Quadcopter is an unmanned aerial vehicle (UAV) or drone with four rotors, each with a motor and propeller. A quadcopter can be manually controlled or can be autonomous. It's also called quadrotor helicopter or quadrotor. It belongs to a more general class of aerial vehicles called multicopter or multirotor.

Small quadcopters are easy to build because of low cost, low inertial force and simple flight control system. Quadcopters provide stable flight performance, making them ideal for surveillance and aerial photography. Other application areas include delivery, land surveys, crop assessment, weather broadcasting, and more. Quadcopters exist in many sizes, from palm-sized ones to those that can carry passengers or heavy cargo.

Civilian use of quadcopters is subject to regulations, which are not mature in many countries. However, quadcopters are an important part of future transportation called Advanced Air Mobility (AAM).

## Discussion

• What's the working principle of a quadcopter?

The main principle behind the flight of a quadcopter is Newton's Third Law of motion, which states that for every action there's an equal and opposite reaction. A quadcopter's propellers push air downwards. This causes an opposite reaction called thrust that pushes the quadcopter upwards against gravity. Air movement comes from Bernoulli's Principle, with larger propeller blades and faster rotation creating more thrust.

When the propellers rotate (for example clockwise), the quadcopter will tend to rotate anti-clockwise. Rotational force is called torque. Helicopters solve this by using a tail rotor. Quadcopters solve this by driving two diagonal propellers clockwise and the other two anti-clockwise. Thus, torque from one pair cancels that of the other.

When each diagonal pair of propellers rotate in opposite directions, their thrusts will be in opposite directions. The quadcopter will not be able to lift up or fly. This is solved by having the blades of each diagonal pair of propellers shaped as mirror images of the other pair. Effectively, all propellers will push air downwards regardless of the direction of rotation.

• What are the components of a quadcopter?

A quadcopter has four motors that drive four propellers. Propeller blades are shaped to create maximum possible thrust. These are attached to the ends of a frame made of aluminium, carbon fibre or balsa wood.

Four Electronic Speed Controllers (ESCs) drive the motors. ESCs are typically placed along the four arms of the quadcopter. They drive the motors by supplying specific voltage and current. Typically, an ESC must be rated at 1.2-1.5 times the maximum current rating of the motor.

Flight Controller (FC) may be considered the "brain" of the quadcopter. It commands the ESCs autonomously or in response to remote commands. It may process inputs from many on-board sensors. For remote control, a quadcopter is equipped with a wireless transceiver.

The power to drive the motors and all electronics comes from a battery via a Power Distribution Board (PDB). Flight time can be estimated from battery capacity and maximum current drawn by all motors.

Other parts include jumper cables, bullet connectors, LEDs and a buzzer. Quadcopters may have a GPS receiver, a camera and many other sensors as required by specific applications.

• How does a quadcopter achieve different movements?

A quadcopter has four different movements:

• Vertical: Quadcopter can move up against gravity or come down in a controlled manner. These movements are called throttle.
• Rotational: Clockwise or anti-clockwise turns about the vertical axis is called yaw.
• Front-to-back Lateral: Tilting along the lateral axis, called pitch, causes the quadcopter's nose to dip or rise. This in turn enables forward or backward motion.
• Left-to-right Lateral: Tilting along this axis, called roll, enables movement towards left or right.

The above imply that a quadcopter has six degrees of freedom: linear movements in along x, y and z directions; rotational movements called yaw, pitch and roll.

The movements are achieved by controlling the rotational speeds of the four motors. This is shown in the figure. For example, with pitch, when back propellers rotate faster than the front ones, the nose dips down and the quadcopter moves forward. With yaw, when two propellers rotate clockwise faster than the other two, the quadcopter turns anti-clockwise.

• What are the different types of quadcopters?

The different types of quadcopters based on their shape are as follows:

• X Quadcopter: A popular design, it's used for aerial photography, videography, First Person View (FPV) racing and acrobatic stunt flying. Frame variations include true X, square, hybrid X and stretched X.
• X-Stretched Quadcopter: Also called V configuration, it's elongated design provides more stability on the pitch axis.
• H Quadcopter: Has a H-shaped frame.
• + Quadcopter: This tracks well when going straight. To go straight or sideways, one motor is driven faster than the other on the opposite arm. Propellers are in an aerodynamically efficient position. It's used in acrobatic flying and FPV stunt flying.
• Y4 Quadcopter: Frame has three arms like a tricopter but rear arm has two motors coaxially mounted for better yaw control. While a tricopter use servo motors, Y4 uses BLDC motors. Hence Y4 has more lifting power and is more durable.
• V-tail or A-Tail Quadcopter: Similar to Y4 but with rear motors mounted at an angle in a V or A shape. It has more yaw control because it uses thrust to turn rather than counter torque.
• What are the applications of quadcopters?

We note the following applications of quadcopters:

• Aerial Photography: Used in filming action sequences and sci-fi scenarios, which makes cinematography a lot simpler.
• Shipping & Delivery: Drone delivery is supported by major corporations including Amazon, UPS, and DHL. Used to transport tiny parcels, groceries, medications, and other items across short distances.
• Geographic Mapping: In difficult-to-reach places like coasts, mountaintops, and islands, quadcopters can collect very high-resolution data and download pictures.
• Disaster Management: After a natural or man-made disaster, quadcopters can quickly negotiate debris and gather information in search of injured patients.
• Precision Agriculture: Quadcopters' infrared sensors may be calibrated to assess the health of crops, allowing farmers to react and enhance agricultural conditions locally with fertilizers or pesticides.
• Weather Forecasting: Quadcopters are being developed to track hazardous and unpredictably changing weather. They can be cost-effectively deployed in storms and tornadoes.
• Wildlife Monitoring: Elephants, rhinos, and big cats were photographed using quadcopters. Drones can function at night thanks to their infrared cameras and sensors.
• Entertainment: In fights and cage matches, quadcopters are utilized to collect recordings and images.
• What are the design considerations for a quadcopter?

