➡​

⬅​

Unmanned Aerial Vehicles

• Unmanned Aerial Vehicles
  • 1. Prerequisites
  • 2. General Motivation
  • Chapter 2 : Drone Types and Use Case
    • 2.1 Rotorcrafts
      • History
      • Applications
      • Challenges
      • Questions
    • 2.2 Fixed wing drones
      • Design Principles
      • History
      • Applications
      • Challenges
    • 2.3 Flapping Wings:
      • History
      • Bird-inspired Flapping-Wing Robots
      • Insect-inspired Flapping Wing Robots
      • Questions
  • Additional Resources
    • Credits:
    • Additional Resources:

1. Prerequisites

To get the most of this module, it is recommended that you have knowledge in:

1. Basic Mechanical Physics
   • Newton’s laws of motion, especially the third law of action and reaction.
   • Concepts of moments and torques.

2. General Motivation

DJI mini pro 5 - a small consumer camera drone. Picture from DJI

Unmanned Aerial Vehicles (UAV) are flying object’s without a pilot and controlled remotely or are autonomous. They are usually referred to as drones. And probably now, when you hear the word drone you are thinking of a small commercial quadcopter people use to take stunning video shots like on the image above? Or maybe you are thinking of drone racing? Or maybe of military drones used more and more frequently in modern war? But, did you know that drones/UAVs are much more than only quadcopters? The first consumer drone entered the market in 2013 - the DJI Phantom 1. In the last decade the drone market got revolutionized and is growing in an incredible pace. More complex mechanics, more stable control and more autonomy. This and the following lectures will give you an overview of different drone types, aerodynamic principles, and what it takes to build and control an UAV.

This module about UAVs aims to give an introduction to aerial robotics and provide an overview over different drone types, their aerodynamical principles and their associated cost and benefits.

Chapter 2 : Drone Types and Use Case

In this chapter we want to give you an overview of different drone types, their flying principle and history. We grouped for that UAVs in three big groups: rotorcrafts or multirotor drones, fixed-wing drones and flapping wing robots.

Overview over the three main drone types: multirotor drone, flapping-wing drone and fixed-wing drone.

2.1 Rotorcrafts

Rotorcrafts are aerial vehicles that generate lift using high speed rotary blades called rotors. They are relatively easy to build, capable of vertical take-off and landing (VTOL), possess high maneuverability (rapid change of velocity vector in multiple directions), but are less energy-efficient for long-range flight than fixed wing vehicle.

The image below shows a collection of some state-of-the-art commercial rotorcrafts. These include tricopters, quadcopter, hexacopters and octocopters, ranging from underactuated to fully actuated systems. Today, rotorcraft UAVs are used in a wide variety of fields including agriculture, search and rescue, infrastructure inspection, cargo delivery, mapping, entertainment, and more.

The figure illustrates various multicopters. From top left: DJI Mavic Air 2, Autel Robotics EVO II, DJI Phantom Pro, CyPhy LVL 1 Drone, Freefly Alta 8, Skydio 2, Voliro T, Yuneec H520E, Yuneec Typhoon H Plus.

The lift force generation principle of a rotorcraft is similar to that of thrust generation using propellers - only the force acts vertically, countering gravity. Each rotor generates both lift and torque. To maintain balance, the system includes an equal number of clockwise and counterclockwise spinning rotors to cancel out rotational torque.

Drone movement is achieved by adjusting the rotational speed of individual rotors. For example increasing all rotors speed equally generates more lift, allowing the drone to ascend. By tilting the drone, the direction of the thrust force becomes misaligned with gravity, allowing the drone to move laterally or to rotate.

Different rotor configurations - both in number and arrangement - serve different operational needs and control strategies. You will learn more about this in the dedicated module about multirotor UAVs.

The figure illustrates four common drone configurations. A classic quadcopter, a hexacopter, octocopter and a co-axial copter. Each shows rotor rotation directions (blue: counterclockwise, green: clockwise) along with a representative commercial model. From left: DJI Mavic Air 2, Yuneec H520E, Freefly Alta 8, OnyxStar HYDRA-12.

History

The first flying rotorcrafts were quadrotors - a machine with four rotors - in 1922 by George de Bothezat. Luckily there exists some footage of that time, which allows us follow the development of rotorcrafts up to the modern age. Below you see a test flight from de Bothezat. The flight was not yet that stable nor high but here we have our first flying rotorcraft!

