10 1016@j Cja 2020 03 036
10 1016@j Cja 2020 03 036
10 1016@j Cja 2020 03 036
PII: S1000-9361(20)30246-6
DOI: https://doi.org/10.1016/j.cja.2020.03.036
Reference: CJA 1617
Please cite this article as: J. Han, Z. Hui, F. Tian, G. Chen, Review on bio-inspired flight systems and bionic
aerodynamics, Chinese Journal of Aeronautics (2020), doi: https://doi.org/10.1016/j.cja.2020.03.036
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Abstract
Humans' initial desire for flight stems from the imitation of flying creatures in nature. The excellent flight
performance of flying animals will inevitably become a source of inspiration for researchers. Bio-inspired flight
systems have become one of the most exciting disruptive aviation technologies. This review is focused on the recent
progresses in bio-inspired flight systems and bionic aerodynamics. First, the development path of Biomimetic Air
Vehicles (BAVs) for bio-inspired flight systems and the latest mimetic progress are summarized. The advances of the
flight principles of several natural creatures are then introduced, from the perspective of bionic aerodynamics. Finally,
several new challenges of bionic aerodynamics are proposed for the autonomy and intelligent development trend of the
bio-inspired smart aircraft. This review will provide an important insight in designing new biomimetic air vehicles.
Keywords: Bio-inspired flight systems, Bionic aerodynamics, Micro air vehicle, Biomimetic air vehicle
*Corresponding author.
E-mail address: aachengang@xjtu.edu.cn
There have been the historical origins of bio-inspired Institute of Technology12, AeroViroment’s
flight systems throughout the development of MAVs. Hummingbird prototype11, and Berkeley’s machine
From the perspective of miniaturization, we believe insect MFI13. The flapping wing can provide more lift
that the development of MAVs can be roughly divided than the fixed wing, and the mass of the flapping wing
into three stages. These three stages also correspond to aircraft has been significantly reduced without
the changes in the key technical indicators and design changing the power system. But because flapping
concepts of MAVs. The first stage refers to the concept consumes extra valuable energy, its flight time is also
and technical demonstration of MAVs from 1996 to significantly reduced compared to rotor and fixed wing.
2002. The main promoter is DARPA's four-year MAVs Nevertheless, the future prospects of the BMAVs are
program. At this stage, the subjects studied are FMAVs inestimable. The bionic concept at this stage has been
and RMAVs. Their technical index is that the aircraft is proven to provide new design ideas for the
about 15 cm in length, about 100 g in mass and about development of MAVs.
20 min in flight time. Representative achievements For the flight, we need the bio-inspired flight system
include: Black Widow 7 from AeroVironment, to bring the MAVs from laboratory to application. The
Microstar8 from Lockheed Sanders, and Kolibri9 from third stage is from 2010 to the present, which pursues
Knick Corporation. In fact, the design concepts of the realization of BMAVs’ flight. In 201514, the British
these FMAVs and RMAVs derived from the direct Army publicly displayed the "Black Hornet" miniature
reduction of traditional large fixed-wing aircraft and unmanned helicopter developed by Norwegian Prox
helicopters in size. Because MAVs cannot carry a lot Dynamics. The "Black Hornet " is about 10 cm long,
of fuel and load like a large aircraft, there are still weighs 16 g (including batteries), has a battery life of
technical bottlenecks that are difficult to break under 25 min, and has a maximum speed of about 18
the technical conditions at that time, which led to the km/h14,15. It can transmit pictures and videos from 800
research of MAVs being in a stagnation period for a m to the handheld surveillance controller. The
long time. At this stage, the development of MAV is hummingbird flapping wing MAV developed by the
far from the originally expected target of DARPA, but DAPAR has reached the required standard for
it provides solid technical support for the future equipping the army. At this stage, the research of
development of MAV and leads the craze of MAVs has benefited from the rapid development of
international research for MAVs7,10. After, it is found advanced materials technology, microelectronics
that the nature has already designed a better MAV that technology and efficient energy technology, and it is
is the flying creature in nature with high flight developing towards miniaturization, automation and
efficiency, low flight noise, strong concealment, good intelligence. At present, the representative applied
mobility and stability. These excellent flight MAVs include the Dragonfly, a robot weighing less
performances of creatures are beyond the reach of than 3 g, developed by Delft University of Technology
modern aircraft. Therefore, learning flight from birds in the Netherlands16, and the smallest and lightest robot
and insects in nature, using the bionics principle to bee in the world, developed by Harvard University17.
design a biomimetic micro air vehicle with small scale, Various new concepts of MAVs have also been
high mobility and high flight efficiency has gradually proposed especially with the breakthrough of artificial
attracted our attention.10 The bio-inspired flight system intelligence and micro-system technology, but the
has begun to appear in aviation history. The use of this concept of bionics has been running through the
bionic concept promoted the second stage of the development of MAVs.
