Loading and Mass & Balance
Mass and balance basics
Every RPA has a maximum take-off mass, an empty mass and a permitted payload. Candidates must understand how to compute the total mass and how to check it against the limit, and how the position of the centre of gravity affects controllability. For multirotors, the centre of gravity should normally lie close to the geometric centre of the rotor layout, and an offset forces the flight controller to hold a steady corrective moment.
Concepts to master
Learn the moment method: moment equals mass multiplied by distance from a reference point, and the centre of gravity is the total moment divided by the total mass. Understand how adding a payload forward or aft shifts the centre of gravity, and how to compensate by relocating the battery. Know the effect of an out-of-balance load on endurance, handling and stability, and the risk of exceeding the structural limits of the airframe or the mount.
Common traps
Questions often mix units, such as grams and kilograms, or forget to include the battery in the total mass. Another trap is assuming that a load within the mass limit is automatically within the balance limit. Watch for payloads mounted on a long arm, where a small mass produces a large moment.
Rule of thumb
Always compute total mass and centre of gravity separately; passing one check does not imply passing the other. Worked example: a 2.0 kg aircraft with its centre of gravity at 0.10 m from the reference gains a 0.5 kg payload at 0.40 m. Total moment is 0.20 plus 0.20 equals 0.40 kg·m, total mass is 2.5 kg, so the new centre of gravity is 0.16 m, a shift of 0.06 m that may exceed the permitted range.