_{De Bothezat 1922 helicopter. First flying quadrotor. Available at: https://www.youtube.com/watch?v=oM6TqjHfC5I}

Due to the difficulty of simultaneously controlling four motor speeds for a human pilot, the development of quadcopters was paused and overtaken by the development of helicopters. Helicopters have a single rotor but need a more complex mechanical structure to balance torques and maneuver. On September 14, 1939, the world’s first practical helicopter took flight in Stratford, Connecticut. The VS-300, designed by Igor Sikorsky led the foundation of controllable rotorcraft. On the footage below you can see some of the early flights of the VS-300. Note the complex mechanical structure necessary for a helicopter to work. Mechanically a quadcopter is much simpler!

_{Igor Sikorsky test flies VS-300. Available at: https://www.youtube.com/watch?v=PnbKZOG2gII}

While helicopters are extremely fascinating vehicules, we don't want to spend more time on them and focus on modern multirotor drones. During the post-war era some development of quadcopters took again place - like the Curtiss-Wright VZ-7 in the 1950s - but the true comeback of multirotors was in the early 2000s with small-scale UAVs.

The Curtiss-Wright VZ-7 machine developed for the U.S. Army in the 1950s. It was retired only a few years later due to insufficient performance. Image from Librairie Images Collège Léodate Volmar.

The rise of compact, efficient microcontrollers, brushless electric motors, and miniaturized inertial measurement units (IMUs) finally solved the core challenge that had hindered quadcopters for decades: stable and responsive electronic control of multiple rotors. Thanks to these advances, flight control could now be fully automated and stabilized by onboard processors rather than a human pilot managing four motors manually. The processors doing this job are usually referred to as autopilot.

Phantom 1 from DJI, released on January 7, 2013. With this drone, camera drones became accessible to a wider audience for the first time.
Image from DJI.

This technological breakthrough sparked a wave of innovation. By the 2010s, several commercial brands entered the market, bringing drones to a wider audience. Chinese company DJI became a dominant player with the launch of the Phantom series in 2013, combining a compact quadcopter frame with integrated GPS, camera stabilization, and user-friendly controls. It was the first consumer drone on the market. Other notable companies like Parrot, 3D Robotics, and Yuneec also contributed to the growing drone ecosystem, offering different designs such as hexacopters and octocopters, tailored for heavier payloads and enhanced stability.

These drones overcame early limitations in battery life, GPS accuracy, and control range through continual improvements in battery technology, GNSS systems, and wireless communication protocols. The result was a rapid evolution from basic remote-controlled flying toys to highly capable autonomous systems used in filmmaking, surveying, agriculture, and more.

Today, multirotor drones take the biggest piece of the drone market and continue to evolve with the integration of obstacle avoidance, collision resilience, machine learning, and swarm coordination, opening up even more applications.

Applications

Nowadays the drone market covers a wide range of different applications with new spin-offs and start-ups continuously pushing the boundaries.

Search and rescue:

On the right, Mike Smith the chief drone pilot accompanied by Jim Cooper, a drone pilot in training.
Image from Murdo MacLeod/The Guardian.

Multirotor drones have become very useful tools in search and rescue operations due to their agility, stability, and ability to access hard-to-reach areas. They allow to search inaccessible and remote areas much faster and safer than a rescue team on the ground and much cheaper than a search team in a helicopter. Equipped with thermal imaging cameras, GPS, and live video feeds, these drones can quickly scan vast terrain where ground teams may struggle to reach. They assist in locating missing persons and pets, delivering emergency supplies, and providing real-time situational awareness to rescue teams.

Aerial photography and mapping:

In the fields of aerial photography and mapping, multirotor drones offer flexibility and precision. They are commonly used by photographers, filmmakers, and surveyors to capture high-resolution images and videos from various angles and altitudes. In combination with GPS and automated flight paths, drones can perform detailed topographic surveys and 3D mapping of landscapes, infrastructure, construction sites or archaeological sites.

Cinematic drone shots—like this one of Lake Bled in Slovenia—have become an integral part of modern photography and filmmaking. Screenshot from DJI Mavic Air Lake Bled, Slovenia by The Leisure Club, available on Youtube.