development for MAVs. Since the flapping-wing MAV Microbat was
The second stage refers to the period from 2003 to designed by the California Institute of Technology, the
the beginning of this century, which is the application of bionics has found a new breakthrough
transformation stage of the design concept for MAVs. for MAVs7,10. The flapping-wing aircraft has become
At this stage, the United States has made the focus of research on bionic aircraft, especially
breakthroughs in the research and development of bionic MAVs18. The excellent flight performance of
MAVs, and a novel MAV (Nano Air Vehicle, NAV) flying animals will inevitably become a source of
with a size within 7.5 cm, a mass of about 10 g, and inspiration for researchers to design aircraft with good
low-speed flight and hovering capabilities has been the aerodynamic performance using the concept of bionics
focus of research by DARPA. 11 It has been found that in the future. In recent years, with the development of
the flight mode of birds and insects employing flapping materials science, electronic technology, biomimetic
wings is more suitable for small-scale objects such as theory, mechanical manufacturing and other related
NAVs to fly at low Reynolds number. Therefore, at disciplines, bio-inspired flight system has become one
this stage, the research focus of MAVs began to change of the most exciting disruptive aviation technologies,
from FMAVs and RMAVs to Biomimetic Micro Air which is also the current international academic
Vehicles (BMAVs). Representative flapping-wing research hotspot. As a complex and highly integrated
micro air vehicle include: MicroBat7 in California intelligent bio-inspired flight system that integrates a
Institute of Technology, Entomopter in Georgia variety of advanced technologies, the development of
Chinese Journal of Aeronautics ·3·
bio-inspired flight system involves multiple disciplines Section 4. And Section 5 gives the prospects and
such as unsteady aerodynamics, micromechanics, challenges of future bio-inspired flight systems.
microelectronics, energy power, and bionics. As of
now, it still needs to solve the extremely challenging 2. Common bio-inspired flight systems with flap
key technologies such as aerodynamics at low ping wing
Reynolds numbers, the design of material structure, the
management of energy, autonomous navigation and 2.1. Bird-like flying vehicles
control, and precision machining3-5,17,18.
In this paper, the development path of Biomimetic There are about 9,000 species of birds in nature19.
Air Vehicles (BAVs) for bio-inspired flight systems Birds show people their excellent flight capabilities
and their latest mimetic progress are summarized. This and inspired people to develop similar bio-inspired
paper divides the current bionic aircraft into two types flight systems. The humans’ initial exploration of
of flapping and non-flapping according to their flying began with imitating birds. In each period of the
different flight modes. In these two states, different history of human civilization, much effort has been
bionic technologies are designed. Then, from the devoted to the invention and design of bird-like aircraft
perspective of bionic aerodynamic, the advances of the or the improvement of the flapping-wing theory.
flight principles of several natural creatures are Especially in the early days of bionic aircraft research,
introduced. Finally, several new challenges of bionic because the key technologies involved in the research
aerodynamics are proposed for the autonomy and of bird-like aircraft are relatively mature, the research
intelligent development trend of the bio-inspired smart of bio-inspired flight systems begin with bird-like
aircraft. Based on the introduction of the development flying vehicles. For example, Smartbird, a
of bio-inspired flight systems, this paper focuses on the flapping-wing vehicle simulating seagull and a
research progress of bionic aerodynamics for milestone for the development of bird-like flying
biomimetic aircraft. Our intention is that this will vehicles, was designed by German FESTO in 201120, 21
provide a resource for researchers engaged in new (as shown in Fig. 1(a)). Smartbird has a wingspan of
biomimetic air vehicles, which will promote their 1.96 m, a length of 1.06 m, and a weight of 450 g. Its
further research interest. The rest of this paper is slewing radius is small. Employing the seagull-like
organized as follows: In Section 2, several common streamlined flapping wings, the parallel link
bio-inspired flight systems with flapping wing are mechanism can control the wingtip torsional movement
introduced, including bird-like aircraft, insect-like while driving the flapping wings. Equipped with a
aircraft, and bat-like aircraft. Section 3 introduces the highly autonomous flight control system, Smartbird
bio-inspired flight systems with non-flapping wing, can fly autonomously. Its head and tail can swing, its
and also introduces some new bionic aircraft currently flexibility is comparable to that of a real bird, and it
developed. The research status of aerodynamic can mix the false with the genuine when flying.
mechanism during biological flight is introduced in
Fig. 1. Classic bird-like flying vehicles (These bird-like aircraft gain soaring power through using flapping wings).
