China Civil UAV Theory Questions — Questions 456–465

Study note: when concave-convex is an option, choose it. Basic blade airfoil types include flat-convex, symmetric, concave-convex, double-convex, S-shaped, and special types; the question's term “asymmetric type” is wrong.

Hover induced velocity is inversely proportional to rotor diameter: the larger the diameter, the gentler the airflow acceleration at the disk, and the smaller the induced velocity.

Multi-rotor aircraft use fixed-pitch propellers and cannot adjust total rotor pitch or tilt angles. Climb/descent: adjust voltage to change rotor speed — higher speed increases lift and the aircraft climbs; lower speed decreases lift and it descends. Horizontal motion: lower the rotor speed on the movement side and raise it on the opposite side, tilting the rotor disk toward the movement direction to create a horizontal lift component. Yaw motion: change the speeds of oppositely rotating rotors, using the difference in reaction torque to yaw.

Study note: large linear velocity, small angle of attack — the angle of attack is largest where linear velocity is smallest. A propeller's angular velocity is the same everywhere, but tip linear velocity is far greater than root linear velocity. To keep lift uniform along the blade, the blade is twisted: the largest installation angle is at the root (hub) and the smallest at the tip. Concepts: angular velocity is how fast the rotation angle changes (angle turned per unit time); linear velocity is speed along a straight line (meters per second or kilometers per hour); angle of attack is the angle between the wing and the airflow direction, which strongly affects lift, drag, speed, and stability.

Li-polymer batteries have high energy density: for the same capacity, a lithium battery is the lightest in weight.

Li-po lithium polymer batteries: (1) nominal voltage 3.7V; (2) full-charge voltage 4.2V; (3) long-term storage voltage 3.8V; (4) discharge protection voltage 3.6V.

To identify battery markings: C is the discharge rate. 25C means the discharge rate is 25 times the rated capacity. Discharge rate is the current required to discharge the rated capacity within a specified time, numerically equal to a multiple of the rated capacity. (Charge rate is a separate rating.)

Multi-axis aircraft use fixed-pitch propellers and control motion by adjusting different blades' speeds. Forward/back and left/right horizontal movement: lower the rotor speed on the target movement side and raise it on the opposite side, tilting the wing lift toward the target direction to create a horizontal lift component. Climb/descent: increase or decrease rotor speed to adjust lift. Left/right turning: change the speeds of normal and reverse propellers, using the difference in rotor reaction torque to yaw.

Decision flow — assess quickly first: check the aircraft's real-time position, remaining power, return distance, and whether the return path is safe. Then act appropriately: (1) if conditions allow, return home — one-key return with full monitoring, raising return altitude if needed to clear obstacles; (2) if conditions do not allow, force-land — choose open ground, keep the aircraft descending steadily, and avoid loss of attitude control. Special note: another question tests what measure is NOT taken at the first moment when voltage is low; in that case, “one-key return” and “forced landing” are measures to take immediately.

Multirotor hover balance requires lift balance, pitch balance, roll balance, and yaw balance; induced drag balance is not a direct hover balance element. Induced drag is the aerodynamic drag produced when a rotor spins at high speed as downwash airflow is generated — a byproduct of lift production and part of the rotor's own aerodynamics. In hover, the aircraft has no horizontal motion and needs no overcoming of horizontal drag (such as friction drag or pressure differential drag), so induced drag mainly affects rotor efficiency (power consumption) rather than directly unbalancing the aircraft.