Fungal and actinomycete spore aerosols measured at different humidities with an aerodynamic particle sizer.

An aerodynamic particle sizer (APS) that uses laser Doppler velocimetry was used to determine aerodynamic diameters of spores of fungal and thermophilic actinomycete species common in mouldy hay, aerosolized at different humidities and temperatures. Results were compared with those obtained from inertial impaction in a cascade impactor. The APS gave slightly smaller measurements than the cascade impactor. Both methods gave aerodynamic diameters generally slightly smaller than the average spore dimensions observed on cascade impactor slides with a microscope. The latter measurements were less than axial dimensions given in the literature. Brief passage of spores through air at 95% relative humidity (RH) and 38 degrees C, compared with 40% RH and 20 degrees C, caused an immediate increase in their aerodynamic diameter and the breaking of chains of spores. Cultures maintained at 75% RH and aerosolized at 98% RH similarly produced larger spore particles than those passed through dry air. These findings have implications for mould-induced asthma and allergic alveolitis since they relate to physical behaviour of airborne spores and particle deposition sites in the lung.     

Comparative ecology of three pteropid bats in Rio Muni, West Africa

Data on morphologic features, habits, stratification, flight, and thermal response are presented for Eidolon helvum, Epomops franqueti , and Micropteropus pusillus from Rio Muni, West Africa. Relative abundance and reproduction activities of these pteropids are concordant with seasonal rainfall and availability of foods in Rio Muni. There are major differences between these sympatric species with regard to roosting sites, foods and feeding behaviour, levels of flight, and aerodynamic properties. Thermal responses vary between the species and are correlated with differences in behavioural thermoregulation.     

Fungal and actinomycete spore aerosols measured at different humidities with an aerodynamic particle sizer

T.M. MADELIN AND H.E. JOHNSON. 1992. An aerodynamic particle sizer (APS) that uses laser Doppler velocimetry was used to determine aerodynamic diameters of spores of fungal and thermophilic actinomycete species common in mouldy hay, acrosolized at different humidities and temperatures. Results were compared with those obtained from inertial impaction in a cascade impactor. The APS gave slightly smaller measurements than the cascade impactor. Both methods gave aerodynamic diameters generally slightly smaller than the average spore dimensions observed on cascade impactor slides with a microscope. The latter measurements were less than axial dimensions given in the literature. Brief passage of spores through air at 95% relative humidity (RH) and 38°C, compared with 40% RH and 20°C, caused an immediate increase in their aerodynamic diameter and the breaking of chains of spores. Cultures maintained at 75% RH and aerosolized at 98% RH similarly produced larger spore particles than those passed through dry air. These findings have implications for mould-induced asthma and allergic alveolitis since they relate to physical behaviour of airborne spores and particle deposition sites in the lung.     

A comparative study of migratory behaviour and body mass as determinants of moult duration in passerines

Birds moult to maintain plumage function through life, but the factors that determine moult duration are poorly understood. In temperate areas, variation in moult duration could be largely associated with between-species differences in migratory behaviour (migrants have less time for moulting after breeding), and body mass (because the aerodynamic cost of rapid moult increases allometrically with body size). Moreover, if the energetic cost of transport favours a smaller body size in migratory species, then the effects of migratory behaviour and body mass on moult duration could be confounded. We conducted a comparative study of the duration of adult complete moult in 48 European passerine species, in relation to body mass and migratory behaviour (sedentary, short-distance migrants and long-distance migrants). Lighter and more migratory species moulted faster than heavier and more sedentary species, but migration was not associated with body mass. If accelerated moult compromises the success of migration, changes in the physiology or phenology of moult in migratory birds are better interpreted as adaptive responses to compensate for such costs.     

Ballooning dispersal in arthropod taxa with convergent behaviours: dynamic properties of ballooning silk in turbulent flows

We present a new model of ballooning behaviour in arthropods in which draglines are regarded as being extendible and completely flexible. Our numerical simulations reveal that silk draglines within turbulent flows can become twisted and stretched into highly contorted shapes. Ballooners are therefore predicted to have little control over their aerodynamic drag and their dispersal within the atmospheric boundary layer. Dragline length is crucial only at lift-off. This prediction runs counter to that of Humphrey who suggested that the length of rigid draglines can be used to control dispersal. In contrast with Humphrey's model, the new model accounts naturally for the large distances travelled by some ballooners.     

