4.4 Weight and Balance: Terms and Limits

Why a Drone Pilot Needs Weight-and-Balance Vocabulary

Small multirotors rarely require a formal load sheet, which tempts candidates to skip this topic. That is a mistake for two reasons. First, Schedule 4 codes it priority A. Second, the moment you fit a heavier gimbal, add a second battery, or strap a spray tank to a 25 kg agricultural airframe, you are doing weight and balance whether you write it down or not — and RPA above 25 kg are individually listed on a RePL by name and maximum take-off weight precisely because their loading matters.

The Weight Terms

The arithmetic is simply:

$\text{Useful load} = \text{Maximum gross weight} - \text{Operating weight}$

Worked example. A survey multirotor has an empty weight of 3.4 kg. Its operating fit is a 1.2 kg flight battery and a 0.4 kg gimbal mount, giving an operating weight of 5.0 kg. Its manufacturer-published MTOW is 7.0 kg. The useful load is 7.0 − 5.0 = 2.0 kg. A 2.3 kg LiDAR sensor therefore cannot be carried — not "can be carried carefully", but cannot be carried, because it puts the aircraft 300 g over MTOW before it leaves the ground.

Weight categories interact with this. That same airframe at 7.0 kg gross sits in the small category (more than 2 kg, not more than 25 kg). If a modification pushed it past 25 kg it would become a medium RPA, and the licensing, landholder and certification consequences change immediately.

The Balance Terms

Balance is about where the weight sits, not just how much there is.

• Datum — an arbitrary reference point, specified by the manufacturer, from which all horizontal distances are measured. It may be the nose, the centre of the airframe, or a point in front of the aircraft. Its position is arbitrary; what matters is that everyone uses the same one.
• Arm — the horizontal distance from the datum to the centre of gravity of an item. Arms behind the datum are positive, arms in front are negative (or the manufacturer places the datum ahead of everything so all arms are positive).
• Moment — the turning effect of a weight about the datum: $\text{Moment} = \text{Weight} \times \text{Arm}$
• Station — a numbered position along the airframe, expressed as its distance from the datum. "Station 45" means 45 units aft of the datum.
• Index unit — a constant divisor applied to moments to keep the numbers small and readable on a load sheet. Dividing every moment by 100 or 1,000 does not change the balance result, only the size of the digits.
• Centre of gravity (CG) — the point through which the total weight acts, found by dividing total moment by total weight.

Finding the Centre of Gravity

$\text{CG} = \frac{\sum(\text{Weight} \times \text{Arm})}{\sum \text{Weight}}$

Worked example. A hexacopter has a datum at the centre of the airframe, with forward arms negative and aft arms positive. Loading is:

CG = −4,200 ÷ 6,300 = −0.67 cm, or about 6.7 mm forward of the datum. If the manufacturer's envelope is ±15 mm, the aircraft is inside it. Move to a 1,300 g camera at the same arm and the moment becomes −18,200, giving a total of −9,800 and a CG of −1.46 cm — outside the envelope, even though total weight of 6,700 g is still below a 7,000 g MTOW. Weight and balance are two separate tests and an aircraft can pass one while failing the other.

Loading Limits and What Breaching Them Costs

Two limits define the safe envelope, and both must be respected:

1. The weight limit — never exceed maximum gross weight.
2. The CG limits — the forward and aft (and, on a multirotor, lateral) boundaries within which the CG must fall.

Exceeding maximum gross weight produces predictable consequences: a higher power requirement to hover, higher motor and ESC current, more heat, shorter endurance, a reduced climb rate, a longer stopping distance, and less spare thrust to recover from a gust. On a hot day at an elevated site those effects compound with density altitude, and the margin that would normally absorb a gust simply is not there.

Flying outside the CG envelope produces different symptoms. The flight controller is tuned for a specific CG, so an off-centre load forces one arm or one axis of motors to run continuously harder to hold the aircraft level. The results are:

• Uneven control response — sluggish in one direction, twitchy in the other.
• Reduced endurance, because the hard-working motors draw more current than the model predicts.
• Premature motor and ESC wear, and higher temperatures on the loaded side.
• In severe cases, loss of stable hover — the controller runs out of authority on the loaded axis and the aircraft drifts or oscillates.

Practical technique for a multirotor

• Mount the battery in its designed position and orientation, and secure it — a pack that shifts in flight moves the CG in flight.
• Balance-test the aircraft before the first flight with a new payload: support it under the manufacturer's CG point and confirm it sits level, or hover it a metre up and watch for a persistent tilt.
• If you must fit an off-centre payload, add ballast on the opposite arm rather than accepting the imbalance, and recheck against MTOW after adding it.
• Recheck after any modification — a heavier propeller set, larger landing gear, or an added parachute all change both weight and balance.