Table 1 Relevant parameters of several typical bird-like aircraft
Relevant parameter
Name Developer Year
Length (cm) Weight (g) Speed (m/s)
Smartbird FESTO, German 2011 106 450
iBird-bot University of California, Berkeley, US 2010 12
Hummingbird DARPA, US 2012 16 10 2.5
Northwestern Polytechnical University,
Dove 2011 60 220 6-10
China
With the maturity of related technologies and autonomy and intelligence, which can be confirmed by
theories, the bird-like flapping-wing aircraft are the relevant parameters of several typical bird-like
developing towards miniaturization, stabilization, aircraft in Table 1. Since 2010, the University of
·4 · Chinese Journal of Aeronautics
California, Berkeley has executed research on some actively adapt to different environments), strong
new flapping-wing aircraft based on the Machine maneuverability (can pass through complicated spaces),
Insect MFI project, and achieved robust autonomous high flight efficiency (low energy consumption for
indoor flight of bionic flapping-wing aircraft by using long-distance flight) and other advantages, and they
robust intelligent control. As shown in Figure 1, this can overcome the shortcomings of high cost, low
series of aircraft mainly includes iBird-bot (Fig.1(b))22 energy utilization, and weak stealth capabilities of
and H2Bird (Fig.1(c))23. They carry the micro-control FMAVs and RMAVs. They have huge development
CPU, camera, gyroscope, acceleration sensor, and potential and application prospects. The current
communication module, and have a total weight of bird-like aircraft mainly imitate the flight mode, that is,
only about 12 g and a payload of more than 2 g24,25. the flapping airfoil generates lift and thrust for flight,
Moreover, the Hummingbird was designed by but the aerodynamic efficiency is still far from that of
American AeroVironment in 2012, it is a Nano MAV, birds. In addition, the research on the sensing, driving
which is a milestone in the history of bionic and control of the bio-inspired flight systems and the
flapping-wing aircraft, and it represents the highest flight mechanism of birds is still in their infancy, so
level of current development of bionic flapping-wing further in-depth research is urgently needed to make
aircraft26 (Fig.1 (d)). The research of Hummingbird is the bird-like aircraft have the excellent flying ability
part of a funded project that is the Nano Flight Program like birds. The bird-like flying vehicle soars like a bird
proposed by DARPA, which cost $ 4 million to in nature. The flapping frequency is generally 10-20
complete in 5 years. It has a wingspan of 16 cm, a Hz28. The lift is generated by the forward speed. This
weight of only 10 g, and flight time of 10-20 min. It low flapping frequency and high wing mass ratio make
can withstand crosswind interference of 2.5 m/s when the coupling relationship between aerodynamics,
soaring at a speed of 10 m/s. Employing the tailless structure and flight mechanics. Therefore, it is an issue
flight mode, it can realize difficult actions such as that cannot be ignored in designing an efficient and
hovering and somersaulting in the sky, and can high-performance bird-like flapping aircraft.
perform reconnaissance tasks in small areas and
indoors. At that time, the Hummingbird has reached 2.2. Insect-like flying vehicles
the technical requirements of the armed forces.
Although the Hummingbird failed to achieve the target In addition to the larger birds in nature, insects are also
of less than 10 cm in size, the US military spoke highly masters of flying. They can fly stably even in the face
of the Hummingbird. The successful development of of strong winds. As some countries have successively
the Hummingbird has opened the way for the research established special research institutions and invested a
of a new generation of bird-like aircraft. The Dove of lot in research funding for MAVs, the insect-like
China27 was reported on September 24, 2011. The aircraft or robotic insects have become another
bionic flapping wings are produced by using the research hotspot after bird-like aircraft. Especially with
streamlined airfoils of birds, and the traditional planar the breakthrough of micro-system technology and
link mechanism drives bionic airfoils. The whole artificial intelligence technology, various insect-like
machine weighs 220 g, the wingspan is 60 cm, and the bio-inspired flight systems such as dragonfly, bee, fly
speed is 6-10 m/s. At that time, limited by the and butterfly have been developed successively.
development level of basic industrial technologies such According to the form of several common insects, the
as domestic microelectronics technology and insect-like flying vehicles can be roughly divided into
micro-electromechanical system technology, the dragonfly-like aircraft, fly-like aircraft, and
bird-like flapping-wing aircraft have relatively slow butterfly-like aircraft. Therefore, we will introduce
development. these insect-like flight systems in detail.
The future bird-like aircraft have the following
characteristics: good aerodynamic performance (can
Fig. 2. Delfly series of dragonfly-like aircraft developed by Delft University of Technology in the Netherlands.
The Delft University of Technology has developed successfully flew in 200530. Delfly II, the
three generations of dragonfly-like aircraft29. As shown second-generation dragonfly-like flight system, has a
in Fig. 2, Delfly I, the first-generation dragonfly-like wingspan of 28 cm and a weight of 16.07 g. It
flight system, has a wingspan of 50 cm and a weight of successfully flew and was controlled by a
21 g, using a V-tail to control flight attitude. It standing-tail31. Delfly Micro16, the third-generation
Chinese Journal of Aeronautics ·5·
dragonfly-like flight system, made a great flying robot is equipped with a camera, which can fly
breakthrough in 2008. It is driven by a traditional forward and hover to perform reconnaissance tasks,
mechanical link structure, equipped with a battery and and its body parts can move like limbs of dragonflies.
camera equipment, with a wingspan of only 10 cm and Moreover, Animal Dynamics received a £1.5 million
a weight of only 3.07 g. This bionic flight system truly grant from the UK Ministry of Defence. Defence
met the concept requirements of a centimeter-level Technology Laboratory in 2015 developed the bionic
MAV, and won the Guinness World Record in 2009. In dragonfly drone “Skeeter” (Fig. 3 (b)). The project is
2014, Delfly Explorer32 with a wingspan of 28 cm and progressing well and the second phase of research has
autonomous obstacle avoidance capability was been enforced. With the development of modern
developed33, and its autonomous flight capability was advanced manufacturing technology, in 2017, the
continuously further improved34. Furthermore, Delfy Draper Company and the Howard Hughes Medical
Nimble (as shown in Fig. 2(e)), a new generation of Institute with the support of the United States
fruit fly-like robot developed by Delft University of Department of Defense combined micro-navigation
Technology in 201835, could accurately reproduce the technology37, synthetic biotechnology and
fast escape action of it even if its rotation axes are not neuroscience technology to modify the gene of the
explicitly controlled. It can perform 360° roll and pitch dragonfly, which makes the dragonfly obtain the ability
flips, which is very flexible. Moreover, dragonfly-like to be controlled and autonomous navigation. As shown
bio-inspired flight systems have also received strong in Fig. 3(c), DragonflEye38, a living robot, is half an
support from the military of various countries. In 2011, insect and half a machinery. This project provides a
Georgia Institute of Technology, with the support of new way for the development of insect-like flight
the United States Air Force, successfully developed a systems.
palm-sized dragonfly robot36 as shown in Fig.3(a). The
chemical muscle to convert the chemical energy of the rear. The reason why this bird-like aircraft is called a
chemical fuel into the kinetic energy of the chemical bat-like aircraft is a small vertical tail. The prototype
muscle to drive the flapping wing to fly. The flew for only 9 s during the first test flight, but after a
Entomopter project was later awarded NASA funding number of improvements, it successfully flew for 25
to demonstrate the possibility of machine insects for a min in August 2002, setting a world record for
new generation Mars probes. flapping-wing aircraft at that time. The second typical
bat-like aircraft is Bat Bot (B2 for short)41, developed
2.3. Bat-like flying vehicles by researchers at the University of Illinois at
Urbana-Champaign and the California Institute of
The bats are the only mammal that can fly like birds. Technology. As shown in Fig. 4(b), the wing
The wing structure of the bat is particularly membrane of B2 uses a silicon-based carbon fiber
complicated, and its wing and elastic skin have evolved reinforced membrane with a thickness of only 56 μm, a
into efficient flight tolls, making the bat a master of wingspan of 47 cm and a weight of 93 g42. The overall
flying. The elastic skin of bats makes the study of system is a flapping machine with 5 of Actuation
bat-like aircraft inevitable to discuss the fluid-structure (DoA). Employing the mechanical coupling structure,
interaction mechanism of flexible structures. Because only one DC brushless motor is used to drive the left
of this, due to the limitations of technologies and and right airfoil flapping at the same time to achieve a
theories, there is relatively few research on bat-like 10 Hz flapping frequency. In order to emphasize the
aircraft. In this paper, two very representative bat-like existence of the functional group joints of bats, the
flight systems are shown to illustrate the development angular motion of each joint is monitored by a motor
prospects of bat-like aircraft in the field of bio-inspired control using a Hall encoder. In Fig. 4(c), B2’s
flight systems. avionics is shown. Due to weight and size constraints,
The first typical bat-like aircraft, Microbat, weighs B2's electronics are custom-made and self-sustained,
only 10 g, developed by the California Institute of including internal computing, sensing, and power
Technology under DARPA funding. As shown in Fig. electronics. The Main Control Board (MCB) hosts a
4(a), Microbat mimics the shape of a bat, with a wing microprocessor with several peripherals for
span of only 23 cm. It uses the simplicity of the basic communication purposes. It can communicate with
crank and pushrods to drive the flapping wing, and terrestrial base stations via DSM2 receiver and
controls the flying attitude through a conventional tail Bluetooth. An Inertial Measurement Unit (IMU) is
with vertical and horizontal stabilizers. For its wing fixed to its ribcage, which monitors attitude
construction, the batten-type structures are combined information during flight. Currently, B2 can complete
with a thin flexible wing, which is easily attached to the roll and pitch flight, and its research is being
the leading edge and root ribs of the flapping wing7. further improved. In 2018, Seth Hutchinson gave a
From Fig. 4(a), the bat-like aircraft Microbat looks detailed introduction to the latest results of his team's
more like a bird. Using a central long body, there are a research on bat-like aircraft at the World Robot
set of flapping wing at the front and a stabilizer at the Conference. A new generation of bat-like aircraft will
soon be available.
