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Building Multicopter Video Drones

Chapter 1. What is a Multicopter?

In the simplest terms, a multicopter uses multiple propellers (rather than a single rotor blade such as on a traditional helicopter) to provide lift. Also, there is no tail rotor (used to provide yaw control and counter the torque put out by driving the main rotor on a helicopter). Multicopters come in many configurations. There are bicopters (two rotors such as on CH-46 helicopters), tricopters, quadrocopters, and so on. In the following image, you can see an example of a quadrocopter as well as two configurations of hexacopters (six rotors):

Let's also think of what a multicopter isn't. We've all heard that dreaded term … drones. A multicopter isn't really a drone in the true sense. Rather, it's defined by the U.S.'s Federal Aviation Administration as an Unmanned Aerial System (UAS). The term UAS covers a wide array of aircraft, from drones to your average hobby radio-controlled airplanes. Most multicopters are piloted in the line of sight (LOS), just as any radio-controlled airplane. This variety is not considered a drone. Technically, a drone both flies outside LOS and has the capability of autonomous flight (autopilot).

With specialized equipment, you can fly a multicopter using a first person view (FPV) camera system, telemetry, and so on, and turn your multicopter into a fully autonomous drone. Therefore, some drones are multicopters, but not every multicopter is a drone.

Most multicopter pilots, however, shy away from the term drone. This is because the term drone evokes images of aerial assassinations using missiles and guns mounted on the aircraft. For instance, in a 2013 article in the Santa Rosa Press Democrat about multicopters, a Santa Rosa police officer was quoted as mentioning "fly-by gang shootings" as crimes just around the corner. Once you know how a multicopter flies, the idea of mounting a weapon system onto a multicopter is physically laughable and ludicrous. Mounting a firearm to a drone provides such a counterforce (kick) that the mental image of the result conjures Wile E. Coyote chasing the Road Runner. The multicopter flying backwards as the bullet stays in one place. Comical indeed!

A multicopter's primary function is for videography, cinematography, and photography. So, what's the difference between a multicopter and a drone? The answer is the guidance system and how you choose to use it.

Still confused? Don't worry, that's what this book is for. Let's dive right in …

How a multicopter moves

This is truly where a multicopter shines. A traditional helicopter is not symmetrical from every angle; therefore, as it flies sideways, the tail wants to swing to the rear and the nose points in the direction of the flight. The wind pushes the tail just like a weather vane. The pilot must counter this by yawing in the same direction of flight, or stability can become an issue. Multicopters are symmetrical from every direction. Therefore, moving sideways has the same feel as forward flight to the pilot.

Just like a traditional helicopter, a multicopter moves forward/backward and from side to side by tilting. Tilting the multicopter changes the direction of the thrust provided by the rotors. For example, by dipping the nose and raising the tail, the direction in which the air flow is pushed is not only down, but also to the rear of the multicopter. If every action has an equal and opposite reaction, pushing air to the rear of the multicopter pushes the multicopter forward. To make one side dip, the speed of the propellers is reduced, and to raise another side, the speed of the motors on that side is increased. The following diagram shows how directional flight is achieved:

It all seems rather simple, right? Increasing and decreasing the speed of each motor provides movement in any direction. The direction is only dependent on the combination of motors that are increased or decreased. There are less moving parts in a multicopter than in a traditional helicopter. The movement in one direction has the same feel as in any direction because the aerodynamics are symmetrical.

So, why do traditional helicopters exist at all? The reason is that the electronics and components of a multicopter have only recently (after the year 2000) begun to be practical and small enough to really make them work. Imagine controlling the speed of each motor independently to control your multicopter by feel. It would be impossible without the help of some very sophisticated electronics. This brings us to the next section.

What's in a multicopter?

We'll go more in depth on each of these parts in Chapter 3, Choosing Your Components. For now, let's get acquainted with what makes up a multicopter.

The airframe

Multicopter frames come in all shapes and sizes, from basic quadrocopters to eight-bladed monster octocopters. They are available for a wide variety of prices too. Sometimes, large frames that cost more are really better. However, this is rare indeed. We will give you more details later, but note that before you choose any components, you should have your purpose in mind. Bigger is not always better, and smaller can't carry much weight. In the following image, you can see a hexacopter frame that retails for under 100 USD and can carry quite a bit of weight. Of course, the components required to fly such a beast can cost you well over 3000 USD (not including the batteries).

Motors and propellers

Motors and propellers are the main propulsion systems for your multicopter. It's truly where the rubber meets the road. These components are, by far, under the greatest strain of any component of your multicopter. Every ounce of weight that your multicopter carries rests on the blades of the propellers. So, as you can tell, strength is a prerequisite.

The bigger are the blades, the more lift there is. However, the bigger the blades, the more is the leverage placed upon the hub of the blades and more strain is exerted. Skimp on the blades and they'll snap, and your whole investment comes crashing to the ground.

