Ornithopter Aerodynamic Experiments

Bird or insect flight has been of great interest to general audience, not only to scientists. Numerous works have been devoted to the study of aerodynamics of insect wings, for example, the hovering flight. It is generally considered that the lift supporting the insect comes from the unsteadiness of the flight. However, the term “unsteadiness” itself is loose and imprecise at all. It may include several unsteady components: the motion of wing, vortex in the flow as well as the surface vorticity. The various contributions are now examined from the perspective of a force element theory to gain more insight into the mechanisms of generating lift.

Purpose This paper aims to provide the international aeronautical community with details of the development of a new disruptive technology for aircraft propulsion. Design/methodology/approach This paper describes the results achieved by a small Austrian aeronautical innovations company in developing a cyclogyro propulsion system capable of vertical launch and efficient forward flight. The research team progressed from concept definition and simulation (2004-2006), through experimental validation and concept demonstration (2006), component optimization (2006-2012), full system demonstration (2012-2014) and examination of ability to scale (both larger and smaller) (2015 onwards). This paper provides details of the results of each of these stages. Findings The research team proved that cyclogyro propulsion can be used for the vertical launch, and that, in forward flight, it has the potential to achieve efficiency beyond the range of conventional fixed wing and rotorcraft. Research limitations/implications This research indicates that the efficiency increases with forward speed within the range achieved in standard wind tunnels (up to 35 m/s). This efficiency appears to be caused by a unique chamber effect within the cyclogyro rotor assembly. Future research should be conducted to analyse this chamber effect in greater detail and to test the cyclogyro rotor for speeds beyond 35 m/s. Practical implications This work indicates that cyclogyro propulsion could have the potential to provide vertical launch, high speed and highly efficient aircraft that have reduced wing span, no external rotors and exceptional agility. This technology could therefore be feasible for vertical take-off and landing aircraft that can safely form densely packed swarms. Social implications It could be researched as an efficiency increase in forward flight completely different to existing propulsion systems. This could open a way for a more efficient air traffic in future and faster reduction of CO2 and NOX emission an allow an environment-friendlier air travelling. Originality/value This paper provides the details of the first cyclogyro aircraft to have flown and will serve the aeronautical community by stimulating the debate on this new disruptive technology.

If you think that fruit flies are tiny, please reconsider. Spectacular radiations of miniaturized beetles, flies and wasps dominate the insect fauna and are characterized by body lengths well below that of the biologist's friend Drosophila melanogaster (∼3 mm). Most remarkably, the smallest flying insect described to date has a length of just 0.16 mm. But just how does flapping flight work at such small scales? The viscous effects of air on a wing increase at smaller sizes and a 1 mm insect moves through the air as would a bumblebee move through mineral oil. Lift forces generated by swirling air around the wing are accordingly impeded given the relative stickiness of the fluid, and alternative mechanisms of force production must be at play. How then do very small insects create the aerodynamic forces necessary to offset body weight and also to generate horizontal thrust to intentionally move about?Xin Cheng and Mao Sun, from Beihang University, China, took on this question by describing the detailed kinematics of forward flight in the tiny wasp Encarsia formosa, which has a wing length of only 0.5 mm. Writing in 1973 in JEB, Torkel Weis-Fogh first described the novel clap-fling aerodynamic mechanism utilized by this species; Cheng and Sun have now built on their earlier computational work on hovering in E. formosa to analyze its forward flight kinematics and associated forces, albeit at the humble airspeed of ∼0.3 m s−1. They identified a distinctive pattern of wing motions very different from that of larger insects (including the fruit fly). First, the wings accelerate backward at very high incidence angles relative to oncoming flow (‘impulsive paddling’), and then close up on the right and left sides while at the same time moving upward. Then in the downstroke, the wings quickly rotate about their posterior edges (which Weis-Fogh referred to as the ‘fling’) and sweep downward and forward, again moving at high incidence angles to the surrounding air.But what do these odd features of wing flapping yield in terms of force production? Cheng and Sun used computer simulations to reveal two means by which the effects of small size and fluid stickiness were largely overcome. Firstly, the fast backward acceleration of the wings at high incidence to the air produces a large thrust but also some vertical force supporting the body weight. Secondly, the ‘flinging’ motion produces a low-pressure region of rapidly swirling air which persists through the downstroke, creating the majority of the vertical force required to offset gravity. Thus, the insects employ distinctive wing motions to offset the otherwise formidable effects of increased viscosity. The associated forces last just milliseconds but, at a wingbeat frequency of ∼350 Hz, are clearly sufficient for routine flight. They may be even more effective during maneuvers, a topic destined for future investigation.Miniaturization is one of the dominant themes in insect evolution and largely underpins the high extant diversity of small insects (and also of their parasitoids). This computational study of aerodynamics in one such lilliputian species suggests that additional fluid dynamic novelties await discovery and analysis by flight biomechanists. Fringed wings with much reduced supporting veins are also well known among the miniaturized fliers, but have largely unknown aerodynamic consequences. Often at the mercy of gentle breezes, these insects live in physical, behavioral and ecological domains supremely distant from our own personal experiences. This aerodynamic study should help us to close that conceptual distance.