flapping motion modes but also depends on their to design and analyze these concepts are discussed,
complicated deformation motion modes. During from structural analysis to aerodynamic analysis and
non-flapping flight state, birds can dynamically change from control to optimization. In addition to their
their wing postures to maximize adaptability to a concept (not repeated here), there are still huge
diverse of flight behavior (e.g., maneuvering, gliding, challenges in the application of the aircraft. In design,
and soaring flight) and flight environments (e.g., dense the passive wing-tip modification devices (e.g.,
jungle, urban, and mountain environments)43,44. winglets, tip sails, wing grids, delta tips, and
Meanwhile, the deformation mode of the bird’s wing multi-winglet)59,60, as the most typical representatives
posture under aerodynamic loading is an extremely of the bio-inspired non-deformable wing techniques,
complicated kinematic coupling process, which mainly have been widely applied in a variety of military and
contains two critical aspects: (A) large-scale geometric civil aircraft. The long-term investigation results show
wing deformations (e.g., wing area, wingspan, chord that the wing-tip devices can effectively decrease the
length, and swept angle); (B) small-scale local tip-vortex strength or move it away in relation to the
deformations of each flexible feather (e.g., bending, aircraft longitudinal axis, to greatly improve flight
twisting, and fluttering phenomenon). For instance, performance and flight safety. Also, some researchers,
swifts actively modify wing sweep to alter sink speed inspired by self-adaptive feather structure features of
and turning rate during maneuvers45. Albatrosses and the birds, have developed some novel biomimetic
petrels enhance wingspan efficiency by extending their control methodologies for the flow and noise over
primary feather structures, to perform gliding behavior airfoils61-69. For example, Wang et al.62 experimentally
efficiently46. Gliding jackdaws dynamically change studied the effects of real feather flap on the flow field
spanwise camber through active and passive characteristics of NACA0012 wings. The results
deflections of their feathers, to reach optimal flight revealed that the real feather flap mounted on the
performance47. Vultures and hawks achieve suction side or pressure side is proved to be beneficial
swiftly-soaring behavior utilizing their slotted to improve the aerodynamic performance of the airfoil
wingtips48,49. Besides, some research results reveal that within a limited range of attack angles. Beierle et al.63
the active and passive deformations of the birds' wings studied the influence of the convex structure on the
under fluid-structure-interaction coupling can also pressure distribution of the bionic wing surface. It was
bring other essential benefits, such as flight stability found that the flow around the convex structure is
augmentation, stall avoidance, drag reduction, noise nearly similar to that around the stagnation point. Ito et
control, etc.50-57 To assimilate the superior aerodynamic al. 65, inspired by an owl's leading-edge serration
benefits of the birds’ wings, the how-to approach for structure, investigated the effects of leading-edge
the design of the bio-inspired wing configuration has serration structure on the aerodynamic performance of
been widely studied in recent decades. As the main the bio-inspired wing at low Reynolds number. It was
lifting surface of each aircraft, a delicately designed found that the leading-edge serration structure can
wing will, no doubt, enhance its flight performance. At effectively suppress noise and improve lift at a high
present, the typical design concepts of bio-inspired angle of attack. Gruber et al.66-68 systematically
wing configurations can be roughly classified into investigated the noise reduction effects of both the
three major categories: (A) bio-inspired serrated and slotted trailing edges on NACA65 wings,
non-deformable wing configurations only considering and the results showed that the two bio-inspired
biological structure elements of the birds' wings, such structures could significantly reduce the noise by about
as slotted wing-tips, feather flaps, non-smooth wing 5dB. Based on the large eddy simulation method, Li et
surface features, serration-shaped leading and trailing al.69 have adequately investigated the flow
edges; (B) bio-inspired deformable wing characteristics and acoustic scattering mechanisms of
configurations only considering basic morphing motion novel owl-based wings with different trailing-edge
features, such as variable-camber deformations, serrations. The numerical investigations indicated that
twisting deformations, variable-sweep deformations, the saw tooth and sinusoidal serrations provide the
folding deformations, and inflatable deformations; (C) most significant noise reduction effects: the maximum
complex bio-inspired wing configurations considering noise reduction is about 9 dB.
both the biological structural elements and morphing
motion modes.
Like natural flight, morphing wings have great
potential for improving the overall performance of the
aircraft. By dynamically adjusting or optimizing shapes
based on various flight conditions, there are many
untapped opportunities beyond current
proof-of-concept demonstrations. Ref.58 discussed the
most prominent examples of the 2D and 3D morphing
wing models. The methods and tools commonly used
·8 · Chinese Journal of Aeronautics
through the leading edge of birds and their head with parallel translation, thus maintaining the high lift of the
large curvature, the local flow velocity increases to dynamic stalling vortex120. The delayed stall phase
form a low-pressure region, which causes the produced most of the lift during flapping121. The
leading-edge suction to generate some driving structure and evolution of the leading-edge vortex are
forces107,108. (C) The effect of inertia. When the wings very complicated and are affected by many parameters
are flapping, the local additional mass effect produces such as Reynolds number, shrinkage frequency,
additional inertial forces. The output power of birds Strouhal number, flexibility and flapping law. For the
during flight is the main subject to study the principles evolution of the leading-edge vortex over time, the
of birds’ flight. At present, a lot of results have been effects of Reynolds number and geometric parameters
achieved by measuring the metabolism inside the bird of wing, many research results have been
muscle109 and external aerodynamics103. These results achieved122-125.
provide a theoretical basis for the research on the new Furthermore, there is another flying master in nature.
