Aircraft Performance, Loading, and Mass-and-Balance
The four forces, applied to a multirotor or fixed-wing UAS
Performance questions test whether you can connect weight, thrust, wing loading, density altitude, and battery state to the operation you are planning. For multirotors, thrust must exceed weight with a reserve margin for maneuvering and gusts; for fixed-wing UAS, stall speed rises with wing loading and with altitude (true airspeed increases for a given indicated speed as density falls).
Concepts to master
- MTOW and payload: every added sensor, battery, or tether reduces endurance and climb margin. Compute payload = MTOW − empty weight − fuel/battery allowance.
- Center of gravity (CG): a UAS has a CG envelope just like an aircraft. Moving a heavy camera forward shifts the CG; exceeding the forward or aft limit degrades controllability and can make the aircraft unrecoverable in a motor-out scenario.
- Mass-and-balance arithmetic: moments = weight × arm (distance from datum); total moment ÷ total weight = CG position. Know how to do this with two or three items.
- Endurance and range: affected by wind (headwind out, tailwind back), payload, temperature, and battery age. Plan a reserve (commonly 20–30% of battery) for the return and a go-around.
- Density altitude effects: reduced lift and thrust, longer takeoff roll for fixed-wing, higher power draw for multirotor.
Worked example
Aircraft empty weight 4.0 kg at arm 0.30 m; battery 1.0 kg at arm 0.20 m; camera 0.5 kg at arm 0.50 m. Moments: 1.20 + 0.20 + 0.25 = 1.65 kg·m; total weight 5.5 kg; CG = 1.65 / 5.5 = 0.30 m from datum. If the aft limit is 0.32 m, you have only 2 cm of margin - adding a heavier camera aft would breach it.
Traps
Forgetting that a heavier payload increases both weight and moment; assuming published endurance is achievable at MTOW in hot, high conditions; and treating battery percentage as linear when voltage sags under load.