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The author flies his homebuilt UAV, the Kestrel-6. (Ed Darack)

Build Your Own Drone

Made from scratch, a Kestrel takes flight.

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Step 2: Better be brainy

Multi-rotors fly with remarkable speed and stability because they have a number of points of thrust, not just one. Each works against and with other thrust points—and with and against gravity—to move the craft along three axes, and, if needed, hold it steady in one position.

A multi-rotor pitches, rolls, yaws, and hovers by varying the speed of its motors (each connected to a fixed-pitch propeller) individually, which varies thrust (for pitching and rolling) and torque (for yawing). This type of aircraft, however, is inherently unstable unless “balanced” by a very powerful flight control computer, one that can analyze aircraft attitude and position, then provide control inputs (as motor speed rate changes) orders of magnitude faster than a human’s ability. Think of trying to balance a baseball atop the tip of a pencil: You’re not really “balancing” it, but constantly moving the pencil under the baseball in a dance with gravity to get a few brief moments of relative stability. But most people don’t have the eye-hand coordination for such a feat. Similarly, until recently, sensors and computers simply couldn’t work fast enough to use multiple thrust points to control a small aerial vehicle.

Over the past few years, the electronics industry has made great strides in the development of micro-electromechanical systems and inertial measurement units. These include tiny, solid-state, multi-axis gyroscopes for spatial orientation and accelerometers to measure change in velocity to guide multi-rotor and other types of aircraft. Manufacturers also produce micro-electromechanical magnetometers for navigation, and pressure sensors (barometers) for altitude determination.

I settled on a company at the forefront of the technology, Hoverfly Technologies. Their HoverflyPRO control module uses 16 parallel processors in its flight control computer to analyze thousands of inputs per second from the onboard three-axis gyroscope, three-axis accelerometer, and digital pressure sensor.

The controller, a printed circuit board that measures just 2.75 by 2.75 inches by 0.5 inch high, takes flight control inputs from a digital receiver (taking commands from a user-controlled transmitter on the ground) and tells the Kestrel to go, stop, and hover.

The board commands the camera to pivot up and down, and side to side, has an altitude-hold function, and overlays vital flight data on live video fed to a ground station—if a video transmission system is mounted to the craft.

I also bought the HoverflyGPS control unit, which, when mated to the PRO board, adds three-dimensional position hold, automatic return-to-home, and waypoint navigation.

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