Inspection:

_{Voliro T for NDT and LPS inspection on a wind turbine and a gaz power plant. Available at: https://youtu.be/Q29N_pTc6kA?si=kTuKMjH5e8VwasCl&t=11}

Multirotor drones are transforming inspection processes across industries by offering a safer, faster, and more cost-effective alternative to traditional methods. They are widely used to inspect infrastructure such as power lines, wind turbines, pipelines, bridges, and telecommunications towers. High-definition cameras, thermal sensors, and LiDAR enable detailed visual and thermal analysis, reducing the need for scaffolding, cranes, or rope access. This not only improves worker safety but also minimizes downtime, reduces cost and enhances maintenance planning.

Transportation:

The last mile in goods delivery is the most cost and time intensive of an entire delivery chain. The idea to deliver goods over urban areas with drones is in the meanwhile already an old story. First companies and test flights occurred already in the 2010s. However, until today, they are struggling with restrictive regulatory laws, safety concerns, noise concerns and limited payload capacities. Nevertheless there exist some success stories, one of them is the Platform 2 from Zipline. They use a hybrid fixed wing and multirotor drone in combination with a droid, a small deliver unit, lowered on a tethered rope to achieve fast, quiet and high precision home delivery.

The Zipline Platform 2 autonomous drone delivery system focused on home delivery. They use a hybrid multirotor and fixed wing drone with a tethered delivery system from air. Image from Zipline.

Companies and research institutions are also developing larger multirotor systems for human transport and urban air mobility, exploring the potential for future drone taxis. However none of them is fully operational up to this day.

Agriculture:

With the advancements in technology, drones offer farmers new and innovative ways to improve efficiency and crop yields. They are used in crop monitoring, yield estimation and to spray fertilizers or pesticides in a targeted manner. This reduces chemical usage, labor cost and can increase crop productivity. An example is the Argas T-50 from DJI, specifically developed for the use in agriculture.

The Argas T-50 from DJI equipped with a sprayer for agricultural use. Image from DJI.

Military:

Drones form an integral part of modern warfare. Most of them being fixed wing drones for higher payload and range, multirotor drones are also used due to their small size and cost. They are mainly used for strategic purposes to spot enemy troops and direct attacks.

Entertainment:

Challenges

While the market and the innovation of multirotor drones grew exponentially over the past decade, there is still a lot of research going on, aiming to make drones more versatile and efficient. We want to provide below a non-exhaustive list with ongoing challenges.

1. Agility, Efficiency, and Autonomy
   Goals:
   • Faster and more responsive flight
   • Reduce energy consumption and decrease need for human control
   • Challenges: limited onboard processing power, battery life constraints, complex control algorithms
2. Aerial Physical Manipulation
   Goals:
   • Grasping or moving objects mid-air
   • Challenges: lightweight but stable design, complex mechanical structure & control
3. Tight or Cluttered Environments
   Goals:
   • Flying through narrow spaces (e.g., buildings, pipelines)
   • Challenges: adaptive design, real-time navigation
4. Landing on complex Surfaces
   Goals:
   • Inclined, moving, or uneven terrain indoors and outdoors
   • Challenges: ensure stable landing without human interaction
5. Drone Swarms
   Goals:
   • Coordinating multiple drones simultaneously
   • Challenges: communication, task distribution, swarm intelligence
6. Robust Obstacle Avoidance
   Goals:
   • Detecting and avoiding dynamic objects in complex environments at high speeds
   • Challenges: reliable sensors and fast processing

Addressing these challenges to improve drone safety, functionality and to expand their application is question of current research.

Questions

TODO…

2.2 Fixed wing drones

A fixed-wing aircraft is a machine that uses a combination of fixed lifting surfaces (wings) and of forward thrust to fly. They generate lift through one or more stationary wings, relying on forward motion provided by a propeller or jet engine. Unlike rotorcrafts they cannot hover or take-off vertically, but are highly efficient for long-distance flight and can carry heavier payloads over extended duration.

• higher lift to drag ratio than multicopters –> energetically more efficient
• fuselage contributes to lift generation and provides space for cargo
• less agile than multicopters

Design Principles

Airplane parts and control surfaces definition. Image from Glenn Research Center

To better understand the challenges and oppourtunities of a fixed-wing UAV, it is helpful to understand the basics of a how an aircraft operates. The image below shows a standard airplane where all major parts and control surfaces are named. We will use these names in the following section to describe the design principles and flight dynamics of a fixed-wing aircraft. Do not hesitate to come back to this overview image to see where a specific part is nomarlly located on an aircraft.

