Most drone operators can fly a Mavic confidently within a week. Understanding why the Mavic stays in the air takes longer, and it’s the difference between a pilot who recognises a developing problem early and one who is surprised by it. This briefing is a tour of the basic principles that the UK Remote Pilot syllabus covers under "UA General Knowledge."

The four forces

Every aircraft — multirotor, fixed-wing, VTOL hybrid — flies because four forces are in balance:

  • Lift — upward, generated by airflow over an aerofoil (or, for multirotors, by spinning propellers).
  • Weight — downward, gravity acting on the aircraft mass.
  • Thrust — forward, from the motors and propellers.
  • Drag — backward, the air resistance the aircraft must overcome.

In level, unaccelerated flight, lift equals weight and thrust equals drag. Climb requires more lift than weight; acceleration requires more thrust than drag.

Lift and the conditions that affect it

Higher air density means more lift for the same airspeed. Three factors degrade air density:

  • Altitude — pressure drops with height, density drops with it.
  • Temperature — hot air expands and becomes less dense.
  • Humidity — humid air is less dense than dry air.

The "hot and high" combination is the classic lift killer. Crisp winter mornings at sea level produce the best lift; muggy summer afternoons at altitude produce the worst. For drone work this matters when operating in elevated terrain, on hot days, or with payload near MTOM.

Primary flight controls

Multirotor and fixed-wing aircraft are fundamentally different in how they steer.

Multirotor

A multirotor has no control surfaces. The flight controller varies individual motor speeds to produce differential thrust, which generates pitch, roll, yaw and height control. In a typical Mode 2 controller layout:

  • Left stick up/down → collective thrust → climb / descend.
  • Right stick forward/back → cyclic pitch → nose down to move forward, nose up to slow.
  • Right stick left/right → cyclic roll → bank left / right.
  • Left stick left/right → yaw → nose left / right.

Fixed-wing

Fixed-wing uses moving control surfaces driven by servos:

  • Elevators — pitch about the lateral axis.
  • Ailerons — roll about the longitudinal axis.
  • Rudder — yaw about the vertical axis.
  • Flaperons / spoilers — increase lift and drag at low speed, for takeoff and landing.

VTOL hybrid

Vertical takeoff and landing aircraft combine both. They take off vertically using rotors, transition through a controlled rotation of thrust vector (either via tilting the rotors or by engaging a separate forward-propulsion motor), then operate as fixed-wing for the cruise portion. The transition is the riskiest phase — managed by the flight controller blending vertical and horizontal thrust.

Sensors and the flight controller

The flight controller is the brain. It receives data from a stack of sensors:

  • Inertial Measurement Unit (IMU) — accelerometers and gyros for attitude and acceleration.
  • Barometric pressure sensor — altitude reference.
  • Compass / magnetometer — heading reference.
  • GNSS receiver — position fix.
  • Optical flow and sonar — ground-relative motion and altitude at low height.

The flight controller fuses these into a coherent picture and outputs commands to the Electronic Speed Controllers (ESCs), which in turn drive the motors.

GNSS — what it gives, what it doesn’t

Most consumer drones use one or more Global Navigation Satellite System constellations:

  • NAVSTAR (GPS, USA)
  • GLONASS (Russia)
  • Galileo (EU)
  • BeiDou (China)

Each has roughly 24 active satellites. A 3D position fix needs a minimum of four; more is better. Typical horizontal accuracy outdoors is 3–5 m.

GNSS gives you position and groundspeed. It doesn’t give you airspeed or wind speed — both of which matter for endurance and control margin in wind. A 10 m/s aircraft with a 9 m/s headwind has 1 m/s of net groundspeed and is burning energy at full throttle.

GNSS limitations to be aware of: weak signals, easily jammed (military exercises, summit weeks, cheap commercial jammers), distorted by ionospheric activity (Kp index above 4 is a flag) or by multipath effects from terrain and structures. What happens when your drone loses GNSS varies by airframe and mode — understanding the manufacturer’s documented behaviour is part of pre-flight preparation.

