7.1 Airframes, Electrical Systems & Propulsion

7.1 Airframes, Electrical Systems & Propulsion

Quick Answer: Treat the airframe as a load-bearing structure that must be inspected, handled carefully, and secured for transport. The electrical/propulsion chain—battery, distribution, ESC, motor, and control electronics—must be intact for Advanced work. Know brushed vs brushless, inrunner vs outrunner, basic IC engine issues, and how redundancy (or lack of it) changes go/no-go when a system is inoperative.

TP 15263 Section 2 (RPAS systems) expects more than “motors spin, drone flies.” Advanced pilots operate near people, in controlled airspace, and with medium aircraft where a structural or electrical failure is a public-safety event, not a lawn ornament inconvenience. This section builds the mechanical and electrical literacy you need for pre-flight, maintenance awareness, and exam scenarios.

Airframes: structure, handling, care, and securing

An airframe is the physical structure that carries aerodynamic loads, propulsion loads, payload, and landing shocks: fuselage or centre frame, arms (multirotor), wings/tail (fixed-wing), landing gear, mounts, and fasteners.

Why structure matters at Advanced level

• Multirotor arms are cantilever beams under continuous vibration and cyclic bending; a hairline crack at a motor mount can propagate in one hard landing.
• Carbon fibre is stiff and light but can hide delamination; injection-moulded plastics creep and crack at bolt holes; aluminum bends and fatigues at stress risers.
• Landing gear and dampers protect the centre stack (flight controller, GPS, battery). Hard landings that “look fine” may have shifted a soft-mounted IMU or cracked a PDB standoff.
• Payload gimbals and cameras add mass, leverage, and wiring chafing paths; loose gimbal screws change CG and can foul props.

Handling and care (field habits)

Securing for transport and site ops

Before road transport or packing:

1. Power down and disconnect the flight battery when the manufacturer and SOPs require it.
2. Remove or fold props if designed for it; otherwise use prop guards only if they do not create a false sense of security for flight.
3. Immobilize arms/gimbals with locks, foam, or cases so nothing frets against carbon edges.
4. Isolate magnetic and RF sources from compass/GPS modules in the same case when practical.
5. Secure the case in the vehicle so a hard stop does not launch a 5–25 kg aircraft into the cabin.

On site, secure the aircraft against wind blow-over when disarmed on a slope or rooftop: a gust can roll a tall multirotor, bend arms, and start an uncommanded motor test if someone thumbs the sticks while “helping.”

Component integrity checklist (airframe)

• Arms: straight, no cracks at roots or motor bells; folding hinges latch positive.
• Fasteners: present, correct length, thread-locked where specified; no stripped inserts.
• Props: matched sets, correct direction (CW/CCW), no nicks at leading edge for high-RPM craft.
• Wiring: strain-relieved at arms; no chafing at carbon holes; connectors fully seated and locked.
• Centre stack: flight controller, PDB/BEC, and ESCs firmly mounted; no loose standoffs.

Any unresolved structural doubt is a grounding decision for Advanced professional work.

Electrical system architecture

Think in energy flow, not marketing diagrams:

Main power path: LiPo/Li-ion pack (or multiple packs) → main power connector / PDB or integrated power board → ESCs → motors.

Avionics / servo path: Same battery (or a dedicated pack) → BEC/regulator or separate rail → flight controller, radio receiver, servos (fixed-wing/heli), GPS, and payload power.

Key components

Electrical integrity for Advanced ops means verifying that connectors are clean and fully seated, wire gauges match current demand, ESCs are cooled and within temperature limits, and there is no evidence of previous overheating (discoloured insulation, soft solder joints, melted XT60/AS150 housings). A “it armed yesterday” history does not replace inspection.

Servos and mixed systems

Fixed-wing RPAS, VTOL hybrids, and camera gimbals use servos. Stalled servos draw large current continuously and can collapse the avionics rail even when propulsion batteries still have energy. Pre-flight should include free movement of surfaces, correct direction of throw, and no binding at full deflection.

Propulsion: electric motors

Brushed vs brushless

Exam expectation: brushless + ESC is the modern multirotor default; brushed motors are not the primary propulsion story for Advanced professional aircraft.