aerodynamic configuration of flapping-wing aircraft.110 As the only mammal flying creature, bats are known as
After living beings’ millions of years of evolution, the three major flying experts in nature, together with
insects have amazing super-maneuvering flight insects and birds. Compared to insects, bats are larger
capabilities and super-high flight stability such as in size and flap less frequently, flying much faster than
hovering, fast taking-off, fast stopping, and fast birds. It has long been thought that bats and birds fly
climbing. The aerodynamics of insects have been with the same principle. However, experimental studies
explored for hundreds of years, especially in the past in recent years have shown that bats have very different
20 years, and with the rapid development of advanced flight patterns from birds126. Brown University and
flow display technology and numerical simulation Lund University are the representative research
technology, systematic and in-depth understanding of institutions that carried out the research on the flight
the typical insect flight mechanism has been mechanism of bats earlier in the world. Muijres and
acquired111-113. Before the 1980s, it was believed that Johansson's research team at Lund University have
the flight principle of insects was the same as that of twice introduced the results of experimental results of
helicopter or birds in gliding. Later, it was discovered the research on the flow mechanism of the bats in
that the steady flow theory could not be used to explain Science. It is found that when the bat is flying at a low
the high lift required for insects. It was necessary to speed, the wings move up very fast and generate lift
study the unsteady flow mechanism of high lift during the whole flapping process; the tail vortex in the
generated by insects114,115. So far, the proposed wake region of the two wings of the birds will merge
high-lift mechanism of flapping-wing flight mainly into a single vortex ring, but the tail vortex in the wake
includes: the clap-fling mechanism114, the delayed stall region of two wings of the bats remain relatively
mechanism116, the rotational circulation mechanism111, independent126,127. The vortexes of wingtip remain
and the wake capture mechanism111. Among them, the stable when the wing flaps upward, and fall off when it
clap-fling mechanism believes that insects' enhanced flaps downward. The direction of vortexes at the root
lift depends on flapping motion114. During the clapping of the wing changed128. As the distance between the
and fling, a low-pressure area is quickly formed in the vortexes and the wing increases, the strength of starting
middle of the two wings. The incoming air generates a vortexes and stopping vortexes decreases, but the total
vortex and does not fall off during fling, resulting in number of vortexes remains unchanged. The stopping
instant high lift117. Many organisms in nature, such as vortex falls off in the near field but forms a new vortex
moths, butterflies, fruit flies, and birds, rely on this in the far field129. The leading-edge vortex is generated
mechanism to generate lift when they take off. Before during slow flight, but it is not generated when
the end of the flapping, the insects can generate strong hovering. When the wings are flapping upward, the
vortexes with different directions in a short time by leading-edge vortex rotates rapidly inside and outside
rapid acceleration or rapid upward motion, and the the wings and moves with the wings together130. This
phenomenon of high lift is called the rotational flying mode is not good for birds, but it is very
circulation mechanism111,118. When insects are flapping efficient and energy-saving for bats, which is likely to
at a high frequency, their wings will encounter the indicate that bats can actively control the generation of
vorticities generated by the previous flapping when the leading edge vortices to obtain better flight
wings change their flapping direction, causing the flow performance130.
speed to increase and generate additional lift. This Kenneth Breuer and Sharon Swartz's team at Brown
mechanism is called the wake capture by Dickinson111. University have also been studying the flight
However, subsequent studies have shown that the mechanism of bats. In 2006, for the first time, they
effect of wake is mainly to reduce lift118,119. The drew eddy current maps of the entire bat wing motion
delayed stall mechanism reveals the high lift cycle131. Subsequently, the static and dynamic
mechanism during insect’s parallel translation. When deformations of bat wings under different aspect ratios,
flapping, the leading-edge stalling vortex on the wing flexibility and loads were measured132. The effects of
surface of the insect does not fall off during the entire parameters such as the degree of wing bending
·12 · Chinese Journal of Aeronautics
deformation, flapping amplitude, bat mass, wing inertia camera was used to measure the flapping frequency
and maneuvering load on aerodynamic characteristics and flapping angle of the flight creatures, which has
and flight performance of bats were analyzed132. It is made a great contribution to the aerodynamic
shown that, as the mass increases, the lift coefficient of characteristics analysis of flying creatures.In order to
the bat and the angle of attack of the wings increase, obtain more accurate flight kinematic parameters, a
and the attenuation of the flapping frequency measurement method combining three high-speed
decreases123. From a physiological perspective, cameras and image processing technology has been
studying the effects of internal factors such as wing proposed. And it is widely used to measure the
membranes, skeletons, and muscles of bat wings on kinematic parameters such as the flapping angle and
mechanical properties, aerodynamic performance, and attack angle of different insects142-144. Moreover, in
energy consumption of bats is the focus of recent order to more realistically reflect the effect of wing
research by this team124-138. These latest research results deformation on flow, and provide dynamic information
provide important basis for the aerodynamic and of the boundary for numerical simulation, it is
structural design of bat-like wings. The bat wings have necessary to measure the dynamic deformation during
many degrees of freedom in structure, and the flight. A series of new measurement technologies have
deformation mechanism is very complicated. Little is been proposed one after another145-149. However,
known about how bats control deformation to achieve directly observing animal flight with the aid of
different flying purposes. These issues are exactly the measurement tools can only understand the flight
key issues that must be faced in the development of mechanics of living things from a macro perspective,
bat-like bio-inspired flight systems, and are also the and it is difficult to reveal the complex flow
current research hotspots of bionic aerodynamics139-141. mechanisms and mysteries behind their flight
performance. Therefore, the emergence of various flow
4.2 Recent progress of aerodynamics in bio-inspire display technologies150-152 has provided a powerful
d flight systems means for studying the mechanism of biological flow,
especially research related to the high-lift mechanism
On the basis of understanding the aerodynamics of of insects and the evolution of their unsteady vortexes.
