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Written by Patrick Sherman
Advanced Flight Technologies
As seen in the March 2017 issue of
Model Aviation.

The most pernicious mistakes that we make flow from the things that we know best. This fundamental truth was brought home to me after my article about the US National Drone Racing Championship appeared in the December 2016 issue of Model Aviation. Shortly after it began arriving in mailboxes, I received an email from a longtime AMA member whom I’ll refer to as Ron—because that’s his name.

Ron wanted to congratulate me on a remarkable “first.” I’d published an article about a major flying competition that included precisely zero photographs of the aircraft being flown. He was right, of course, and I had to stop and think about how that happened.

He also added that photos of racing quadcopters were relatively scarce in other publications and he just wanted to get a good look at one. I pointed out that racing quadcopters are small and fast, making it a challenge to capture a photograph of one during competition. They also tend to be ugly machines, with gangly limbs, exposed wiring, and scars from the frequent crashes that are an inevitable part of the sport.

However, none of that explained why I, a professional journalist with decades of experience, had failed to provide a photograph of the subject of my story. I then realized the source of my mistake—I know what a racing quadcopter looks like, and having been an active multirotor pilot for the past six years, I understand how the design evolved from the early tab-and-slot airframes.

To me, describing and illustrating quadcopters would be like reporting on the Daytona 500 and including a detailed description of a car. Ron comes from a different background in the broader aeromodeling community, and racing quadcopters are a legitimately new and mysterious type of aircraft worthy of a detailed explanation. So, Ron, this one is for you!

The racing quadcopter has been designed and built for a single purpose—to go fast—resulting in a rugged, functional design.

X Marks the Spot

Although most drone racing organizations allow for multirotor configurations mounting three, four, and six motors, by far and away, the most common configuration is the quadcopter: an airframe with four propellers mounted on individual limbs in the shape of an X. It has fixed-pitch propellers, and the motors are rigidly attached to the limbs.

All maneuvering is accomplished by means of differential thrust to control pitch and roll, and differential torque to control yaw. Quadcopters, as they are commonly known, are capable of hovering and moving in any direction, as is a helicopter. Movement through the air is achieved by generating thrust in excess of the lift necessary to keep the aircraft aloft—and can they move!

Speeds in excess of 60 miles per hour are common in competitive racing, and some advanced prototypes are capable of exceeding the 100 mph speed limit established by the FAA for all UAS under the FAA’s Part 107 (federal drone registration requirements). This fact was, presumably, established outside of the the US.

Racing quadcopters are powered by three- and four-cell LiPo batteries similar to those used in other electric flying activities; however, they tend to have high discharge ratings to accommodate the demands of racing. It is not uncommon for an aircraft flown at race speeds to deplete a battery in less than three minutes.

Each motor is independently controlled by its own ESC that is connected to the aircraft’s flight control system (its brain), which is responsible for taking control inputs from the pilot’s radio and translating them into specific instructions for the motors to speed up or slow down.

The radios themselves would be familiar to any RC pilot: two gimbal-mounted sticks with a variety of switches and knobs assigned to additional channels, transmitting a spread spectrum signal in the 2.4 GHz band. Unlike other types of drones, successfully flying a racing quadcopter is almost entirely dependent upon the individual pilot’s skill.

Keep It Simple

Throughout the past several years, multirotor flight controllers have become increasingly sophisticated with sensors that include accelerometers, barometric altimeters, GPS receivers, optical-flow cameras, ultrasonic and infrared range-detection systems, and even machine vision. These features make the aircraft more stable, easier to control, and enable autonomous flight-control modes such as return-to-home.

None of these exist on racing quadcopters. Instead, racers rely on simple gyroscopes—one for each axis—to interpret the movement of the aircraft in pitch, roll, and yaw relative to the pilot’s input. Consequently, racing quadcopters are capable of extremely aggressive, acrobatic flight performance.

When I first started flying multirotors seven years ago, this was the only type of available flight controller. With my own focus on controlled, stable flight to support aerial observation of events on the ground, I rejoiced when accelerometers were added to the suite of sensors because they allowed the aircraft to return to a level attitude when I moved the sticks back to the center position.

There are at least as many racing quadcopter configurations as there are racing quadcopter pilots. Even standard airframes are often heavily modified by the pilots, based on experience and personal preferences.

A typical racing quadcopter lacks even this fundamental capability. It will flip and roll indefinitely until it interacts with an external object—such as the earth’s surface.

Needless to say, using such a control scheme to guide a remotely piloted aircraft through 5-foot gates while traveling at 80 mph requires no small amount of skill. These pilots are as skilled as anyone who has ever competed in an AMA sanctioned event. Oh, did I mention that they do this wearing blindfolds?

In the Pilot’s Seat

Okay, so they don’t actually wear blindfolds, but they do wear FPV video goggles. Rather than watching the aircraft from the edge of the field as RC pilots have done for decades, racing quadcopter pilots fly from the perspective of their aircraft.

Each multirotor carries a camera with a wide-angle lens onboard, mounted between the two forward limbs. It is connected to a video transmitter, which sends a real-time video stream to the pilot, who typically watches it through goggles that completely obscure his or her view of the outside world.

To maintain overall situational awareness, a pilot works with a spotter—a person responsible for maintaining visual contact with the aircraft and advising the pilot of any hazards that the limited field of view prevents him or her from perceiving.

Although FPV flying has been the source of some controversy in the past within the AMA membership, it is now a well-established mode of operation. The requirements for FPV operation within the AMA Safety Code are outlined in Document 550, which has been revised several times to keep up with this fast-changing technology.

Needless to say, the extreme speed that racing quadcopters are capable of reaching affects how the FPV system is implemented for this particular application.
Taking a close look at a racing quadcopter, you will notice that the camera appears to be angled up, well above the horizon, when the aircraft is in level flight.

Of course, these multirotors spend little time in level flight. Instead, they pitch over to angles that can exceed 45°, converting most of the total
lift generated by their propellers into thrust and hurtling them downrange with startling speed.

-Patrick Sherman

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