MULTI-ROTOR 101

By Oliver Volkmann, published in Drone 360

 

1. THE BASIC ANATOMY OF MULTI-ROTOR AIRCRAFT:

So you like drones, but if you had to admit it, you probably don’t know that much about them. Not to worry, we’ve got you covered.

2. SO, WHAT IS A MULTI-ROTOR AIRCRAFT?

LIKE THE NAME SAYS, a multi-rotor aircraft is any aircraft that uses multiple propellers to generate vertical lift.

Multi-rotors are often referred to as vertical takeoff and landing (VTOL) platforms. They can hover in place and fly independently of their forward orientation. The ease of control makes multi-rotors a great choice for model aircraft beginners. Generally, a multi-rotor consists of a central core to which a number of arms are attached. The end of each arm houses one or two brush-less electric motors that each drive a fixed-pitch prop.

3. HOW MANY DIFFERENT KINDS ARE THERE?

IN ADDITION TO fixed-wing and helicopter models, you can choose from several different drone types, including:

■ Quadcopter (four motors)
■ Tricopter (three motors)
■ Hexacopter (six motors)
■ Y6 (two motors on each of the three arms)
■ X8 (two motors on each of four arms)
■ Octocopter (eight motors)

4. WHAT’S UNDER THE HOOD?

FIRST, PICTURE A MULTI-ROTOR hovering in front of you with zero wind conditions. To maintain that position and altitude, its air­frame has to be oriented so the combined thrust of the rotors is constant and vertically centered under the unit’s center of gravity. In addition, the combined thrust of the rotors has to be equal to the weight of the aircraft. Any outside effect disturbing the hover position has to be countered by an adjustment to the direction of the combined thrust delivered by the rotors. To understand the challenge of that task, imagine balancing a broomstick on the tip of your finger: It requires constant repositioning of your finger under that broomstick’s center of gravity to keep it from falling. Tricky, right?

Well, multi-rotor copters make these adjustments automatically by continuously redistributing relative thrust values across the rotors.

If the right-hand side of the platform drops, the unit increases relative thrust from the rotors located on that side of the airframe until it’s level again. We don’t have the multitasking power to manage more than two motors for such a balancing act.

Luckily, we live in an age of powerful, tiny computers and sensors. Every multi-rotor copter has a flight controller, which consists of an inertial measurement unit (IMU) and computer. Gyroscopes and accelerometers inside the IMU measure the cop­ter’s orientation and movement at rates as high as 800Hz (800 times per second). The computer evaluates the information, then calculates and makes thrust adjustments for each motor, all without user input.

Typically, the flight controller makes those adjustments by sending instructions to electronic speed controllers (ESCs), themselves small computers that control how fast a motor spins a rotor. The faster the rotor turns, the more thrust it generates. Unlike pitch-controlled thrust (used on conventional helicopters), most electrically propelled multi-rotors use this hyper-accurate control over rotor speed to adjust relative thrust among the rotors.

By Oliver Volkmann, published in Drone 360