Also, the bigger the blade, the stronger the motor must be in order to counteract the torque required to turn the blade. It might seem like the motor doesn't truly have to deal with a lot of resistance. But that's just not true. If a propeller is moving enough air to lift a couple-dozen pounds into the air, there is a lot of wind resistance that the motor must counteract. Faster motors are weaker. It's a giant balancing act to figure out how to get the right blades, motors, and so on, to lift your payload for the longest time possible.

The following image shows carbon fiber blades attached to motors that spin at 480 RPM per volt (KV):

The electronic speed control

The electronic speed control (ESC) is a marvelous invention. This truly makes flying multicopters possible. Electric motors require more voltage to start spinning than to keep spinning at their lowest speed. Also, as you apply more voltage, they don't necessarily speed up on an even curve. The ESC spikes the voltage to start the motor and eases it back to keep them spinning at a low speed on a low throttle. Also, as you apply more throttle input, the ESC accelerates the motor evenly. Furthermore, most ESCs can be programmed to any curve you like. A real tech head can have a field day just programming ESCs. Don't let that scare you though … most ESCs are preprogrammed with the necessary settings to get you going without ever needing to dive in.

ESCs are the pass-through from battery to motor. They must be carefully balanced with the motor to give enough power to the motor and not burn out. Putting an underpowered ESC in your multicopter can cause crashes … or even fires. You must have an individual ESC for every motor. The following image shows all the six ESCs tied down to the hub of this multicopter:

The guidance system (the brain)

Now this is where the real magic happens. To fly a multicopter, it literally takes tens-of-thousands of calculations per second to sense whether you're going up or down or whether you are moving, tilting, or rotating, all the while adjusting your motors to counteract these forces to keep your multicopter stable. There are several aspects to the guidance system.

Most guidance systems have the same set of sensors nowadays. The main difference from system to system is how fast the calculations are done and the algorithms that are used in the firmware. Yes … I said firmware. These are literally flying computers.

In the following image, you can see the three main components of the DJI WooKong-M guidance system (this system has been the industry standard for multicopters for several years, so we'll use it as an example):

The circular module in the given image is a dual-purpose antenna. It senses both the direction (using a compass) and the GPS location (by using a 6-12 satellite lock system). This provides extremely accurate positional data for the brain to do its work.

In the other section of the photo, you'll see two grey boxes. The one in the background is the sensor box. This includes a 3-axis accelerometer and gyros to determine the pitch, roll, and yaw movement several thousand times a second. Additionally, it contains a barometer to indicate altitude. The box in the foreground is the main brain that takes all this information and your control inputs (coming in on the left side from the radio receiver), compares that to the GPS and compass data to create an accurate impression of what the drone is doing (as well as what you wish it to do), and sends speed information to the ESCs (out on the right side), and in turn to the motors to move your drone properly and in a stable fashion. Like I said … this is where the magic happens.

Furthermore, there are more add-ons that you can get to interface with your guidance system. Camera gimbals can hook in the WooKong-M (and most other systems), and as the multicopter tilts to move, the camera can actually stay level. Also, onscreen displays (OSD) can be hooked in for your camera transmitter (allowing you to see all the telemetry, including battery life, attitude (orientation), height, and so on, on a viewing monitor while seeing what your camera sees). The following image shows the autonomous flight add-on. This unit communicates with the airborne multicopter from the ground using an iPad. You can actually click on a Google Earth map … and the multicopter will fly there. Or, you can even draw out preprogrammed flight paths. These add-ons are what truly transform a multicopter into a drone.

Camera gimbals and transmitters

The term camera gimbal is a short way of saying, "a fancy device that keeps the camera leveled and reduces vibration no matter what the multicopter does within reason." So yeah … camera gimbal is much shorter. Generally, these systems hook in to your guidance system and are tuned by the pilot (you) to work properly. Gimbals (good ones at least) are not cheap; the more weight that you want to carry and the more movement you want makes the price go up exponentially. The gimbal in the following image is capable of carrying a DSLR camera. It's made by Photoship One, and a new one retails for around 800 USD.

Furthermore, it's important to have a good transmitter. It's unheard of for any videographer to blindly shoot a video in a general direction while hoping to capture what he/she wants in a great way. It's unheard of because it's ludicrous, and the first sign that you've hired the wrong pilot. Transmitter/receiver systems are generally not all that expensive, and you can expect it to be one of the smallest investments you'll make. Be careful though. These can drain power rapidly if you get the wrong one.

Radio systems

Your radio is the primary interface between a human and a machine. It's important to get a good one that feels right for you. The buttons should be easy to find, and it should (above all) be reliable. FM transmitters are a way of the past. Futaba, with their FASST, and Spektrum/JR (with DSMx) are the waves of the present and future. No longer do you need to worry about competing transmitters, calling out a channel, or severe fading. The new generation of transmitters/receivers are digitally paired, and have LOS ranges in terms of miles. The following is an image of the Spektrum DX7s transmitter and the AR-8000 receiver with a satellite: