China Civil UAV Theory Questions — Questions 306–315

Li-po lithium polymer battery: nominal voltage 3.7V, full-charge voltage 4.2V, long-term storage voltage 3.8V–3.9V, discharge protection voltage 3.6V.

Study note: flight controller hardware goes at the center; GPS goes on top. The GPS antenna must receive satellite signals from the sky, so to avoid blockage by the airframe and other equipment it should be installed on top of the aircraft.

The “controller” held by ground personnel is the ground remote transmitter, not the flight controller.

Study note: the DuPont cable connects to the flight controller. The thickest 2 red/black wires connect to the battery, the medium 2–3 wires connect to the motor, and the thinnest 3 wires connect to the flight controller.

A brushless ESC drives the motor through an electronic commutation circuit and can regulate motor speed, not fail to regulate it; and UAVs commonly use brushless AC motors. So option A is wrong. Three-phase AC motors receive AC converted from battery DC by a brushless ESC as motor input. So option B is correct. A brushed ESC can drive a DC motor directly and regulate speed conveniently through its control circuit, but driving an AC motor is inconvenient. So option C is wrong.

This question mainly tests multi-rotor motor-propeller matching. A 3-inch prop is 2/3 smaller in diameter than a 1060/1047 prop, so to maintain the same lift its speed must far exceed that of a KV900–1000 motor. Under 3S: KV900–1000 motors pair with 1060 or 1047 props (9-inch props also work); KV1200–1400 pair with 9050 (9-inch) to 8×6 props; KV1600–1800 with 7-inch to 6-inch props; KV2200–2800 with 5-inch props; KV3000–3500 with 4530 props. Under 2S: KV1300–1500 with 9050 props; KV1800 with 7060 props; KV2500–3000 with 5×3 props; KV3200–4000 with 4530 props.

Study note: large-low, small-high. Large props need large torque, so use low-kV motors; small props need small torque, so use high-kV motors. A large-diameter propeller needs greater torque to produce enough lift, and a low-kV motor provides greater torque for a large-diameter propeller. A high-kV motor gives higher speed but less torque on a large-diameter propeller and cannot meet the lift requirement. When choosing a motor and propeller, consider the UAV's weight, expected flight time, and required lift to select the most suitable combination.

A motor's kV value is the no-load RPM increase per 1 volt added: it is the RPM gained at no load for each additional 1V. A 1000kV motor gains 1000 RPM per volt.

UAV motors are mainly of two types: (1) coreless brushed DC motors, mostly used on toy aircraft; (2) three-phase AC brushless motors, mostly outrunner in design, widely used in the UAV industry for high efficiency, low energy consumption, low noise, extra-long life, and high reliability. Brushless motors require brushless ESCs, so option B is correct.

The ESC must handle peak currents during takeoff, landing, and acceleration, so it needs about 5 times the stable-flight current. Choosing a 50A ESC is safest. Note: the 10A here is the stable-flight continuous output current, not the battery's maximum output current.