Mass, Balance and Endurance After a Sensor Swap
A 12 kg maximum take-off mass multirotor is used for agricultural survey. The operator replaces the original 900 g multispectral sensor with a 1 650 g sensor and a 250 g gimbal adapter. The aircraft's empty mass is 6.2 kg, the battery is 3.6 kg, and the original payload was 900 g, giving a take-off mass of 10.7 kg. The flight controller's battery-consumption model was calibrated for the original configuration. The aircraft's arms are 550 mm from the centre, and the new sensor is mounted 60 mm forward of the original sensor's centre of gravity. The original centre of gravity was 8 mm forward of the geometric centre. The maximum take-off mass is 12 kg and the centre-of-gravity limit is plus or minus 25 mm from the geometric centre.
1. Compute the new take-off mass and the new centre-of-gravity position, and state whether the flight is legal.
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Mass: the original take-off mass was 10.7 kg. The new sensor is 1 650 g and the adapter is 250 g, so the new payload is 1 900 g, which is 1 000 g more than the original 900 g. The new take-off mass is 11.7 kg, which is below the 12 kg limit but leaves only 300 g of margin.
Centre of gravity: the original aircraft, excluding the payload, had a moment such that the total centre of gravity was 8 mm forward of the geometric centre with a 900 g payload at some reference position. The change is a 1 000 g increase in payload mass, with the new sensor 60 mm forward of the original, plus a 250 g adapter. Taking moments about the original centre of gravity: the added 1 000 g at 60 mm forward contributes 60 000 g·mm forward, and the 250 g adapter, if mounted at the sensor, contributes 15 000 g·mm forward, for 75 000 g·mm. Dividing by the new total mass of 11 700 g gives about 6.4 mm forward shift. The new centre of gravity is about 14.4 mm forward of the geometric centre.
That is within the plus or minus 25 mm limit, so the flight is legal on mass and balance, but the margin is reduced. I would verify the adapter's actual mounting position and re-weigh the aircraft rather than rely on the calculation.
2. The flight controller's endurance model was calibrated for the original configuration. What is the practical effect, and how do you plan the sortie?
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The model predicts endurance from battery consumption per unit time, which scales with mass and with the power required to hover and cruise. A 9.3 percent mass increase from 10.7 to 11.7 kg raises induced power, and the added 1 000 g is concentrated forward, which may require the flight controller to hold a pitch trim that further increases power. The model will therefore over-predict endurance, and the error grows with wind and manoeuvring.
I would not trust the displayed endurance until it is recalibrated. I would fly a controlled test sortie in calm conditions, record actual consumption against the model, and derive a correction factor. Until then I would apply a conservative reserve: plan to land with at least 30 percent state of charge, not the 20 percent used previously, and shorten the survey legs accordingly.
I would also check the centre-of-gravity effect on handling. A forward centre of gravity increases pitch stability but costs control authority and power in hover. If the aircraft shows a persistent nose-down trim or reduced pitch response, I would move the battery aft within its allowed range to restore the centre of gravity toward the original position, then re-verify the mass and balance.
3. What documentation and airworthiness checks are required before returning the aircraft to service?
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I would record the modification in the aircraft's technical log: the new sensor mass, the adapter mass, the new empty mass, the new take-off mass, the centre-of-gravity calculation and the weighing result. I would verify that the maximum take-off mass is not exceeded and that the centre of gravity is within limits for the full range of battery positions and payload configurations.
I would check the structural attachment of the new sensor and adapter, including the fastener torque and any load-spreading provisions, because a 1 900 g payload on a 550 mm arm generates significant bending loads in gusts. I would verify that the electrical supply and data interface are compatible and that the added current does not exceed the payload bus rating.
I would update the operations manual and the risk assessment to reflect the new mass and endurance, and I would recalibrate or re-validate the flight controller's endurance model. Finally, I would confirm that the change is within the scope of the aircraft's authorization; if the special certificate specifies a maximum take-off mass or a payload configuration, a material change may require notification or amendment before flight.