Building a Quadcopter

A quadcopter hovers when four propellers throw enough air downward to support its weight, and it stays pointed the right way only because two of them spin one way and two the other.

A pilot commands four things, up and the three rotations, and a fixed set of sums and differences of the four motor speeds delivers each one without disturbing the others.

Treating a propeller as a disc that accelerates the air through it explains where thrust comes from, why hovering is expensive, and why a big slow propeller beats a small fast one.

A quadcopter's motors turn electrical power into a spinning magnetic field with no rubbing contacts, and one number, the Kv rating, ties the voltage you feed them to the speed you get out.

The electronic speed controller is the motor's missing brushes: it senses where the rotor is and switches battery current through the three phases in time, taking a throttle number and giving back a precise speed.

A lithium battery's cell count sets the voltage, its capacity and C rating set how much current it can give and for how long, and everything between it and the motors is a fight against the resistive loss that grows with the square of the current.

A tiny chip holds a gyroscope that measures how fast the craft is turning and an accelerometer that feels which way is down, and each is trusted for what the other cannot do.

A complementary filter trusts the gyroscope over short times and the accelerometer over long ones, and the single blend it makes gives a tilt estimate that is neither drifting nor shaking.

A quadcopter holds itself steady by measuring the error between what it is doing and what it was told to do, and correcting it with three terms that answer the present, the past and the future of that error.

The frame carries the thrust, survives the crashes, and above all must be stiff enough that its own flexing does not feed vibration into the sensor the control loop depends on.

A handheld transmitter sends the four stick channels and a handful of switches to a receiver on the craft, and the qualities that matter are latency, range, and what the craft does the instant the link drops.

A camera, a video transmitter and a pair of goggles put the pilot inside the craft, and the whole chain is judged by one thing above picture quality: how little it delays the view.

The parts become a craft in a fixed order, each step checked before power reaches it, and the first flight is a careful sequence of bench tests that ends, if nothing is wrong, with a gentle hover.