Inrunner vs outrunner

• Inrunner: rotating part (rotor) is inside the motor can; typically higher RPM, often used with gearboxes or on some fixed-wing setups.
• Outrunner: the outer bell rotates; high torque at lower RPM, ideal for direct-drive propellers on multirotors.

Most camera multirotors use outrunner brushless motors sized so prop RPM and torque match hover thrust without a gearbox. Mismatched props (too large, too much pitch) overheat motors and ESCs even if the battery “still has percentage left.”

ESC role (do not skip)

The ESC interprets throttle commands from the flight controller and drives three-phase windings. Modern ESCs also report telemetry (RPM, current, temperature). A failing ESC can cause:

• One motor at wrong thrust → yaw/roll couple the FC fights until battery or altitude runs out
• Sudden stop → immediate loss of control authority on that axis
• Desync under load → intermittent thrust, often worst at high throttle or high DA

For Advanced site work near people or in controlled airspace, motor/ESC anomalies on the ground hover check are a no-go, not a “see if it settles.”

Internal-combustion (IC) engines — overview

Some larger fixed-wing, agricultural, or long-endurance RPAS use piston or rotary IC engines (glow, gasoline, heavy fuel) or hybrid generators charging a battery bus. Systems-level points for the exam:

• Fuel quality, filters, and leaks are fire and power-loss hazards.
• Vibration is higher than pure electric; mounts, avionics isolation, and prop tracking matter more.
• Restart after flameout may be impossible in the time/altitude available—plan glide or ballistic recovery, not “it will catch again.”
• Mixture, temperature, and carb icing concepts transfer from manned aviation at a simplified level.
• Maintenance intervals and two-person verification culture (later chapter) apply as strongly as for electric systems.

Even if your day-to-day aircraft is a quadcopter, Advanced syllabus coverage of IC avoids blank stares when a scenario mentions fuel systems or hybrid gensets.

Redundancies and risks of inoperative systems

What redundancy buys you

Hexacopters and octocopters are not magic: losing one motor may allow a controlled descent if weight and control laws allow, but do not assume automatic safety. Manufacturer documentation defines whether continued flight is approved.

Inoperative systems — Advanced risk logic

Treat “inoperative” as a formal condition, not a shrug:

1. Identify the failed system (compass, one ESC, RTK, strobe, return-to-home radio, landing gear servo).
2. Check MEL-like guidance in the manufacturer manual / your SOPs: is dispatch allowed with limitations?
3. Assess mission environment: controlled airspace, near people, EVLOS, medium RPA, night lighting requirements—higher risk environments demand higher serviceability.
4. Decide: repair, defer with documented limitation, or cancel. Advanced professional culture prefers cancel over “quick orbit to test.”

Examples:

• Inoperative navigation lights when night ops require them → no-go.
• Single remaining battery path on a dual-path aircraft with known intermittent connector → no-go for public-area work.
• Compass unavailable / ATTI only near people or obstacles → usually no-go (see Section 7.4).
• One motor weak on ground run-up → no-go; do not “trim it out” in airspace shared with crewed traffic.

Electrical system integrity as a control measure

For Advanced operations, electrical integrity is part of safety assurance thinking: connectors, strain relief, correct fusing or manufacturer protection, clean power to the flight controller, and verified fail-safes (low-battery RTH thresholds that actually match real voltage sag under hover load). Lab-perfect resting voltage means little if a high-resistance connector drops 0.5 V under climb current.

Pre-flight systems flow (integration)

1. Airframe visual and tactile inspection; secure transport locks removed; props correct and tight.
2. Electrical connectors seated; no damaged insulation; ESCs/motors free of debris.
3. Power-on sequence per manufacturer; listen for abnormal ESC tones or binding.
4. Control check on ground: correct motor mapping, servo directions, kill/disarm works.
5. Hover check in a safe volume: symmetric thrust, no vibration spike, telemetry currents sane.
6. If any anomaly — resolve or cancel; do not export risk into controlled airspace or near people.

Bottom line: Airframes fail from neglect and transport abuse; propulsion fails from heat, mismatch, and connector resistance; avionics fail from brownouts and vibration. Advanced pilots inspect the whole chain, understand motor/ESC types, respect IC hazards where present, and treat inoperative systems as formal risk decisions—especially when redundancy is absent.