Wing Geometry:
The wings are the part which contribute usually the most to the lift generation of a plane. As we have already seen in the section about aerodynamic lift, there are many factors having an impact on the dynamics of a wing. While wing design is an entire topic for itself, we want to introduce here a few more of the most important design parameters of an airfoil.
Despite the pressure drag (also form drag) and friction drag (also parasite drag), there exists a commonly used third type of drag: induced drag. Induced drag is drag that is "induced" by the lift generation. The high pressure difference between the top and bottom of the wing during lift generation, causes air to "spill" around the wing. This in turn, causes vortex generation leading to an increase in the total drag.

Illustration of aircraft with swept and tapered wings.

The aspect ratio is defined as:

\(AR = \frac{s^2}{A} = \frac{s}{c}\)

where:
\( s \): span [m]
\( A \): wing area [m²]
\( c \): chord length [m]

The aspect ratio (AR) of a wing is an important aerodynamic parameter that influences the efficiency (lift to drag ratio) of a wing. A high aspect ratio reduces induced drag and generally leads to a higher lift-to-drag ratio, and a better glide angle. Gliders, for instance, have wings with high aspect ratios (usually around 30).

Effect of different aspect ratios on lift coefficient. Schema from Raymer, 1992, Aircraft design: a conceptual approach.

Effect of taper ratio on lif distribution along the span compared to optimal elliptical distribution. Schema from Raymer, 1992, Aircraft design: a conceptual approach.

Taper ratio is defined as the ratio of the tip chord length to the root chord length:

\(\lambda = \frac{c_t}{c_r}\)

where:
\( \lambda \): taper ratio
\( c_t \): tip chord length [m]
\( c_r \): root chord length [m]

Tapering is used to improve the lift-to-drag ratio and achieve a lift distribution closer to the ideal elliptical distribution, which minimizes induced drag. An elliptical lift distribution provides the best aerodynamic efficiency but is difficult and expensive to manufacture due to complex wing shapes. A properly designed tapered wing can approximate this behavior, reducing excessive lift near the wingtips and improving overall efficiency of up to 6%.

Wing tips offer another way to reduce induced drag by preventing high-pressure air from beneath the wing to flow around the tip toward the low-pressure region above the wing. Various wing tip configurations - such as sharp, cut-off, hoerner, or winglets - are used to reduce the energy lost in vortex formation and thus decrease induced drag. The worst case aerodynamically is a simple rounded wing tip, which allows the air to "escape" easily around the tip.

Different wing tips to prevent vortex formation and reduce induced drag. Illustration from Raymer, 1992, Aircraft design: a conceptual approach.

• dihedral angle
  Balances at what angle of attack an aircraft is longitidunal stable.

Control Surfaces:

Three priniciple axis of an aircraft: Roll, Pitch and Yaw.Glenn Research Center

A change in yaw of the aircraft is achieved by deflecting the rudder on the tail to the right or left. Glenn Research Center, NASA.

To understand the rotations of an aircraft we must first define the different axes. The yaw axis is defined to be perpendicular to the wings and point downwards from the center of gravity of the plane. A yaw motion is a movement of the nose of the aircraft from side to side.
The pitch axis is perpendicular to the yaw axis and is parallel to the plane of the wings and directed towards the right wing tip. A pitch motion is an up or down movement of the nose of the aircraft.
Fianlly, the roll axis is perpendicular to the other two axes and is directed towards the nose of the aircraft. A rolling motion is an up and down movement of the wing tips of the aircraft.

In flight, rotations are produced with the use of control surfaces which are located around the aircraft. By deflecting a control surface an aerodynamical force acts on it which induces a torque around the center of gravity of the plane.
A yaw motion is created by turning the rudder to the left or right which as a result turns the plane to the right or left.
The pitch of the aircraft is changed by deflecting the elevators at the tail: if they are deflected downards, the camber increases leading to an increase in the lift force at the tail, which in turn pitches the nose of the plane downwards.
Similarily, a roll rotation is achieved with the ailerons of the wings. By tilting the aileron of one wing downwards while tilting the one of the other wing upwards, the lift of the two wings is no longer balanced which leads to a roll motion.

A change in pitch of the aircraft is achieved by deflecting the elevators up or down. Glenn Research Center, NASA.

A change in roll of the aircraft is achieved by deflecting the ailerons on the wings in opposite direction up or down. Glenn Research Center, NASA.

