Unmanned aircraft: technical and performance limitations
Classes and mass
MTOM (maximum take-off mass) includes battery, payload and anything you have bolted on - it is not the empty weight. The Open category classes hinge on it: C0 below 250 g, C1 below 900 g with a defined impact-energy limit, C2 below 4 kg, C3 and C4 below 25 kg (C4 has no automation beyond stabilisation). C1, C2 and C3 aircraft carry direct remote identification and geo-awareness, and C2 provides a low-speed mode that unlocks the 5 m A2 separation.
Performance
Every performance figure is conditional. Endurance falls with payload, cold, wind and aggressive manoeuvring; a claimed control range assumes perfect line of sight and an uncongested band, which is why legal VLOS, not the brochure, sets your practical distance. Density altitude - hot days and high ground - reduces lift and thrust. Battery capacity sags in the cold, and GNSS accuracy degrades near buildings and under heavy canopy; magnetic interference triggers compass errors.
Traps: confusing empty weight with MTOM - adding 30 g of prop guards to a 240 g aircraft pushes it out of C0. Assuming the advertised range is usable. Assuming maximum speed is a limit you should fly at. Forgetting that the 120 m limit is measured from the closest point of the surface, not from your launch point.
Rule of thumb: plan endurance at no more than about two-thirds of the advertised figure, keep batteries warm until launch, and never fly below the manufacturer's stated minimum temperature - cold is the most common cause of an uncommanded landing.