Breast Feathers as an Air-current Sense Organ for the Control of Flight Behaviour in a Songbird (Carduelis spinus)

Analysis of the flight behaviour of siskins (Carduelis spinus) was made possible by tethering them with a harness to a flight balance in a wind tunnel. It could be shown that the breast feathers work as an air-current sense organ influencing the siskins' flight pattern and the aerodynamic parameters.     

The ideal and the feasible: physical constraints on evolution

The laws of physics and the properties of the physical environment impose constraints on evolution. Structures and processes that may be imagined cannot in some cases be evolved, because they are physically impossible. This paper explores the consequences of the particulate nature of matter and of light; of the wave nature of light and sound; of the laws of diffusion and heat exchange; of the mechanical properties of materials; of limits to aerodynamic and hydrodynamic performance; and of the behaviour of electricity.     

The mechanical power requirements of avian flight

A major goal of flight research has been to establish the relationship between the mechanical power requirements of flight and flight speed. This relationship is central to our understanding of the ecology and evolution of bird flight behaviour. Current approaches to determining flight power have relied on a variety of indirect measurements and led to a controversy over the shape of the power-speed relationship and a lack of quantitative agreement between the different techniques. We have used a new approach to determine flight power at a range of speeds based on the performance of the pectoralis muscles. As such, our measurements provide a unique dataset for comparison with other methods. Here we show that in budgerigars (Melopsittacus undulatus) and zebra finches (Taenopygia guttata) power is modulated with flight speed, resulting in U-shaped power-speed relationship. Our measured muscle powers agreed well with a range of powers predicted using an aerodynamic model. Assessing the accuracy of mechanical power calculated using such models is essential as they are the basis for determining flight efficiency when compared to measurements of flight metabolic rate and for predicting minimum power and maximum range speeds, key determinants of optimal flight behaviour in the field.     

Analysis of the collapsibility of the upper airway in a spectrum of sleep-disordered breathing: a modelling approach

Using a simplified model of the upper airways with two independent collapsible elements (nostrils and hypo-pharynx), we calculated the cross-sectional area of these two elements, taking into account pressure drops. We experimentally measured flow and pressure in the fossa and hypo-pharynx in various syndromes. This allowed us to compare the behaviour of the area supplied by our model with the aerodynamic resistance that is often used to analyse upper airway flow limitation events. We showed that nostril and hypo-pharyngeal areas are better correlated than the resistance values and thus concluded that the pressure divided by the square of the flow is a better parameter for analysing flow limitation in upper airways than resistance. Owing to its simplicity, our model is able to supply the area of the collapsible element in real time, which is impossible with more sophisticated models.     

CFD Based Determination of Aerodynamic Effects on Birds with Extremely Large Dihedral

Using a sophisticated aerodynamic method, the effects of extremely large dihedral on the aerodynamic characteristics of birds are determined. With this method, it is possible to generate solutions for the addressed aerodynamic problem, which shows a high complexity due to interference effects caused by dihedral and pronounced 3-dimensional flow properties as well as due to complex wing geometries. From the obtained results it follows that extremely large dihedral has substantial effects on the aerodynamic force characteristics. There are significant changes in the lift, the drag and the side force, thus affecting the flight performance. Furthermore, the obtained results show that the aerodynamic rolling and yawing moment characteristics are influenced by extremely large dihedral to a high degree. This is a significant outcome for lateral-directional stability because the aerodynamic rolling and yawing moment characteristics have a determinative influence here.     

Flight of the dragonflies and damselflies

This work is a synthesis of our current understanding of the mechanics, aerodynamics and visually mediated control of dragonfly and damselfly flight, with the addition of new experimental and computational data in several key areas. These are: the diversity of dragonfly wing morphologies, the aerodynamics of gliding flight, force generation in flapping flight, aerodynamic efficiency, comparative flight performance and pursuit strategies during predatory and territorial flights. New data are set in context by brief reviews covering anatomy at several scales, insect aerodynamics, neuromechanics and behaviour. We achieve a new perspective by means of a diverse range of techniques, including laser-line mapping of wing topographies, computational fluid dynamics simulations of finely detailed wing geometries, quantitative imaging using particle image velocimetry of on-wing and wake flow patterns, classical aerodynamic theory, photography in the field, infrared motion capture and multi-camera optical tracking of free flight trajectories in laboratory environments. Our comprehensive approach enables a novel synthesis of datasets and subfields that integrates many aspects of flight from the neurobiology of the compound eye, through the aeromechanical interface with the surrounding fluid, to flight performance under cruising and higher-energy behavioural modes.This article is part of the themed issue ‘Moving in a moving medium: new perspectives on flight’.     

