7.1 Electrical Terms and RPAS Electrical Components

Five Terms That Underpin the Whole Unit

REES — Electrical and electronic systems for RPAS — is the second-largest knowledge unit in the RePL syllabus with nine Schedule 4 topics, and it opens with vocabulary.

Two relationships tie them together and both appear in RPA work:

$V = I \times R \qquad\text{(Ohm's law)}$

$P = V \times I \qquad\text{(Power)}$

Worked example. A 6S LiPo at a nominal 22.2 V drives a multirotor drawing 40 A in the hover. Hover power is 22.2 × 40 = 888 W. If the same aircraft peaks at 90 A during an aggressive climb, peak power is 22.2 × 90 ≈ 2,000 W. That difference is why a battery's continuous and burst current ratings are separate numbers, and why a pack sized only for hover current sags badly under a climb.

Hertz turns up in three places on an RPAS: the radio frequency of the C2 link (2.4 GHz, 5.8 GHz, 900 MHz), the flight controller's loop rate, and the pulse-width-modulation frequency at which an ESC switches its output to the motor.

The Power Chain

Power flows in one direction through a recognisable chain:

Flight battery → power distribution board → electronic speed controllers (ESCs) → motors

with taps off that main bus feeding the flight controller, receiver, telemetry module, gimbal and payload.

The components Schedule 4 names

• Electronic speed controller (ESC) — converts the battery's direct current into the timed three-phase output that spins a brushless motor, and sets its speed on command from the flight controller. One ESC per motor. ESCs run hot and are a common failure point.
• Battery eliminator circuit (BEC) — a voltage regulator that steps the high pack voltage down to the 5 V or 12 V that the receiver, flight controller and servos need, "eliminating" the need for a separate receiver battery. A BEC failure kills the low-voltage electronics while the motors still have power — an ugly failure mode.
• Receiver and remote receivers — decode the C2 uplink. "Remote receivers" are secondary receiver units mounted elsewhere on the airframe with their own antennas, giving diversity so the airframe cannot shadow the signal in every orientation at once.
• Telemetry module — encodes and downlinks aircraft state.
• Flight batteries — the main energy source (see the battery sections that follow).
• Receiver battery — a separate small battery powering the receiver on airframes that do not use a BEC. Redundant power for the control electronics.
• Circuit breakers and fuses — protective devices that open the circuit on an overcurrent, protecting wiring from a fire.
• Servomechanisms (servos) — position-controlled actuators. Rare on a multirotor, essential on a fixed-wing for ailerons, elevator and rudder, and used everywhere for gimbals, release mechanisms and spray valves.
• Aerials/antennas — radiating elements for the C2, telemetry, payload and GNSS links.
• GPS receivers — decode satellite signals for position (see the GNSS section).
• Altimeters — barometric for pressure height, and radio, radar, laser or acoustic altimeters for true height above the surface, used for terrain following and precision landing.
• Collision avoidance sensors — vision, infrared, ultrasonic or lidar sensors that detect obstacles. Note the limits: most vision systems fail on featureless surfaces, in low light, and against thin objects such as wires.

Redundancy, Back-ups and Consequences of Failure

Redundancy means providing more of a component than the aircraft strictly needs, so that a single failure is survivable. Back-up means providing an alternative path or mode. Schedule 4 asks about both, and about the consequences of a malfunction and the remedial action.

The pattern is worth internalising because it is how the questions are built: a symptom is described and you are asked for the consequence or the action. Two of them are worth memorising verbatim:

• A quadcopter that loses one motor cannot maintain controlled flight. Three motors cannot simultaneously balance thrust and torque. The aircraft yaws and descends. The pilot's only meaningful input is where it comes down.
• A hexacopter that loses one motor can usually continue level flight with reduced authority by redistributing thrust across the remaining five. It should still be landed at once.

Pre-flight and Symptom Recognition

Most electrical failures announce themselves before they become emergencies. Build these into the pre-flight and the post-flight:

• Warm or hot motors, ESCs or battery after landing. Heat is wasted power. A single component consistently hotter than its neighbours points to a bearing, a mis-set ESC, a damaged winding, or an out-of-balance airframe forcing that arm to work harder.
• Voltage sag under load. A pack that drops sharply when power is applied and recovers slowly is either under-specified for the aircraft's current draw or near the end of its cycle life.
• Intermittent link or telemetry. Check antenna connectors and routing before blaming the environment; a coaxial connector loosened by vibration is a very common cause.
• Corroded, chafed or discoloured wiring and connectors. Discolouration around a connector means it has been running hot — a high-resistance joint, which by Ohm's law dissipates power as heat exactly where you least want it.
• Blown fuse or tripped breaker. Never reset and fly. A protective device operates because something drew more current than it should; find the cause first.

Record every one of these in the technical log. A "warm motor 3" note that appears three flights in a row is a maintenance finding, not a coincidence.