China Civil UAV Theory Questions — Questions 286–295

GPS positioning requires at least 4 satellites. Because signals are unstable, redundancy is needed for flight safety, so 4–5 satellites ensure basic safety.

Study note: when concave-convex is an option, choose it. The concave-convex propeller cross-section is uneven, which lets the blade generate higher lift and thrust: the air above the rotating blade flows faster than below, producing lift, and the concave-convex design exploits this better while improving airflow and thrust. S-shaped blades have a cross-section like the letter “S” and produce a swaying effect as they rotate, giving flexibility under different flight conditions. Symmetric blades are symmetric about the centerline, with a high lift-to-drag ratio (less drag for the same lift) and a stable pressure center that generates substantial lift from root to tip across the speed range; their symmetry avoids swaying, suiting missions that need stability.

Study note: flight controller hardware goes at the center; GPS goes on top. UAV flight controller hardware includes angular rate sensors, and mounting them at the aircraft's center yields more accurate data for sensing flight status.

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.

Study note: large linear velocity, small angle of attack. A propeller's angular velocity is the same everywhere, but tip linear velocity is far greater than root linear velocity. 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).

Study note: large linear velocity, small angle of attack. This question tests multi-axis propeller structure. 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. Therefore the root lift coefficient is greater than the tip lift coefficient. 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.

By Newton's third law (action and reaction), each spinning motor drives its propeller and applies a reaction torque to the airframe. To cancel the reaction torque, multirotor propellers use an alternating layout of different rotation directions in corresponding pairs, so the motors' reaction torques cancel each other.

A balance charger works as follows: when the battery is connected, the charger immediately detects each cell's voltage and adjusts charging current accordingly. If a cell is low, the charger raises current to replenish it quickly; if a cell is high, it lowers current to avoid overcharging. All cells thus charge evenly, avoiding individual overcharge or undercharge. This significantly extends battery life by ensuring every cell is fully charged, improves the tool's endurance by raising the pack's total stored energy, and prevents overheating, improving battery safety.

Multi-axis aircraft power systems mainly use electric motors, which are simple, light, and convenient. Drive motors are brushed or brushless; brushed motors are less efficient and have gradually fallen out of use in UAVs.

Outrunner motors have high torque, suit larger propellers, and are efficient, so they are widely used on electric UAVs; inrunner motors spin fast and suit electric ducted fans (or models with gearboxes). A stepper motor converts electrical pulse signals into angular or linear displacement: each input pulse turns the rotor by one angle or step, so it cannot serve as a multi-axis aircraft drive motor.