Drones
What you’ll learn
- How a quadcopter creates lift and steers
- What the flight controller does
- Key parts, and staying safe and legal
Four propellers
A quadcopter has four motors. Two props spin clockwise and two anticlockwise, so their twisting forces cancel out.
| To do this | Change motor speeds like this |
|---|---|
| Climb / descend (throttle) | All faster / all slower |
| Tilt forward (pitch) | Back motors faster than front ones |
| Tilt sideways (roll) | One side faster than the other |
| Rotate (yaw) | Clockwise pair faster than anticlockwise pair (or the reverse) |
Tilting points some of the thrust sideways, which moves the drone.
The flight controller
A drone is unstable on its own; it would flip in a fraction of a second. The flight controller reads an IMU hundreds or thousands of times a second and runs PID loops to adjust every motor and keep it level. Open-source firmware such as Betaflight, ArduPilot and PX4 does this. ArduPilot and PX4 also handle GPS navigation and missions.
Main parts
- Frame, 4 brushless motors and props
- ESCs (electronic speed controllers), one per motor
- Flight controller, radio receiver
- LiPo battery (see Safety)
Safety and rules
Example: why yaw works
The clockwise props push the drone’s body anticlockwise, and vice versa. Speed up the clockwise pair (and slow the other pair to keep total lift the same) and the body turns anticlockwise.
Example: a PID loop for roll
error = desired_roll - measured_rollcorrection = Kp*error + Ki*sum(error) + Kd*(change in error)left_motors = throttle + correctionright_motors = throttle - correctionThe same loop runs for pitch and yaw, many hundreds of times a second.
Which motors?Easy
To move forward, which motors spin faster? The back two, so the drone tilts nose-down and thrust pushes it forward.
Why opposite spins?Medium
What would happen if all four props spun the same way? Their reaction torque wouldn’t cancel, so the whole drone would spin continuously.
Look up the rulesHard
Find your country’s official drone rules. What weight requires registration, and what’s the maximum allowed height?
Quick check
Finished this lesson?