Propulsion:
To generate lift, a fixed-wing aircraft needs to generate thrust. While big aircrafts usually use a type of jet engine to produce thrust, smaller aircrafts and UAVs often use propellers for their thrust generation. Although less efficient at high speed, they are easier to implement and offer equal efficiency at lower speeds where most fixed-wing UAVs operate.

Launch and Recovery:
Since stand-alone fixed-wing aircraft are not capable to to vertical take-off and landing (VTOL), their take-off and landing is more complex than for rotorcrafts. Big fixed-wing aircraft almost always use a runway to build up enough speed to produce enough lift to take off. A runway is also used for the landing to deaccelerate once the wheels touched the ground.
Smaller fixed-wing drones usually require a smaller take-off speed. While a lot of them are launched by throwing them in the air by hand, there are some which make use of catapult to generate take-off. To recover smaller fixed-wing drones and UAVs they usually land on their belly.

History

The history of modern havier-than-air flight starts in the late 18th century with George Cayley from York, England. Recognised by many as “The Father of Aeronautics” he successfully identified in 1799 the four forces: lift, weight, drag and thrust and how they are linked together. Towards the end of his life, in 1852, he created a glider that successfully did the first human gliding flight.

In the second half of the 19th century the German Otto Lilienthal made thousands of repeated human gliding flights. He is known for having formulated the first aerodynamic equations before he died in 1896 from injuries as a result of a crash from 15 meters during one of his flights.

The early history of aviation ends with the famous Wright brothers at the beginning of the 20th century where they made the first recognised “sustained and controlled heavier-than-air powered flight”. It was them who made airplanes steerable by adding lateral control surfaces and laid the foundation for the developments towards modern aviation in the 20th century.

_{Early Flight Vintage Films. Footage of failed flight attempts and the successful flights of the Wright Brothers (last clip). Available at: https://www.youtube.com/watch?v=W6y3bsBXrHc}

In the decades that followed, driven by rapid innovation, aviation transformed from dangerous experiments to a cornerstone of our modern society. The development of lightweight, durable materials, new propulsion systems, and aerodynamic improvements to wing and aircraft design enabled aircraft to fly farther, faster, and higher. The two world wars acted as accelerators of innovation, leading to more powerful engines, higher payloads, jet engines, and the first all-metal aircraft, among other things. Today, fixed-wing aircraft are used in a variety of areas, from large civil aircraft up to fighter jets.

A modern fighter Jet the Sukhoi Su-57 of the Russian Air Force. Photograph taken by Maxim Maksimov. Wikimedia.

An Airbus A320 from the British Airways while landing at Zurich Airport. One of the most used civil airplanes today. Wikimedia.

While big fixed-wing aircrafts offer a big variety of interesting designs, we want to focus here on UAVs.
Unmanned fixed-wing aircrafts already had their beginning during World War I with unmanned remotely controlled aircrafts over radio-frequency. In 1917 the British developped their first UAV Aerial Target around the same time as the US the Kettering Bug - a forerunner of modern-day missiles. Since then fixed-wing UAVs became important for military and civil use, where they often cover similar fields of applications. Their main use case is in long-range missions in surveillance and mapping.

Applications

1. Mapping surveying
2. Surveillance and Security
3. Agriculture
4. environmental monitoring
5. Disaster management
6. Cargo & delivery

Challenges

1. VTOL capabilities
2. autonomous navigation
3. Energy management
4. miniaturization
5. Safety and Airspace integration

2.3 Flapping Wings:

A flapping wing drone is an aircraft where lift and thrust generation and maneuvers are obtained by the actuation of flapping wings. They seek to imitate the flapping-wing flight of birds, bats and insects and are also known as ornithopter.

Flapping-wing robots can be split into three groups based on their size and weight: large-scale over 100g, small-scale between 1g and 100g and insect-size flappers below 1g. Despite the weight the different flapping-wing systems differ in the frequency of flapping, which is faster for small- and insect-scale robots, their hover capacity which decreases or vanishes for large-scale systems and their type of actuation used which usually are conventional electric motors for large-scale system and electro-static actuators for insect-scale systems.

While bird-inspired flapping wing drones usually incoorporate a tail providing lateral control surfaces used for stability and maneuverability, most insect-inspired flapping wing drones do not have a tail.

