China Civil UAV Theory Questions — Questions 316–325

With a stable-flight continuous output current of 5A, the motor's continuous output power is 5A, so the ESC's rated current should be 4–5 times the motor power: a 20–30A ESC should be chosen. Option C fits. Note: the current here is the stable-flight continuous current, not the battery's maximum output current.

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, adjacent propellers on a multirotor spin in opposite directions, so their reaction torques cancel each other.

Study note: when diagonal is an option, choose diagonal. This question tests motor-failure handling: the usual response is to stop the diagonal motor, because stopping the diagonal motor balances the aircraft's dynamics, so it can stay stable while continuing its mission or landing safely.

An ESC's current rating directly determines the power and current of the motor it can drive. An ESC marked “30A” can sustain a maximum stable operating current of 30 amps.

An ESC's BEC 5V means the ESC's BEC (battery eliminator circuit) voltage is 5 volts. A BEC is a circuit module that converts battery voltage to the operating voltage needed by the ESC, receiver, servos, and other electronics. It outputs 5V through the red and black DuPont wires, so while the ESC drives the motor it can also power the flight controller or other equipment.

A motor's kV value is the no-load RPM increase per 1 volt added. A 1000kV motor gains 1000 RPM per volt.

Study note: diameter first, then height, both in mm. A brushless motor model number's first two digits are the stator coil diameter in millimeters; the last two digits are the stator coil height (excluding the shaft) in millimeters. A 2208 motor has a stator diameter of 22 millimeters.

Study note: diameter first, then height, both in mm. A 3810 motor has a stator diameter of 38mm and a stator height of 10mm.

A motor's kV value is the no-load RPM increase per 1 volt added. A 500kV motor gains 500 RPM per volt at no load.

Motor speed is measured per minute. 3000 RPM is 50 revolutions per second, which matches our understanding of UAV rotor speeds. 3000 revolutions per hour would be less than 1 per second — obviously too slow. 3000 per second is obviously too fast.