Aerodynamics and remotely piloted aircraft performance
Lift, drag and the environment
A multirotor generates lift through rotor thrust; lift depends on air density, rotor speed and disc area, so anything that thins the air - heat, altitude, humidity - reduces performance. Induced drag is greatest at low speed and in the hover; translational lift makes forward flight more efficient, which is why hovering in one spot burns battery fastest. Fixed-wing RPA trade the same four forces but live with stall speed and the need for a launch and recovery area.
Battery and power realities
Lithium batteries sag under load, and voltage falls as charge depletes; capacity is not linear with time. Endurance is reduced by payload, wind, aggressive manoeuvring, cold temperatures and high density altitude. Maximum endurance speed is slower than maximum range speed. Plan reserves - a common planning figure is to land with at least 20–30% remaining.
Worked example / rule of thumb
On a 35 °C day at 1000 ft elevation, density altitude may exceed 3000 ft. Expect a noticeably slower climb and shorter endurance: derate payload and plan shorter legs. Rule of thumb: for every significant increase in density altitude, assume less lift, less climb and less time.
Traps
Confusing ground speed with airspeed in wind; assuming return-to-home climbs over terrain it has not mapped; forgetting compass/GPS degradation pushes the aircraft into a less stable attitude mode; overlooking that a tailwind outbound becomes a headwind home.