In this paper, detailed development of a nonlinear aeroelastic coupled trim model of a twin-cyclocopter in forward flight is presented. Twin-cyclocopter consists of two cycloidal rotors as main thrusters and a conventional nose rotor for pitch-torque balance. It is shown that five control inputs (mean and differential rpm, mean and differential phase offset of cyclorotors, rpm of nose rotor) are needed to balance three moments and two forces on cyclocopter in forward flight while forces along lateral direction remain balanced at all stages. In this coupled trim procedure, blade aeroelastic response equations and vehicle trim equations are solved together by simultaneously updating control inputs and blade response. To obtain the blade response and forces for a given set of control inputs, an aeroelastic model of cyclorotor and an aerodynamic model of the conventional nose rotor in forward flight is developed. The nonlinear aeroelastic model of the cyclorotor is developed by coupling unsteady aerodynamic model of cyclorotor in forward flight with a geometrically exact beam based structural framework capable of predicting large bending and torsional deflections of rotor blade. Towards this, complex aerodynamics of the cyclorotor is thoroughly investigated and various underlying phenomena, such as dynamic virtual camber, effects of near and shed wake and leading-edge vortices are rigorously modeled. A modified Double Multiple Streamtube (D-MS) model is implemented to capture the complex dynamic inflow characteristics of cyclorotor in forward flight. The present model is validated with previously published in-house experimental data on the performance of a trimmed cyclorotor at different forward speeds.

By analyzing high-speed video of the fruit fly, we discover a swimminglike mode of forward flight characterized by paddling wing motions. We develop a new aerodynamic analysis procedure to show that these insects generate drag-based thrust by slicing their wings forward at low angle of attack and pushing backwards at a higher angle. Reduced-order models and simulations reveal that the law for flight speed is determined by these wing motions but is insensitive to material properties of the fluid. Thus, paddling is as effective in air as in water and represents a common strategy for propulsion through aquatic and aerial environments.

7 - A Period of Reappraisal: — Measurements of Man-Power and Studies of Bird and Insect Flight

Despite a century of manned flight, when it comes to aerobatics, we're still a long way behind insects. And even though their agility can be extremely frustrating when evicting an unwanted guest, insects can teach us a great deal about the complex forces that keep them aloft and the mechanisms that control them. Michael Dickinson is fascinated by all aspects of insect flight: from the aerodynamic forces they generate to the complex neural systems that control each intricate wing beat. But it hadn't been possible to correlate variations in insect wing beats and the muscles that control them,with the effects they have on a flight path until recent technical developments allowed Claire Balint, a student in the Dickinson lab, to simultaneously monitor flight muscle activity and wing movements to see how flies steer their erratic path(p. 3813).But flies are notoriously mobile; the only way for Balint to collect the high speed flight recordings she needed to analyse the insect's wing movements was to tether the insect in a flight arena while focusing three ultra-high speed cameras on the flapping insect to capture its intricate manoeuvres. And by skilfully inserting microscopic electrodes into five of the insect's flight muscles, she could record flight muscle activity simultaneously as they flew into a mild headwind. But Balint also needed to measure the aerodynamic forces generated by each wing beat before she could begin correlating the insect's muscle activity with the forces generated. Digitising the positions of both right and left wings over a total of almost 870 wingbeats, Balint was able to calculate the forces generated by the wings as they moved through the air, as well as measuring them on a scaled-up mechanical model, before beginning the painstaking task of correlating the forces that keep the insect aloft with individual muscle activity.Balint explains that `although a reasonably robust theory exists for predicting the forces resulting from an arbitrary change in wing motion, the link between aerodynamically relevant changes in wing kinematics and the activity of specific steering muscles was less clear'. She and Dickinson also knew that certain muscle groups were responsible for three individual aspects of the insect's wing beat movements; the downstroke deviation, the dorsal amplitude and a shift in the ventral amplitude known as `mode'. Undaunted by the enormous amount of data generated by the flapping insects, Balint began analysing the aerodynamic forces generated by the insects according to each of these wing beat features, to see how they affected the insect's aerodynamic performance. Thanks to this novel approach, the team soon realised that specific sets of muscles were responsible for different aspects of the insect's aerobatic armoury. `The basalare muscles primarily control lift and roll by varying the downstroke force, the muscles of pteralae III and I control thrust and yaw by controlling the upstroke force and an unknown muscle group controls lift and roll by varying the upstroke force inclination',explains Balint.Having found that it's the flies' ability to control several force generating mechanisms in concert that allows them to dodge and weave so effectively, Dickinson is keen to know more about the elusive insect's flight and pursuit strategies.

