6.2 Payload, Centre of Gravity and Pre-Flight Checks

Payload, Centre of Gravity and Pre-Flight Checks

Adding a payload — a camera, a sensor pod, or a delivery package — changes the way an RPA flies. Weight increases the power required to hover, and a shifted centre of gravity (CoG) changes how the aircraft responds to control inputs. This section covers payload and CoG effects, the performance margin that wind and density altitude eat into, and the full pre-flight checklist a remote pilot must complete before every flight.

Payload Weight and Centre of Gravity

Payload is anything attached to the aircraft that was not part of its baseline design — cameras, gimbals, LiDAR units, crop-spray tanks, or parcels. Two effects matter:

1. Total weight — more weight means more lift required, higher motor current, shorter endurance, and reduced authority to recover from turbulence. Always respect the manufacturer's maximum payload and maximum take-off weight (MTOW); exceeding it is illegal under CASR Part 101 and unsafe.
2. Centre of gravity (CoG) — the point through which gravity acts on the aircraft. The flight controller is tuned for a specific CoG location. A payload that shifts the CoG forward, aft, or laterally causes:
   • One side or axis of motors to work harder, reducing efficiency and endurance.
   • Uneven control response — the aircraft may feel sluggish in one direction and twitchy in the other.
   • In extreme cases, the controller cannot maintain stable hover or level flight.

Balance the aircraft so the CoG sits at the manufacturer-specified location — usually marked on the airframe or documented in the flight manual. Hang-test or balance-test methods apply for multirotors and fixed-wing RPA respectively.

Battery Position and Balance

The battery is often the heaviest single item on a small RPA, so its position has a large influence on CoG. Always:

• Mount the battery in the designated position and orient it as shown in the manual.
• Use the correct battery type and capacity — a larger battery may fit physically but shift the CoG and exceed MTOW.
• Secure the battery so it cannot shift in flight; a sliding battery changes CoG mid-flight and can destabilise the aircraft.

Wind and Density Altitude: Performance Margin

Every RPA has a performance margin — the spare lift and control authority available above what is needed to hover. Two environmental factors reduce that margin:

• Wind — flying into a headwind reduces ground speed and extends time to return; crosswinds push the aircraft toward obstacles; gusts require sudden power spikes. Always check the forecast against the aircraft's documented maximum wind speed and never fly at or above that limit.
• Density altitude — high temperature, high humidity, and high elevation all reduce air density, so the propellers produce less lift per revolution. On a hot day at an inland launch site, the aircraft may need noticeably more power to hover, shortening endurance and reducing the margin to recover from a gust.

Plan flights with margin: launch with enough battery reserve to return against the wind, and reduce payload or delay the flight when density altitude is high. A heavily loaded aircraft on a hot, windy day has very little spare performance — that is when accidents happen.

The Full Pre-Flight Checklist

A structured pre-flight check is mandatory, not optional. Use a written checklist every time — memory is not reliable. The table below sets out the items to verify before takeoff.

Hover Check

Immediately after takeoff, perform a hover check before transiting away:

• Hold a stable hover at eye level, a few metres in front of the pilot.
• Confirm control response — pitch, roll, yaw, and throttle all react correctly and proportionally.
• Watch for abnormal vibration or oscillation that was not visible on the ground.
• Check for unexpected drift — a steady drift with no wind input suggests a sensor or compass problem.
• Confirm the C2 link is solid and the ground station telemetry (height, battery, satellite count) is sensible.

If anything is abnormal, land immediately and investigate. A problem found at the hover check is almost always cheaper and safer than one found downrange.

Post-Flight Log Review

After landing, complete the loop:

1. Power down the aircraft and record the flight time and any incidents in the flight log.
2. Note any anomalies — link drops, compass warnings, unusual vibration, or battery behaviour — for maintenance follow-up.
3. Inspect the airframe, props, and motors for damage acquired during the flight.
4. If the operation is conducted under a ReOC, log the flight as required by the operator's procedures.

A disciplined post-flight review catches developing faults early and keeps the aircraft legal and airworthy for the next sortie.