We are witnessing the advent of a new era of robots - drones - that can autonomously fly in natural and man-made environments. These robots, often associated with defence applications, could have a major impact on civilian tasks, including transportation, communication, agriculture, disaster mitigation and environment preservation. Autonomous flight in confined spaces presents great scientific and technical challenges owing to the energetic cost of staying airborne and to the perceptual intelligence required to negotiate complex environments. We identify scientific and technological advances that are expected to translate, within appropriate regulatory frameworks, into pervasive use of autonomous drones for civilian applications.

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Science, technology and the future of small autonomous drones

Flies are among the most agile flying creatures on Earth To mimic this aerial prowess in a similarly sized robot requires tiny, high-efficiency mechanical components that pose miniaturization challenges governed by force-scaling laws, suggesting unconventional solutions for propulsion, actuation, and manufacturing To this end, we developed high-power-density piezoelectric flight muscles and a manufacturing methodology capable of rapidly prototyping articulated, flexure-based sub-millimeter mechanisms We built an 80-milligram, insect-scale, flapping-wing robot modeled loosely on the morphology of flies Using a modular approach to flight control that relies on limited information about the robot's dynamics, we demonstrated tethered but unconstrained stable hovering and basic controlled flight maneuvers The result validates a sufficient suite of innovations for achieving artificial, insect-like flight

http://science.sciencemag.org/content/sci/340/6132/603.full.pdf

Controlled Flight of a Biologically Inspired, Insect-Scale Robot

Recent progress in flapping wing aerodynamics and aeroelasticity

1. Introduction 2. Fixed, rigid wing aerodynamics 3. Flexible wing aerodynamics 4. Flapping wing aerodynamics.

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Aerodynamics of Low Reynolds Number Flyers

Nomenclature Ar = rotor disk area CD = sectional drag coefficient CD0 = zero-lift drag coefficient Clα = lift-curve slope CP = power coefficient CPi = induced power coefficient CP0 = profile power coefficient CT = thrust coefficient c = chord length D = drag force D.L . = disk loading L = lift force m = mass P.L . = power loading SF = separated flow T = rotor thrust V = local wind velocity perceived by flap W = weight W f = final weight Wo = gross takeoff weight α = blade section angle of attack η = efficiency μ = dynamic viscosity ρ = air density σ = rotor solidity = flapping amplitude (peak to peak)

Challenges Facing Future Micro-Air-Vehicle Development

Dimensionless numbers are important in biomechanics because their constancy can imply dynamic similarity between systems, despite possible differences in medium or scale. A dimensionless parameter that describes the tail or wing kinematics of swimming and flying animals is the Strouhal number, St = fA/U, which divides stroke frequency (f) and amplitude (A) by forward speed (U). St is known to govern a well-defined series of vortex growth and shedding regimes for airfoils undergoing pitching and heaving motions. Propulsive efficiency is high over a narrow range of St and usually peaks within the interval 0.2 < St < 0.4 (refs 3-8). Because natural selection is likely to tune animals for high propulsive efficiency, we expect it to constrain the range of St that animals use. This seems to be true for dolphins, sharks and bony fish, which swim at 0.2 < St < 0.4. Here we show that birds, bats and insects also converge on the same narrow range of St, but only when cruising. Tuning cruise kinematics to optimize St therefore seems to be a general principle of oscillatory lift-based propulsion.

Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency

Insect wings are complex structures that deform dramatically in flight. We analyzed the aerodynamic consequences of wing deformation in locusts using a three-dimensional computational fluid dynamics simulation based on detailed wing kinematics. We validated the simulation against smoke visualizations and digital particle image velocimetry on real locusts. We then used the validated model to explore the effects of wing topography and deformation, first by removing camber while keeping the same time-varying twist distribution, and second by removing camber and spanwise twist. The full-fidelity model achieved greater power economy than the uncambered model, which performed better than the untwisted model, showing that the details of insect wing topography and deformation are important aerodynamically. Such details are likely to be important in engineering applications of flapping flight.

https://science.sciencemag.org/content/sci/325/5947/1549.full.pdf

Details of Insect Wing Design and Deformation Enhance Aerodynamic Function and Flight Efficiency

SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization,
that dragonflies fly by using unsteady aerodynamic mechanisms to generate
high-lift, leading-edge vortices. In normal free flight, dragonflies use
counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing
downstroke, attached flow on the forewing upstroke, and attached flow on the
hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats
with highly separated downstroke flows, with a single LEV attached across both
the fore- and hindwings. We use smoke visualizations to distinguish between
the three simplest local analytical solutions of the Navier–Stokes
equations yielding flow separation resulting in a LEV. The LEV is an open
U-shaped separation, continuous across the thorax, running parallel to the
wing leading edge and inflecting at the tips to form wingtip vortices. Air
spirals in to a free-slip critical point over the centreline as the LEV grows.
Spanwise flow is not a dominant feature of the flow field – spanwise
flows sometimes run from wingtip to centreline, or vice versa –
depending on the degree of sideslip. LEV formation always coincides with rapid
increases in angle of attack, and the smoke visualizations clearly show the
formation of LEVs whenever a rapid increase in angle of attack occurs. There
is no discrete starting vortex. Instead, a shear layer forms behind the
trailing edge whenever the wing is at a non-zero angle of attack, and rolls
up, under Kelvin–Helmholtz instability, into a series of transverse
vortices with circulation of opposite sign to the circulation around the wing
and LEV. The flow fields produced by dragonflies differ qualitatively from
those published for mechanical models of dragonflies, fruitflies and
hawkmoths, which preclude natural wing interactions. However, controlled
parametric experiments show that, provided the Strouhal number is appropriate
and the natural interaction between left and right wings can occur, even a
simple plunging plate can reproduce the detailed features of the flow seen in
dragonflies. In our models, and in dragonflies, it appears that stability of
the LEV is achieved by a general mechanism whereby flapping kinematics are
configured so that a LEV would be expected to form naturally over the wing and
remain attached for the duration of the stroke. However, the actual formation
and shedding of the LEV is controlled by wing angle of attack, which
dragonflies can vary through both extremes, from zero up to a range that leads
to immediate flow separation at any time during a wing stroke.

/pdf/dragonfly-flight-free-flight-and-tethered-flow-35vxeiljgv.pdf

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

In the absence of much passive stability, flying insects rely upon active stabilisation, necessitating the provision of rich sensory feedback across a range of modalities. Here we consider from a sensory perspective what quantities flying insects measure, in order to ask from a mechanical perspective why they should want to do so. We consider each of the sensory modalities separately and uncover three general principles. Firstly, we find that insects have evolved to measure changes in kinematic state, rather than absolute state. For example, although the antennae may be loosely thought of as airspeed sensors, we show that they are configured as a sophisticated adaptive sensing system which is much more appropriate for measuring changes in airspeed than absolute airspeed. Secondly, we find that insect sensory systems are tuned to sense self-motion components in specific directions. For example, certain visual interneurons of flies operate as matched filters that are tuned to detect the optic flow fields induced specifically by rotation about one particular axis. Thirdly, we find that insects commonly combine sensory input from across modalities to form composite, multi-modal quantities which they use as feedback to the control system. For example, certain individually identified descending interneurons combine input from the compound eyes, ocelli, antennae, and cephalic wind-sensitive hairs into one composite signal which is then used in flight control. We infer from these three general organisational principles that insects are configured to sense excitation of their natural modes of motion. This natural-mode sensing hypothesis: (1) explains why insects should want to sense changes in state rather than absolute state; (2) predicts what specific directions of motion they should sense, and (3) specifies how sensory input from different modalities should be combined.

Sensory Systems and Flight Stability: What do Insects Measure and Why?

SUMMARY Here we provide the first formal quantitative analysis of dynamic stability
in a flying animal By measuring the longitudinal static stability derivatives
and mass distribution of desert locusts Schistocerca gregaria , we
find that their static stability and static control responses are insufficient
to provide asymptotic longitudinal dynamic stability unless they are sensitive
to pitch attitude (measured with respect to an inertial or earth-fixed frame)
as well as aerodynamic incidence (measured relative to the direction of
flight) We find no evidence for a `constant-lift reaction9, previously
supposed to keep lift production constant over a range of body angles, and
show that such a reaction would be inconsequential because locusts can
potentially correct for pitch disturbances within a single wingbeat The
static stability derivatives identify three natural longitudinal modes of
motion: one stable subsidence mode, one unstable divergence mode, and one
stable oscillatory mode (which is present with or without pitch attitude
control) The latter is identified with the short period mode of aircraft, and
shown to consist of rapid pitch oscillations with negligible changes in
forward speed The frequency of the short period mode (approx 10 Hz) is only
half the wingbeat frequency (approx 22 Hz), so the mode would become coupled
with the flapping cycle without adequate damping Pitch rate damping is shown
to be highly effective for this purpose — especially at the small scales
associated with insect flight — and may be essential in stabilising
locust flight Although having a short period mode frequency close to the
wingbeat frequency risks coupling, it is essential for control inputs made at
the level of a single wingbeat to be effective This is identified as a
general constraint on flight control in flying animals

Graham K. Taylor

Papers

Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency

Details of Insect Wing Design and Deformation Enhance Aerodynamic Function and Flight Efficiency

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

Sensory Systems and Flight Stability: What do Insects Measure and Why?

Dynamic flight stability in the desert locust Schistocerca gregaria.