History

Flapping-wing drones have roots in early aviation, as the most intuitive approach to create a flying machine was to get inspiration by nature: birds and insects. Attempts to create flapping-wing aerial vehicle date back to the ancien Greek legend of Daedalus and Icarus and the work of Architas 400 BC. In the 15th century Leonardo da Vinci sketched designs for bird-like flying machines. In the 1990s a research team around James DeLaurier developed a piloted ornithopter that was flying in 2006 for 40s.

Daedalus fixing wings onto the shoulders of Icarus. A painting of the flemish painter Pieter Thjis from the 17th century.

Ornithopter designed by James DeLaurier which was flying for 40s with its flapping wings in 2002. Institute for Aerospace Studies via Wikimedia.

But the modern era of flapping-wing drones started in the late 20th century with the advancements in materials science and lightweight electric motors allowing for smaller insect- and bird-scale robots. Until today it is mainly a research topic with potential applications in agriculture, search-and-rescue, entertainement and environmental monitoring.

You might now rightfully ask yourself: Why with the maturity of very efficient fixed wing drones and very agile multirotor drones is the research nowadays still interested in flapping wing robots?

Well there are several interesting opportunities when working with flapping wing drones. From a neuroscience point of view, FWFR serve as a robotic platform to explore control algorithms used by birds and insect, offering insights into biological flight. Aerodynamically, flapping wings offer an advantage over fixed wings or propeller at small scale which lose efficiency due to low Reynolds numbers. Lastly, oscillating wing motion produces less noise than fast spinning propellers and producing a more natural sound that tends to have a higher acceptance in human environments.

Bird-inspired Flapping-Wing Robots

Bird inspired flapping-wing robots produce lift and thrust by flapping their wings. The flapping motion consists mainly of an up-and-down motion typically at moderate frequencies. Lift and thrust are principally generated during the downstroke of the wing. Birds also use their tail to produce control forces for stability and maneuvering during flight. In nature, bird flight varies vastly from agile short distance flight (as in small birds) to extremely efficient long distance flights over thousands of kilometers when birds commute between the northern and southern hemisphere.

Look at the impressive footage below from a sparrowhaw. Despite flying at around 50km/h at top speed, it manages to take sharp turns, to maneuver in confined spaces and even to temporairily tuck the wings to pass through narrow gaps.

_{How sparrowhawks catch garden birds. Agile maneuvers of a hawk in slow motion. Available at: https://www.youtube.com/watch?v=Ra6I6svXQPg}

There are several challenges when trying to mimick a bird with a robot. An obvious one is to replicate the agile flying maneuvers which require a complex wing and tail mechanismn to achieve multiple degree of freedoms together with robust control algorithmns in a complex aerodynamical regime. Additionally, long-range flights require a hybrid strategy allowing to change between flapping and gliding modes. Another major challenge is the trade-off between a lightweight robot with yet powerful actuators, capable of providing sufficient force and torque. Despite that, specific maneuvers like take-off and landing are a big challenge for flapping robots.

Below you find some the key characteristics for bird-insipired flapping-wing robots:

• forward flight
• larger scale
• flapping frequency 2-20Hz
• wings flap mainly up and down (vertical plane)
• tail control
• passively stable
• glide-capable

Bird-inspired Flappers

RoboFalcon P-Flap BionicSwift

RoboFalcon: The RoboFalcon developped by Ang Chen and his team is equipped with a wing morphing mechanismn to achieve fast rolling agility in flapping level flight. The robot has wingspan of 1.2m and a weight of 600 grams.

Source: 

A. Chen, B. Song, Z. Wang, D. Xue and K. Liu, "A Novel Actuation Strategy for an Agile Bioinspired FWAV Performing a Morphing-Coupled Wingbeat Pattern," in IEEE Transactions on Robotics, vol. 39, no. 1, pp. 452-469, Feb. 2023.

P-Flap: This 700 gramm flapping-wing robot developped by Raphael Zufferey and his team includes a grasping mechanismn on the claw which grasp a branch within 25 miliseconds. This allows the robot to perch and land autonomously on a branch.

Source: 

Zufferey, R., Tormo-Barbero, J., Feliu-Talegón, D. et al. How ornithopters can perch autonomously on a branch. Nat Commun 13, 7713 (2022).

BionicSwift: Commercial flapping wing robot developped by festo. This 42 grams robot has a wingspan of 68 centimeters and is a highly agile flyer.

Source: 

Festo (2021).

