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News & Notes

Multirotor 101

We answer 5 questions you have about multirotor drones

June 14, 2016

Basic Anatomy
Multirotor aircraft include most of these components.
thinkstock.com
 1  So, what is a multirotor aircraft?

Like the name says, a multirotor aircraft is any aircraft that uses multiple propellers to generate vertical lift. Multirotors are often referred to as vertical takeoff and landing (VTOL) platforms and multicopters. They can hover in place and fly independent of their forward orientation. The ease of control makes multirotors a great choice for model aircraft beginners. Generally, a multirotor consists of a central core to which a number of arms are attached. The end of each arm houses one or two brushless electric motors that each drive a fixed-pitch propeller.


 2  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 three arms)
  •  X8 (two motors on each of four arms)
  •  Octocopter (eight motors)
Illustrations Rick Johnson
 3  What's "under the hood" that makes multirotors easy to operate?

First, picture a multirotor 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, multirotor 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 multirotor 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.
 4  How do I fly these things?

Operator control of a multirotor is facilitated by means of remote control, of course, which is most commonly done via a conventional radio transmitter. But many models also allow for transmission of commands via laptop, tablet, or smartphone using specially designed computer programs or apps.

Multirotors are equipped with a small, lightweight radio that receives instructions from a radio transmitter on the ground and passes these instructions to the flight controller. Using the transmitter, you can issue instructions to your multirotor. So, for example, if the wind is blowing to the left, you can steer against the wind by instructing the multirotor to fly to the right. If you want to change altitude, you can increase or decrease overall thrust until you hit your desired altitude. It’s as easy as it sounds, and the ease of piloting a multirotor is a major factor in its growing popularity.
Start small
It may be tempting to jump in and buy an advanced and costly drone, but it's OK to perfect your skills on a fun-sized, low-cost multirotor first.
Micro Aerial Projects LLC
 5  How much should I spend on a drone and what do I get for my money?

Small units start at around $30 apiece, and, at that price, they’re pretty much disposable. Granted, the cheaper the unit, the fewer sensors and integrated features you’ll get. But these low-cost, entry-level multirotors are great for beginners. Because let’s face it: It’s not a question of “if” but  “when” you’ll experience your first crash. The Dromida Verso is cheap, fun to fly, and recovers quickly from a crash.

For a few dollars more, you can get a small copter capable of doing flips and rolls — and equipped with a small camera, like the Blade Inductrix 200, to record your flights. Typically, multirotors under $350 don’t come with the onboard sensors you really need to automatically maintain your unit’s position. Get ready for sometimes frustrating trial and error before you get familiar enough with the controls to do smooth, more accomplished flying. Typical flight times are in the range of 7 to 15 minutes, and many of these platforms are not capable of carrying additional payload.

Starting at around $350, you can purchase a ready-to-fly (or RTF) multirotor equipped with more advanced flight controllers that incorporate more sensors, such as global navigation satellite system (GNSS) receivers, which significantly shorten the learning curve to safely fly multirotors. The advantage of GNSS flight modes is that at any point, you can let go of the controls and the flight controller will take over and maintain the aircraft’s position (within the limits described in the owner’s manual). Another great advantage of GNSS-equipped multirotors is that they often come with a “return to launch/home” feature, a failsafe that automatically brings the multirotor back to its first GPS fix, typically the launchpoint. GNSS flight modes should always be reserved for outdoor flying in environments with a clear view of the sky, because they rely on satellite signals to calculate position.

In the $350 to $1000 range, you’ll find a variety of multirotor platforms with myriad options. Some come with stabilized camera mounts (primarily for action cameras), some with integrated cameras, and some include the ability to automatically execute preprogrammed waypoint missions. The most common flight modes include: altitude hold (altitude maintained by the onboard flight controller), GPS position hold (position maintained by flight controller), and stabilize/attitude mode (only stability maintained by flight controller). Flight times in this price bracket range normally from 10 to 20 minutes with a full payload. Check out the Traxxas Aton or or Xiro Xplorer if this is your price range.

Above $1,000? You’re in the prosumer market with customizable multirotor platforms, greater payload capacity, and longer flight times (say 20 to 28 minutes).  But first things first: Invest time in developing your flying skills before you invest money in a high-end unit like the Phantom 4.


Note: A version of this story appeared in the Drone360 March/April 2015 issue.


Featured image: Oliver Volkmann