1. Drones 101

The parts

A quadcopter is four motors with propellers on arms, a battery, and a small computer called the flight controller. AnaTele works with flight controllers running ArduPilot, the open-source autopilot used in everything from hobby quads to survey drones.

ArduPilot numbers the motors of an "X" quad the way the figure shows. Motor 1 is front-right, not front-left, which surprises most people. Diagonal pairs spin the same way: M1 and M2 counter-clockwise (CCW), M3 and M4 clockwise (CW). Each spinning propeller twists the frame the opposite way. With two of each direction, the twists cancel and the drone doesn't spin.

Why this matters in an investigation

If a motor is wired to the wrong output, or a propeller is on the wrong motor, the drone usually flips on takeoff. AnaTele reads SERVOn_FUNCTION, FRAME_CLASS and FRAME_TYPE from the log so that every motor number it shows matches this picture.

Three angles: roll, pitch, yaw

The attitude of a drone is which way it's facing and how it's tilted. Three angles describe it:

• Roll: tilting left or right, like a plane banking.
• Pitch: nose up or nose down.
• Yaw: turning left or right while staying level. This is the drone's heading.

A quadcopter has no rudder or wings, so it moves by tilting. To fly forward it pitches its nose down. Part of the thrust then points backwards and pushes it forward. That is why "is the tilt correct?" is the most important question in chapter 5.

Thrust, weight and hover

To hover, the motors must push up exactly as hard as gravity pulls down.

The simulator's drone weighs 1.6 kg, so its weight is

\[ W = m g = 1.6\,\text{kg} \times 9.81\,\text{m/s}^2 \approx 15.7\,\text{N}. \]

ArduPilot records the motor output needed to hover as MOT_THST_HOVER. In the simulated flights it is 0.38: 38 % of the usable range. More output gives more thrust and the drone climbs. Less output and it descends.

Motors also have limits. Below MOT_SPIN_MIN (0.15 here) they can stall. Above MOT_SPIN_MAX (0.95) there's no headroom left. A motor that sits at one of these limits is saturated. Chapter 6 shows why that's an important clue.

Check your understanding

The drone is hovering and you want it to roll right. Which motors must push harder?

Answer

The left ones (M2 back-left and M3 front-left). Pushing up harder on the left tips the drone to the right.