The essence of quadcopter design is to generate enough thrust to carry the payload. Payload depends on the application. Thrust is provided by good propeller design. Their blades are shaped to minimize drag and maximize thrust. Each rotor produces thrust and torque about its center of rotation. There's gravity. There's also drag in the opposite direction to the flight. These are main forces to consider during design.

Good flight stability and control are essential. Safety and durability are important.

Brushless DC (BLDC) motors are commonly used for quadcopters. These consist of a permanent magnet which rotates around a fixed armature. Compared to brushed DC motors, we get more torque per weight, reduced noise, increased reliability, longer lifetime and increased efficiency.

For remote/radio control (RC), we need to select the correct wireless technology. This determines the range of communication and control.

For power, Lithium Polymer (LiPO) and Lithium Polymer High Voltage (LiHV) batteries are commonly used for quadcopters. They should be shaped for neat mounting within the frame. Battery capacity determines flight time. Higher capacity generally means heavier batteries and hence there's a trade-off.

• How to calculate the thrust of a quadcopter?

Payload is weight in addition to the "bare weight" of the quadcopter consisting of its essential components. For a typical drone, the average payload is 0.3-2 kg. A cargo drone can carry 20-200 kg. In either case, thrust should be sufficient to deliver the payload.

To get the quadcopter airborne, the quadcopter needs a certain amount of thrust. Thrust is the force normal to the plane of the propeller. Thrust from a single motor $$r$$ for $$r \in [1..4]$$ is given by $$T = ρA{V_r}^2$$, where $$ρ$$ = real-time air density (kg/m3), $$A$$ = cross-sectional area of the propellers, $$V_r$$ = instantaneous peripheral velocity of rotors (m/s).

Total thrust is includes thrust from all four rotors. In the simple case of upwards throttle (vertical take-off), this should exceed gravitational force.

• What are range and endurance of a quadcopter?

Endurance is defined as the total time that a quadcopter stays in the air on a full tank of fuel. For a quadcopter, the battery is the fuel. 2 hours, 31 minutes, and 30 seconds is the longest endurance a quadcopter achieved. This record was created by Ferdinand Kickinger of Germany in 2016. It used low discharge rate and high-capacity lithium-ion batteries. He stripped the airframe of non-essential weight to reduce power draw and extend endurance. To calculate the endurance in minutes, take the battery's capacity in amp-hours. Divide it into the average current draw and then multiply it by 60.

Range is defined as the distance travelled by an aerial vehicle on a full tank of fuel. A high-end consumer drone can have a drone of about 2.5-4.5 miles (4-8 km). Mid-level consumer drones have a range of about 0.25-1.5 miles (0.4-3 km). DJI Mavic 2 Pro is the longest range drone.

• Which type of control system is used in a quadcopter?

Quadcopters require flight control software and hardware elements. These allow it to be controlled remotely. It's either done by a pilot or autonomously by an onboard computer. Flight dynamics are highly variable and non-linear. Maintaining attitude and stability may require continuous computation and readjustment of aircraft's flight systems.

Elements on the ground form a Ground Control System (GCS) and use a modem and datalink for communication. Quadcopter collects information from an Air Data System (static and dynamic pressure), GNSS receiver or Attitude and Heading Reference System/AHRS (roll, pitch, and yaw data). A Flight Control Computer (FCC) uses this data to guide the quadcopter to its next waypoint. The FCC also operates payloads and communicates with the GCS.

MATLAB/Simulink are the tools to carry out the simulation, how a quadcopter operates. The three controllers are Feedback Linearization with PD Controller (FBL+PD), Feedback Linearization with LQR (FBL+LQR), and PID Controller are implemented as separate Simulink models and simulated.

## Milestones

1907

The first quadcopter is created by brothers Jacques and Louis Bréguet.

1917

The first pilotless aircraft is developed after the outbreak of World War I. It's named as Ruston Proctor Aerial Target. It's a radio-controlled pilotless airplane, based on RC technology from the inventor Nikola Tesla. The goal of the Aerial Target is to act as a flying bomb, which can pilot into enemies.

1920

Quadcopters are developed by engineers to solve the problems that helicopter pilots had with making vertical flights. The first one is named with the Omnichen 2 by Etienne Omnichen.

1935

The British develop "Queen Bee", a radio-controlled target drone for radio-controlled unmanned aircrafts.

1937

The U.S. Navy starts experimenting with radio-controlled aircaft. This results in the development of Curtiss N2C-2 Drone.

1958

Curtis Wright VZ7 is a VTOL quadrotor helicopter designed by the Curtiss-Wright company for the US Army. It is a "Flying Jeep".

1981

The Israeli IAI Scout drone is operated in combat missions by the South African Defence Force against Angola during Operation Protea.

1999

The Bell Boeing Quad Tiltrotor is a fixed quadcopter combining with the tilt rotor concept for a C-130 size military transport.

2013

Amazon proposes the use of commercial drone technology in delivery system.

2018

Airbus is developing a battery-powered quadcopter to act as an urban air taxi, initially without a pilot, but potentially autonomous in future.

2021

A Cessna 172 of Canadian Flyers International Inc. registered as C-GKWL collides with a drone operated by the York Regional Police while on approach to Buttonville Municipal Airport.

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