China Civil UAV Theory Questions — Questions 466–470

Term explanations: rotor induced drag — when a rotor moves through air, the pressure difference between the airfoil's upper and lower surfaces produces lift while accelerating air downward; this downward airflow exerts a reaction force on the rotor, and its component in the rotor's direction of travel is induced drag. Simply put, it is the resistance hindering the rotor's advance, created while the rotor produces lift and sets surrounding air in motion. Rotor reaction torque — when a rotor spins, Newton's third law produces an equal and opposite reaction moment: the rotor reaction torque. For example, a multirotor's rotors spin in different directions precisely to cancel reaction torque and keep the aircraft stable. Hub moment — the hub connects the rotor to the airframe. Hub moment is the moment produced at the hub by rotor lift, drag, and other forces, mainly composed of flapping moment, lead-lag moment, axial forces, and similar hub moments, affecting control and stability. Blade lead-lag moment — as a blade rotates, periodic aerodynamic forces and its own elasticity tend to make it swing back and forth in the blade plane; this swinging produces a moment at the hub: the blade lead-lag moment. It affects rotor system dynamics and aircraft stability, but is generally not among the main forces and moments considered for a multirotor in flight. Why multirotors disregard blade lead-lag moment: multirotor structures are relatively simple, with fairly rigid rotor connections; blade design focuses mainly on providing lift and controlling attitude, without the strict lead-lag requirements of helicopters and other complex aircraft. Multirotor hub structures are usually simple, constraining blade lead-lag strongly, so lead-lag moments are relatively small and have little effect on the aircraft's overall forces and moments in actual flight. Multirotor flight control mainly adjusts motor speeds, focusing on lift changes and reaction torque balance, with low sensitivity to blade lead-lag moment. In flight control system design and operation, blade lead-lag moment is usually not a main consideration because it affects attitude and position control relatively little.

The wording “only depends on” is wrong.

Study note: reaction torque deflects opposite to rotation (opposite-opposite).

Study note: when you see hub, choose hub. As a propeller rotates, relative airflow speed increases with radius; twisting the blade gives different installation angles at different radii, ensuring fairly uniform aerodynamic force across radii and avoiding excessive or insufficient local stress. Linear velocity is smallest at the hub, so the largest installation angle or pitch is needed there.

In low-altitude, low-speed environments, aircraft are mainly subject to friction drag, interference drag, induced drag, and pressure differential drag. Shock wave drag (④) is significant in high-speed flight; circulation drag (②) is not a standard drag type. So option C is correct.