Aerodynamic interference between the main and tail rotor can have a strong negative influence on the flight mechanics of a conventional helicopter. Significant unsteadiness in the tail rotor loading is encountered under certain flight conditions, but the character of the unsteadiness can depend on the direction of rotation of the tail rotor. Numerical simulations, using Brown's vorticity transport model, of the aerodynamic interaction between the main and tail rotors of a helicopter are presented for a range of forward and lateral flight trajectories. Distinct differences are predicted in the behavior of the system in left and right sideward flight that are consistent with flight experience that the greatest fluctuations in loading or control input are required in left sideways flight (for a counterclockwise rotating main rotor). These fluctuations are generally more extreme for a system with tail rotor rotating top-forward than top-aft. Differences are also exposed in the character of the lateral excitation of the system as forward flight speed is varied. The observed behavior appears to originate in the disruption of the tail rotor wake that is induced by its entrainment into the wake of the main rotor. The extent of the disruption is dependent on flight condition, and the unsteadiness of the process depends on the direction of rotation of the tail rotor. In intermediate-speed forward flight and right sideward flight, the free stream delays the entrainment of the tail rotor wake far enough downstream for the perturbations to the rotor loading to be slight. Conversely, in left sideward and quartering flight, the free stream confines the entrainment process close to the rotors, where it causes significant unsteadiness in the loads produced by the system.

Insects, birds, and bats that power and control flight by flapping their wings perform excellent flight stability and maneuverability by rapidly and continuously varying their wing motions. This article provides an overview of the state of the art of vortex-dominated, unsteady flapping aerodynamics from the viewpoint of diversity and uniformity associated with dominant vortices, particularly of the relevant physical aspects of the flight of insects and vertebrates in the low- and intermediate-Reynolds-number ( Re) regime of 100 to 106. After briefly describing wing morphology and kinematics, we discuss the main vortices generated by flapping wings and the aerodynamic forces associated with these structures, focusing on leading-edge vortices (LEVs), wake vortices, and vortices generated by wing motions over a broad Re range. The LEVs are intensified by dynamic wing morphing in bird and bat flight, producing a significantly elevated vortex lift. The complex wake vortices are the footprints of lift generation; thus, the time-averaged vortex lift can be estimated from wake velocity data. Computational fluid dynamics modeling, quasi-steady models, and vortex lift models are useful tools to elucidate the intrinsic relationships between the lift and the dominant vortices in the near- and far-fields in flapping flight.

The paper describes a methodology to extract linearized state-space dynamic inflow model from a generic and high fidelity aerodynamic analysis for single and coaxial helicopter rotor systems. Both hover and forward flight are studied including maneuvering and the results are demonstrated by a free-wake analysis. The proposed methodology which is founded on a Single Frequency Analysis is computationally efficient and provide the entire frequency-domain behavior of the wake inflow states. Besides being able to incorporate any advanced computational tool, the present methodology takes advantage of exact wake states and thus adequately encompasses high frequency responses. The paper also extends the methodology to the case where states of higher harmonics are required and to the case of heaving, rolling and pitching rotor disc. For a single rotor system the results resemble the well known classical dynamic inflow model. For coaxial rotor system in hover, the high mutual couplings between the rotors are clearly demonstrated and affect the resulting system matrices. An important feature of the present methodology lays in the fact that the entire formulation is non-dimensional and the in-flow analysis is completely separated from the loads analysis, which enables a consistent generalization of the results.