Effect of slotted wing tips on yawing moment characteristics

The aerodynamic yawing moment characteristics of bird wings with slotted tips are dealt with. Emphasis is placed on the effect of sweep which the separated feathers constituting the wing tips show and which can reach significant values. Reference is made to basic aerodynamic characteristics of wings with sweep which yields a stabilizing yawing moment significantly larger than that of unswept wings. Then, the yawing moment characteristics are determined for a wing, the features of which are considered as representative of bird wings with sweep in their slotted tips. A sophisticated aerodynamic procedure is used for obtaining results of high precision. It is shown that the sweep in the slotted wing tips yields a stabilizing yawing moment of significant magnitude, considerably increasing with the lift coefficient. To make the significance of wing tip sweep for the ability to generate yawing moments more perspicuous, a wing modification the slotted tips of which are unswept is considered for comparison. It turns out that this wing shows yawing moments which are substantially smaller. A physical insight into the effect of slotted wing tip sweep on the aerodynamic yawing moment characteristics is provided by showing the underlying mechanism. From the results presented in this paper it follows that the sweep in slotted wing tips provides a substantial contribution to the aerodynamic yawing moment and, thus, to yaw stability. It may be concluded that this is an essential reason why there is sweep in the slotted tips of bird wings.     

Aerodynamic performance of a hovering hawkmoth with flexible wings: a computational approach

Insect wings are deformable structures that change shape passively and dynamically owing to inertial and aerodynamic forces during flight. It is still unclear how the three-dimensional and passive change of wing kinematics owing to inherent wing flexibility contributes to unsteady aerodynamics and energetics in insect flapping flight. Here, we perform a systematic fluid-structure interaction based analysis on the aerodynamic performance of a hovering hawkmoth, Manduca, with an integrated computational model of a hovering insect with rigid and flexible wings. Aerodynamic performance of flapping wings with passive deformation or prescribed deformation is evaluated in terms of aerodynamic force, power and efficiency. Our results reveal that wing flexibility can increase downwash in wake and hence aerodynamic force: first, a dynamic wing bending is observed, which delays the breakdown of leading edge vortex near the wing tip, responsible for augmenting the aerodynamic force-production; second, a combination of the dynamic change of wing bending and twist favourably modifies the wing kinematics in the distal area, which leads to the aerodynamic force enhancement immediately before stroke reversal. Moreover, an increase in hovering efficiency of the flexible wing is achieved as a result of the wing twist. An extensive study of wing stiffness effect on aerodynamic performance is further conducted through a tuning of Young's modulus and thickness, indicating that insect wing structures may be optimized not only in terms of aerodynamic performance but also dependent on many factors, such as the wing strength, the circulation capability of wing veins and the control of wing movements.     

On the Evolution of Feathers from an Aerodynamic and Constructional View Point

The evolution of birds and feathers are examined in terms ofthe aerodynamic constraints imposed by the arboreal and cursorialmodels of flight evolution. The cursorial origin of flight isassociated with the putative coelurosaurian ancestry of birds.As presently known, coelurosaurs have a center of mass locatedin the pelvic region and an elongated pubis that is ventrallyor anteriorly directed. Both of these characteristics make itdifficult to postulate an origin of flight that would involvea gliding phase because the abdomen cannot be flattened intoan aerodynamic shape. Moreover, the cursorial model must counteractgravity using the hindlimb and, thus, selection for the powerrequirement for lift-off would not focus on the forelimb. Therefore,if the hypothesis proposing a coelurosaurian ancestry of birdsis to remain viable, it must be via an as yet undiscovered taxonthat is compatible with the morphological and aerodynamic constraintsimposed by flight evolution. The arboreal model, currently centers around non-dinosauriantaxa and is more parsimonious in that early archosaurs haveshort pubes that do not preclude an aerodynamic body profile.Moreover, the arboreal proavis uses gravity to create the airflowover the body surfaces and is, thus, energy efficient. Considerationof the initial aerodynamic roles of feathers and feather designare consistent with a precursory gliding phase. Whether avianancestry lies among coelurosaur theropods or earlier archosaurs,we must remain mindful of the complex aerodynamic dictates ofgliding and powered flight and avoid formalistic approachesthat co-opt sister taxa, with their known body form, as functionalancestors.     