biological flight, researchers began to use experimental For example, the classical direct momentum
techniques and numerical simulation methods to carry integration method is used to estimate the steady load
out aerodynamic research on bio-inspired flight of the bionic structure153-155, and this method has been
systems. The experimental techniques can often popularized for the estimation of unsteady loads156,157.
provide flight parameters for numerical simulations, Recently, the volume measurement based on phase
which are mainly carried out for the research on some average Particle Image Velocimetry (PIV) has been
simplified bionic flight. Therefore, the research object successfully implemented. The method uses flow field
here mainly refers to a simplified moving wing to velocity measurement to estimate the lift and drag
mimic a bio-inspired flight system. However, characteristics of small aspect ratio moving wings158.
numerical simulation would reduce the experimental Minotti159 and Mohebbian160 et al. also developed a
cost and quickly obtain the aerodynamic results. The method for calculating the unsteady aerodynamic of
research object began to transfer from a simple model flapping wings from near-field flow data for PIV
to a bionic model. Both methods aim to reveal the systems. Recently, Lentink et al. successfully built a
mechanism of biological flight and develop a novel test system for measuring the propulsion performance
biologically-inspired flight system. The experimental of birds in free flight based on the above methods,
techniques and numerical simulation methods used in which provides a very promising technology for future
the current aerodynamic research of bionic flight unsteady effects of calibrating the flight propulsion
systems are summarized in this section, which will performance of natural animals in the future161.
provide researchers who are engaged in the research of Furthermore, with the advent of three-dimensional
bionic flight systems with quick referable and available full-field time-resolved PIV testing technology, the
research methods. method of estimating pressure based on vortex
For the study of aerodynamics for bionic flight dynamics has also begun to be applied in the study of
systems, experimental techniques have always been bionic flow mechanisms162,163. The 4D-PTV is another
favored. Here, we present a series of experimental interesting load testing technology under the
research schemes for aerodynamic problems of bionic Lagrangian framework. In the near future, this
flight systems, which will help researchers engaged in technology will provide a clearer vortex topology and
experimental research on bionic flight systems to the higher prediction accuracy of instantaneous load for
choose the experimental method that suits themselves. the flow field164. In addition to measuring the
In the early research of animal flight mechanism, direct kinematic and morphological parameters of flying
observation and measurement of flapping flight organisms for flight experiment research, the
parameters of living animals are the main research measurement of dynamic parameters and visualization
methods. In early experiments, only a high-speed of flow fields are also very important. Although free
Chinese Journal of Aeronautics · 13 ·
flight of living animal specimens is the best object, the and calculates aerodynamic loads based on the
experimental conditions required are very complicated, instantaneous speed, shape, and attitude angle of the
training of living specimens is also very steady state. The quasi-steady method works well in
time-consuming and laborious, and the number of cases where the unsteady effects are not significant. In
samples and space are limited. The living specimens in the experiments of insect166, wasps167, and
the free flying state are the best test subjects for flight, hummingbirds168, the thrust required for flight and the
but the experimental conditions required are very aerodynamics of rigid flapping-wing aircraft169 have all
complicated, training of living specimens is also very obtained consistent results with the experiments.
time-consuming and labor-intensive, and the number of Because creatures have strong flexibility, various
samples and space are limited. And the development of movement modes, and large changes in attitude during
bio-inspired flight systems also needs to develop flight, how to accurately simulate complex motion
related aerodynamic testing technology. Therefore, in boundaries has become one of the keys to biological
summary, it is important to test the flow mechanism motion and bionic flow simulation. Therefore, there are
and aerodynamic using a simplified bionic model. The two main techniques to deal with the rigid move
bionic experimental model needs to focus on the boundary in the bionic the numerical simulation of
simulation of low Reynolds number flow environment, unsteady flow. One is to ignore the autonomous and
as close as possible to the characteristics of living body passive deformation of biomimetic organisms, and
motion. Secondly, a model support mechanism that can abstract their complex motions into the simple motions
accurately simulate the movement of flying animals is of bionic models, such as the heaving and pitching
needed. The accuracy of the bionic motion simulation motion of the bionic flapping wing170 and wave motion
will directly affect the experimental test results and of bionic fishes. Another is to give the kinematic
their credibility. Generally, the movement mechanism parameters of creatures in free flight state using
of the model needs to be optimized and considered in high-speed cameras as the motion boundary conditions
combination with the specific form and measurement for numerical simulation of unsteady flow171. By
scheme of the bionic model165. It is usually necessary combining the bionic or the measured kinematic
to combine the specific form of the bionic model and parameters with the unsteady NS equation, not only the
the measurement scheme to optimize the model's numerical simulations of insect lift172 and wing-body
kinematic mechanism. The above experimental interference effects173 under hovering conditions have
schemes have provided a wealth of experience for the been completed, but also the research on the flow
study of bio-inspired flight systems. It can be seen that mechanism of insects under complex flight conditions
there are still many problems worthy of discussion and has been successfully implemented174.
urgency to be solved in the experimental research of In recent years, the Lattice Boltzmann Method
aerodynamics for the bio-inspired flight system. (LBM) with simple programming, easy integration of
Developing an experimental system capable of new physical models, convenient handling of complex
detecting actual flight dynamic data is a new challenge boundaries, and natural high parallelism has also
for modern experimenters. become popular in bionic flow simulations175, 176.