Insect-inspired Flapping Wing Robots

Next to birds, insects are one of the most versatile and diverse fliers on the planet. While their size ranges from a few centimeters to only a fraction of a millimeter, they manage to hover in place, fly in strong winds, and develop swarm intelligence. Researchers around the world try to understand how insects achieve this. One way of doing so is by mimicking them in robots to better understand the underlying physics and the control techniques used.

We invite you to have a look at the video below from the YouTube channel Ant Lab. They do an incredible job of making stunning video shots of a wide variety of insects. The video below shows several fascinating insects taking off in super slow-motion. Thanks to that, you can see well the complex but rhythmic flapping of the wings, the diversity of the wing structures, and how some of them manage to fly despite seeming unstable in the air. While this does not yet talk about UAVs, it is a great way to understand the motivation and goal behind the research to develop such tiny flying robots.

_{23 Insect Species in Slow-motion flight. Available at: https://www.youtube.com/watch?v=gDI5g3rd0Ls}

Insect inspired drones, try to replicate those kind of flying behaviors at their small scale. From an engineering point of view this miniaturization is extremely challenging, since on a robot, often lighter than a single gram, you must fit an actuator, a processor and a power system. This being said there does not yet exist a small-scale flapping wing flying robot completely mastering all of these challenges. Another challenge is to control those small UAVs. Compared to larger UAVs, the small sized insect inspired drones operate at different reynold number, where viscous forces dominate and standard aerodynamic models fail to model the dynamics. However, research suggests that aerodynamically flapping-wing flight becomes the preferred solution compared to propellers at a very small scale.

Before showing you a few examples of existing robots, the main characteristics of insect-inspired flight can be summarized as follows:

• hovering capability
• small scale and ultra lightweight designs
• high flapping frequency, typically ranging from 20 Hz to 300 Hz
• wings motion mainly in the horizontal plane with wings flapping forward and backward (horizontal plane)
• tailless flight control, relying on wing modulation for stability and maneuvers
• very agile, but typically inherently unstable
• high power demand relative to weight

Insect-Size Flapping-Wing Drones

RoboBee RoboFly SoftFly PMN-PT-Flyer

RoboBee: This 80mg flapping-wing robot with 35mm wingspan was loosely modeled on the morphology of flies. Built by Kevin Ma it uses piezo-electric artificial flight muscles and is powered using a tethered wire. Closed-loop controlled it can hover and make controlled flight maneuvers.

Source: 

Kevin Y. Ma et al., Controlled Flight of a Biologically Inspired, Insect-Scale Robot. Science 340, 603-607(2013).

Laser RoboFly: This laser powered RoboFly capable of a wireless take-off with a weight of only 190mg was developped by Johannes James. It's powered by photovoltaic cell receving energy from a laser. As soon as the cell leaves the laser beam, the actuators stop and hence this robot is only capable of take-off but no sustained flight.

Source: 

J. James, V. Iyer, Y. Chukewad, S. Gollakota and S. B. Fuller, "Liftoff of a 190 mg Laser-Powered Aerial Vehicle: The Lightest Wireless Robot to Fly," 2018 IEEE International Conference on Robotics and Automation (ICRA).

SoftFly: This SoftFly uses biomimetic artifical muscles that are capable of deformation to withstand external impact. With a weight of 120mg and a wingspan of 660mm is capable of stable flight. It uses a dielectric elastomer as an actuator and is powered using thin thethered wire.

Source: 

Chen, Y., Zhao, H., Mao, J. et al. Controlled flight of a microrobot powered by soft artificial muscles. Nature 575, 324–329 (2019)..

Pmn-pt flyer: This robot called pmn-pt flyer after its actuator type - a pmn-pt cantilever. Developped by Takashi Ozaki and his team this robot uses wireless power-transmission, weighs 1.8g and has a wingspan of 100mm. It is able to take-off but no longer distance flight is possible since it needs to stay in proximity of the energy transmitting antenna.

Source: 

Ozaki, T., Ohta, N., Jimbo, T. et al. A wireless radiofrequency-powered insect-scale flapping-wing aerial vehicle. Nat Electron 4, 845-852 (2021).

Questions

TODO…

Additional Resources

Credits:

This course page was created by Lisa Romana Schneider, MSc in Robotics at EPFL, and funded by IEEE RAS and EPFL.

Additional Resources:

Raymer, D. P. (1992). Aircraft design: A conceptual approach (2. ed). American Institute of Aeronautics and Astronautics.