AERODYNAMICS,THERMOREGULATION, AND THE EVOLUTION OF INSECT WINGS: DIFFERENTIAL SCALING AND EVOLUTIONARY CHANGE

We examine several aerodynamic and thermoregulatory hypotheses about possible adaptive factors in the evolution of wings from small winglets in insects. Using physical models of Paleozoic insects in a wind tunnel, we explore the potential effects of wings for increasing gliding distance, increasing dispersal distance during parachuting, improving attitude control or stability, and elevating body temperatures during thermoregulation. The effects of body size and shape, wing length, number, and venation, and meteorological conditions are considered. Hypotheses consistent with both fixed and moveable wing articulations are examined. Short wings have no significant effects on any of the aerodynamic characteristics, relative to wingless models, while large wings do have significant effects. In contrast, short wings have large thermoregulatory effects relative to wingless models, but further increases in wing length do not significantly affect thermoregulatory performance. At any body size, there is a wing length below which there are significant thermoregulatory effects of increasing wing length, and above which there are significant aerodynamic effects of increasing wing length. The relative wing length at which this transition occurs decreases with increasing body size. These results suggest that there could be no effective selection for increasing wing length in wingless or short-winged insects in relation to increased aerodynamic capacity. Our results are consistent with the hypothesis that insect wings initially served a thermoregulatory function and were used for aerodynamic functions only at larger wing lengths and/or body sizes. Thus, we propose that thermoregulation was the primary adaptive factor in the early evolution of wings that preadapted them for the subsequent evolution of flight. Our results illustrate an evolutionary mechanism in which a purely isometric change in body size may produce a qualitative change in the function of a given structure. We propose a hypothesis in which the transition from thermoregulatory to aerodynamic function for wings involved only isometric changes in body size and argue that changes in body form were not a prerequisite for this major evolutionary change in function.     

On mathematical modelling of insect flight dynamics in the context of micro air vehicles

We discuss some aspects of mathematical modelling relevant to the dynamics of insect flight in the context of insect-like flapping-wing micro air vehicles (MAVs). MAVs are small flying vehicles developed to reconno?tre in confined spaces. This requires power-efficient, highly-manoeuvrable, low-speed flight with stable hover. All of these attributes are present in insect flight and hence the focus on reproducing the functionality of insect flight by engineering means. Empirical research on insect flight dynamics is limited by experimental difficulties. Force and moment measurements require tethering the animal whose behaviour may then differ from free flight. The measurements are made when the insect actively tries to control its flight, so that its open-loop dynamics cannot be observed. Finally, investigation of the sensory-motor system responsible for flight is even more challenging. Despite these difficulties, much empirical progress has been made recently. Further progress, especially in the context of MAVs, can be achieved by the complementary information derived from appropriate mathematical modelling. The focus here is on a means of computing the data not easily available from experiments and also on making mathematical predictions to suggest new experiments. We consider two aspects of mathematical modelling for insect flight dynamics. The first one is theoretical (computational), as opposed to empirical, generation of the aerodynamic data required for the six-degrees-of-freedom equations of motion. For this purpose we first explain insect wing kinematics and the salient features of the corresponding flow. In this context, we show that aerodynamic modelling is a feasible option for certain flight regimes, focusing on a successful example of modelling hover. Such modelling progresses from the first principles of fluid mechanics, but relies on simplifications justified by the known flow phenomenology and/or geometric and kinematic symmetries. This is relevant to six types of fundamental manoeuvres, which we define as those flight conditions for which only one component of the translational and rotational body velocities is nonzero and constant. The second aspect of mathematical modelling for insect flight dynamics addressed here deals with the periodic character of the aerodynamic force and moment production. This leads to consideration of the types of solutions of nonlinear equations forced by nonlinear oscillations. In particular, the mechanism of synchronization seems relevant and should be investigated further.     

