China Civil UAV Theory Questions — Questions 426–435

Torque vs. moment: (1) Definition — moment is the physical quantity of a force's turning effect on an object, equal to force times moment arm. Torque is a special case of moment: the moment that twists an object (usually a shaft or rod) under force, describing torsional mechanical behavior. (2) Direction — moment is a vector whose direction follows the right-hand screw rule, perpendicular to both force and moment arm directions. Torque direction also follows the right-hand screw rule, but unlike moment it is usually the same as or opposite to the shaft's rotation direction. (3) Applications — moment is widely used in physics, mechanics, aerospace, and other fields to describe and analyze rotation. Torque is mainly used in mechanical engineering and material mechanics, especially when analyzing torsion of shafts, rods, and similar objects.

A multi-axis UAV is a UAV with three or more rotor axes.

Study note: when you see angular velocity/angular rate, choose it (GPS is the first choice for position). Angular rate sensors (angular rate gyroscopes) sense the UAV's angular rate (angular velocity) about its body axes. Attitude sensors sense the UAV's pitch, roll, and heading angles. Altitude and airspeed sensors (air data) sense flight altitude and airspeed. Position sensors sense the UAV's position.

China's Civil Aviation Flight Rules, Article 80: flight time is the time from when the aircraft begins moving under its own power from the loading point in preparation for takeoff until it stops moving at the unloading point at the end of the flight. Takeoff time is the instant the aircraft begins its takeoff roll with wheels moving; landing time is the instant the landing roll ends. Helicopter flight time is the time from when the rotor starts turning to when it stops turning. Helicopter takeoff and landing times are the instants the main wheels (skids) leave and touch the ground.

Study note: when two short options and one long option appear, choose the longest. Single-rotor with tail rotor unmanned helicopters, coaxial twin-rotor unmanned helicopters, and multi-axis UAVs are all classified by aircraft structure.

Excessive wind speed greatly affects aircraft stability and shooting quality. Strong wind can unbalance the aircraft, causing shaky footage or failed shots. Wind speed changes also affect flight path and shooting results, making aerial photography unstable.

Heading angle is the angle of the aircraft rotating about its own vertical axis, also called yaw angle. In multi-axis aerial photography, heading changes directly affect camera pointing and shooting direction: if heading changes, camera pointing changes too, possibly causing the shot to miss its target or rotate, seriously affecting aerial photography quality. Pitch and roll changes also affect aerial photography to some extent. Pitch is rotation about the aircraft's lateral axis, mainly affecting climb/descent and camera elevation angle. Roll is rotation about the longitudinal axis, mainly affecting left-right movement and banking. But compared with heading, pitch and roll affect aerial photography less.

Study note: when you see a forward-protruding gimbal, choose it. A forward-protruding gimbal extends the camera equipment forward, so during full upward-tilt shooting it effectively prevents other parts of the aircraft from blocking the lens, ensuring the shot is not spoiled by aircraft structure.

In option A, with motor power unchanged, larger blade diameter and greater total blade pitch increase motor load and slow rotation, so payload cannot increase. In option B, with blade diameter unchanged, lower motor power reduces blade rotation speed and thus lift; smaller total blade pitch further reduces lift. This cannot increase payload. In option C, with total blade pitch unchanged, greater motor power and larger blade diameter both increase lift, so payload may increase.

An autogyro needs a takeoff roll, so it is not a helicopter. A helicopter is an aircraft that flies using lift and thrust provided by one or more horizontally rotating rotors.