula L = Wg. This formula describes an aircraft in a steady state, but it does not apply to accelerating or decelerating descents or to accelerating or decelerating climbs.

In a steady climb, a fixed-wing aircraft is in equilibrium: lift equals weight and thrust equals drag. When transitioning from level flight into a climb, lift is briefly greater than weight, but in a stable steady climb, lift and weight are equal. Options B and C do not match the force balance in a steady climb.

Study note: primary surfaces are aileron, elevator, rudder; auxiliary surfaces are slats, flaps, trim tabs. The elevator, rudder, and all-moving horizontal tail are primary control surfaces, so options A and B are excluded.

Study note: primary surfaces are aileron, elevator, rudder; auxiliary surfaces are slats, flaps, trim tabs. An aircraft's primary control surfaces include the ailerons, elevator (or all-moving horizontal tail), and rudder, which control rotation about the longitudinal, lateral, and vertical axes respectively. Primary control surfaces are mounted on the trailing edges of the wing and tail. When deflected, they change the aerodynamic characteristics of the wing and tail, thereby controlling the aircraft.

Study note: any option saying 'all are fitted' is wrong. Vertical takeoff and landing: (1) vertical takeoff and landing is divided into rotorcraft vertical takeoff and landing and fixed-wing vertical takeoff and landing; (2) vertical takeoff and landing is the mainstream launch and recovery method for rotorcraft UAVs; (3) fixed-wing vertical takeoff methods include: using a tail stand to support the aircraft and taking off under engine power, and configuring an engine for vertical takeoff; (4) fixed-wing vertical landing methods include: fitting a dedicated landing engine and decelerating for landing under the combined thrust of the main engine and the dedicated engine, and recovering in a vertical attitude, decelerating for landing under the vertical component of engine thrust.

Study note: of two short options and one long one, choose the longest. Arresting-net recovery: (1) recovering UAVs with an arresting-net system is currently one of the more common recovery methods for small UAVs worldwide; (2) an arresting-net system usually consists of an arresting net, an energy absorption device, and automatic guidance equipment; (3) the energy absorption device is connected to the arresting net, and its function is to absorb the energy of the UAV hitting the net, so that the UAV does not keep bouncing on the net after contact and get damaged; (4) the automatic guidance equipment is generally a television camera placed behind the net, or an infrared receiver mounted on the arresting-net frame, which promptly reports deviations in the returning UAV's route to the ground station, for recovery with an arresting net.

A flat-bottom airfoil has a certain camber on its upper surface and a fairly flat lower surface, so the upper surface camber is greater than the lower surface camber.

Study note: look for 'engine stopped'. Parachute recovery is a fairly common recovery method. The process is: (1) the parachute consists of two stages, a main canopy and a drogue canopy (also called a drag canopy); (2) after the UAV completes its mission, the ground station sends a remote-control command to the UAV, the engine goes to idle, the aircraft slows down and descends; (3) at a suitable flight altitude and speed, the drogue canopy opens, sharply slowing the aircraft and causing it to descend, and by this point the engine has stopped; (4) when the UAV descends to a certain flight altitude and speed, the recovery control system signals the main canopy to open, first in a reefed, inflating state, and after a time the main canopy fully inflates; (5) the UAV hangs under the main canopy and lands slowly, and a ground-contact switch under the aircraft closes, separating the main canopy from the UAV.

When only the ailerons are deflected for a left turn, the aircraft rolls left into a left bank, and the relative airflow comes from the left, producing left sideslip (the relative airflow to the left of the nose). Options A and C do not match the sideslip direction.