Based on CFD method and FW-Hpds equations, a numerical method for predicting aeroacoustic characteristics of coaxial rotor is established. In order to accurately simulate the flowfield of coaxial rotor, several techniques including moving-embedded grids, implicit LU-SGS scheme, ROE-MUSCL scheme and Spalart-Allmaras turbulence model are adopted to solve the RANS equations. The aeroacoustic characteristics of coaxial rotor in forward flight are calculated and analyzed by using FW-Hpds equations. The calculated acoustic characteristics of the AH-1/OLS model in forward flight for validation are in good agreements with the experimental data. The acoustic characteristics of coaxial-rotor in forward flight including the sound pressure time history and directivity patterns are investigated in detail. Some meaningful conclusions are obtained from the calculated results. The counter-rotation of upper and lower rotors has significant effects on the sound pressure time history and leads to different phases of peak value at different observation points. The noise radiation intensity of coaxial rotor in forward flight exhibits an approximately symmetrical distribution on both sides of the forward flight direction, which is one of the important differences between the coaxial rotor and the single rotor.

Avian flight is guided by optic flow-the movement across the retina of images of surfaces and edges in the environment due to self-motion. In all vertebrates, there is a short pathway for optic flow information to reach pre-motor areas: retinal-recipient regions in the midbrain encode optic flow, which is then sent to the cerebellum. One well-known role for optic flow pathways to the cerebellum is the control of stabilizing eye movements (the optokinetic response). However, the role of this pathway in controlling locomotion is less well understood. Electrophysiological and tract tracing studies are revealing the functional connectivity of a more elaborate circuit through the avian cerebellum, which integrates optic flow with other sensory signals. Here we review the research supporting this framework and identify the cerebellar output centres, the lateral (CbL) and medial (CbM) cerebellar nuclei, as two key nodes with potentially distinct roles in flight control. The CbM receives bilateral optic flow information and projects to sites in the brainstem that suggest a primary role for flight control over time, such as during forward flight. The CbL receives monocular optic flow and other types of visual information. This site provides feedback to sensory areas throughout the brain and has a strong projection the nucleus ruber, which is known to have a dominant role in forelimb muscle control. This arrangement suggests primary roles for the CbL in the control of wing morphing and for rapid maneuvers.

In this paper, we show an approach to elucidate the free flight of an insect using a simulation. We modeled a fly, Drosophila, by using aerodynamics, body dynamics, and control theory. The modeled virtual insect performs free flight in virtual space generated by a computer. We simulated the free flight of a virtual insect having two dimensions and two degrees of freedom. The flight pass and flight velocity of the virtual insect during a free flight were calculated by Newton's equations of motion. The aerodynamic force generated by the flapping motion of the virtual insect was estimated by using the blade element theory. An optimal regulator theory was used as a control law. The flight pass from the initial position to the target position and the wing motion was obtained from the results of the free flight simulation of the virtual insect. We can presume the wing motion of an insect in free flight by using the flight simulation of a virtual insect. These results have suggested that the approach based on the simulation is effective in elucidating the free flight of an insect.

HOVERING FLIGHT FOR A MICROMECHANICAL FLYING INSECT: MODELING AND ROBUST CONTROL SYNTHESIS

Flying insects are thought to achieve energy-efficient flapping flight by storing and releasing elastic energy in their muscles, tendons, and thorax. However, 'spring-wing' flight systems consisting of elastic elements coupled to nonlinear, unsteady aerodynamic forces present possible challenges to generating stable and responsive wing motion. The energetic efficiency from resonance in insect flight is tied to the Weis-Fogh number (N), which is the ratio of peak inertial force to aerodynamic force. In this paper, we present experiments and modeling to study how resonance efficiency (which increases withN) influences the control responsiveness and perturbation resistance of flapping wingbeats. In our first experiments, we provide a step change in the input forcing amplitude to a series-elastic spring-wing system and observe the response time of the wing amplitude increase. In our second experiments we provide an external fluid flow directed at the flapping wing and study the perturbed steady-state wing motion. We evaluate both experiments across Weis-Fogh numbers from1<N<10. The results indicate that spring-wing systems designed for maximum energetic efficiency also experience trade-offs in agility and stability as the Weis-Fogh number increases. Our results demonstrate that energetic efficiency and wing maneuverability are in conflict in resonant spring-wing systems, suggesting that mechanical resonance presents tradeoffs in insect flight control and stability.