8.1 Multirotor Airframe and Component Layout

Knowing Your Aircraft Physically

The category unit for a multirotor RePL is RBKM — Aeronautical knowledge and operation principles: Multirotor — and its first topic is deceptively practical: know the typical components of the aircraft and where its electrical components sit. This is not trivia. A pilot who cannot point to the ESC on their own airframe cannot interpret a hot-ESC warning, and a pilot who does not know where the compass module is will keep launching from surfaces that upset it.

The Mechanical Airframe

A useful discipline is to run this table as a physical walk-around, always in the same order — body, arms, motors, propellers, battery, gear — so a missed item is obvious.

The Electrical Layout

Schedule 4 topic 1(b) asks specifically for the location and function of the electrical components.

• Electronic speed controllers (ESCs) — one per motor, mounted either inside the centre body on a power distribution board or out on the arms close to their motor. Arm-mounted ESCs get better airflow cooling; body-mounted ESCs are better protected. Function: convert DC into the three-phase output that drives the brushless motor at a commanded speed.
• Receiver and antenna — normally in the centre body, with antennas routed out through non-conductive parts of the shell and, ideally, set at 90 degrees to each other for diversity. Function: decode the C2 uplink.
• Gyros / Inertial Measurement Unit (IMU) — on or near the flight controller, on vibration-isolating mounts. Function: measure rotation rates and accelerations to derive attitude.
• Flight controller — the centre of the aircraft, mounted as close to the centre of gravity as the design allows. Function: fuse sensor data, run the stabilisation loops, and command each ESC.
• Battery — in its mounting on the centre body, positioned by the designer to place the CG correctly.
• Battery eliminator circuit (BEC) — on the power distribution board or built into an ESC. Function: step pack voltage down to the 5 V or 12 V the low-voltage electronics need.
• GNSS sensor / antenna — mounted high, often on a mast or on top of the shell, and as far from the power wiring as possible.

Why the GNSS module sits where it does

Two reasons, and they are both examinable. First, the GNSS antenna needs a clear view of the sky, so anything that shadows it — a payload, a folded arm, an operator's hand — costs satellites. Second, the GNSS module on most airframes also carries the magnetometer (compass), and the compass must be as far as physically possible from the motor currents, ESC switching noise and the battery's high-current leads. A compass sitting 20 mm above a power distribution board carrying 60 A is measuring the aircraft, not the earth.

That is why the tall, awkward-looking GNSS mast on a heavy-lift airframe exists, and why removing or shortening one "to make it fit in the case" is a genuine airworthiness change rather than a convenience.

Configurations and Their Consequences

Multirotors are classified by motor count, and the count buys redundancy rather than performance.

The coaxial note is worth remembering: stacking two rotors on one axis does not double thrust. The lower rotor operates in the disturbed, downward-moving air from the upper one, so a coaxial pair typically produces around 75–80 per cent of two separated rotors. You buy compactness and redundancy, and you pay in efficiency.

The Physical Pre-Flight

Turning the layout into a check that catches real defects:

1. Body — press around arm joints and mounting points, looking and listening for cracking. Check that any inspection panel is closed and latched.
2. Arms — on a folding airframe, confirm each arm is fully extended and its lock is engaged. A partially latched arm can fold under load. Check for play at the hinge.
3. Motors — spin each by hand. Free, smooth rotation with no grinding, no lateral rock, and no catching. Grit in a bearing is audible before it is a failure.
4. Propellers — verify correct CW/CCW assignment against the hub markings, check for nicks and cracks, and confirm they are fully seated and locked. Retire damaged blades; do not sand them smooth.
5. Battery — correct type and capacity for the airframe, correctly oriented, latched, and confirmed by cell-checker before fitting.
6. GNSS mast and antennas — upright, undamaged, connectors seated, not shadowed by the payload.
7. Landing gear — secure, uncracked, and — if retractable — cycled to confirm full travel and correct locking.
8. Payload and gimbal — mounted at the designed station, secure, cables routed so they cannot foul a propeller or a retracting leg.

Two symptoms deserve special attention because they reveal problems the walk-around misses. A motor consistently hotter than its neighbours after landing points to a bearing, an ESC or an out-of-balance load on that arm. New vibration in the hover — visible as jelly in the video or as attitude jitter on the telemetry — points to a damaged or unbalanced propeller, a loose motor bolt, or a degraded IMU mount, and it corrupts the accelerometer data the flight controller depends on. Both go in the technical log.