Of course, it is unrealistic to use only experimental Combining it with the Immersed Boundary Method
methods in the research of bionic aircraft, and it is (IBM)177 to deal with moving boundaries and large
extremely convenient to use numerical simulation deformation has become an alternative to overcome the
methods to provide reliable design data for the shortcomings of body-fitting meshes. It has been
development of bio-inspired flight systems. Especially applied to unsteady flow simulation of 2D or 3D rigid
in recent years, with the development of various flapping wings178,179, rigid dragonfly180 and butterfly181.
numerical simulation methods, numerical simulation This numerical method overcomes the difficulty of
methods have also provided a reference for the study of traditional computational fluid dynamics meshing,
bio-inspired flight systems. The numerical simulation which is useful for solving aerodynamic simulations of
methods for solving biological movements have also bio-inspired flight systems with complex motion. Not
experienced continuous improvement and a long only the simulation of unsteady flow for the bionic
development process. According to the development of model has been calculated, but also the simulation of
numerical simulation methods for bio-inspired flight maneuvering flight closed-loop control of the bionic
systems, we summarize the research status and existing aircraft has been performed182. In order to make full
problems of each numerical method. This will provide use of the parallel characteristics of LBM, our research
assistance for the further study of numerical simulation team has developed a three-dimensional IB-LBM
methods. Since it was discovered that the theory of unsteady solver for supercomputers. We have carried
steady aerodynamics could not explain the formation out large-scale numerical simulation of bionic flapping
mechanism of high lift in biological flight, the wing or flapping multi-wing. The effects of individual
quasi-steady theory began to be used to calculate the distance183 and flapping frequency184 on the thrust
aerodynamics of biological flight. The quasi-steady performance of flapping multi-wing are discussed, and
method ignores wing motion and flow field changes, the effects of kinematic parameters185 and geometric
·14 · Chinese Journal of Aeronautics
parameters186 on the thrust performance of the flapping for bionic flow of bionic flying vehicles, it is necessary
wing are studied. The refined vortex structure and its to improve the high-precision simulation method of
evolution process were obtained. LBM has shown great bionic flow with complex moving boundary at low
application potential in the study of bionic flow Reynolds number, construct a nonlinear dynamic finite
mechanisms and the design of bionic aircraft. One element model and the framework of efficiently
point that needs special attention here is that because solving the aerodynamics for flexible bionic structures,
the wings of a creature are deformed during flight, how and construct large-scale nonlinear parallel simulation
to simulate the unsteady flow mechanism considering method.
flexible deformation is the current research hotspot. (3) As a complex and highly integrated intelligent
The flow mechanisms and aeroelastic effects contained system that integrates a variety of advanced
in biological flexible wings are very important for technologies, the development of bio-inspired flight
understanding the superb flying skills of insects, birds systems involves multiple disciplines. In this paper,
and bats. Among many numerical simulation methods based on the introduction of the development of MAVs,
for dealing with moving boundaries and flexible large focuses on the research progress of bionic
deformations, the IBM has shown great advantages in aerodynamics, and focuses on the bionic flow
large deformation nonlinear fluid-structure interaction mechanism, key scientific problems and research
simulations187. Tian Fangbao's team at the University methods were sorted out. It is hoped that the design
of New South Wales has made a huge contribution to concept of the bionic aerodynamics and bio-inspired
the study of this method188-190. They applied the flight systems will be combined to provide power for
improved method to the study of the flow the innovation of various bionic aircraft.
characteristics of the bionic wing, and obtained many
promising results191,192. These works provide a certain
reference for the development of bio-inspired flight Acknowledgements
systems. Currently, improving the reliability of
fluid-structure interaction methods in biological flight This work was supported by the National Natural
and bionic flow simulation is an important Science Foundation of China (Nos. 11872293,
development direction193. In summary, with the 11672225 and 11602199), China Postdoctoral Science
continuous expansion of computing resources, using Foundation (No. 2017M623184), the National Key
numerical methods to study the aerodynamics of bionic Laboratory of Science and Technology on
flight systems has become the focus of future research. Aerodynamic Design and Research of China
(No.6142201190408), the Key Laboratory of
5. Conclusions Aerodynamics Noise Control of China
(Nos.1801ANCL20180103, 1901ANCL20190108),
The bio-inspired flight systems represent the most Australian Research Council (Nos. DP200101500 and
exciting and challenging direction for the development DE160101098), and the Program of Introducing
of future flying robots194. With the higher requirements Talents of Discipline to Universities of China (known
of autonomous and intelligent bionic aircraft, the as the “111” Program, No. B18040).
bionic aerodynamics related to the bio-inspired flight
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