Datalink and the radio frequencies

The control link between the pilot’s controller and the aircraft, plus telemetry and payload data, runs on radio frequencies. Most consumer drones use:

  • 2.4 GHz — the dominant control band. Maximum legal output in the UK is 100 mW. Shared with Wi-Fi, Bluetooth and a lot of other consumer equipment. Doesn’t penetrate obstacles well.
  • 5.8 GHz — commonly used for video transmission. 25 mW maximum. Even worse at penetrating obstacles.

Sources of radio frequency interference to watch for during planning include High Intensity Radio Transmission Areas (HIRTAs), mobile telephone masts, urban Wi-Fi clusters, outside broadcast vans, military bases, and other UAS operators. If the datalink breaks, the aircraft enters its programmed failsafe behaviour (usually return-to-home or hover-and-land). What that behaviour actually does — especially when combined with GNSS loss — should be known cold by the pilot before launch.

Motors

Drone motors are brushless DC (BLDC) electric motors. Key ratings:

  • Voltage rating — the maximum safe voltage.
  • Current draw — amperes consumed during operation.
  • KV rating — RPM per volt without load. High KV motors spin fast with less torque (racing); low KV motors spin slower with more torque (lift-heavy commercial).
  • Thrust output — in grams or kilograms.

The stator is the stationary component containing windings; the rotor is the rotating component with permanent magnets. Current through the stator generates a magnetic field that interacts with the rotor magnets, producing rotation. Each motor is driven by an ESC that converts the flight controller’s digital signal into the right power waveform.

Batteries: the energy budget

Two chemistries dominate drone batteries:

  • Lithium-ion (Li-ion) — good energy density, modest peak discharge current, often managed by an internal Battery Management System (BMS), and safer to charge. Better for endurance, less suited to bursty thrust demand.
  • Lithium Polymer (LiPo) — high peak discharge current, strong instantaneous thrust, shorter total endurance, charged externally with a balance charger. Higher fire risk if abused.

Capacity is rated in milliampere-hours (mAh). Pack voltage depends on cell count in series: a typical 4S LiPo pack has 4 cells at ~3.7 V each, totalling ~14.8 V nominal. Cells in parallel increase capacity rather than voltage.

Thermal runaway

The headline LiPo / Li-ion failure mode. A damaged, overcharged or short-circuited cell overheats; the heat propagates to adjacent cells; those cells overheat in turn; the result is a cascading fire that is very difficult to extinguish (lithium reacts with water, so standard fire extinguishers can make it worse — Class D extinguishers are the correct choice).

Mitigations are mostly procedural: charge in fireproof bags or boxes, never charge at extremes of temperature, monitor charging supervised, check for swelling or gaseous smell before charging, dispose of any pack showing physical damage. Most working operators charge in a steel ammo box or LiPo-rated charging bag and keep a Class D extinguisher on hand.

Temperature effects

Cold reduces both capacity (less flight time) and peak discharge current (less thrust margin). Hot accelerates ageing and increases swelling and fire risk. Most modern smart batteries are designed to operate optimally between 15 and 30°C. Outside that band, performance drops or safety margins shrink.

The operational envelope

Every airframe has a documented envelope: maximum speed, maximum endurance, maximum wind tolerance (including gusts), temperature range, MTOM (maximum takeoff mass), centre of gravity tolerances. The pilot’s job, every flight, is to plan inside that envelope with margin. Drones don’t fail catastrophically at the envelope edge — they fail at conditions a step beyond the envelope, when the wind picks up, the temperature drops, or the payload was heavier than briefed.

Pre-flight checks address the slow drift of equipment condition: compass calibrated, propellers free of chips or cracks, everything secure, battery fully charged with margin for conditions, telemetry working, LEDs in correct state, multiple GPS fix achieved, correct flight mode selected. Every operator runs their own version of the checklist, but every version has the same intent: catch the small issues before they become big ones.