The effects of wind and posture on the aerodynamic performance during the flight stage of skiing

Numerical simulation is conducted to evaluate the wind and posture effects on the aerodynamic performance of a skier during the flight stage. Both steady and unsteady models are applied on a 2D geometry. Using the Fluent code, the fundamental equations of fluid flow are solved simultaneously. In particular we focus on the influence of wind speed and direction on aerodynamic forces with several different postures of the skier in steady modeling. For a chosen case, the unsteady models are used to predict the transient characteristics of streamline distributions and aerodynamic forces. It is found that the skier's postures, wind speed, and direction play a significant role. The wind condition affects the pressure force (the form drag) on the skier and makes it a resistance or thrust regarding wind directions. The optimized posture with a minimization of resistance under a facing wind is determined as a moving-forward body of the skier. The unsteady modeling reveals that the wake around the skier and aerodynamic forces are strongly dependent on time. This initial study not only provides a qualitative and theoretical basis for the athletes to understand the effects of wind and postures, and then to optimize their postures according to the wind condition during the flight stage of skiing, but also builds the foundation for the systematic study of skiing process with more advanced CFD models in the future.     

They seem to glide. Are there aerodynamic effects in leaping prosimian primates?

Leaping primates often assume a horizontal position while airborne. When the limbs are spread out in such maneuvers, skin folds between the upper limbs and the trunk are exposed. This has led to the assumption that the animals make use of aerodynamic forces for either gliding, steering, or braking before the landing. In terms of physics, aerodynamic lift or aerodynamic drag can cause the described effects. As coefficients of lift and drag are unknown for flying primates, we have calculated those values that give the animals either a 5% gain or loss in leaping distance. These turn out to be in the range of values for cylinder-shaped "blunt" (unstreamlined) bodies. A significant influence of aerodynamic forces on the flight path can therefore be assumed. The smaller-bodied species (e.g., galagos) are more strongly influenced by their great surface areas. Although frontal areas scale positively allometrically with respect to body mass, air speed gains importance in the larger-bodied species (e.g., sifakas). They cover absolutely greater distances and have the higher takeoff velocities. The actual importance of lift and drag cannot be derived from our theoretical calculations but must be determined experimentally.     

The aerodynamic costs of warning signals in palatable mimetic butterflies and their distasteful models

Bates hypothesized that some butterfly species that are palatable gain protection from predation by appearing similar to distasteful butterflies. When undisturbed, distasteful butterflies fly slowly and in a straight line, and palatable Batesian mimics also adopt this nonchalant behaviour. When seized by predators, distasteful butterflies are defended by toxic or nauseous chemicals. Lacking chemical defences, Batesian mimics depend on flight to escape attacks. Here, I demonstrate that flight in warning-coloured mimetic butterflies and their distasteful models is more costly than in closely related non-mimetic butterflies. The increased cost is the result of differences in both wing shape and kinematics. Batesian mimics and their models slow the angular velocity of their wings to enhance the colour signal but at an aerodynamic cost. Moreover, the design for flight in Batesian mimics has an additional energetic cost over that of its models. The added cost may cause Batesian mimics to be rare, explaining a general pattern that Bates first observed.     

MORPHOLOGY AND DISPERSAL POTENTIAL OF WIND-DISPERSED DIASPORES OF NEOTROPICAL TREES

Morphological and aerodynamic traits affecting mean potential dispersal distance are quantified for wind-dispersed diaspores of tree species on Barro Colorado Island, Panama. The sample includes 34 species in 16 families and represents six aerodynamic groups. Mass and area (maximum cross section) each vary over six orders of magnitude among the species. In contrast, wing-loading, defined as weight divided by area, varies over only one order of magnitude, as does the rate of descent. While the regression of rate of descent on the square root of wing-loading is significant overall, the slopes vary significantly among five aerodynamic groups. At comparable wing-loading values, diaspores of fluffy kapok fall faster than four other aerodynamic groups and rolling autogyros fall faster than non-rolling autogyros. Assuming the diaspores are released from their typical tree height and experience a mean windspeed of 1.75 m sec^−-1, the expected mean dispersal distance varies among the 34 species from 22 to 194 m. Rate of descent is weakly correlated with shade tolerance of seedlings for a subset of 18 species; rate of descent is more strongly correlated with the log of dry mass of seed for all 34 species. Given these wide differences in dispersal potential, any generalizations about tropical trees that use wind dispersal are of dubious value.